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The Moons of the Gas Giants Astronomy 311 Professor Lee Carkner Lecture 20 The Group of Seven There are seven large (diameter >2000 km) satellites in the solar system Each is a distinct world of its own Jupiter’s Lovers Io, Europa and Callisto were Jupiter’s lovers in Greek mythology Saturn was the king of the Titans The Galilean Moons Discovered by Galileo in 1610 Studied by Voyager 1 and 2, HST and Galileo (the spacecraft) Orbits of the Galilean Moons All are tidally locked to Jupiter in a 1to-1 ratio 1 The periods of the orbits of the 3 inner moons are in a 1:2:4 ratio Formation of the Galilean Moons The inner parts of the nebula were hotter than the outer The inner 3 satellites experienced tidal heating and differentiated into a rocky core and an icy crust The Interiors of the Galilean Moons Galileo Visits the Galilean Moons The Galileo spacecraft arrived at Jupiter in 1995 and studied the moons of Jupiter for 8 years Io Io is the most volcanically active world in the solar system These changing forces squeeze and flex Io producing heat The interior heat has also produced a differentiated interior Volcanism on Io Io has no impact craters Volcanoes produce plumes of material that extend up to 280 km above the surface Volcanoes can be very long lived Io’s Plasma Torus Io’s volcanoes put lots of ions into its orbit The ions are effected by Jupiter’s magnetic field producing a plasma torus Europa Europa’s surface is covered with a layer of ice Water flows up to the top continually resurfacing Europa Tidal flex may also crack the surface Evidence for Warm Oceans on Europa Europa has ice rafts where the surface has been broken up and reassembled Europa also has smooth areas where water has flowed up and re-frozen Galileo magnetometer measurements indicate that Europa has a variable magnetic field Models for the Interior of Europa Ganymede Ganymede is the largest moon in the solar system Ganymede should have little tidal heating due to its distance from Jupiter Ganymede must have had more geological activity in the past The Surface of Ganymede Ganymede shows 2 types of surface features Old dark terrain New bright terrain Callisto Callisto is the most distant Galilean moon from Jupiter It has experienced the least tidal heating Callisto shows few signs of interior or surface activity Large impact basin Valhalla Titan The second largest moon in the solar system Only moon with an atmosphere Why does Titan have an atmosphere? Titan’s Atmosphere Titan has a thicker atmosphere than the Earth Titan’s atmosphere may have originally been composed of ammonia (NH3) and methane (CH4) 90% of the atmosphere is N2 Chemicals in Titan’s Atmosphere Titan’s atmosphere also contains hydrocarbons (composed of H and C) and polymers (long chains of H, N and C) Titan may have the necessary organic material to form the building blocks of life Triton Triton is in a decaying, highly inclined (23 degrees), retrograde orbit Triton shows evidence of geologic activity When Triton was first captured it was probably in a highly eccentric orbit which resulted in tidal heating Triton’s Atmosphere Triton has a very thin nitrogen atmosphere (1.6 X 10-5 atmospheres of pressure) A little bit of nitrogen evaporates to produce the atmosphere Summary The six large moons of the gas giants resemble the terrestrial planets of the inner solar system They can have volcanoes, atmospheres, and evidence of resurfacing In general they are cold and have rocky interiors and icy exteriors Some produce internal energy through tidal heating Europa and Titan may possibly have the conditions for life to exist Summary: Io and Europa Io Strong tidal heating produces massive volcanism Volcanism produces powerful outgassed plumes, sulfurous surface and plasma torus of ions Europa Icy surface shows evidence for water flowing up from interior May have a warm subsurface ocean due to tidal heating Summary: Ganymede and Callisto Ganymede Shows both old dark terrain and bright new terrain Must have had more internal heat to drive geologic activity in the past Callisto No tidal heating results in no differentiation Fairly uniform mixture of icy and rock with many craters Summary: Titan and Triton Titan Large size and low temperatures results in an thick atmosphere Atmosphere composed of nitrogen, methane, hydrocarbons and polymers Triton Has a decaying, inclined retrograde orbit Thin atmosphere and surface activity
