the Moon
By far biggest moon of terrestrial planet
- ~ 1/4 diameter of Earth
- Comparable to largest Galilean satellites
Density (mass and radius) 3.4 g/cm3 -- low!
- less than terrestrial planets
- mostly rock, little Fe core
Rocky body
- many craters (impact structures -- like Mercury
Lack of atmosphere
- 2.4 km/s escape velocity (low!, Earth’s is 11 km/s)
- High T ( ~300 K) => gas escapes vg 68
- No weathering -- long-lived features
- craters as old as 4 billion y. still visible
- oldest craters on earth about 100 million y. old