Science 3210 001 : Introduction to Astronomy

Lecture 4 : Overview of the Solar System, The Sun, and the Inner Planets

Robert Fisher

Items

First Midterm in two weeks.

Homeworks / textbooks

Review Week 2

Celestial Sphere

Zenith, Nadir, Meridian, Equinox, Solstice

Retrograde Motion

Review Week 3

Kepler’s Three Laws

Newton’s Three Laws

Spectra -- Continuum, Absorption, Emission

Today’s Material

A Few Comments about the Primary Colors and Color Photography

Overview of the Solar System Planets, Moons, Rings, Asteroids, Comets… Fundamentals of Planetary Physics

The Inner Solar System The Cratered Worlds of Mercury and the Moon Venus and Mars Earth

A Few Comments About Color Theory and Color Photography

The Primary Colors

There are three primary colors precisely because the human eye has three types of cone photoreceptor cells, each sensitive to one band of light.

Cone Cell Color Response

Each of the the three types of cone cells has a different biochemical makeup with a different color response curve :

Solar Spectrum

The reason why our rod cells have a peak absorption at roughly 500 nm in wavelength is simply because the solar spectrum peaks at that same wavelength :

First Color Photograph

James Clerk Maxwell produced the first color photograph in 1861 using three images photographed on black and white positive films, filtered through each of the primary colors :

Overview of the Solar System

Overview of Solar System

The Sun. In a sense, the sun is the solar system. 99.9% of the total mass of the solar system, and also the source of the vast majority of energy.

Sun Compared with Planets

A 3D computer rendering of the sun and the planets, all compared to-scale :

Overview of Solar System

The Planets

Inner planets are close to the sun -- consisting of relatively warm rocky bodies, and thin to non-existent atmospheres.

Outer planets are further from the sun -- they have rocky cores and enormous gaseous atmospheres which constitute the bulk of the planet.

Comparison of Inner Planets

A 3D computer rendering of the inner planets and the moon, all compared to-scale :

Comparison of Outer Planets

A 3D computer rendering of the outer planets, compared with the inner planets, all to-scale :

Question

Which planet can never be seen on the Meridian at midnight? A) Mercury B) Mars C) Jupiter D) Saturn

Overview of Solar System

Moons / Rings

A moon is simply a natural satellite of a planet. There are over 100 known moons -- most around Jupiter and the outer planets.

The largest moon in the solar system (Jupiter’s Ganymede, discovered by Galileo) is larger than Mercury. If it were orbiting the sun, it would be classified as a planet.

Some of the largest moons (most noteably Saturn’s Titan) have atmospheres. Others show signs of active geological activity, including one (Jupiter’s Io) that shows direct signs of volcanic activity.

Ganymede

Ganymede, as photographed by the Galileo space probe.

A Theoretical Model of Interior of Ganymede

Io

Jupiter’s moon Io is volcanically active, spewing sulfur plumes.

Overview of Solar System - Rings

All four of the outer solar system planets have ring systems, though Saturn’s is by far the most spectacular.

These rings are the remnant material left over from the formation of the moons surrounding the planet, and are made up of enormous numbers of icy rocks.

In a sense, the moon/planet systems are a kind of “mini solar system,” and the rings are analogous to a scaled-down version of the asteroid belts in our own solar system (which we will discuss in just a moment).

Artist’s Conception of Saturn’s RIngs

Saturn Imaged by the Cassini Spacecraft

Saturn’s Rings Imaged by Voyager

Overview of Solar System - Minor Bodies

Dwarf Planets - a very new category.

According to the new International Astronomical Union (IAU) specification laid down last year, planets are officially defined to meet three criteria : Major bodies orbiting the sun (or another star).

Large enough to be spherical in shape.

Have swept their neighborhood clear.

Dwarf planets satisfy the first two criteria, but fail the third. Examples include Pluto and the asteroid Ceres.

Disk Clearing / Gap Formation

A large enough body will sweep up the material its neighborhood. Here, for instance, is the result of a simulation of the early solar system, before it had been cleared of gas and dust :

Planet

Star

Disk (in greyscale)

Gap

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Overview of Solar System - Minor Bodies

Asteroids

The asteroids were once thought to have been a “broken-up” planet, but they actually contain far too little mass to constitute a planet. They are the left-over products of planet formation.

They range in size from the spherically-shaped Ceres (almost 1000 km in radius) down to much smaller bodies barely a few kilometers across.

The Asteroid Belt - Minor Bodies

Most asteroids are clustered between the orbits of Mars and Jupiter. It is thought that Jupiter’s enormous gravitational force kept these bodies from coalescing into a rocky planetary core.

Vesta, Ceres, and the Moon

Side-by-side comparison of two of the largest asteroids (Vesta and Ceres) and the Earth’s moon :

Question

Why are the larger bodies in previous image rounder than the smaller ones?

The Asteroid Eros

Eros was visited by the spaceprobe NEAR Shoemaker, which began its orbit around the asteroid on February 14, 2000.

Eugene Shoemaker (1928 - 1997)

Barringer (or Meteor) Crater in Arizona

Shoemaker’s great legacy was to establish that catastrophic impacts do occur throughout the solar system.

Overview of Solar System - Minor Bodies

In addition to the asteroid belt, another belt of bodies orbits the sun beyond the orbit of Neptune -- the Kuiper Belt.

It is thought that Pluto is in fact a Kuiper belt object -- one of the largest.

Because of their great distance from the sun, Kuiper Belt Objects are much harder to detect than asteroids. 800 objects have been detected to date.

Kuiper Belt Objects

Like the case of the asteroid belt, the Kuiper Belt is “debris” left over from the early solar system. In this case, they have been missed by Neptune’s “sweeping” of its neighborhood.

Overview of Solar System - Meteoroids, Dust, and the Solar Wind

Meteoroids, Dust, Solar Wind

The trail of comets is filled with tiny meteoroids. When the Earth’s orbit intersects a comet’s trajectory, we experience a meteoroid shower on Earth. These happen at regular dates on the calendar each year.

The collision of rocky bodies over the course of history of the solar system produces smaller bodies, down to dust-sized particles. These dust-like particles are responsible for the zodiacal light effect.

The outer layers of the sun continuously blow away a stream of charged particles, referred to as the solar wind.

Magnetic Field

The magnetic field surrounding a magnet can be visualized using iron filings.

Solar Atmosphere

The outer layers of the sun are incredibly active, powered by the Sun’s intense magnetic field.

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Fundamentals of Planetary Physics

The properties of the planets are largely determined by a few crucial physical parameters -- its mass, rotational rate/inclination, and surface temperature.

The combination of mass and surface temperature, for instance, will determine the atmospheric content of the planet.

Larger bodies tend to be more geologically active than smaller bodies.

More rapidly rotating bodies tend to have stronger magnetic fields.

Planetary Magnetic Fields

The interior of the planets contains electrically conducting iron and nickel, which flow like fluids over very long timescales.

Planetary rotation sets these fluids in motion and generates currents, which in turn generate magnetic fields surrounding the planet.

Earth’s Magnetic Field

Earth has the strongest magnetic field of the inner planets in the solar system.

This is believed to be important in protecting life on Earth from harmful charged particles from the Sun and elsewhere.

Earth’s Aurora

Charged particles from the solar wind become trapped in the Earth’s magnetic field and stream down to the poles, generating the phenomenon we see on earth as the Aurora.

Why are Some Moons and Planets Geologically Active and Others Dormant?

Geological activity requires a source of energy.

That source of energy is the heat interior to a body.

The amount of heat energy contained in a body is proportional to its volume.

The rate at which heat energy is lost is proportional to its surface area.

Consequently, smaller bodies cool more rapidly than larger ones. As a result, smaller bodies tend to be less geologically active.

Examples of Planetary / Lunar Interiors

Atmospheric Physics

In a gas in thermal equilibrium each molecule shares the same kinetic energy. This means that lighter molecules must be moving faster on average to have the same kinetic energy as heavier molecules.

H2

O2

Does Gravity or Heat Win?

Gravity exerts an inward pull on atmospheric molecules. Kinetic energy (in the form of heat) causes them to want to escape.

H2

O2

Gravity

Atmospheric Physics and the Giant Planets

The fate of a molecule is determined by the planet’s mass and the temperature of the atmosphere -- the larger the mass, the more species it can retain.

What about the most common element, hydrogen, and its molecular counterpart H2?

The Earth, and all inner solar system planets, lack the sufficient mass to retain H2.

The outer solar planets do have sufficient mass to retain H2.

This observation explains how the outer solar system planets grew to become giant planets.

How do Interplanetary Space Probes Work?

The night-time launch of Voyager 1 on a Titan-Centaur rocket in 1977.

How do Interplanetary Space Probes Work?

A space probe in orbit about the sun must obey the same laws laid down by Kepler, which apply to all orbiting bodies.

Planet 2

Planet 1

Space Probe

Sun

Question

Is it possible to send a satellite to the sun? What would its orbit look like?

Gravitational Slingshot Effect

If a spacecraft were to encounter a stationary body, its final speed must equal its initial speed.

Gravitational Slingshot Effect

The same encounter, viewed from the reference frame of the sun, looks very similar, except that the spacecraft has acquired the motion of the planet. It has gotten an “assist” from the planet.

Planetary Grand Tour

An rare alignment of the planets (occuring once every 200 years) permitted the Voyager 1 and 2 space missions a “Grand Tour” of the solar system using the gravitational slingshot effect.

Cratered Worlds : Mercury and the Moon

The cratered world of the Mercury and the moon share many properties in common with one another.

Both are too small to have any substantial atmosphere.

Without any atmospheric erosion, their surface records the earliest period of formation and subsequent history.

Mercury

Mercury is the smallest of the rocky inner planets, and is the least similar to Earth

Orbits the sun at .4 AU, and has a slow rotational speed with extreme seasons

Mercury Interior

Knowing the mass of Mercury, scientists have modeled the interior.

Among the rocky planets it is unusual for its very large iron core -- possibly due to an early impact.

Moon

The Earth’s moon is the nearest body in the solar system, with a radius about 25% that of the Earth, and a mass about 1% of the Earth.

The surface of the moon has been extensively studied by several unmanned and manned exploration missions.

Without weathering, the surface is ancient in comparison to the Earth -- almost 4 billion years old.

Scientists have concluded that the moon most likely originated from a giant impact early in the history of the solar system.

The cratering history indicates that impacts peaked in the distant past -- around 4 billion years ago.

Surface of the Moon

Daedelus Crater on Far Side of Moon

Venus

Venus is often referred to as Earth’s “sister planet” because of the similarity in orbit, size, mass, and rocky composition.

Venus orbits the sun at a distance of .73 AU from the sun.

Venus’s radius is 95% that of the Earth, and its mass is 82% that of Earth. Both planets have a molten core and a rocky composition, and nearly equal surface gravities.

Venus Photographed by Magellan Spaceprobe in Optical

Venus Imaged by the Magellan Spaceprobe in Radio (false color)

Venus vs. Earth

Despite these similarities, Venus is greatly dissimilar from the Earth in other respects :

Venus’s thick atmospheric surface pressure is some 90 times that of the Earth, made up almost entirely of carbon dioxide.

Venus’s surface temperature is incredibly hot -- nearly 900 degrees Fahrenheit (!) -- hot enough to melt lead. Clearly, no surface water exists.

Venus has no moon.

Venus’s rotational axis is has a tilt of just 3.4 degrees as opposed to 23.5 degrees for the Earth.

Venus is a very slow rotator -- A Venusian day is one Venusian year (243 Earth days).

Venus lacks a magnetic field.

Venusian Atmosphere

The Venusian Atmosphere is hottest at the surface, but already by the upper cloud layer the temperature is much higher than on Earth.

80 F 620 F 940 F

Mars

The “red planet” Mars is the current focus of NASA’s unmanned interplanetary missions, because it is believed to have once harbored a warm, moist Earth-like phase -- possibly even life.

There are several similarities between Earth and Mars.

Mars orbits the sun at 1.5 AU.

Its axis is tilted at 25 degrees.

Its day is nearly identical to one Earth day.

Mars vs. Earth

Mars is much smaller than the Earth, with a radius about half that of Earth, and a mass of about a tenth the Earth’s.

The surface temperature today is far below freezing.

Even if one could warm water ice on Mars today, it would go directly into a gaseous state without becoming liquid because of the thin atmosphere.

It has two tiny moons, Phobos and Deimos.

While tilt is similar to that of Earth today, the tilt angle oscillates wildly over tens and hundreds of millions of years.

It has only a weak magnetic field in its crust, and lacks a magnetic core.

Next Week -- Global Warming, and the Outer Planets

Next week we will continue our discussion of Mars and the Earth, comparing the geology of the two planets.

We will discuss the consequences of enriching our atmosphere with carbon dioxide -- is Earth destined to become like Venus?

We will also begin to cover the Outer Planets.

First midterm in two weeks!