5B Satellites of the outer solar system The almost-planets

5B Large Moons

5B Galilean Satellites

5B Interiors

5B Surfaces

5B Io

5B Io Glowing in the Dark... Galileo images show glowing gases, hot volcanos, in Jupiter’s shadow Volcanic glow suggests volcanos are too hot to be “ordinary” basalt magma

5B Eruptions

5B

5B Europa

5B 100 x 140 km

5B 32 x 42 km

5B Eroded appearance at highest resolution: Few impact craters 1.7 x 4 km

5B Ganymede

5B Voyager: 2 terrain types on Ganymede: cratered and grooved

5B Intense faulting in grooved terrain 79 x 57 km

5B But some young areas are very smooth… Surface age: 3.5 b.y. or 1.0 b.y.? 54 x 90 km

5B Cratered terrain: also complex

5B Callisto Multi-ring basins Bright fresh craters Many, many, other craters… km across

5B Zooming in on Callisto

5B Big surprise at high resolution: –Eroded appearance –Few small craters 55 x 44 km

5B Dark material between craters is very smooth at small scales- Why? Sublimation? 13 x 13 km

5B Comparative planetology

5B Surface Processes Io –Volcanism (endogenic) –No craters (young surface) Europa –Tectonics (endogenic) –Cryovolcanism (endogenic) –Few craters (young surface) Ganymede –Tectonics (endogenic) –Some craters (older surface) Callisto –Mostly Craters (very old surface)

5B Energy Recall, for endogenic processes to occur energy is needed. However, the Galilean satellites are small, so they have little internal heat (original or radioactive). They are far from the Sun, so get little solar energy. Where does the energy come from?

5B Tidal Heating

5B Titan

5B Titan’s surface will be revealed with radar mapping by the Cassini spacecraft Its atmosphere will be measured with the Huygens probe Its thick atmosphere harbors complex chemical reactions For this reason, it is interesting to study as an early-earth analog and it may harbor life The surface may consist of large ethane seas

5B Triton

5B Triton is in a retrograde motion. This implies it was captured. Combine this fact with their compositional similarities and Pluto’s uniqueness leads some to postulate a similar origin for the two bodies. The existence of an atmosphere on Triton can be seen in the wind streaks on the surface Triton’s surface is relatively young, there are few craters.

5B

5B MirandaAriel Umbriel TitaniaOberon Thalassa Despina Larissa Proteus Prometheus Janus

5B Small Satellites The remaining outer planet satellites are small bodies of rock and ice They have little internal heating and little endogenic activity Because of their old surfaces, they are generally heavily cratered Metis Adrastea Amalthea Thebe

5B Small Satellites – some are quirky Mimas’s large crater Hershel was almost enough to shatter the small moon Iapetus is two-faced with a dark leading hemisphere (3% reflectivity) and a bright trailing hemisphere (50%) Miranda’s surface is extensively modified with great valley systems (10 km deep) and peculiar trapezoidal mountain ranges

5B Comparative Planetology 2 In general, the bigger a planetary body is, the more energy is available to modify it –Small bodies are irregularly shaped and covered only with craters (exogenic) –Slightly bigger bodies are spherical –Bigger bodies are spherical and differentiated –The largest bodies are spherical, differentiated, and have endogenic processes which modify their surfaces