Ages of surfaces - activity
We use crater counts to estimate relative ages
- higher crater density implies older surface
- cf. Moon
- Only case where age as a function of crater density has been calibrated
- radioactive dating of Moon rocks from known locations Fig. 7
- Elsewhere (i.e. near Jupiter), we expect a similar pattern
- steady rate of crater production over last 3 b. y.
- “Heavy bombardment” phase in the early years of the Solar System
- Actual rates at Jupiter not known
- expect some increase near Jupiter because of gravitational focusing
- Callisto more heavily cratered than Lunar Maria
- age comparable to oldest (most heavily cratered) surfaces on the Moon, Mars, and Mercury
- little surface activity over last 4 b. y.