1. Test question 12.1 • What is so special about the orbits of Io, Europa, and Ganymede • xTheydon’t change over time • xTheyare perfectly circular • vTheirorbits are in resonance • xTheyare not in the same plane

2. Test question 12.2 • The energy source for Io’s volcano • vAcombination of tidal distortion and dissipation • xRadioactiveisotopes • xBombardmentby icy comets • xSolarirradiation

3. Test question 12.3 • There are hardly any impact craters on Europa whereas they are densely packed on Callisto because: • xEuropawas captured recently but Callisto formed long ago, • xEuropawas protected by Jupiter’s gravity, • xCallisto’ssurface is made of fine sands which can easily be reshaped by relatively small meteorites D) vEuropahas a warm ocean which flood its surface from time to time

4. Test question 12.4 • The structure of Saturn’s ring is • xAflat and solid disk • xAgaseous disk span out of its envelope • vAcollection of sub-meter size particles • xAnoptical illusion created by the refraction of light through Saturn’s atmosphere

5. Test question 12.5 • Titan’s mass is less than that of the Moon. Yet it has a thick atmosphere whereas Moon is without it. This difference is due to • vTitan’ssurface is much colder • xTitanhas many more active volcano to replenish its atmosphere • xMoon’satmosphere is removed by solar wind • xMoonhas no nearby rings to protect it from Sun’s ultraviolet radiation

6. Test question 12.6 • Spiral density waves in Saturn’s ring are induced by • xCollisionsinside the rings • xGiantimpacts from meteorites • xPlanetquake inside Saturn • vMoonsaround the ring

7. Test question 12.7 • Uranus rings have width no more than tens of km because • xTheyare too thinly populated • vTheyare confined by the perturbation of shepherding moons • xTheyare glued together as coherent entities • xUranus’ gravity dominates their motion