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 1- to-1 ratio 1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 has been studying the moons from Jupiter orbit ever since    

Galileo at Io

Surfaces of the Galilean Moons

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 (NH 3 ) and methane (CH 4 )   90% of the atmosphere is N 2 

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 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