ConceptChecks
ConceptCheck 13-1: You can determine the diameter of the satellite, even though you cannot see the physical extent of the satellite. As the satellite is increasingly blocked or occulted by the planet (from our view), we receive less and less of the satellite’s reflected light. The time it takes for the satellite’s light to completely fade away tells you the transit time, and, combined with the satellite’s speed, the diameter can be calculated.
ConceptCheck 13-2: No. In order to measure the masses of Jupiter’s moons, the gravitational deflection of a spacecraft by these moons had to be measured.
ConceptCheck 13-3: Because the average density of each moon decreases with distance from Jupiter (Table 13-1). This implies a warmer interior in an early Jovian nebula, where rock and metal grains would form the bulk of solid moon-forming material. Farther away from Jupiter, water would freeze with solid grains and help build moons with a lower average density. This is analogous to the formation of the solar system discussed in Section 8-5.
ConceptCheck 13-4: It is not enough for Jupiter to exert a constant tidal force on Io; that would be like squeezing a ball into one deformed shape and keeping it there. A varying tidal force is necessary, and this arises due to Europa and Ganymede rhythmically tugging the long axis of Io’s orbit back and forth by about ½°.
ConceptCheck 13-5: No. Jupiter has its own magnetic field that comes from electric currents within its interior. As Jupiter’s field sweeps over Io, electric currents and a magnetic field are induced. This induced magnetic field is a separate and much weaker field than Jupiter’s.
ConceptCheck 13-6: The orbital resonance of Io, Europa, and Ganymede causes variations in the tidal forces exerted onto these moons by Jupiter. These tidal forces generate heat, but are stronger on Io because it is closer to Jupiter than Europa.
ConceptCheck 13-7: The induced magnetic field comes from the induced electric current in Europa created by Jupiter. For this current to flow through Europa’s water-interior, the water must be liquid and therefore not solid ice. When water comes in contact with rock (probably Europa’s rocky mantle), the water dissolves some of the rock’s minerals, making the water electrically conductive. Modeling of Europa’s induced magnetic field points to a large subsurface ocean.
ConceptCheck 13-8: Callisto has an unexpected induced magnetic field: unexpected because this indicates a liquid ocean, yet at Callisto’s temperature any interior water should be frozen. However, ammonia allows water to remain a liquid at temperatures where pure water would freeze.
ConceptCheck 13-9: Ninety-five percent of Titan’s atmosphere is nitrogen, which is thought to come from either ultraviolet sunlight breaking down ammonia or comet impacts generating nitrogen from ammonia. Ammonia in water also lowers the temperature at which that water will freeze, and this allows water below the surface to act like lava in cryovolcanoes.
ConceptCheck 13-10: Cassini discovered lakes using radar. By reflecting radio waves, radar can be used to map the height of surface features. The lakes show up as very flat regions, and tributary channels support this interpretation.
ConceptCheck 13-11: Voyager found that temperatures and pressures on Titan are just right for methane to exist as a gas, liquid, or solid. Titan’s methane cycle depends on these changes between liquid and gas, just as Earth’s hydrologic cycle depends on changes between liquid and gas phases of water.
ConceptCheck 13-12: If Jupiter formed like a mini solar system as expected, any moon that formed with Jupiter would have a prograde orbit (in the same directions as Jupiter’s rotation), and would also orbit approximately in Jupiter’s equatorial plane. The retrograde moons (orbiting in the direction opposite to Jupiter’s rotation at high inclinations to Jupiter’s equator) must have a different origin, and the capture of asteroids is most likely.
ConceptCheck 13-13: Hyperion’s average density is less than half that of water. The only obvious way to account for Hyperion’s size then, is to assume it is very “porous” and has a lot of empty space inside.
ConceptCheck 13-14: Cassini passed through the plumes and detected dissolved minerals (or salts). The simplest interpretation is that a large subsurface ocean partially dissolves minerals from a rocky mantle.