Solar System - General Questions

Q1. What are the properties which distinguish the inner planets from the outer

planets?

A1.

Inner Planets Outer Planets

Close to Sun Far from Sun

Orbits closely spaced Orbits widely spaced

Small Large

High density Low density

Rocky in composition Gaseous composition

A2. All the planets in the solar system orbit the sun in the same direction . The orbits of the planets

all lie in approximately the same plane. The orbital motion of the planets and most of the moons and

the rotation of most of the planets all occur in the same direction. The planets are divided into 2

groups: small, dense terrestrial planets close to the sun and gas giant planets far from the sun. Most

of the mass of the solar system is contained in the sun but most of the motion is contained in the

planets.

Q2. What are the systematic properties of the solar system?

A3. The solar system is flat (all planets orbit in about the same plane) and the motion generally

follows a common direction (most planets, moons, and asteroids orbit and revolve in the same

direction). There are significant differences between the inner planets (small and dense) and the

outer planets (large with low densities).

Q3. How is the solar system organized? That is, what general properties does it have?

A4. Venus rotates backwards, but very slowly. Uranus rotates sideways. Several small moons orbit

their planets in the backward direction. Comets orbit the sun with fairly random directions and

orientations.

Q4. Describe the exceptions to the pattern of systematic motion in the solar system.

A5. The solar system consists of the sun and all objects whose motion is controlled by the sun’s

gravity. Those objects include the 9 major planets, moons at most of the planets, asteroids (or minor

planets), comets, and assorted debris that floats amongst the larger objects.

Q5. How is the term "solar system" defined? What sorts of objects exist in the solar

system?

A6. Terrestrial planets are small, dense, and close to the sun. Giant planets are large, have low

density, and are far from the sun.

Q6. Compare the properties of the terrestrial and giant planets.

A7. Density is defined as the mass of an object divided by its volume. It is a measure of how tightly

packed material is in the object. In our experience, less dense objects float in water while denser

objects sink.

Q7. Define and explain the concepts of density and temperature.

Temperature is a measure of how fast the atoms in an object are moving. The slower the atoms

move, the lower the temperature. At absolute zero, all atomic motion has stopped.

Q8. Define the term "solar system". How is it different from the universe?

A8. The solar system consists of the sun and all objects whose motion is controlled by the sun. The

sun is an ordinary star, just like all the other stars visible in the night sky. Hence, the solar system is

but a very tiny object in our galaxy of 100 billion stars, which is but one of billions of galaxies.

Q9. In general, how can we tell if a newly discovered object is a part of the solar

system?

A9. If the motion of the newly discovered object is controlled by the sun, it is a member of the

solar system. To determine if it is a member of the solar system, we must study its motion to

determine if it goes around the sun or not.

Solar System - Terrestrial Planets Questions

Q1. Compare volcanism as it occurred on Mars to that which is observed on Earth

and the Moon. How do we know that all the volcanoes on Mars are extinct?

A1. Volcanoes on Mars built mountains just as they did on Earth, as opposed to volcanism

on the Moon which erupted from many cracks and fissures to fill low lying areas without

building mountains. Volcanoes on Mars are much bigger than Earth’s volcanoes, because on

Earth a single hot spot makes many volcanoes as the crust moves past the hot spot. We know

that all of Mars volcanoes are extinct because all of them have impact craters on their flanks.

Q2. How can we determine the relative age of a planetary surface from remote

observation? How old are the surfaces of Mercury, Venus, and Mars?

A2. The relative age of a surface is determined from the density of craters in it. The greater

the number of craters per square mile, the older the surface. Mercury has a very old, cratered

surface. Venus has a very young surface with few craters. Mars is split into two regions, one

very old and one relatively young.

Q3. Why are Venus’ and Mars’ atmospheres so very different from ours?

A3. Venus has always been too hot to have liquid water, due to its proximity to the sun.

When carbon dioxide is released into its atmosphere from volcanoes, it merely builds up in

the atmosphere. On Earth carbon dioxide dissolves in the oceans and does not accumulate in

the atmosphere. The extra greenhouse effect on Venus’ atmosphere has run away to produce

a very hot environment.

Mars is a smaller planet than Earth, and its weak gravity is not able to hold its atmosphere

permanently. Now that Mars’ volcanoes are all extinct (also because it is a small planet and

has lost its internal heat), the atmosphere is slowly dissipating. Hence, it is very thin today.

Q4. Describe the four processes that shape the surfaces of solid planets. Give an

example of a planet or moon that has been significantly affected by each process and

describe the effect it has had on the planet or moon.

A4. The four processes which shape the surfaces of solid planets are: cratering (impacts not

only create the craters but they also pulverize the rocks and spread them around the new

craters), volcanism (the outflow from volcanoes covers large areas of the surface with new

rocky material), tectonism (horizontal plate motion creates new surface material at spreading

centers and recycles old surface material where plates collide), and erosion (the action of

wind and water grinds away at the surface features and generally smooths them out).

Q5. Describe the physical properties (temperature, pressure, composition) of the

atmospheres of Venus, Earth, and Mars.

A5. The atmospheres of both Venus and Mars are mostly CO2 while Earth's is N2 (78%)

and O2 (22%). Surface temperatures range from 860 oF at Venus to about 40

oF at Earth and

about 0 oF at Mars. Surface pressures on Venus are 90 times those on Earth, while the

pressure at Mars' surface is only 0.8% of Earth's surface pressure.

Q6. If we find one part of a planet heavily cratered and another part lightly cratered,

what can we conclude about the two parts of the planet?

A6. Assuming that equal numbers of meteorites land on all parts of a planet, the lightly

cratered surface must be much younger than the heavily cratered surface. Without additional

evidence we cannot say what may have formed the young surface.

Q7. For each of the terrestrial planets (plus the Moon), compare the relative size of the

core.

A7. Compared to its size, Mercury's core is the biggest in the solar system. The Moon's is

the smallest and Mars' is also relatively small. Earth's and Venus' cores are in between these

extremes in relative size.

Q8. Pick any single, large surface feature on Mercury, Venus, or Mars: Name it and

describe its characteristics and origin.

A8. Examples: Valles Marineris is a large rift canyon on Mars longer than the entire US is

wide. Olympus Mons is a huge volcano on Mars, much larger than any mountain on Earth.

The Caloris Basin is a large impact crater on Mercury which is almost 1,000 miles across.

Maxwell Montes is a large mountain range on Venus that may be volcanic or may be related

to plate motion.

Q9. How have the surfaces of each of the terrestrial planets (plus the Moon) been

affected by each of the four major surface-forming processes?

A9.

Mercury Venus Earth Moon Mars

Cratering lots very little very

little

lots lots on half of

surface

Volcanism maybe lots spotty lots in selected

regions

lots on other half

Tectonics no some, maybe

some horizontal

motion

lots no one region, but no

horizontal motion

Erosion no no lots no some, mostly from

blowing dust

Q10. What two factors determine whether or not a planet will be able to retain an

atmosphere? Explain how they compete with each other.

A10. The competing factors which determine whether or not a planet can retain an

atmosphere are its gravity and the temperature of the atmosphere. For higher temperatures,

the atoms in the atmosphere move faster and can more easily escape the bonds of gravity.

Q11. Describe volcanic activity as it has occurred on each of the terrestrial planets and

the Moon. What are the similarities and what are the differences?

A11. On the Moon and Mercury, volcanism occurred only early in the history of the planet

as general oozing from cracks in the crust. The lava released this way filled low lying basins

and did not form volcanic mountains. On Mars there was some of this general volcanism, but

there are also very large volcanic mountains. These mountains built up to huge sizes because

the crust remained stationary over hot spots. Volcanism on Earth is concentrated at the

convergent boundaries between plates where subduction returns material to the mantle and

hot spots develop. Isolated hot spots in the middle of plates do not build volcanoes to the

same degree as on Mars because of the motion of the plates over the hot spot. A few volcanic

plains exist on Earth as well. Volcanism on Venus has been global, recovering the entire

surface within the last few hundred million years. Volcanic mountains of various sizes and

types as well as volcanic plains exist across the planet.

Q12. How have the surfaces of the small terrestrial planets evolved differently from

those of the larger terrestrial planets?

A12. Small planets cooled quickly and formed a single very thick plate for the crust. This

crust was dominated by vertical motion and heat loss by conduction, and had no horizontal

motion. Larger planets cooled more slowly and their crust separated into multiple plates

which moved horizontally. Heat was lost primarily through this tectonic motion.

Q13. For each of the 4 terrestrial planets (including the Moon) that we have studied,

identify the major geologic process which has shaped its surface and describe what

effect it has had on the surface.

A13. The most significant geologic process on each of the planets is: Mercury — cratering;

Venus — volcanism; Earth — plate tectonics; and the Moon — cratering.

Q14. For each of the processes mentioned (cratering, volcanism, tectonics & erosion),

describe a planet which has been strongly affected by the process.

A14. The Moon and Mercury (and part of Mars) have been strongly affected by cratering.

Their surfaces are covered by a large number of impact sites. Volcanism has been important

on the Moon, Venus, and Mars (and possibly Mercury). The outpouring of molten material

from the interior has covered large parts of the surface of these bodies with fresh material.

The Earth has been strongly affected by tectonics. Where plates meet, mountain ranges and

trenches are formed; at spreading centers, ridges of new crustal material are seen. Erosion has

also been important on Earth and Mars, gradually wearing away the sharp surface features

from other processes.

Q15. Why is the core of Mercury so large and the core of Mars so small?

A15. A planet forms from whatever material can condense into solid particles at its position

in the solar cloud. Mercury formed close to the sun where the temperature was high. Many of

the common rocky materials could not condense as solid particles at those temperatures. Thus,

iron (which could condense there) became a more prevalent part of Mercury. Mars, on the

other hand, formed farther from the sun where it was cooler. That allowed a greater variety of

rocky material to condense to participate in its formation. Thus, iron was a smaller fraction of

all the material going into Mars.

Q16. Compare the interior structure of Mercury, Earth, and Mars.

A16. Mercury has a large (for its size) iron core. Earth has a much smaller core which is

partly solid and partly liquid. Mars core is even smaller -- the planet is not completely

differentiated. For each planet, the lighter silicate rocks float in the mantle above the core.

Q17. Describe the physical properties (composition, temperature and pressure) in the

atmospheres of the terrestrial planets.

A17. Venus atmosphere is mostly CO2, has a surface temperature of 860oF, and 90 times

Earth pressure. Mars is also mostly CO2, but has a surface temperature of only -20oF and a

surface pressure of only 0.8% Earth's. Earth's atmosphere is composed mostly of N2 and O2,

and has an average temperature of about 60oF.

Q18. Compare the role of volcanism on the surface of the terrestrial planets.

A18. Venus has had such extensive volcanism that the entire surface has been recoated

within the last few hundred million years. Mars had extensive volcanism in the past, but

predominantly in the northern hemisphere. Volcanism on Earth occurs primarily at plate

boundaries and at isolated hot spots.

Q19. What are the two possible sources of the atmospheres of terrestrial planets?

A19. New atmospheric gases are released on terrestrial planets by volcanic activity and, in

their very early history of the solar system, by numerous comet impacts.

Q20. How is the age of a rock determined?

A20. Since all radioactive material decays with a unique half-life (the length of time

required for half a sample of the material to decay), the ratio of daughter elements (the decay

product) to the original material tells you how many half-lives have past since the rock

formed.

Solar System - Giant Planets Questions

Q1. Why are the rock/ice cores of the gas giant planets so much larger than any of the

terrestrial planets?

A1. Planet formation begins as small solid particles stick together to form progressively larger and

larger particles. Farther from the sun where the temperature was cooler, more kinds of material

could exist in solid form. As a result, the outer planets had more raw material to grow from and

became much larger than the inner planets.

Q2. Describe in very general terms the surface appearance of each of the gas giant

planets.

A2. All the gas giant planets have banded atmospheres, with wind patterns parallel to their

equators. Those on Jupiter are the most obvious, but faint impressions of bands can be seen on all

the gas giants. Oval cyclonic storms also are ubiquitous to the gas giant planets. Some, such as the

Great Red Spot on Jupiter, seem very stable while others come and go rather quickly. Uranus and

Neptune have a characteristic blue color from methane clouds high in their atmospheres.

Q3. Compare and contrast the interior structure of the gas giant planets.

A3. All the gas giant planets have cores of rock and ice that are about the same size. Jupiter and

Saturn have extensive layers of metallic atomic hydrogen outside the cores (Jupiter’s is much larger

than Saturn’s). All of the gas giant planets have molecular hydrogen layers on the outside, although

the layers for Uranus and Neptune are rather small compared to those on Jupiter and Saturn.

Q4. What satellite missions have been sent to the gas giant planets?

A4. Pioneer 10 & 11 flew past Jupiter and Saturn. Voyager I and II also flew past Jupiter and Saturn.

Voyager II went on to Uranus and Neptune. Galileo is on the way to Jupiter now.

Q5. Describe the major constituents of the solar system, and place them in the broader

context of the universe at large.

A5. The solar system consists of the sun, and all objects whose motion is controlled by the sun.

That includes the nine major planets, about 50 satellites of the planets, asteroids or minor planets,

and many, many comets. The solar system is a very tiny part of the overall universe. In a scale model

where the solar system is about a mile across, the nearest star is about 2000 miles away. Our galaxy

alone contains more than 100 billion stars.

Q6. Explain why the giant planets radiate more energy than they receive from the sun.

What observation of Saturn confirms a part of the explanation?

A6. Both planets are still shrinking slowly. As matter falls inward, energy is released. On Saturn,

helium condenses and falls as rain in the upper interior. This falling motion also generates heat. The

atmosphere of Saturn is depleted of about half its helium (in comparison with Jupiter), an

observations which supports the second process.

Solar System Formation Questions

Q1. What is chemical differentiation and what effect has it had on the planets?

A1. Chemical differentiation occurs in a liquid or gas when heavy material sinks and less dense

material rises. In the early history of the terrestrial planets, when they were entirely molten, this

allow the heavy, dense materials (such as iron and nickel) to sink to the center to form dense cores

while lighter materials rose to the surface to form a less dense mantle.

Q2. Why did the terrestrial planets chemically differentiate after they formed? What

are the consequences of this event?

A2. Once the terrestrial planets had formed, they quickly melted as result of large scale impacts

and radioactive heating. When the entire planet was molten, heavier material sank toward the

center and lighter material rose to the surface. As a result, most of Earth's iron is in the core and our

surface is composed predominantly of silicate rocks.

Q3. Why does a collapsing cloud of gas become flattened? Be sure to describe

completely the reasons why this happens.

A3. If the cloud is rotating, the centrifugal force of rotation will impede the collapse of material

near the equator more than it will for material near the axis of rotation. Centrifugal force is the force

"created" because matter "wants" to move in a straight line instead of rotating. Since it always

points away from the axis of rotation, it is exactly opposite gravity for matter near the equator, but

in a different direction from gravity (and weaker also) for matter near the axis of rotation. Thus

matter near the axis will collapse faster and the cloud will become flattened at the poles.

Q4. Why are planets that formed close to the sun made of rocks and iron while those

that formed far from the sun mostly gaseous?

A4. Planets form from tiny solid particles that collide and stick together. Close to the sun, the

temperature was high and only a few materials like iron and rock could remain solid. Further from

the sun, the temperature was much lower and various ices could also condense to participate in

planet formation. Hence, the planets close to the sun are made of dense rock and iron while those

further away contain large quantities of ices, most of which have melted and turned to gases.

Q5. What role does rotation play in explaining the properties of planets that form in a

solar system?

A5. As a collapsing cloud gets smaller, it will begin to spin faster and faster. The increasing rate of

spin will cause the outward force of rotation to become stronger. Because this force is strongest at

the equator (where the material spins fastest), the outward force there will work against gravity and

slow down the collapse. Near the axis of rotation, the outward force will be less effective in slowing

the collapse. These regions will collapse faster, and the cloud will become flattened. Eventually,

material will collect in a thin disk surrounding the central concentration of matter. The central

concentration becomes the star while planets form in the disk. This explains why the orbits of the

planets all lie in about the same plane and move in the same direction.

Q6. What role does temperature play in explaining why planets close to the sun are

small and dense?

A6. Planets form as small solid particles stick together. Temperature determines what kind of

material is solid. Close to the sun, where the temperature is high, only a few materials could be solid

– mostly silicate rocks and iron. Further from the sun, where the temperature is much lower, water

could also freeze. Thus, the inner planets are made of dense and relatively scarce material while the

outer planets are both larger and less dense.

Q7. In general, how do planets form from the material that surrounds a forming

star?

A7. As described above, planets form as small solid dust particles gently bump into each other and

stick together. Repeated collisions cause these particles to gradually become larger and larger,

building up the planets once piece at a time.

Q8. The giant planets undergo a second process that allows them to become even

bigger. Describe this process and explain how it affects the nature of the planet.

A8. Because the outer planets form from the very common ices, they become larger than the

inner planets. Because they are bigger, they also have stronger gravities. This strong force of gravity

is enough to pull in individual atoms from the surrounding gas in the disk. This makes them even

bigger, and gives them an overall gaseous composition.

Q9. Why do planets melt soon after they form? What affect does this have on the

planet?

A9. New planets become quite hot as a result of both the numerous collisions with other objects

they experience and the relatively high level of radioactivity they possess. After a planet has melted,

its heavier material will sink to the center while lighter material will float to the outside.

Q10. What is the role of temperature in planet formation? How did the temperature

depend upon distance from the forming sun during planet formation?

A10. Temperature determines which materials can form solid particles in a gas. Since only solid

particles participate in planet formation (see next question), the value of the temperature at a given

location determines which material, and how much material, is available for planet formation at that

location. Close to the forming sun, the temperature was very high and only a few materials could

form solids. The temperature rapidly dropped with distance from the sun until at the distance of

Jupiter the hydrides such as water could freeze.

Q11. What role did gas play in the formation of giant planets?

A11. The giant planets had much more raw material to work with because the hydrides are much

more plentiful than are the silicates and metals. They were able to grow large enough for their

gravity to be strong enough to begin to attract and hold individual atoms of gas from the

surrounding cloud. This allowed them to become much larger, since there is so much more gas

present than solids. This process was much more effective for Jupiter and Saturn, where the overall

density of material was higher, and less important for Uranus and Neptune, which are further from

the sun.

Q12. Why are all the planets differentiated? How did differentiation occur during the

formation of the solar system; that is, what caused it?

A12. As the planets formed, a great deal of heat built up inside the planets. This heat came both

from the violent collisions which were forming the planets and from the radioactive material which

was incorporated into the planets. This internal heat was enough to melt the planet. In the liquid

interior of the planet, heavy material (metals) sank to the center while lighter material (rocks)

floated to the surface. This resulted in a differentiated planet.

Q13. What event stops planet formation?

A13. When the sun has completed its formation, a wind of matter is blown away from the sun.

This wind of matter strips away any remaining gas and dust from which planets might form. Without

additional raw material to work with, planet formation is stopped.

Q14. Why is the biggest planet in the middle of the solar system?

A14. The largest planet in the solar system had to form far enough from the sun for ices to be in

solid form so that its solid core could grow to large size and close enough to the sun for the density

of gas to be high so that a lot of gas could be attracted by the gravity of the large core.

Q15. Why are the gas giant planets so much larger than the terrestrial planets? Why

are Uranus and Neptune not as large as Jupiter and Saturn?

A15. The temperature in the gas cloud which surrounded the sun as it formed became

progressively cooler at greater distances from the sun. Beyond the so-called frost line water could be

solid, thereby dramatically increasing the amount of material in solid form. The gas giant planets

formed in this region, while the terrestrial planets formed closer to the sun where only the rarer

rocky materials could participate in the formation of the planets. Still further from the sun the

overall density was much less, so the growth of the planets was slower. By the time Uranus and

Neptune became big enough for their gravity to attract gas atoms, very little time was left before the

forming sun blew the remaining gas away. Hence, they did not grow as large as Jupiter or Saturn.

Q16. Why did the planets chemically differentiate soon after their formation? What is

the consequence of this event for planets today?

A16. After their formation, the planets were hot enough to completely melt. During this molten

phase, the heavier materials sank to the center while the lighter, less dense materials floated to the

surface. As a result the planets today have dense cores, usually rich in iron, and lighter mantles.

Q17. Explain why temperature is so crucial to the formation of solids from a gas. How

does this effect explain the difference in composition between Mercury and Earth?

A17. Any material changes from a liquid to a solid at some exactly specified temperature. For

example, water freezes at exactly 32 oF. Planet formation begins as small solid particles stick

together and gradually become larger and larger. Since Mercury formed closer to the sun than did

Earth, where the temperature was higher, only iron and few of the rocky materials were solid there.

At Earth’s distance from the sun, many more rocky compounds had solidified.

Q18. Describe the processes which occur to create planetesimals (small planets) in the

disk surrounding a forming star.

A18. Planet formation begins when microscopic particles condense from the gas as the

temperature cools. These small particles collide and stick together, gradually growing through a

process known as accretion. Still later, still larger objects collide and aggregate as the planets begin

to grow.

Q19. Why is Jupiter the biggest of the planets (Hint: There are two reasons)?

A19. Jupiter was able to grow larger because it is the first planet (in distance from the sun) to form

where water ice could condense into small particles. since there is much more ice than the metallic

and rocky materials in the solar cloud, that provided much more material to make Jupiter than the

inner planets had available. Even further from the sun, the overall density of the cloud was smaller,

and thus the planets are smaller also.

Once the protoplanet Jupiter became large enough, its gravity began to attract gas atoms to the

planet, making it much bigger still. This second process of growth resulted in the final giant planet

we see today. Again, the drop in overall cloud density prevented more distant planets from growing

equally large.

Q20. Which is the first common material to form solid grains in a cooling cloud of gas?

Why is the formation of ice so much more important?

A20. Iron is the common material with the highest freezing temperature. It condenses first in a

cooling cloud. Because oxygen is a thousand times more abundant than iron or any other heavy

material, when it reaches its freezing temperature, a great deal more material can freeze than for

any other substance.

Q21. How do dust grains grow to form small planets?

A21. Small planets form as tiny microscopic dust grains stick together when they gently bump into

each other as they orbit the still forming sun. These particles gradually grow through more and more

collisions. This process continues on an ever increasing scale until a small planet has formed.

Q22. In what two ways does the existence of planets like Earth depend upon the

occurrence of supernovae explosions?

A22. The incredible temperatures that occur during a supernova explosion allow nuclear reactions

that consume energy to occur. These nuclear reactions produce heavy elements that would never be

produced in normal stars. The explosion distributes these products into space, where they are much

later incorporated into new generations of stars and planets.

The rapidly expanding cloud of debris from the explosion can also stimulate star and planet

formation if it passes through a nearby giant molecular cloud. The interaction causes pieces of the

giant molecular cloud to be compressed, which initiates the collapse that leads to the formation of a

star, along with its attendant planets.

Space Exploration Questions

Q1. What are some of the reasons given for spending huge sums of money to explore

the solar system?

A1. Among some of the reasons offered to justify the space program are: (1) to generate or

maintain national prestige, for example, we can do something you cannot do, (2) to satisfy the

challenge of exploration, as also motivates the exploration of remote locations on Earth, (3) to gain

scientific knowledge of other places in th solar system, and (4) to develop applications and products

which are useful for everyday life and ordinary people.

Q2. If a new, nearby planet were discovered, describe the overall strategy that should

be employed to explore it.

A2. First, we should send a satellite to fly by the planet to study its general properties. Such a

mission gives us enough general information to determine what aspects require more detailed

investigation during the next phase of exploration. Such explorations are generally undertaken by

orbiting spacecraft, short term landers, and atmospheric probes. Next comes intensive study with

long term landers, often equipped with some kind of roving vehicle for studying other locations.

Finally will come the utilization of the resources or properties of the planet for other purposes.

Q3. Describe briefly one of the major space missions to explore the planets (name of

mission, place visited, type of mission, etc.).

A3. Many missions could be mentioned. Three examples are the Viking mission to Mars that sent 2

orbiters and 2 landers to map the surface, study the atmosphere, and analyze the surface; the

Voyager mission that sent 2 fly-by spacecraft to visit Jupiter, Saturn, Uranus, and Neptune; and the

Magellan mission that sent an orbiter to Venus to map its surface.

Q4. Why was the Apollo program to the Moon undertaken? From a scientific

perspective, what was wrong with it?

A4. The primary motivation for the Apollo program was to maintain national prestige -- to beat the

Russians to the Moon. But the program probably sent humans to the Moon before there was

adequate scientific justification, and we stopped the scientific exploration of the Moon too soon to

gain full benefit from the huge effort and expense of the Apollo program.

Q5. In general terms describe the state of exploration of the solar system.

A5. We have performed fly-by reconnaissance missions of all planets but Pluto. More detailed

investigations have occurred at Venus, the Moon, and Mars. Only the Moon has been visited by

humans.

Q6. Describe in general terms the spacecraft exploration of the Moon.

A6. Many spacecraft have been sent to the Moon, with many failure especially in the early days.

These missions have included fly-bys, hard and soft landers, and ultimately humans. Once the Apollo

mission ended, no spacecraft visited the Moon for nearly 20 years, until rather recently we began to

study the Moon again.

Q7. Spacecraft study of a planet can be done with either a fly-by, orbiter, or lander

type mission. What are the relative advantages and disadvantages of each type of

mission?

A7. Fly-by spacecraft are simple and cheap, but provide very limited opportunity to observe their

target. Orbiting spacecraft provide long-term observations of their target, but still are capable only

of remote observations. Spacecraft which land on their target can take much more complete

observations of their target, but can observe only a limited region of the target.

Q8. In what way have satellites fundamentally changed the study of the solar system,

compared to other branches of astronomy? What are some of the reasons, both

scientific and non-scientific, for exploring the solar system?

A8. Astronomy has traditionally been an observational science. We have been required to study

astronomical bodies from a great distance, unable to probe and test them directly. Satellite

exploration of the solar system has now changed that! We have visited all planets but one, along

with several other smaller bodies in the solar system, and have returned samples of the Moon to

Earth-based laboratories for further study. The original reasons for sending satellites to explore

other bodies in the solar system had more to do with maintaining national pride than with scientific

discovery. The development of technology for other uses, and the potential for exploitation of

resources on other planets, have also sometimes motivated our exploration.

Q9. If you were designing the ideal program to explore a planet by spacecraft, what

types of missions would you suggest for a logical development of our knowledge?

A9. The first mission to explore a planet should be a brief reconnaissance by a fly-by spacecraft to

determine the general properties of the planet so that further studies are properly directed. This

mission can be followed by an orbiter, which conducts a long term, more detailed study of the

planet. Then may come a lander or atmospheric probe to directly sample and measure the material

of the planet. Rovers or long term exploration of the planet may follow, with utilization of the

planet’s resources in the distant future.

Q10. Describe generally the 4 logical steps for exploring a planet. How far have we

gotten through this process for any bodies in the solar system?

A10. 1. Fly-by the planet to see what it's like.

2. Orbit the planet for long term observations

3. Explore (e.g., with a roving vehicle on the surface) to gather detailed information

4. Utilize the planet for human gain

All planets but Pluto have been visited at least once, but we have not reached step 4 for any planets

and have done only a little of step 3 at the Moon.

Q11. Discuss the role that politics plays in decisions regarding the American space

program. Give an example of how politics affected the Apollo program to the Moon.

A11. Decisions about which space missions to pursue are often made more on the basis of politics

than on scientific merit (e.g., "no Buck Rogers, no bucks"). Funding for each year of the development

of a spacecraft must be approved by Congress, and a mission can be canceled at any stage by that

process if the political system loses interest in the project. For example, the Apollo mission to the

Moon was initiated so we could "beat the Russians" to the Moon. Once we got there (before they

did), we quickly lost interest in the project and later scientific missions were canceled.

Venus Questions

Q1. Why can the surface of Venus not be photographed from space? How is the surface mapped?

A1. The atmosphere of Venus is 100% cloudy – no images of the surface can be obtained from space.

However, radio waves do penetrate through the atmosphere, so radar techniques can be used to

map the surface. A radar unit sends out radio waves and receives the reflected beam. By timing how

long it took the waves to return, a map of surface features can be made.

Q2. How do we know that the entire surface of Venus is very young geologically? What is the

cause?

A2. We know the surface of Venus is very young (geologically) because there are very few craters on

the surface. The older a surface is the more craters it will have. Venus’ surface has been recently

covered by volcanic outflows, which have covered the entire surface.

Q3. Why are small craters absent from the surface of Venus? How are large craters on Venus

different from similar sized craters on other planets?

A3. Small meteors will burn up entirely in Venus’ dense atmosphere. Thus, small craters will not

form on Venus. All large craters on Venus have been flooded by lava, which indicates that molten

material is available just under the surface.

Q4. What single process is responsible for the formation of Venus’ surface as we see it today?

How do we know that its surface is relatively young?

A4. Virtually all of the surface of Venus has been covered recently by volcanic outflows. We know

the surface is relatively young because very few craters are seen on the surface.

Q5. Of the four processes which can shape the surface of a planet, three have had a negligible role

on Venus. Describe the observations which support that statement.

A5. Craters are very rare on the surface, as shown in radar maps. There is no evidence of folding

where plates collide or ridges where plates separate, so plate tectonics has not had a major effect on

the surface. Photos from the surface show sharp cornered rocks which show little erosion, which

suggests that erosion has not played a major role in shaping the surface.

Q6. Explain how radar is used to map the surface of Venus. Why must we resort to that technique

to "see" the surface of Venus?

A6. Radar operates by sending out a beam of radio waves and recording the return beam which

bounces off an object. By measuring the length of time required for the reflection to return and the

wavelength of the returning waves, we can determine which point on the surface of the planet a

given reflection came from. Since Venus’ surface is always cloud covered, ordinary photographs do

not reveal its surface. Fortunately, radio waves are not affected by clouds.

Earth Questions

Q1. What observations give us information about the structure and material of the

interior of Earth?

A1. We learn about the interior of Earth by studying the arrival times of earthquake waves at

many different places around the surface of Earth. When, and even whether, the waves arrive

at a particular location tell us how the waves traveled through the interior of Earth. For

example, if only P (or pressure) waves are observed at one place (meaning the S or shear

waves were absorbed somewhere in between the earthquake and the station), that tells us that

liquid material must exist on the path the waves followed.

Q2. Describe the structure and material of the interior of Earth.

A2. Earth contains a core of dense material, probably iron, that occupies about half the

radius of the planet. This core is solid on the inside and liquid on the outside because of the

increasing pressure at the center of Earth. A less dense mantle of silicate rocks surrounds the

core.

Q3. What is the difference between an S and a P wave? What does their arrival (or lack

thereof) tell us about the interior of Earth?

A3. Pressure or P waves are waves that oscillate in the direction the wave is traveling in,

while shear or S waves oscillate perpendicular to the direction of travel. S waves cannot

travel through a liquid. Since there is always a shadow zone in which S waves are not

received from earthquakes, this observation suggests that there is a liquid portion to the core

of Earth. The size of the shadow zone tells us how large this liquid core is.

Q4. When referring to the average density of Earth, what does the concept of

uncompressed density refer to? What does its value tell us about Earth?

A4. The uncompressed density of a planet is the density it would have if its interior were not

compressed by the force of gravity. It is, therefore, a better measure of the properties of the

material from which the object is made than the observed density of the planet. The

uncompressed density of Earth is about 4.5 times the density of liquid water, significantly

higher than the average density of rocks found at the surface of Earth. This result tells us that

Earth must contain significant amounts of materials that are denser than rocks.

Q5. How can we tell that the outer core of the Earth is liquid? Be sure to explain any

terminology or concepts you use in your answer. Why do we believe that the inner core

is solid?

A5. S-type earthquake waves (waves whose vibrations are perpendicular to the direction of

motion of the wave) can not penetrate through a liquid medium. This type of earthquake

wave is not received by stations on the far side of Earth — which indicates that a liquid

region is located somewhere between the station and the site of the earthquake. By mapping

precisely those regions that do not receive S-type waves, geologists can pinpoint the size of

the liquid core of Earth.

While we do not have direct observations to prove that the center of Earth’s core is solid, our

models and computations indicate that it is. As one approaches the center of the Earth, both

the temperature and the pressure increases. With increasing pressure, the minimum

temperature required to melt the iron core also increases — at a faster rate than does the

actual temperature. At some distance from the center, the actual temperature becomes less

than the minimum temperature to melt iron and the core freezes into solid form.

Q6. Why do we suspect that the very center of Earth is solid even though the outer

layers of the core are liquid?

A6. Deeper layers of Earth are subjected ever greater temperature and pressure. We suspect

the inner core is solid because of the greater pressures that exist there. The properties of iron

indicate that iron subjected to such conditions of high pressure and high temperature returns

to the solid state in spite of the high temperature.

Q7. How can earthquake waves tell us whether a planet has a molten core?

A7. Of the two types of earthquake waves, only P (pressure) waves can pass through a liquid.

The other type, S or shear waves, cannot pass through liquids. Hence if only one type can

penetrate through the centre of a planet we know that there must be liquid somewhere inside

the planet.

Q8. Describe the contents and properties of the interior of Earth. Mention such

properties as size, mass, and density of different regions, what they are made of, and

whether they are solid or liquid.

A8. Earth’s interior consists of a dense iron core and a less dense silicate mantle. The radius

of the core is about half the radius of Earth, but accounts for only 17% of its volume and 33 %

of its mass. The core consists of two sections – a solid inner core and a liquid outer core –

both composed mostly of iron.

Q9. Explain in general why convection occurs? Describe the process of convection in

the context of Earth’s mantle.

A9. Convection occurs when heat is added to a fluid faster than it can move through the fluid.

Eventually a hot bubble of fluid begins to rise through its cooler surroundings and deposits its

heat at the top of the fluid. In Earth’s mantle, heat from the core is added to the base of the

mantle. Hot bubbles of rock rise to the top of the mantle, spread out horizontally, and then

fall back toward the center of Earth.

Q10. How do earthquakes help us learn about the interior of Earth? What is the basic

structure of Earth’s interior?

A10. Earthquakes cause vibrations in the Earth which can pass through the interior and be

detected on the opposite side of Earth. By studying exactly when different kinds of waves

arrive at different locations on Earth, we can learn about the material they passed through on

their way. For example, if there is a significant delay in the arrival of waves at one station

compared to a nearby station, it tells us that some material in the path of those waves slowed

down their passage but did not exist along the path the other waves took. The analysis of

many earthquakes allows us to gradually piece together a three-dimensional picture of Earth’s

interior. It consists of a two part iron core that is solid on the inside and liquid on the outside,

a large mantle of less dense rocky material surrounding the core, and a very thin crust on the

top.

Q11. What are the sources of energy which keep the centre of Earth warm?

A11. The interior of Earth is warm because of the energy trapped there during the formation

of the planet and because of the release of energy by radioactive materials which are

concentrated there.

Q12. Describe the structure of the core and mantle of Earth.

A12. The core of Earth is composed of nearly pure iron. It occupies about half the radius of

Earth, but only 17% of the volume. Because it is very dense, it accounts for about one third of

the mass of Earth. The inner portion of the core is solid while the outer portion of the core is

molten. The less dense, rocky mantle which surrounds the core accounts for most of the rest

of the material of Earth.

Q13. Why is it important to have a network of seismic stations to study the structure of

Earth’s interior?

A13. When an earthquake occurs, its waves travel through Earth in all directions. These

waves can be recorded at a network of seismic stations around Earth. The arrival times and

characteristics of the waves tell us how fast the waves traveled through different parts of the

interior. Without a network of stations we would not be able to deduce where the waves

originated or what path they followed through the interior of Earth.

Q14. The observed density of Earth is 5.5, while the uncompressed density is only 4.5.

What is an uncompressed density? Why is it less than the observed density?

A14. For any large object, gravity squeezes the material of the object to a smaller size than

the material would naturally have. This compression increases the observed density of the

object. If you have a good model of the interior of the object, it is possible to predict what the

density of the object would be without the natural compression due to gravity. This predicted

density is the uncompressed density. It is smaller than the observed density because the

uncompressed object (without the effects of gravity)is larger than the observed object.

Q15. What are the two types of seismic waves that travel through the Earth? How can

we use them to discover that the interior of Earth contains a liquid region?

A15. All waves, including seismic waves, are one of two types: transverse waves which

oscillate in a direction perpendicular to the direction of the wave motion and longitudinal

waves which oscillate in the same direction as the wave motion. For seismic waves,

transverse waves are referred to as S waves while longitudinal waves are called P waves. S

waves are not able to move through the body of a liquid. Since S waves are not observed to

penetrate the central core of Earth even though P waves do, we can conclude some part of the

core is liquid.

Q16. Describe the interior structure of the Earth. Compare this structure to that of

Mercury and the Moon.

A16. The interior of Earth consists of a solid inner core and a liquid outer core, both mostly iron,

surrounded by a mantle of silicate rocks. Mercury's core is a much larger fraction of the total planet

than is Earth's, but the Moon has little if any differentiated core. The cores of both Mercury and the

Moon are probably solid because the small bodies have lost their heat content.

Earth - Surface Questions

Q1. How is crust created and destroyed on Earth?

A1. New crust is created on Earth where two plates of old crust separate. Molten material

from the interior of Earth wells up to fill the vacancy left by the separating plates, cools, and

becomes new crust. When two plates of crust collide, one is pushed down into the interior of

Earth while the other is crumpled as it rides up on top of the other plate.

Q2. How do we believe the Hawaiian Islands were produced? In what way do they

provide evidence in support of the concept of plate tectonics?

A2. The Hawaiian Islands, and the undersea mountains to their west, represent a long chain

of volcanic mountains that we believe have been created by a single hot spot in the mantle of

Earth. Different mountains are produced as the crust of Earth moves over this stationary hot

spot. This idea is supported by the observation that each island in the chain is older than the

preceding one as you move from the eastern end of the chain toward the west.

Q3. Describe and explain the variety of geologic processes which occur when two plates

collide.

A3. When two plates collide, one is usually driven downward under the other, its material to

be re-absorbed into the mantle. The other plate is crumpled at the point of collision, forming

folded ranges of mountains along the boundary. Cracks and weak spots in this folded plate

allow hot material from inside Earth to rise to the surface in volcanoes. Motion inside Earth

as the plates slip past each other cause earthquakes.

Q4. How are mountain ranges formed on Earth? Why are they often associated with

earthquake zones?

A4. Mountain ranges are formed on Earth when two plates collide. One plate is subducted

while the other rides on top and is buckled to form mountains. As the two plates slide against

each other, stress and pressure are occasionally released by jerky motion of the plates -- what

we experience as an earthquake.

Q5. What are the two different (although related) processes that create mountains on

Earth? How do they build mountains? Long after the mountain is formed and its shape

altered by other processes, how could we tell which process was responsible for its

creation?

A5. Mountains can be created either by volcanic processes or by the collision of two tectonic

plates. When a volcano erupts, molten rock form the interior flows onto the surface. It may

build up in a pile forming a mountain. When plates collide, one side rides on top of the other.

The buckling of this plate creates tall ridges that become mountain ranges. We can tell the

difference between these types of mountains from the type of rock present. The cooled lava is

a distinctive type of rock, easily recognized long after the end of the volcano.

Q6. For each of the four processes that can shape the surface of a planet, describe how

it has affected the current appearance of Earth.

A6. Cratering has had only a minimal effect on Earth. Volcanism has covered only relatively

localized regions of Earth, often in association with tectonic activity. Plate tectonics through

both the creation of new crust at spreading centers and the destruction of crust during plate

collisions has had a profound effect on Earth’s surface. Erosion by air and water has also had

a profound effect on changing Earth’s surface appearance.

Q7. Why are the continental plates higher than those which make up the ocean floor?

A7. The continental plates are both thicker and less dense than the plates which make up

ocean floors. Since all the plates are floating on the top of the mantle, the less dense ones

float higher just like a light piece of wood floats higher in the water compared to a denser

object.

Q8. Why is there horizontal plate motion on the surface of Earth?

A8. Horizontal plate motion occurs because of the convective currents in the Earth’s mantle.

When hot rising mantle material reaches the top of the mantle, it spreads out and moves

horizontally before sinking back into the deep mantle. Pieces of the crust are carried along on

this horizontal motion of the mantle.

Q9. What is the evidence for horizontal plate motion on the surface of Earth?

A9. There are many lines of evidence which support the concept of horizontal plate motion.

Among them are the sequential ages of the islands in the Hawaiian island chain (and several

similar chains in the Pacific Ocean) as a result of a plate moving over a single hot spot

volcano and the similarities of rock strata and fossils between the tips of south America,

Africa, and Antarctica from the time that those regions were joined together in Pangaea.

Q10. How is Earth's crust being recycled? Discuss both the destruction and the

creation of the crust.

A10. Because of plate motion of the crust, crustal material is being subducted back down to

the mantle where plates collide and one plate is pushed down under the other. Where plates

are separating, mantle material rises to the surface to create new crust.

Earth - Atmosphere Questions

Q1. What are the primary chemical constituents of Earth’s atmosphere? What is

unusual about its composition, compared to other planets? Why is this unusual material

present?

A1. Earth’s atmosphere is made mostly of nitrogen and oxygen gases. The unusual part of

this is the presence of free oxygen in the atmosphere. Free oxygen is very reactive, and

should have disappeared long ago. Its presence suggests that some mechanism is

continuously creating more oxygen for the atmosphere. Of course, that process is life.

Photosynthesis by plants releases oxygen into the atmosphere.

Q2. Describe the thermal structure (changes in temperature with altitude) of Earth’s

atmosphere. What unusual feature does our atmosphere have in this regard, compared

to other planetary atmospheres?

A2. As you move upward from the surface of Earth, the temperature cools for several

kilometers before it reverse and begins to increase (in the stratosphere). This increase

ultimately reverses into another decline in temperature, which in turn changes into an

increase in temperature at high altitude. The net effect is that the temperature oscillates back

and forth over a fairly narrow range of values.

Q3. What are the astronomical causes of climate changes, such as the periodic

occurrence of ice ages?

A3. Periodic climate changes are caused by an effect known as the Milankovich cycle. The

combined effect of small changes in the eccentricity of Earth’s orbit around the sun, the

precession or wobbling of Earth’s axis of rotation, and small changes in the tilt of Earth’s

axis of rotation can cause the subtle changes in climate that lead to an ice age.

Q4. Earth’s atmosphere is composed mostly of nitrogen and oxygen. Why is there so

little carbon dioxide in our atmosphere, compared to other planets? Why is there so

much oxygen?

A4. Even though carbon dioxide is the most abundant gas released in volcanic processes, it is

not the most abundant gas in our atmosphere because it slowly dissolves in the liquid water

on Earth and ultimately forms carbonate rocks. The existence of free oxygen in our

atmosphere is purely a result of biological activity, specifically photosynthesis.

Q5. What happened to Earth’s original primary atmosphere? Where did the present

secondary atmosphere come from? What happened to all the carbon dioxide that

should be in our atmosphere?

A5. The original atmosphere of Earth was lost when the solar wind of the early sun stripped

away all the gas from around Earth. A secondary atmosphere was slowly released by

volcanoes as gases were released from the molten rock. Most of this gas is carbon dioxide,

but it is gradually dissolved in the oceans and precipitates out as carbonate rock instead of

building up in the atmosphere.

Q6. What three factors may cause Earth’s climate to go into an ice age?

A6. The 3 factors that must combine to send Earth’s climate into an ice age are: (1)

precession, or a change in the direction Earth’s axis points in space; (2) nutation, or a slight

wobble in the inclination of Earth’s axis of rotation with respect to the plane of its orbit; and

(3) a slight change in the eccentricity or shape of its orbit around the sun.

Q7. How did the earth's atmosphere become rich in molecular oxygen (O2)?

A7. Oxygen in Earth’s atmosphere is the result of biological activity on Earth. Plants release

oxygen as a by-product of photosynthesis. Since oxygen so readily reacts with many different

materials, it would quickly disappear if it were not constantly being replenished.

Q8. Describe the changes in the temperature in Earth’s atmosphere with increasing

altitude.

A8. As you rise from the surface of Earth the temperature decreases until it reaches a local

minimum at about 6 miles (10 km). The temperature then begins to increase up to about 35

miles (60 km), where it begins to decrease again. Finally at about 60 miles (100 km) it begins

to increase again for the rest of the way up until it finally merges with the interplanetary

medium.

Q9. Earth’s atmosphere is described as consisting of 4 layers. What are these 4 layers?

Describe a distinguishing characteristic for each layer.

A9. From the surface upward, the layers in Earth’s atmosphere are: troposphere

(distinguished by the water cycle, which controls our weather); stratosphere (distinguished by

the ozone layer); mesosphere (the top of the greenhouse where CO2 radiates energy to space);

and thermosphere (so thin it is easily heated by sunlight).

Q10. How does the thermal structure of Earth’s atmosphere compare to that of Venus’

atmosphere?

A10. Venus’ surface is much hotter than Earth’s. The atmospheres of both planets cool with

increasing altitude above the surface. Between sea level and about 75 km altitude, Earth’s

atmosphere cools and heats in several layers. In the same span, Venus’ atmosphere becomes

progressively cooler. At about 75 km they have both reached the same temperature.

Q11. What event do astronomers believe caused the extinction of the dinosaurs? What

evidence supports this idea?

A11. Astronomers believe that dust and steam raised into the atmosphere by the impact of a

large asteroid cooled the climate enough to cause the extinction of the dinosaurs and most

other species 65 million years ago. Several lines of evidence point in this direction. The most

persuasive is that the layer of rock deposited during that period, wherever it is found on Earth,

is rich in the element iridium. Iridium is extremely rare on Earth, but is much more abundant

in iron-rich meteorites.

Q12. What controls the temperature of Earth’s atmosphere? Describe both the physical

principle and the sources of energy.

A12. The temperature of any object is determined by the balance of input energy and emitted

energy. The temperature of the object rises or falls until the object is able to radiate as much

energy as it receives. The sources of energy for our atmosphere are sunlight and heat leaving

the surface of Earth from the interior. Each layer of the atmosphere heats up until it can

radiate as much energy as it receives form the sun and the layers below.

Q13. What are the major constituents of Earth’s atmosphere? In what way are they

unusual for an atmosphere?

A13. Our atmosphere is composed primarily of nitrogen (78%) and oxygen (22%). While

nitrogen is a common gas for an atmosphere, free oxygen molecules are quite unusual.

Oxygen is a very reactive substance and normally is quickly consumed in reactions with other

substances in the environment. Its presence in our atmosphere is the result of continual

production of more oxygen by plant life on Earth. Without that production mechanism, our

atmosphere would be quickly cleaned of all its free oxygen molecules.

Moon Questions

Q1. In considering the origin of the Moon, why is unlikely that Earth captured the Moon as it

passed us?

A1. The Moon is so large relative to the Earth that it would be very difficult for Earth's gravity to

capture it as it moves past us. This is analogous to a human being trying to catch a bowling ball.

Q2. How does the impact theory for the origin of the Moon explain general properties of the

Moon?

A2. If a large object impacted on Earth and splattered debris into Earth orbit to form the Moon,

most of the impactor's core would fall to the center of Earth, leaving the Moon with a very small

core. Most of the volatile metals (like gold and silver) would be vaporized, leaving the Moon

deficient in those elements.

Q3. How does the co-accretion (or double planet) theory suggest the Moon formed? What are the

major deficiencies of this theory for the origin of the Moon?

A3. The co-accretion theory suggests the Moon formed as a separate object in close association

with the forming Earth. That is, they formed at the same time and at the same distance from the sun.

This theory fails to explain why the Moon’s core is so much smaller than Earth’s. It also fails to

explain the more subtle differences in the chemical composition at the surface, for example, the

deficiency on the Moon of volatile metals like gold and lead.

Q4. Describe how the Moon formed according to the currently accepted theory. How does it

explain the properties of the Moon?

A4. The currently accepted theory for the formation of the Moon suggests that a Mars-sized body

collided with Earth. Some of the material of the colliding body and the surface of Earth were blasted

into orbit around Earth, and later collected together to form the Moon. Most of the core material of

the impactor would fall to the center of Earth, producing a very small core for the Moon. Most of the

volatile metals would be evaporated by the impact and lost.

Q5. Explain the concept of resonance

A5. Resonance occurs when a small force is applied at the same point in a cycle of motion. For

example, a child kicks in the air at the back of each swing on a playground swing. This small force

gradually increases the amplitude of the swing to a large effect.

Q6. Why do we believe that the core of the Moon is very small?

A6. The average density of the Moon is only barely larger than the average density of a typical rock.

Since material in a core should be significantly denser than rock, the core of the Moon cannot be

very large or the overall density would be larger.

Moon Surface Questions

Q1. How do we know that most of the Moon’s craters formed very early in its history?

A1. By comparing the density of craters (for example, number of craters per million square miles) on

different parts of the Moon’s surface with the ages of the rocks found in those regions, we can

determine that most of the craters formed very early in the history of the Moon.

Q2. What formed the dark regions on the face of the Moon? Other than being darker, how are

they different in appearance from the lighter regions?

A2. The dark regions on the Moon are lava flows from the early history of the Moon. They have far

fewer craters than other regions on the Moon, because the lava covered the earlier craters.

Q3. Describe the events which occur during the formation of a medium sized crater.

A3. The energy of motion of the impactor is released on contact in an explosion which destroys the

impactor and excavates a crater about 10 times bigger than the original object. The material

expelled from the crater is hurled upward, and then falls back to the surface to form a raised ejecta

blanket surrounding the new crater. Bedrock under and around the crater is shattered, and the rock

closest to the crater may even be melted by the energy released in the explosion.

Q4. How can we determine the relative age of different parts of the Moon from Earth-based

observations alone?

A4. The older a region is, the greater the number of craters it will have. Relative ages can be

determined by just counting the number of craters in a given area.

Q5. Why are all the maria on the side of the Moon that faces Earth?

A5. Maria formed as lava oozed to the surface through cracks formed in the crust of the Moon by

giant impacts. Because of Earth’s gravitational attraction, the crust is thinner on the side of the

Moon which faces Earth. With a thinner crust it was more likely that cracks produced during an

impact would reach down to the mantle where molten material was present.

Q6. Describe the processes which occur during the formation of a crater. Why do some craters

have flat-bottomed floors?

A6. When an object strikes the surface of a planet, it is moving so rapidly that the force of the

collision causes it to explode. The energy of this explosion blows a great deal of material up into the

sky — both the impactor and some of the surface material. Shock waves from the explosion also

shatter the rock under and around the new crater, and push the surrounding material into a raised

rim around the crater. The material blasted out of the crater falls back to the surface to form an

ejecta blanket around the crater. In some cases enough of this material falls back into the crater to

give it a flat bottom as it partially fills in the new crater.

Q7. Describe the properties of the material found on the surface of the Moon. What role have

meteorites played in determining these properties?

A7. The surface material on the Moon is composed of ordinary rock broken into a fine powder or

dust. The rocks on the surface of the Moon were ground down by the countless impacts of tiny

meteorites.

Q8. How is the age of a rock determined? Explain the reasoning behind your procedure.

A8. The age of a rock can be determined by measuring the amount of a radioactive substance in the

rock compared to the amount of its decay product in the rock. This ratio tells us how many half lives

(the length of time required for half of the radioactive substance to decay) have occurred since the

rock formed. Laboratory measurements of the half life then allow us to find the age of the rock in

years.

Q9. How is a central mountain peak formed in a crater?

A9. When a large meteorite impacts another body, the energy of motion of the impactor causes an

explosion which excavates a crater far larger than the original body. When the extra weight of the

material from the crater is removed, the remaining crust rebounds by springing upward. If this

rebound is frozen in place as the crust re-cools, the central peak will remain.

Q10. How do we know the maria are much younger than the highlands on the Moon?

A10. The relative age of any solid planetary surface can be determined from the relative density of

craters on the surface. The highlands have many more craters per unit area than do the maria. That

means that the surface is much older, which has allowed the number of craters to build up over a

longer time interval.

Q11. How can a resonance affect the motion of an object? Describe a resonance in the solar

system, and describe how the motion has been altered.

A11. A resonance occurs when a small force acted repeated at the same point in the cycle of motion

of an object. Even though the force is very small, its effect accumulates over time to produce a

noticeable change in the motion of the object. The force of Earth acting on the "heavy" side of the

Moon has slowly pulled that side to always face Earth as the Moon orbits Earth. The action of the

sun on Mercury has produced a similar result there, except that Mercury rotates three times for

every two orbits around the sun.

Q12. Why is the front side of the Moon different in appearance from the back side?

A12. When the Moon was forming, its interior was pulled slightly off center because of the strong

pull of gravity of Earth. As a result, the crust on the side which faces Earth is thinner than the crust

on the back side. That made it easier for giant impacts on our side of the Moon to crack the early

crust so that maria could be formed. There also appear to be more giant impacts on our side of the

Moon, perhaps as a result of the gravitational focusing of incoming material by Earth.

Q13. Describe the sequence of events when a large meteor collides with the Moon to create a

crater.

A13. Because of the violence of the impact, the meteor explodes on impact. This explosion blasts

out a crater about 10 times larger than the original meteor. This explosion also pushes surrounding

surface layers upward, producing a raised rim around the new crater. Powerful shockwaves travel

downward into the crust of the Moon, shattering the rocks under the impact. The heat generated by

the impact may melt the rocks left in the crater. Material blasted out of the crater falls back to the

surface to form an ejecta blanket around the new crater.

Q14. Why is the surface of the Moon covered by a thick layer of powdery dust?

A14. ince the Moon has no atmosphere, even the tiniest meteor strikes the surface at high speed.

These tiny specks of rock create tiny craters in the surface material of the Moon and chip off tiny

flakes of material. The accumulation of these flakes over billions of years is the regolith or dust

found on the surface of the Moon.

Mercury Questions

Q1. What spacecraft observations do we have of Mercury? What limitations were present for

those observations?

A1. The only spacecraft that has visited Mercury is Mariner 10, which made three fly-by passes of

the planet in the mid-1970's. Unfortunately, each fly-by observed the same side of the planet. In

addition, these observations are limited by the relatively crude technology available at that time.

Q2. Compare and contrast the interior structure of the Moon and Mercury.

A2. While both Mercury and the Moon are made of rocky surfaces and iron cores, the proportions

of the two parts are entirely different. Mercury has a very large iron core, while the Moon has a very

small one.

Q3. Why does Mercury not have an atmosphere when Titan, a similar sized body which orbits

Saturn, does have one?

A3. The presence of an atmosphere depends on the balance between gravity (which holds the

atmosphere) and temperature (rapid motion of atoms allows them to escape). Mercury is close to

the sun, so the temperature is quite high. Titan is far from the sun and has a very cold atmosphere.

This Titan is able to hold onto its atmosphere while Mercury cannot.

Q4. Why is Mercury both one of the hottest and one of the coldest planets in the solar system?

A4. Because Mercury does not have an atmosphere, the heat gained from the Sun during the

daytime is quickly lost at night. Since Mercury is very close to the Sun, the day side is heated to a

high temperature. At night the surface cools to very low temperatures.

Q5. How is the age of a rock determined? Explain the reasoning behind your procedure.

A5. The age of a rock can be determined by measuring the amount of a radioactive substance in the

rock compared to the amount of its decay product in the rock. This ratio tells us how many half lives

(the length of time required for half of the radioactive substance to decay) have occurred since the

rock formed. Laboratory measurements of the half life then allow us to find the age of the rock in

years.

Q6. Why do we think Mercury may have a molten core? Why does that surprise us?

A6. Mercury's weak magnetic field indicates that some part of its core remains molten, since

magnetism is generated by currents in a fluid iron core. That is surprising since we would have

expected a small planet like Mercury to have lost its internal heat and solidified throughout by now.

Q7. Describe how scarps were formed on Mercury. Are there any similar features elsewhere in

the solar system?

A7. Scarps formed on Mercury's surface when it cooled very quickly soon after formation. As it

cooled, the planet shrank slightly and the stress was relieved when the surface cracked in various

places. While the same process has not occurred elsewhere in the solar system, similar looking

cracks are often associated with earthquake faults on Earth. Valles Marineris on Mars is also a (much

larger) crack caused by expansion or up-thrusting of the surface.

Q8. How can a resonance affect the motion of an object? Describe a resonance in the solar system,

and describe how the motion has been altered.

A8. A resonance occurs when a small force acted repeated at the same point in the cycle of motion

of an object. Even though the force is very small, its effect accumulates over time to produce a

noticeable change in the motion of the object. The force of Earth acting on the "heavy" side of the

Moon has slowly pulled that side to always face Earth as the Moon orbits Earth. The action of the

sun on Mercury has produced a similar result there, except that Mercury rotates three times for

every two orbits around the sun.

Q9. Compare the interior structure of Mercury to that of Earth. What observation gives us this

information about Mercury? How is the difference in structure between Mercury and Earth

explained?

A9. Mercury has a much larger core, in proportion to its size, than does Earth. This conclusion is

reached by comparing the average density of Mercury to that of Earth, after compensating for the

compression caused by Earth’s greater gravity. The large core of Mercury occurred because it

formed closer to the sun, where the temperature was higher. Fewer rocky materials could condense

to help form a planet there, although iron could still easily condense under those conditions.

Q10. Why doesn't Mercury have an atmosphere?

A10. Mercury is so close to the sun that any atmosphere it had would be very hot, which means that

atoms in its atmosphere would be moving very rapidly. However, Mercury is also a rather small

planet, which means that its gravity is not very strong. It does not have an atmosphere because its

gravity is too weak to hold onto a high temperature atmosphere.

Q11. What is the origin of the jumbled hills on Mercury?

A11. The jumbled hills originated as a result of the giant impact which created the Caloris Basin on

Mercury. The seismic waves created by this impact were so powerful they traveled all the way

around Mercury. When they reconverged on the side of Mercury exactly opposite the impact site,

they we sufficiently strong to break the surface into large blocks. This area of disruption became the

jumbled hills.

Mars Question

Interior

Q1. Why do we suspect that Mars has a small core? Explain your reasoning.

A1. Since the average density of Mars is only slightly greater than the average density of a typical

rock (3.8 for Mars versus about 3 for a typical rock), it could not have a significant core of dense

material without raising its overall density to a higher value.

Atmosphere Questions

Q1. Mars is frequently engulfed in global dust storms. Why do these storms occur?

A1. In the winter on Mars, a significant fraction of the atmosphere condenses onto the polar ice cap.

The resulting low pressure over the pole causes a strong wind as air moves from the opposite pole

(where summer heat is causing that ice cap the evaporate). These winds moving from one polar cap

to the other cause global dust storms.

Q2. Why is Mars’ atmosphere so thin today, compared to Earth’s?

A2. A terrestrial planet’s atmosphere is released by volcanoes. In the case of Mars, its gravity is not

strong enough to permanently retain an atmosphere. When the volcanoes became extinct, the

atmosphere continues to leak away but no new gases were released to replace those that were lost.

Q3. Why are there large seasonal changes in the atmospheric pressure on Mars?

A3. Mars is far enough from the Sun and cold enough for CO2 to condense into ice during the

Martian winter. So much condenses that the atmospheric pressure changes appreciably.

Correspondingly, when the ice cap sublimes in the summer the atmospheric pressure increases

dramatically.

Q4. Describe the evolution of Mars atmosphere over time.

A4. Mars once had a dense, warm, humid atmosphere. While volcanoes were active, new gasses

were released to replace those lost from the atmosphere due to the weak gravity or which

condensed on the surface. When the volcanoes died, that replacement mechanism was lost. Water

was gradually lost by the process outlined in the previous question and as permafrost in the soil. The

weak gravity of the planet was unable to hold onto the atmosphere, which slowly evaporated from

the planet.

Q5. Why is Mars’ atmosphere so very thin today? What evidence from surface features is there

that Mars once had a dense atmosphere?

A5. Mars is a small planet with weak gravity. Its atmosphere slowly leaks away to space. Now that

all of its volcanoes are extinct, there is no source of new gas to resupply what has left. Dry river

channels visible on the surface indicate that Mars’ climate was once warmer than today. If its

atmosphere had always been as thin as we see today, liquid water could not have flowed on the

surface.

Mars - Surface Questions

Q1. Part of Mars’ surface is relatively young, while the other part is much older. How do we know?

Why or how did this occur?

A1. We can tell that the northern half of Mars is relatively young because there are very few impact

craters there, compared to the southern half of the planet. The surface of the northern hemisphere

of Mars has been covered relatively recently by volcanic outflows from the many volcanoes found

there.

Q2. Mars has many volcanoes? How do we know they are all extinct? Why are they so much

larger than those found on Earth?

A2. We can tell that all the volcanoes of Mars are extinct because all of them have impact craters on

their flanks. If there had been recently active, at least a few of them would not have impact craters

on them. They become large, compared to Earth’s volcanoes, because there is no plate tectonics on

Mars. A volcano on Earth is constantly "cut off" as the crust moves past a single hot spot in the

mantle. Instead of single large volcano, plate tectonics on Earth produces a chain of smaller

volcanoes.

Q3. What evidence is there that liquid water once existed on the surface of Mars? What had to be

different about Mars in the past for liquid water to exist on the surface?

A3. Dry river channels on Mars’ surface suggest that liquid water once existed there. However, the

present atmosphere of Mars would not allow liquid water to exist on the surface. The atmosphere is

so thin that water would evaporate very quickly from the surface. The existence of dry river channels

on Mars suggests that the atmosphere must have been much denser in the past.

Q4. What evidence is there at erosion has occurred on Mars? Why would we not expect it to

continue today?

A4. The most obvious evidence of erosion on Mars are the dry river beds at various places. It is also

possible that the dust storms may produce a very mild erosion. The liquid erosion is not possible

today on Mars because the atmosphere is too thin to allow liquid water to exist on the surface.

Q5. What evidence is there that there has been no plate tectonic activity on Mars? What caused

the huge canyon system on Mars?

A5. The volcanoes on Mars are very large, compared to a typical terrestrial volcano, which indicates

that the crust did not move during the formation of the large volcanoes. However, there is evidence

of vertical motion or swelling associated with volcanoes. As one region expanded, adjacent regions

cracked from the stress created by the uplift. These cracks are visible as a huge canyon system on

Mars that stretches for thousands of miles.

Q6. In what two ways are craters on Mars different from those on the Moon?

A6. The ejecta of some craters on Mars shows evidence of fluid flow instead of explosive ejection.

This suggests that permafrost in surface layers on Mars melted at impact and the water carried loose

particles away from the crater. Some craters on Mars also show the effects of erosion, as blowing

dust fills them in. Neither of these phenomena are seen on the Moon.

Q7. Why are volcanoes on Mars so much larger than any on Earth?

A7. Since there is no plate tectonics on Mars, a volcanic vent remains in one location and builds a

single volcanic mountain. On Earth, several mountains can be made by a single vent as crustal plates

move over it. This plate motion prevents any one volcanic mountain from getting very large on Earth.

Q8. Describe the composition of Mars’ ice caps. What effect does their formation each winter

have on the rest of Mars?

A8. Mars’ ice caps are made of a combination of water ice and carbon dioxide ice (dry ice). So much

dry ice condenses each winter to reduce the air pressure over the pole by as much as 20 %. The

pressure difference between the poles (one freezing while the other "melts") causes a condensation

flow of air from one pole to the other.

Q9. Describe the two types of dry river channels on Mars. How do we believe they occurred?

A9. Some river channels on Mars show many tributaries in a highly developed system of channels,

reminiscent of young river systems on Earth. Other channels are huge outflow channels which seem

to have been carved by single, massive floods. In both cases, we believe that the flow of water

occurred when permafrost was melted (perhaps by impacts or volcanism) and reached the surface in

springs.

Q10. Give a general description of the surface appearance of Mars.

A10. Mars is heavily cratered in the southern hemisphere and heavily volcanic in the northern

hemisphere. One major volcanic highland is known as the Thasis bulge, which cause a long fracture

canyon on its edge. Dry river channels are found, along with extensive evidence of dust erosion.

Polar caps form at each pole during the winter.

Jupiter Questions

Q1. Why did the Galileo spacecraft take so much longer to get to Jupiter than did the

Voyager spacecrafts?

A1. After the Challenger accident, it was decided to launch Galileo with a small conventional rocket

instead of from the shuttle. As a result, it had to circle the sun three times to make close passes by

Venus and Earth in order to gain enough speed to reach the outer solar system. Voyager, on the

other hand, was launched with the much larger Saturn V rocket on a direct path to Jupiter.

Q2. Describe the two components of the Galileo spacecraft. What were they designed to

observe?

A2. The Galileo spacecraft consists of both an atmospheric probe and an orbiter. The probe plunged

straight into the atmosphere of Jupiter to observe the properties of the atmosphere (temperature,

composition, cloud particles, etc.). The orbiter continues to orbit Jupiter, taking observations of the

planet and its moons.

Q3. Describe the spacecraft exploration of Jupiter. What missions have visited? What

did they do there, that is, what kind of mission were they?

A3. Jupiter has been visited by 5 spacecraft. Pioneer 10 and 11 flew past Jupiter in the early

1970's, Voyager I and II flew past in the late 1970's, and Galileo arrived in the 1990's. Galileo

consists of both an atmospheric probe and an orbiter that continues to obtain observations of

Jupiter today.

Jupiter - Interior Questions

Q1. The interior of Jupiter is divided into three layers. Describe the physical properties

of the material in each layer. In this context, what is meant by the term metallic?

A1. From the outside, the layers are composed primarily of liquid molecular hydrogen,

liquid atomic metallic hydrogen, and liquid rock and ice. Metallic hydrogen occurs under

very high pressure, when the hydrogen atoms are able to conduct electricity by sharing their

electrons.

Q2. Why is the core of Jupiter a mixture of rock and ice while Earth is just rock? Why

is Jupiter's core so much bigger than Earth?

A2. The type of solid material available to form a planet depends upon the temperature at

that location in the cloud. The temperature of the cloud diminishes with increasing distance

from the sun. At Jupiter's distance from the sun, the temperature was low enough that the ices

could solidify. Earth is too close to the sun for that to have happened, so Earth is all rocky

material while Jupiter's core contains both rock and ice. When ice solidifies, there is a great

deal more solid material available to form a planet, since the ices are much more common

than rocky material. Hence, Jupiter's core is much larger than Earth.

Q3. How do the gas giant planets radiate more energy than they receive from the sun?

A3. Two mechanism of heat generation are present. The gas giants (except Uranus) are still

radiating some heat as they continue to slowly collapse. In addition, helium condenses into a

liquid and falls toward the interior somewhere in the molecular hydrogen layers. This falling

rain gains enough energy as it falls to significantly heat up the gas it falls through. This heat

eventually escapes from the surface of the planet, making it warmer than it would otherwise

be.

Q4. Describe the structure of the interior of Jupiter? What unusual properties does the

matter inside Jupiter have?

A4. Jupiter has a core of rock surrounded by ice. Outside the core is a large region of atomic

hydrogen which has the unusual property (for hydrogen) of being a good conductor of

electricity. For this region it is referred to as metallic hydrogen. Outside this region is a

region of more normal molecular hydrogen.

Q5. What two types of observations give us information about the interior structure of

Jupiter?

A5. We can learn about the interior of Jupiter both from the average density of the planet and

from measurements of the flattening of the planet, which measures the interior response to

Jupiter’s rotation.

Q6. Compare the core of Jupiter to Earth. Why is Jupiter’s core such a small

percentage of the overall planet?

A6. Jupiter’s core is about the same size as Earth’s core, but contains between 3 and 30 times

as much material. Obviously, the density of Jupiter’s core is quite a bit higher than ours,

because of the greater compression of a more massive planet. Even at the high end of the

range of possible masses, the core of Jupiter is still only about 10% of the mass of the whole

planet. The additional material was attracted to Jupiter during its formation, because its core

had grown large enough for its gravity to be strong enough to attract gas from the

surrounding cloud. Earth never made it that far.

Q7. What is meant by the term "metallic hydrogen?" What observable consequences

does the layer of metallic hydrogen have for Jupiter?

A7. A metal is defined as a material which conducts electricity effectively. Hydrogen

becomes electrically conductive under very high pressures. The layer of metallic hydrogen

inside Jupiter creates the very strong magnetic field observed around Jupiter

Q8. What causes convection? What effect does convection have on the surface

appearance of Jupiter?

A8. Convection occurs when heat is added too rapidly into a material. Convective motion

occurs to transport the excess heat away from the source. Convection in the outer layers of

Jupiter brings hot material toward the surface. Molecules in this hotter material have different

colors than the cooler layers that are descending. These color differences cause the banding

seen on the surface of Jupiter.

Jupiter - Io Questions

Q1. What is the source of heat that produces the volcanic activity on Io?

A1. Io’s orbit around Jupiter is locked in a 2:1 resonance with Europa’s orbit. Every other orbit they

line up at the same place in Io’s orbit. This causes Io’s orbit to become slightly elongated at that point.

The changing tides on Io caused by its changing distance from Jupiter cause the moon to slightly

expand and contract internally. The friction created by this internal motion heats the interior, which is

released by volcanic activity.

Q2. Why is Io the most volcanic object in the solar system? Explain the mechanism

which causes the volcanism.

A2. Io’s orbit is in a two-to-one resonance with Europa’s orbit around Jupiter. This resonance has

pulled Io’s orbit into a slightly elongated shape. When Io is closer to Jupiter it’s shape is distorted by

the tidal forces from Jupiter; when it is further away it’s shape relaxes somewhat. This internal flexing

of Io causes friction, which leads to the build up of heat, which in turn is released by numerous

volcanoes.

Q3. Why is Jupiter’s moon Io volcanically active?

A3. Its orbit has a 2 to 1 resonance with Europa’s orbit just outside its orbit. The repeated tugs of

Europa at a fixed point in Io’s orbit gradually have pulled its orbit into an elongated shape. As the

distance between Jupiter and Io changes during Io’s orbit, the pull of gravity it feels from Jupiter also

changes, causing the shape of the moon to be alternately squeezed and expanded. The internal friction

created by this changing shape generates internal heat, which powers the volcanic activity on the

moon.

Q4. What two lines of evidence tell us that Io is an extremely volcanically active moon?

A4. We know that Io is extremely volcanically active because we have seen numerous plumes from

currently erupting volcanoes and we have not seen any impact craters anywhere on the surface.

Jupiter - Europa Questions

Q1. What evidence is there that Europa has a liquid water ocean?

A1. The most compelling evidence for a liquid ocean on Europa is the appearance of the surface. It is

extremely smooth, as if it is unable to support the weight of tall features. It has many criss-crossing

ridges and grooves from the constant shifting of the surface. Pieces that look like icebergs are also

found at several locations on the surface.

Q2. How do we know that Europa is composed primarily of rock even though its

surface is entirely ice-covered?

A2. Evan though its surface is ice-covered, Europa’s average density is 3.0 — which is about the

density of a typical rock and too high for ice.

Jupiter - Ganymede Questions

Q1. Describe the surface appearance of Ganymede, both globally and locally (that is,

both on a large scale and on a small scale). Has the surface been recently active? How

do we know?

A1. On a large scale, Ganymede appears to be rather blotchy, with large vaguely round dark section

interspersed with lighter terrain. On a small scale, there are numerous sections of parallel grooves

perhaps caused by motion of the surface as the icy surface cooled. Any activity that produced these

features had to occur in the early history of the solar system because all parts of the surface of

Ganymede are extensively cratered.

Jupiter's Moons Questions

Q1. In what way do the four large moons of Jupiter represent a miniature solar system?

A1. The two outer moons are larger and composed mostly of ice, while the two inner ones are

somewhat smaller and composed mostly of rock. This pattern is similar to the pattern of terrestrial and

giant planets in the solar system. The large moons around Jupiter formed from a gas disk orbiting the

planet in much the same way that the planets formed from a gas disk orbiting the sun.

Q2. Describe (briefly) the unique or distinguishing characteristics of each of the seven

large moons in the solar system.

A2.

Moon – maria, lava-filled basins

Io - active volcanoes

Europa – ice-covered oceans

Ganymede – intersecting grooves, dark spots

Callisto – crater covered, possible ocean

Titan – dense atmosphere

Triton – geysers, rigid ice surface, frost covered

Q3. Compare Ganymede and Callisto, the two icy moons of Jupiter. How do their

surfaces differ, both in appearance and in the processes which have occurred? How do

their interiors differ?

A3. Callisto’s surface is completely covered by craters and shows no signs of other geologic activity.

While Ganymede also has some craters, it’s surface also shows a complicated system of parallel

ridges and grooves and large areas of different colors and materials. These observations suggest that

Ganymede has undergone some plate tectonic activity. The interiors of Callisto and Ganymede also

differ: Callisto is undifferentiated and has no core, while Ganymede has a dense core surrounded by a

mantle.

Q4. For any one of the Galilean moons of Jupiter, describe its surface appearance and

the geologic processes which formed it.

A4. Io’s surface is pock-marked by many dark volcanic features. It’s surface is yellowish orange with

no impact craters because of the intense volcanic activity. Europa’s surface is very smooth and cris-

crossed by many lines. It’s icy surface bears a strong resemblance to pack ice on Earth – solid ice

floating on a liquid ocean. Callisto’s surface is impact-scarred and fairly dark. Ganymede’s surface

also shows many impact craters, but has a more mottled appearance with large dark spots scattered

across the surface. Systems of parallel grooves also cris-cross the surface. These features are probably

the result of the shrinking of the surface as the moon cooled long ago.

Q5. Define the four different types of moons in our solar system.

A5. Moons in the solar system can be classified as tiny (less than 100 km in diameter), intermediate

( between 100 km and 1000 km in diameter), large rocky (larger than 1000 km in diameter and

composed mostly of rocky material), and large icy (larger than 1000 km in diameter and composed

mostly of icy material).

Q6. Describe the 4 types of satellites in the solar system. Name each of the large

satellites and give a very short, one word or phrase description of each.

A6. The four types of satellites are: tiny (less than 100 km; irregular in shape), intermediate (up to

1000 km in radius); large rocky (density greater than 2); and large icy (density less than 2). The large

rocky mons are our Moon (cratered), Io(volcanic), and Europa (smooth). The large icy moons are

Ganymede (largest), Callisto (cratered), Titan (atmosphere), and Triton (ice volcanoes).

Saturn Questions

Q1. What two processes allow Saturn to radiate more energy than it receives from the

sun? What observation supports the existence of one of these processes?

A1. Saturn produces some energy from the slow release of gravitational energy as it continues to

shrink at a very gradual (by human standards) pace. Even more energy is released when droplets of

liquid helium form inside Saturn and begin to "rain" on deeper layers. These falling droplets release

gravitational energy and heat the material through which they fall. This model of energy production

inside Saturn is supported by the observation that there is less helium at the surface of Saturn than

there is at the surface of either Jupiter or the sun. The outer layers of Saturn are slowly being depleted

of their supply of helium as it falls to deeper layers.

Saturn's Moons Questions

Q1. Why does the large moon of Saturn have an atmosphere even though an equally

large moon at Jupiter does not?

A1. Saturn’s moon Titan has virtually the same gravitational force as Jupiter’s moon

Ganymede. Hence they should both be able to hold an atmosphere equally. However,

Ganymede is just enough closer to the sun that any atmosphere it might have would be

warmer than an atmosphere at Titan. Since the warmer atoms move faster, they are harder for

a gravitational force to hold on to. Thus, an atmosphere at Ganymede is much more likely to

escape because the individual atoms would be moving faster.

Q2. For any one of the intermediate-size moons of the solar system, describe its unique

or interesting characteristics

A2. Examples include: Mimas (with a crater almost large enough to have shattered the moon),

Enceladus (most reflective body in the solar system), Iapetus (one side whiter than snow, the other

side darker than coal), Hyperion (largest non-spherical object in the solar system with chaotic

rotation), Phoebe (an extremely dark surface which reflects only 5 % of the light that strikes it), and

Miranda (features large angular regions that may represent blocks of material that have recently

reformed the moon itself).

Saturn's Rings Questions

Q1. Explain the concept of resonance.

A1. Resonance occurs when a small force is applied at the same point in a cycle of motion. For

example, a child kicks in the air at the back of each swing on a playground swing. This small force

gradually increases the amplitude of the swing to a large effect.

Q2. Why do planetary rings exist? Why do they not exist at the terrestrial planets?

A2. Planetary rings exist inside the Roche limit of their planet. The Roche limit is the minimum

distance a liquid body could survive in orbit around the planet without being torn apart by the tides

from the planet. Ring particles represent material that was prevented from collecting together to form

a moon because of their position. Terrestrial planets have such weak forces of gravity that their Roche

limits are within or very near their atmospheres. Any particles inside their Roche limits will quickly

fall into the planet.

Q3. Why should the lifetime of planetary rings be fairly short? Why are they still

present?

A3. Planetary rings are composed of fairly small particles. Collisions among these particles will

quickly begin to spread them out – after each collision one particle will move closer to the planet

while the other will move further away. Ring particle should be lost both by falling into the planet and

by escaping from their orbits. Rings are still present because of two factors. Some rings have

shepherding moons which keep the particles from wandering off. Even with this mechanism, there

must also be a source of new ring particles – boulders within the rings that are slowly being ground up

as particles collide with them.

Q4. Compare and contrast the properties of the rings at Jupiter and Uranus.

A4. The rings of Jupiter form broad sheets and are composed of very tiny particles, while the rings of

Uranus are confined to very narrow bands and are composed of relatively large, dark particles. Both

ring systems are very faint.

Q5. Why should rings be short lived phenomena? Why do they instead survive for long

periods?

A5. Collisions between ring particles should cause them to gradually spread out, with some particles

eventually falling into the planet and others escaping outwards. Ring particles are maintained both by

the effects of shepherd moons which keep them confined to a particular orbit and are manufactured as

larger bodies are ground up within the rings.

Q6. What is the Roche limit? What is its role in ring formation?

A6. The Roche limit is the distance from a planet where the internal gravity of a liquid body would

no longer be strong enough to hold it together against the tidal forces of the planet it orbits. The rings

of the giant planets all lie inside the Roche limit of their planet, indicating that material in that zone

was prevented from collecting together to form satellites but instead became the rings we see today.

Q7. Why do we believe that a given ring particle will last only a short time in its orbit?

What two things can happen to make rings more long lived?

A7. The orbits of ring particles are not stable because collisions can so easily redirect their motion.

The net effect of collisions among ring particles is to spread the rings out -- both toward the planet

where the particles collide with the planet and away from the planet where they are eventually lost to

space. Rings can be preserved with by the presence of shepherding moons which confine the ring

particle orbits or by the presence of larger bodies in the rings which resupply the rings with small

particles as they are ground up.

Q8. How can a resonance affect the motion of an object? Describe a resonance in the

solar system, and describe how the motion has been altered.

A8. A resonance occurs when a small force acted repeated at the same point in the cycle of motion of

an object. Even though the force is very small, its effect accumulates over time to produce a

noticeable change in the motion of the object. The force of Earth acting on the "heavy" side of the

Moon has slowly pulled that side to always face Earth as the Moon orbits Earth. The action of the sun

on Mercury has produced a similar result there, except that Mercury rotates three times for every two

orbits around the sun.

Q9. How do we know that Saturn’s rings are composed of small particles? Give two

different observations, one ground-based and one from a satellite.

A9. Ground-based observations of stars that pass behind the rings, but blink in and out of view, show

that the rings are neither solid nor gaseous. Satellite observations of the rate at which the particles

cool off after they enter Saturn’s shadow tells us that the ring particles are quite small, only about a

centimeter across on average.

Q10. Suppose you discovered a giant planet like Jupiter in another solar system and

found that it had no moons. Would you expect it to have rings? Explain why or why not.

A10. Without moons it would be extremely difficult for a planet to keep a ring system. Moons act to

shepherd or contain the ring particles so they do not wander away to be lost from the ring system.

Without a moon to keep ring particles from being lost the rings will quickly disappear.

Q11. How can we measure the size of individual rings particles? What is different

about the results found on Jupiter and Saturn?

A11. The sizes of ring particles can be measure by how quickly they cool off as they enter the

planet’s shadow (smaller particles will cool faster) or by how much light is scattered into the forward

direction (that is, away from the sun) compared to the backward scattering (large particles produce

more backward scattering). Jupiter’s ring particles are much smaller on average than Saturn’s ring

particles.

Q12. Why are rings so incredibly thin, compared to their diameter?

A12. Again, collisions play a key role. If one or both of the particles which collide have some vertical

motion before the collision, the collision will tend to cancel out some of the vertical motion. Over

time, the motion of the particles becomes more and more uniform in the plane of the rings.

Q13. Describe the overall appearance of Saturn’s rings, as observed by the Voyager

spacecraft.

A13. Saturn’s rings contain thousands of tiny ringlets within the broad band of the rings visible from

Earth. Even in the dark division between the main rings, there are dimmer ringlets. There is also a thin

braided "F" ring outside the main ring system, confined by two shepherding moons. Dark spokes are

also seen to rotate with the rings of Saturn.

Q14. Describe a typical ring particle in the Saturnian ring system.

A14. A typical particle in the rings of Saturn is the size of a small pebble and composed of water ice.

Q15. Why are most ring systems very thin?

A15. If a ring particle is in a tilted orbit (compared to the average of all orbits) it is likely to collide

with another particle every time it passes through the plane of the rings. These collisions will tend to

cancel out the vertical motion of the orbit until everything is moving in the same plane.

Q16. Why do terrestrial planets not have ring systems?

A16. Planetary rings usually occur inside the Roche limit, defined as the distance at which a liquid

moon would be broken up by the tidal forces of the planet. The terrestrial planets are so small that

their Roche limits are within the outer reaches of their atmospheres, where ring particles quickly burn

up.

Q17. What happens if there is a small moon just outside a planetary ring?

A17. A small moon just outside a ring acts as a shepherd for the ring particles, keeping them confined

to the ring. As a ring particle catches up with the moon, it is pulled forward by the gravity of the moon.

This extra speed allows it to move to a slightly small orbit. The result is a sharp outer edge to the ring,

and a longer lifetime for the ring.

Uranus Neptune & Pluto Questions

Q1. How were Uranus, Neptune, and Pluto discovered?

A1. Uranus was discovered completely by accident as William Herschel mapped the heavens.

After Uranus was discovered, astronomers watched its orbit very carefully. Over time they noticed

that it was not following the path predicted by the known forces of gravity from the sun and other

planets. Predictions were made that another , unknown planet must be causing the deviations from the

predicted path. These predictions led to the discover of Neptune.

The same process of watching and predicting was followed for Neptune. While none of the predicted

planets have ever been found, they did motivate astronomers to search the heavens for additional

planets. One of these systematic searches led to the discovery of Pluto.

Q2. What observation tells us that Uranus and Neptune are composed mostly of water?

A2. While the average density of the planet gives some hint that they are composed mostly of water,

more detailed evidence comes from comparing their radius and mass with models composed of

different substances.

Q3. Why are Uranus and Neptune blue instead of the reds, browns, and yellows that

are typical of Jupiter and Saturn?

A3. The blue color of Uranus and Neptune is caused by methane in their atmospheres which absorbs

red and yellow, but reflects blue.

Uranus Questions

Q1. Describe how seasons are different on Uranus (with its sideways rotation)

compared to Earth.

A1. When the pole points toward the sun that part of the planet will be in perpetual sunlight (for

many years) and will experience an intense summer. At the same time the opposite pole will be in

darkness and winter. When the axis of rotation is more nearly sideways to the sun, the equatorial

regions will experience regular day/night cycles and a moderate summer.

Q2. How are the rings of Uranus different in appearance from those at Saturn? What is

thought to cause this difference in appearance?

A2. The rings of Uranus are widely separated from each other and narrow in width. The rings of

Saturn cover a broad expanse of radius, but are divided into thousands of finely divided ringlets.

Saturn also has one narrowly defined ring outside the main belt of rings. A narrow ring is the result of

tiny moons on either side of the ring that shepherd the ring particles to keep them from spreading out.

Q3. What component of Uranus’ atmosphere gives it its blue color? Why?

A3. Uranus appears blue to us because of the methane in its outer atmosphere. Methane absorbs red

light, leaving the blue to be reflected by the atmosphere.

Q4. How were the rings of Uranus first discovered?

A4. The rings of Uranus were discovered just before a star was scheduled to pass behind Uranus, an

even called an occultation. Before the occultation caused by the planet, the light of the star was briefly

interrupted several times by an unknown series of objects. When the same sequence of disappearance

was observed after the primary occultation, it was realized that rings must account for the temporary

blockages of light from the star.

Neptune Questions

Q1. How was Neptune discovered?

A1. After observing the motion of Uranus for approximately 50 years, astronomers concluded that its

motion could not be explained by the effects of gravity of the known objects in the solar system. The

motion could only be explained if the gravity of another planet was included in the calculation. These

calculations allowed astronomers to predict the precise location of this unseen planet, and Neptune

was quickly discovered exactly where it had been predicted.

Pluto Questions

Q1. What is the significance of the discovery of Pluto’s moon?

A1. Once the moon was discovered and its orbit determined, we were able to determine the mass of

Pluto and able to estimate the size of Pluto more accurately than before. With this information, we

found that Pluto is an icy body that is much smaller than many had thought – much too small to affect

the orbits of the other planets.

Q2. Why was the mass and size of Pluto unknown for many years? How were they

finally determined?

A2. Pluto is so tiny and distant that its size cannot be directly observed with ground-based telescopes.

The mass of a planet can only be determined by its gravitational influence on another body, and no

nearby body was known for Pluto until its moon was discovered in 1978. Once its moon was

discovered, the orbital motion could be used to determine its mass. Eclipses of the moon by Pluto also

allow astronomers to determine its size.

Q3. What observations finally allowed astronomers to determine the mass of Pluto?

Why did the result surprise them?

A3. Pluto’s mass was finally measured after the motion of its newly discovered moon was analyzed.

It turns out the Pluto is much lighter than astronomers had previously thought. It is much too small to

cause any significant deviations in the motions of Neptune.

Pluto Questions

Q1. What is the significance of the discovery of Pluto’s moon?

A1. Once the moon was discovered and its orbit determined, we were able to determine the mass of

Pluto and able to estimate the size of Pluto more accurately than before. With this information, we

found that Pluto is an icy body that is much smaller than many had thought – much too small to affect

the orbits of the other planets.

Q2. Why was the mass and size of Pluto unknown for many years? How were they

finally determined?

A2. Pluto is so tiny and distant that its size cannot be directly observed with ground-based telescopes.

The mass of a planet can only be determined by its gravitational influence on another body, and no

nearby body was known for Pluto until its moon was discovered in 1978. Once its moon was

discovered, the orbital motion could be used to determine its mass. Eclipses of the moon by Pluto also

allow astronomers to determine its size.

Q3. What observations finally allowed astronomers to determine the mass of Pluto?

Why did the result surprise them?

A3. Pluto’s mass was finally measured after the motion of its newly discovered moon was analyzed.

It turns out the Pluto is much lighter than astronomers had previously thought. It is much too small to

cause any significant deviations in the motions of Neptune.

Meteors Questions

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Q1. What is the difference between a primitive meteorite and a differentiated one?

Give an example of each.

A1. A primitive meteorite is one whose material has never been melted. Its composition

reflects whatever small grains were present during its formation. Small chondrules within the

meteorite are the remains of the dust particles out of which it formed. Many stony meteorites,

including the carbonaceous chondrites, are primitive meteorites. A differentiated meteorite is

one that comes from a body large enough to have chemically differentiated into an iron core

and a rocky mantle. The obvious example of a differentiated meteorite is the iron class

meteorites.

Q2. What is meant by the term "primitive meteorite"? How can they be used to

determine the age of the solar system?

A2. A primitive meteorite is one that has never been completely melted. Thus, the material

inside it has been preserved since the material in the solar system began to form into solid

bodies. Primitive meteorites are identified by the presence of chondrules inside them, which

are the rounded grains of dust out of which the solar system began to form.

Q3. Describe what is happening when we observe a typical "shooting star" in the

night sky.

A3. A shooting star is a small bit of rock which enters Earth's atmosphere at high speed and

burns up due to the friction it experiences as it moves through the atmosphere. We see the hot

air left behind the moving meteor.

Q4. Describe the three basic types of meteorites.

A4. The 3 types of meteorites are: (1) stony (composed mostly of rocky material but often

containing small bits of metallic material), (2) iron (almost pure iron alloy), and (3) stony-

iron (a roughly 50-50 mix of rock and metal).

Q5. What do we see when a small meteor enters the atmosphere? What is happening

to cause this?

A5. When a small meteor enters the atmosphere, the friction it experiences with the upper

atmosphere heats it and vaporizes it. The column of hot air created by the passing meteor is

visible as a bright streak of light which moves rapidly across the sky.

Q6. Describe the three types of meteorites. What is the significance of the existence of

chondrules in some meteorites?

A6. Stony meteorites are composed predominantly of rocky material, although most of

them have small pieces of metallic iron embedded in them. Iron meteorites are essentially

pure metallic iron-nickel. Stony-iron meteorites are an equal mix of metal and rock. The

stony meteorites are by for the most common type of meteorite.

Chondrules are small roundish inclusions in some stony meteorites. They are the partially

melted remains of the original grains of dust from which all objects in the solar system

formed. Hence, they represent the oldest, most primitive material available for study in the

solar system.

Q7. How do we know that the material in a cabonaceous chondrite has never been

geologically altered? Why is that important?

A7. Carbon compounds are easily destroyed at even moderate temperatures. If the material in a

carbonaceous chondrite meteorite had ever been subjected to any significant geologic process, the

carbon compounds would have evaporated and been lost. The absence of such events in the past

history of a meteorite tells us that it is unaltered since the formation of the solar system. These objects

give us direct information about the conditions in the solar nebula as the solar system formed.

Asteroids Questions

Q1. What are the similarities and differences between asteroids and comets? Where

do the comets we see come from?

A1. Asteroids are rocky bodies, mostly in nearly circular orbits between Mars and Jupiter. Comets

are icy bodies, usually observed in highly elongated orbits. Comets that we see come from either the

Kuiper Belt (a donut-shaped ring in the plane of the solar system) or the Oort Cloud ( a large spherical

cloud outside the solar system).

Q2. How can the chemical composition of an asteroid be determined from ground-

based observations?

A2. The chemical composition of an asteroid can be found by comparing the spectrum of reflected

sunlight from the asteroid to the reflected spectrum of various meteorites on Earth. When a good

match is found, we know the composition of the asteroid is similar to that of the meteorite.

Q3. How can the size and shape of an asteroid be determined from ground-based

observations?

A3. There are two ways to find the shape of asteroids. If several people observe the passage of the

asteroid in front of a star, their separate measurements of when the star disappeared and reappeared

tell us what the size and shape are. If an asteroid passes close to Earth, radar waves can be bounced

off the asteroid. The returning waves tell us the size and shape of the asteroid.

Q4. Why do most asteroids occur in a narrow belt in the solar system?

A4. Most asteroids lie in orbits just inside the orbit of Jupiter. They are small bodies which were

prevented from ever collecting together because of the disturbing influence of Jupiter's force of

gravity. Jupiter's influence kept their orbits "churned up," so that gentle collisions which build larger

bodies did not happen as often.

Comet Questions

Q1. What are the similarities and differences between asteroids and comets? Where

do the comets we see come from?

A1. Asteroids are rocky bodies, mostly in nearly circular orbits between Mars and Jupiter.

Comets are icy bodies, usually observed in highly elongated orbits. Comets that we see come

from either the Kuiper Belt (a donut-shaped ring in the plane of the solar system) or the Oort

Cloud ( a large spherical cloud outside the solar system).

Q2. Why do comets have a relatively short lifetime? Where do the comets we see today

come from?

A2. Every time a comet comes close to the sun, more of the frozen gases evaporate from

the nucleus and are lost to space. After just a hundred passes around the sun, most comets

will have completely dissipated. Since comets have relatively short lifetimes when their

orbits bring them close to the sun, there must be a reservoir of comets in orbits that always

remain outside the solar system. These reservoirs are called the Kuiper Belt (a donut-shaped

zone of comets just outside the orbit of Neptune) and the Oort Cloud (a spherical cloud of

comets that stretches halfway to the nearest star).

Q3. Describe the three parts of a comet, when the comet is close to the sun.

A3. Comets consist of a nucleus (a small solid ball of dirty ice), a coma (a large, thin,

roughly spherical ball of gas and dust which have evaporated from the nucleus and surround

it), and a tail (gas and dust which have been blown out of the coma and trail away in the

direction opposite to the sun).

Q4. What are the differences between long period comets and short period comets?

A4. A short period comet has an orbit around the sun of less than 200 years while a long

period comet takes more than 200 years to orbit the sun once.

Q5. Where do the comets we see come from?

A5. Comets come from either the Kuiper belt (a donut shaped cloud of comets in orbits

from the outer edge of the planetary orbits out to a few hundred AU) or the Oort cloud (a

spherical cloud of comets that reaches 100,000 AU or more from the sun).

Q6. What are the two types of tails a comet may have? Why do they point away from

the sun?

A6. The dust tail is driven away from the comet by radiation pressure from the sun. It

gently curves as the orbit of the dust particles slowly falls behind the comet. The ion tail is

driven away from the comet by the solar wind of particles streaming away from the sun.

Since both tails are driven away from the comet by forces coming from the sun, they both

point generally away from the sun.

Q7. Describe the properties and appearance of the nucleus of a comet.

A7. The nucleus of a comet is small (generally only a few miles across), irregular in shape,

and very dark (reflecting only a few % of the light which strikes its surface).

Q8. Describe the Oort cloud. What is its significance?

A8. The Oort cloud is a spherical swarm of cometary nuclei in roughly circular orbits at

distances from a few hundred to several hundred thousand astronomical units from the sun. If

one of these objects is disturbed from its circular orbit and falls into the solar system, it will

appear to us as a long period comet when it is close to the sun.

Q9. Describe the appearance of a comet when it is close to the sun. How does the sun

affect the appearance?

A9. When close to the sun, a comet consists of a tiny, solid nucleus surrounded by a large

spherical gaseous coma with a long gas and dust tail that stretches away from the sun. The

light of the sun works to heat the nucleus of the comet, causing some of the ice in the nucleus

to evaporate. The evaporating ice also releases some of the dust trapped in the ice. These

released materials form the coma and tail of the comet.

Q10. Describe the three components of a comet when it is close to the sun.

A10. A comet near the sun consists of a tiny (5-10 mile) solid nucleus surrounded by a

huge (100,000 mile) coma of gas and dust which has evaporated from the nucleus, with a

long (millions of miles) tail of gas and dust streaming away from the coma in the direction

away from the sun.

Q11. Why must there be a reservoir of comets?

A11. Each time a comet passes near the sun, some of its gas evaporates. After about 100

passes, a typical comet will have completely evaporated and disintegrated. If there was not a

reservoir of comets outside the solar system continually supplying fresh comets to the solar

system, they would have long ago vanished from our part of the solar system.

Q12. Describe the Kuiper Belt. What is believed to be the origin of comets in the

Kuiper Belt?

A12. The Kuiper Belt is a donut-shaped ring of comets that exist in nearly circular orbits

just outside the solar system. The Kuiper Belt extends from just outside Neptune’s orbit for a

few hundred astronomical units. Comets in the Kuiper Belt are believed to have formed there,

in the outer fringes of the disk from which all objects in the solar system formed.

Q13. Compare and contrast the Kuiper belt and the Oort cloud.

A13. The Kuiper belt is a fairly flat distribution of comets in the plane of the solar system

from just outside the orbit of Pluto to a few hundred AU. The Oort cloud is a spherical

distribution of comets which lie many thousands of AU's from the sun.

Q14. What direction does the tail of a comet point? Why?

A14. The tail of a comet points generally away from the sun because it is formed of matter

that is pushed away from the comet by the light and wind of matter flowing from the sun.

Q15. Why is it important to send satellites to study comets instead of just observing

them from Earth?

A15. When comets approach the sun they are hidden inside a veil of evaporated gases that

prevent us from directly observing the solid nucleus. Nuclei are so small that they would be

hard to study from Earth even if we could see them. And comet nuclei are covered by a layer

of dark material that prevents us from directly studying the main body of the nucleus.

Q16. Describe the properties of the nucleus of Halley's Comet, as observed by the

Giotto spacecraft.

A16. The nucleus of Halley's Comet is about 5 x 10 miles in size and covered with a very

dark substance. Two large jets and several smaller jets of evaporating gas were seen.

Q17. Describe the orbits and origin of short period and long period comets.

A17. Long period comets originate in the Oort cloud at distances of a few tens of thousands

of AU. Their very elongated orbits have random orientations compared to the plane of the

solar system. Short period comets originate in the Kuiper belt a few hundred AU from the sun

and have orbits aligned approximately with the plane of the solar system. Their orbits have

usually been modified by a close gravitational interaction with Jupiter.

Telescopes Questions

Q1. What is meant by the term "national observatory"? How does one get to use a

telescope there?

A1. A national observatory is one that is available to anyone in the country to use and is supported

by the federal government. To obtain time on a telescope at a national observatory, you prepare a

proposal that describes what you want to observe and what you expect to learn from the observation.

These proposals are evaluated by teams of scientists to determine which are the most scientifically

important. The top proposals are awarded free time on the telescopes.

Q2. The human eye, photographic film, and CCD’s are all detectors used in

astronomy. What are their relative advantages and disadvantages?

A2. While the human eye is a wonderfully versatile detector, it is connected to the human brain

which often has preconceptions of what will be seen. As a scientific instrument it is unreliable

because you cannot show me what you have seen, but can only describe it as seen through the filter of

the brain. Images recorded on film can be evaluated by everyone and are recorded without bias, but

film is not very sensitive to light. CCD’s are much more sensitive than film, and record their images

electronically so they can be processed by computers and shared around the world

Q3. What are the two basic types of telescopes? Why are all large telescopes of just

one type?

A3. Telescopes can be either reflecting (with a mirror forming the image) or refracting (with a lens

forming the image). Large refracting telescopes are impractical because large lenses, which can only

be supported around the edge, tend to sag and distort the image. In addition, different colors are bent

to a focus at different positions as they pass through thick lenses. Mirrors do not suffer from either of

these drawbacks.

Q4. For astronomical purposes, what are the two most important powers of a

telescope? What are their definitions, uses, and limitations?

A4. The most important powers of a telescope are light gathering power and magnifying power.

Light gathering power is a measure of how bright an image will be in the telescope, and is determined

by the diameter of the objective lens or mirror. Large light gathering power is required to make dim

objects detectable. The limits are higher cost for larger telescopes and greater engineering difficulties

in making precise instruments larger.

Magnifying power determines how large the image will appear in the telescope. It is useful in

examining the fine details of resolved objects. Unfortunately, turbulence in our atmosphere sets a

limit on useful magnifications for ground-based observations.

Q5. Why are large refracting telescopes not feasible?

A5. A refracting telescope uses a large lens to gather light and form an image. The weight of a large

piece of glass causes it to sag. Because a lens can only be supported around its edge, it is impossible

to maintain an accurate surface on a large lens in a refracting telescope. Large pieces of glass also

bend different colors by different amounts. This causes color fringes around the images formed by

any large lens.

Q6. Explain magnifying power and light gathering power for telescopes. Why is light

gathering power more important for most astronomical applications?

A6. The magnifying power of a telescope describes how large an image it forms. Light gathering

power describes how much light is gathered to form the image. Since stars are too far away for any

telescope to magnify them into a discernable image, light gathering power is more important to

astronomers because it determines whether a dim object can be seen at all.

Q7. Describe one of the advances in technology which has revolutionized astronomy in

the last decade.

A7. Charge-coupled devices (CCDs) are electronic devices that have replaced film for taking

pictures because they are many times more sensitive to light than the best films. Adaptive optics

allows astronomers to adjust the image formed by a telescope to compensate for the blurring effect of

our atmosphere. Next generation telescopes have very large mirrors that are lighter and more accurate

than traditional thick mirrors.

Q8. What is a false color image, as used in astronomy?

A8. In a false color image, different colors are used to represent changes in some property of the

image. For example, different colors might be used to represent different levels of brightness in a

photograph or different colors might be used to represent different elevations in a map showing the

surface of another planet.

Q9. Describe some (at least two) reasons for placing telescopes in orbit around Earth.

A9. Many wavelengths of light do penetrate through the atmosphere at all. The only way to observe

them is from above the atmosphere. Even visible light, which does penetrate the atmosphere, is

blurred by its passage through the atmosphere. Much sharper images can be obtained from orbit.

Some light is absorbed at any wavelength, so images in space are always brighter than those seen on

the ground.

Q10. What is meant by the term "light gathering power", as applied to telescopes?

What determines how much light gathering power a telescope has? Why does it matter

to astronomers?

A10. Light gather power defines how much light is gathered by the telescope, and therefore, how

bright an object will appear in the telescope. This is important to astronomers because greater light

gathering power allows them to observe dimmer objects. The light gathering power is determined by

the size (diameter) of the main lens or mirror of the telescope. The greater the area of the main lens or

mirror, the greater the light gathering power.

Q11. What are the two most important reasons for placing telescopes in satellites?

A11. A telescope in a satellite is above Earth’s atmosphere. This allows it to observe wavelengths,

such as the ultraviolet, that are completely absorbed by the atmosphere. Images obtained above the

atmosphere are also much sharper than those obtained with ground-based telescopes because they do

not have to look through the blurry atmosphere.