The presence of an atmosphere on a small planetary body the size of the Moon is surprising. Loss of material by escape would have depleted the atmosphere over the age of the solar system. Since these objects are not large enough to possess, or to sustain for long, a molten core, continued outgassing from the interior is not expected. However, it is now known that four small bodies in the outer solar system possess substantial atmospheres: lo, Titan, Triton, and Pluto. These atmospheres range from the very tenuous on lo (of the order of a nanobar) to the very massive on Titan (of the order of a bar). The atmospheric pressures on Triton and Pluto are of the order of 10 μbar. Perhaps the most interesting questions about these atmospheres concern their unusual origin and their chemical evolution. lo is the innermost of the four Galilean satellites of Jupiter, the other three being Ganymede, Europa, and Callisto. All the Galilean moons are comparable in size, but there is no appreciable atmosphere on the other moons. The first indications that lo possesses an atmosphere came in 1974 with the discovery of sodium atoms surrounding the satellite and the detection of a well-developed ionosphere from the Pioneer 10 radio occultation experiment. The Voyager encounter in 1979 established the existence of active volcanoes as well as SOa gas. These are the only extraterrestrial active volcanoes discovered to date, and they owe their existence to a curious tidal heating mechanism associated with the 2:1 resonance between the orbits of lo and Europa.
An Impacting Descent Probe for Europa and the Other Galilean Moons of Jupiter
