Venus, Earth and Mars

Lecture 14

•Telescope Observing lab following class

•Maps are available at the front of the classroom

•You may want to stop for a snack on the way, we probably won’t do a lot of observing before about 7:30 due to sunlight (although we may look at the Sun if we get set up early enough).

• If weather (ie, clouds) prevents us from seeing enough tonight, we will likely try again later in the semester.

Announcements

•Comparative Planetology is the study of planets as groups, comparing their similarities and differences. We will take this approach to studying the solar system, starting with the planets most similar to our own.

•We Will compare several features of these planets:

• Interiors

• Surfaces

• Atmospheres

• Magnetospheres

• Moons

Comparative Planetology

The Early HistoryThe terrestrial planets formed 4.6 billion years ago from the inner solar nebula.

Four main stages of evolution:

Most traces of bombardment (impact craters) now destroyed on Earth by later geological activity

Two sources of heat in planet’s interior:

• Potential energy of infalling material

• Decay of radioactive material

Earth’s InteriorDirect exploration of Earth’s interior (e.g. drilling) is impossible.

Earth’s interior can be explored through seismology:

earthquakes produce seismic waves.

Two types of seismic waves:Pressure waves:

Particles vibrate back and forth

Shear waves:

Particles vibrate up and down

Seismology

Seismic waves do not travel through Earth in straight lines or at constant speed.

They are bent by or bounce off transitions between different materials or different densities or temperatures.

Such information can be analyzed to infer the structure of Earth’s interior.

Venus and Mars

Two most similar planets to Earth:

• Similar in size and mass • Atmosphere

• Similar interior structure• Same part of the solar system

Yet, no life possible on either one of them.

Interior

Basic structure:

Solid crust

Solid mantle

Liquid core

Solid inner core

interior gets hotter towards the center.

Earth’s core is as hot as the sun’s surface; metals are liquid.

Melting point = temperature at which an element melts

(transition from solid to liquid)

Melting point increases with increasing

pressure towards the center

=> Inner core becomes solid

The Rotation of Venus

• Almost all planets rotate counterclockwise, i.e. in the same sense as orbital motion.

• Exceptions: Venus, Uranus and Pluto

• Venus rotates clockwise, with period slightly longer than orbital period.

Possible reasons:

• Off-center collision with massive protoplanet

• Tidal forces of the sun on molten core

Mars• Diameter ≈ 1/2 Earth’s diameter

• Very thin atmosphere, mostly CO2

• Rotation period

= 24 h, 40 min.• Axis tilted against orbital plane by 25o, similar to Earth’s inclination (23.5o)

• Seasons similar to Earth Growth and shrinking of polar ice cap

• Crust not broken into tectonic plates

• Volcanic activity (including highest volcano in the solar system)

The Active Earth

About 2/3 of Earth’s surface is covered by water.

Mountains are relatively rapidly eroded away by the forces of water.

Earth’s surface geology is much more dynamic than Venus and Mars.

Tectonic PlatesEarth’s crust is composed of several distinct tectonic plates, which are in constant motion with respect to each other Plate tectonics

Evidence for plate tectonics can be found on the ocean floor

… and in geologically active regions all around the Pacific

Plate TectonicsTectonic plates move with respect to each other.

Where plates move toward each other, plates can be pushed upward and downward formation of mountain ranges, some with volcanic activity, earthquakes

Where plates move away from each other, molten

lava can rise up from below volcanic activity

Earth’s Tectonic History

History of Geological Activity

Surface formations visible today have emerged only very recently compared to the age of Earth.

The Surface of VenusEarly radar images already revealed mountains, plains, craters.

Venera 13 photograph of surface of Venus:

Colors modified by clouds in

Venus’s atmosphere

More details from orbiting and landing spacecraft:

After correction for atmospheric

color effect:

Radar Map of Venus’s Surface

Surface features shown in artificial colors

• Scattered impact craters

• Volcanic regions

• Smooth lava flows

Lava Flows

Young, uneven lava flows (shown: Lava flow near Flagstaff, AZ) show up as bright regions on radar maps.

Surface Features on Venus

Smooth lowlands

Highland regions:

Maxwell Montes are ~ 50 % higher than Mt. Everest!

Craters on VenusNearly 1000 impact craters on Venus’s surface:

Surface not very old.

No water on the surface; thick, dense atmosphere

No erosion

Craters appear sharp and fresh

Volcanism on Earth

Volcanism on Earth is commonly found along subduction zones

(e.g., Rocky Mountains).

This type of volcanism is not found on Venus or Mars.

Shield Volcanoes

Found above hot spots:

Fluid magma chamber, from which lava erupts repeatedly through surface layers above.

All volcanoes on Venus and Mars are shield volcanoes

Shield Volcanoes

Tectonic plates moving over hot spots producing shield volcanoes Chains of volcanoes

Example: The Hawaiian Islands

Volcanism on Venus

Sapas Mons (radar image)

2 lava-filled calderas~ 400 km (250 miles)

Lava flows

Volcanic Features on Venus

Baltis Vallis: 6800 km long lava flow channel (longest

in the solar system!) Coronae: Circular bulges formed by volcanic activity

Aine Corona

Pancake Domes:

Associated with volcanic

activity forming coronae

Some lava flows collapsed after molten lava drained away

Lakshmi Planum and Maxwell Mountains

Radar image

Wrinkled mountain formations indicate compression and wrinkling, though there is no evidence of plate tectonics on Venus.

Tales of Canals and Life on Mars

Early observers (Schiaparelli, Lowell) believed to see canals on Mars

This, together with growth/shrinking of polar cap, sparked imagination and sci-fi tales of life on Mars.

We know today: “canals” were optical illusion; do not exist!

No evidence of life on Mars.

The Geology of Mars

Giant volcanoes

Valleys

Impact craters

Vallis Marineris

Reddish deserts of broken rock, probably smashed by

meteorite impacts.

The Geology of MarsNorthern Lowlands: Free of craters; probably re-surfaced a few billion years ago.

Southern Highlands: Heavily cratered; probably 2 – 3 billion years old.

Possibly once filled with water.

Volcanism on Mars

Volcanoes on Mars are shield

volcanoes.

Olympus Mons:

Highest and largest volcano

in the solar system.

Volcanism on Mars

Tharsis rise (volcanic bulge):

Nearly as large as the U.S.

Rises ~ 10 km above mean radius of Mars.

Rising magma has repeatedly broken through crust to form volcanoes.

Hidden Water on MarsNo liquid water on the surface:

Would evaporate due to low pressure.

But evidence for liquid water in the past:

Outflow channels from sudden, massive floods

Collapsed structures after withdrawal of sub-surface water

Splash craters and valleys resembling meandering river beds

Gullies, possibly from debris flows

Central channel in a valley suggests long-term flowing water

Hidden Water on Mars

Gusev Crater and Ma’adim Vallis:

Giant lakes might have drained repeatedly through the Ma’adim Vallis into the crater.

Ice in the Polar Cap

Polar cap contains mostly CO2 ice, but also water.

Multiple ice regions separated by valleys

free of ice.

Boundaries of polar caps

reveal multiple layers of dust, left behind by

repeated growth and melting of

polar-cap regions.

Evidence for Water on Mars

Large impacts may have ejected rocks into space.

Galle,

the “happy face crater” Meteorite ALH84001:

Identified as ancient rock from Mars.Some minerals in this meteorite were deposited in water Martian crust must have been richer in water than it is today.

Atmospheres

Atmospheric composition severely altered (

secondary atmosphere) through a combination of two processes:

1) Outgassing: Release of gasses bound in compounds in the interior through volcanic activity

Terrestrial planets had primeval atmospheres from remaining gasses captured during formation

2) Later bombardment with icy meteoroids and comets

The Structure of Earth’s Atmosphere

The ozone layer is

essential for life on Earth since it protects the atmosphere

from UV radiation

Composition of Earth’s atmosphere is further influenced by:

• Chemical reactions in the oceans,

• Energetic radiation from space (in particular, UV)

• Presence of life on Earth

The temperature of the atmosphere depends critically on its albedo = percentage of sun light that it reflects back into space

Depends on many factors, e.g., abundance of water vapor in the atmosphere

Human Effects on Earth’s Atmosphere1) The Greenhouse Effect

Earth’s surface is heated by the sun’s radiation.

Heat energy is re-radiated from Earth’s surface as infrared radiation.

CO2, but also other gases in the atmosphere, absorb infrared light

Heat is trapped in the atmosphere.

This is the Greenhouse Effect.

The Greenhouse Effect occurs naturally and is essential to maintain a comfortable temperature on Earth,but human activity, in particular CO2 emissions from cars and industrial plants, is drastically increasing the concentration of greenhouse gases.

Global Warming

• Human activity (CO2 emissions + deforestation) is drastically increasing the concentration of greenhouse gases.

• As a consequence, beyond any reasonable doubt, the average temperature on Earth is increasing.

• This is called Global Warming

• Leads to melting of glaciers and polar ice caps

( rising sea water levels) and global climate changes, which could ultimately make Earth unfit for human life!

Human Effects on the Atmosphere2) Destruction of the

Ozone LayerOzone (= O3) absorbs UV radiation, (which has damaging effects on human and animal tissue).

Chlorofluorocarbons (CFCs) (used, e.g., in industrial processes, refrigeration and air conditioning) destroy Ozone.

Destruction of the ozone layer as a consequence of human activity is proven (e.g., growing ozone hole above the Antarctic);

UV image

Extremely inhospitable:

96 % carbon dioxide (CO2)3.5 % nitrogen (N2)Rest: water (H2O), hydrochloric acid (HCl), hydrofluoric acid (HF)

4 thick cloud layers ( surface invisible to us from Earth).

Very stable circulation patterns with high-speed winds (up to 240 km/h)

Extremely high surface temperature up to 745 K (= 880 oF)

Very efficient “greenhouse”!

UV image

The Atmosphere of Venus

The Atmosphere of MarsVery thin: Only 1% of pressure on Earth’s surface

95 % CO2

Even thin Martian atmosphere evident through haze and clouds covering the planet

Occasionally: Strong dust storms that can enshroud the entire planet.

The Atmosphere of Mars

Most of the Oxygen bound in oxides in rocks

Reddish color of the surface

History of Mars’s AtmosphereAtmosphere probably initially produced through outgassing.

Loss of gasses from a planet’s atmosphere:

Compare typical velocity of gas molecules to escape velocity

Gas molecule velocity greater than escape velocity

gasses escape into space.

Mars has lost all lighter gasses; retained only heavier gasses (CO2).

Earth’s Magnetic Field

• Convective motions and rotation of the core generate a dipole magnetic field

• Earth’s core consists mostly of iron + nickel: high electrical conductivity

The Role of Earth’s Magnetic FieldEarth’s magnetic field protects Earth from high-energy

particles coming from the sun (solar wind).

Surface of first interaction of solar wind with Earth’s magnetic field = Bow shock

Region where Earth’s magnetic field dominates = magnetosphere

Some high-energy particles leak through the magnetic field and produce a belt of high-energy particles around Earth: Van Allen belts

The Aurora (Polar Light)

As high-energy particles leak into the lower magnetosphere, they excite molecules near the Earth’s magnetic poles, causing the aurora

A History of VenusComplicated history; still poorly understood.

Very similar to Earth in mass, size, composition, density, but no magnetic field Core solid?

Heat transport from core mainly through magma flows close to the surface ( coronae, pancake domes, etc.)

Solar Wind InteractionSolar wind interacts directly with the atmosphere, forming a Solar wind interacts directly with the atmosphere, forming a bow shock and a long ion tail. bow shock and a long ion tail.

COCO22 produced produced

during outgassing during outgassing remained in remained in atmosphere (on atmosphere (on Earth: dissolved in Earth: dissolved in water).water).

Any water present Any water present on the surface on the surface rapidly evaporated rapidly evaporated → feedback → feedback through through enhancement of enhancement of greenhouse effectgreenhouse effect

The Moons of Mars

Phobos

Deimos

Two small moons: Phobos and

Deimos.

Too small to pull themselves into spherical shape.

Very close to Mars; orbits around Mars faster than Mars’ rotation.

Typical of small, rocky bodies: Dark grey, low density.

Probably captured from outer asteroid belt.

Earth’s Moon

Earth has an unusually large moon… details next time.

We will discuss the Moon and Mercury

Read Units 37 and 38

For Next Time