Introduction to the Solar System

In general terms we want to answer the following questions:
Why is the Earth geologically active while the moon isn't?
Why are small rocky planets in the inner solar system and large gas giants (w icy moons) in the outer solar system?

I. Formation of the Solar System.

A. Brief history of the Universe:
    Big Bang ~18 Gy ago.
    Galaxy formation ~12 Gy (material clumps).
    Stars form and explode continuously in large clusters
    Solar system forms ~4.6 Gy
    Sun will explode in ~5.4 Ga.

B. Some points of solar system formation:
     Stars form from cloud of dust.
    Gravitational collapse, possibly triggered, makes disk.
    Nuclear reaction forms star.
    Remaining material in disk makes binary star or solar system.

II. Comparing the Moon and Earth.

A. The Moon:
Two units - grey, heavily cratered (anorthosite), the highlands.
- dark, smooth areas (basalt), maria.
We can tell dark areas are younger without samples by counting the number of craters per unit areas - less craters means younger.
Basic history: Highlands formed from segregation of magma ocean at 3.2 Ga. Radioactive heating causes episode of basaltic volcanism, forming the maria.

B. The Earth:
Has abundant examples of geologic activity:
Erosion - water and wind,
Volcanism,
Tectonism - faulting and folding.
Most of the latter two can be directly tied to plate tectonics:
Earth is the only planet with current plate tectonics.
Plate tectonics is driven by mantle convection.
Mantle convection is caused by radioactive heating.
Plate tectonics removes heat efficiently.
Planets with high volume to surface area ratios retain their heat, and hence their geologic activity, longer. This is why the Earth is active and the Moon is not.

III. The other terrestrial (rocky) planets. Their size should allow prediction of their level of geologic activity.

A. Mercury:
Slightly larger than the Moon, is heavily cratered like the Moon.
Thrust faults indicate planet contracted overtime.
Has a high density, possibly indicative of a large iron core.

B. Venus:
Nearly same size as Earth, and has quite a bit of geologic activity.
Dense CO2 atmosphere cause high surface temperature and pressure.
No water and low surface winds means no erosion.
Average surface age ~500 m.y.
Some unique terrains: tesserae - highly deformed areas; coronae - volcano tectonic plateous surrounded by a topographic moat.

C. Mars:
In between Earth and Moon in size, and has a mix of very old and very young geologic units.
Largest volcanoes in the solar system.
Once had running water (and may still have), as indicated by channels and valley networks.
Wind erosion still takes place.

IV. The outer planets and samples of their satellites.
Farther out in solar system each planet gets larger portion of nebular disk but is farther from the Sun. End result - large gaseous planets with icy satellites.

A. Jupiter:
Largest planet in the solar system: 1000x volume of Earth, 1/1000 mass of the sun. The Galilean satellites from farthest to closest:

1. Callisto:
Slightly larger than Mercury, but ice-rock mixture
Largest impact basin in solar system - Valhalla

2. Ganymede:
Largest moon in the solar system, 8% larger than Mercury.
Has old terrain like Callisto, but also slightly younger grooved terrain.
May have liquid water in interior, has magnetic field.

3. Europa:

Looks like fractured white ball.

            Mean density (3030 kg m-3) indicates mostly rock.
            Few impact craters because of geologic activity related to tidal interaction with Io
            May have liquid ocean near surface.
            Analog to plate tectonics on Earth.

4. Io: Same size as our moon, but most volcanically active moon in solar system due to tidal interaction with Europa.

B. Saturn:
1. The rings:
1,000,000 km across only a few meters thick. Made of millions of ice chunks 1 cm-1m in diameter.
Particles are inside Roche limit and tidal forces prevent coalescence of a moon.
2. Most of the moons are mostly ice and heavily cratered.
3. Enceladus - has recent ice flows, possibly as a result of interaction with Dione.
4. Titan - only moon with a dense atmosphere. Atmosphere has many organic molecules and possibly methane oceans. Could there be life?

C. Uranus:
1. Planet spins and rings and moons orbit at 90o angle with ecliptic.
2. Most moons heavily cratered.
3. Miranda (a moon) has add ridged structures termed coronae (not related to coronae on Venus). Origin is unknown.

D. Neptune:
1. Largest moon, Triton, is probably a captured planetesimal and has very few craters. Triton has odd-shaped cantaloupe terrain.
2. Triton has active ice "volcanic" plumes
3. Triton is in a retrograde orbit and will eventually smash into Neptune

E. Pluto and Charon - can be thought of as a binary planetary system; moon is very close and 1/8 size of Pluto.

F. Comets and Asteroids - the icy and rocky leftovers from formation of the solar system.