Galilean Satellites
- Density pattern just like planets around Sun
- disk remained after Jupiter formed Z 83
- material closer to Jupiter was hotter
- Jupiter very hot and luminous at birth
- gravitational energy converted to heat
- cf. condensation sequence fig. 20-3
- IO and Europa mostly rock
- Ganymede and Callisto 50:50 rock and ice
- Surfaces
- we see reflected sunlight at shorter wavelengths (i.e. visible)
- but thermal, black-body radiation in the infrared
- Callisto infrared spectrum => surface T ~ 155 K
- cf. water ice freezes at 32 F = 0 C = 273 K
- Look at features in reflectance spectra
- water ice on Callisto, Ganymede, and, especially, Europa
- no ice on Io, but sulphur dioxide (SO2) frost