ASTRONOMY 161

ASTRONOMY 161Introduction to Solar System Astronomy

Class 12

Solar System SurveyMonday, February 5

Key Concepts

(1) The terrestrial planets are made primarily ofrock and metal.

(2) The Jovian planets are made primarily ofhydrogen and helium.

(3) Moons (a.k.a. satellites) orbit the planets; somemoons are large.

(4) Asteroids, meteoroids, comets, and Kuiper Beltobjects orbit the Sun.

(5) Collision between objects in the Solar Systemcause impact craters.

Family portrait of the Solar System:

Mercury, Venus, Earth, Mars, Jupiter, Saturn,Uranus, Neptune, (Eris, Ceres, Pluto):

My Very Excellent Mother Just Served Us Nine(Extra Cheese Pizzas).

The Solar System:List of Ingredients

Ingredient

SunJupiterother planetseverything else

Percent of total mass

99.8% 0.1%

0.05% 0.05%

The Sun dominates the Solar System

Jupiter dominates the planets

Object1) Sun2) Jupiter3) Saturn4) Neptune5) Uranus6) Earth7) Venus8) Mars9) Mercury

Mass330,000

3209517151.0

0.820.110.055

Object

10) Ganymede11) Titan12) Callisto13) Io14) Moon15) Europa16) Triton17) Pluto

Mass

0.0250.0230.0180.0150.0120.0080.0040.002

A few words about the Sun.

The Sun is a large sphereof gas (mostly H, He –hydrogen and helium).

The Sun shines because itis hot (T = 5,800 K).

The Sun remains hotbecause it is poweredby fusion of hydrogento helium (H-bomb).

(1) The terrestrial planets aremade primarily of rock and metal.

Mercury, Venus, Earth, & Mars.

The terrestrial planets are:low in mass (< Earth mass)high in density (> 3900 kg/m3).

Water = 1000 kg/m3

Air = 1 kg/m3

Rock = 3000 kg/m3

The mass of a planet is determined byNewton’s version of Kepler’s Third Law,applied to a satellite (natural or artificial).

The average density is mass divided byvolume (for a sphere, V = [4π/3] r3).

The density of terrestrial planets is greaterthan that of rock, reflecting the presence ofextremely dense metal cores.

(2) The Jovian planets aremade primarily of hydrogen and helium.

Jupiter, Saturn, Uranus, &Neptune:

The Jovian planets are:

high in mass (> 14 Earth masses)

low in density (< 1700 kg/m3).

Jovian planets are made mainlyof light elements like hydrogenand helium.

Pluto: The “Oddity” (Now a “Dwarf Planet”)

Pluto is very low in mass and moderate in density(about 2000 kg/m3). It is surmised that Pluto ismade of mixture of ice and rock.

Spectra, again!

The spectrum of reflected light tells us what anobject is made of (at least on the surface).

Example: The spectrum of Pluto is similar to that ofmethane ice (frozen CH4).

(3) Moons (a.k.a. “satellites”) orbitthe planets; some moons are large.

The current moon count:Mercury = 0Venus = 0Earth = 1Mars = 2

Jupiter = 63Saturn = 56Uranus = 27

Neptune = 13

Ceres = 0Pluto = 3Eris = 1

Jovian planets are moon-rich; others are not.

The Giant Moons(moons bigger than Pluto)

Earth:Jupiter:

Saturn:Neptune:

The MoonIoEuropaGanymedeCallistoTitanTriton

Giant Moons and Dwarf Planets

The giant moons are low in density compared to theterrestrial planets.

Another Size Comparison

(4) Asteroids, meteoroids, comets,and Kuiper Belt objects orbit the Sun.

Asteroids: made of rock and metal, less than1000 km across. (Most asteroids are in orbitbetween mars and Jupiter.)

Meteoroids: made of rock and metal, lessthan 300 meters across. (When a meteoroidenters Earth’s atmosphere, it produces ameteor.)

Comets: made of dirtyice, a few kilometersacross. (Comets havetails of gas and dustwhen they come near theSun.)

Kuiper Beltobjects:made of ice,hundreds ofkilometersacross. (The“KuiperBelt” liesbeyond theorbit ofNeptune.)

(5) Collisions between objects in theSolar System cause impact craters.

The orbits of the planets are well separated; planets donot collide with each other.

Smaller objects, though, frequently collide with planetsand moons.

When a meteoroid,asteroid, or cometstrikes a terrestrialplanet (or moon), itblasts out an impactcrater.

The Moon is heavilycratered; Earth hasfew craters, due toerosion andgeological activity.

The few impact craterson the Earth havebeen smoothed outby erosion, glaciers,lava flows, etc.

A heavily crateredworld is ageologically “dead”world.

The current state of the Solar Systemcontains clues to its history.

Terrestrial planets are close to Sun

a = 0.4 A.U. 1.5 A. U.

Jovian planets are far from Sun

a = 5 A.U. 30 A. U.

Just coincidence, or an important clue?

All planets revolve in the samedirection (counterclockwise,seen from above the North Pole).

Just coincidence, or an important clue?

Most (but not all) planets rotate in the same direction(counterclockwise, seen from above the North Pole).

Uranus and Pluto are “sideways”, Venus is “upside-down”.

Could this also be a clue?

Few closing questions:

1) How do we know the mass of Mars? Mass ofVenus?

2) Which one is denser: Moon or Earth?

3) Which one is denser: Moon or Pluto?

4) Can density of a planet be lower than density ofwater?

5) What are the seasons on Uranus?