The innermost satellites of Jupiter, in order of distance from the planet, are Amalthea, Io, Europa, Ganymede, and Callisto. The latter four were discovered by Galileo in 1610 and are collectively referred to as the Galilean
satellites. Amalthea was discovered by Barnard in 1892. They all have nearly circular prograde orbits lying almost exactly in Jupiter’s equatorial plane. Our knowledge of the Galilean satellites increased considerably as a con-
sequence of the flybys of Voyagers 1 and 2 on March 5, 1979 and July 9, 1979, respectively, and the Galileo mission (1995–2000) has yielded a further enormous jump in our knowledge of these bodies. We now know as much
about the surfaces and interiors of the Galilean satellites as we do about some of the terrestrial planets in our inner solar system. These satellites are very different from one another and from the terrestrial planets; many of
the physical processes occurring in their interiors and on their surfaces are unique to these bodies.
From Appendix 2, Section D, it can be seen that Io is only slightly larger and more massive than the Moon. Its similarity to the Moon extends no further, however; the Voyager and Galileo missions showed Io to be the
most volcanically active body in the solar system. During the flybys of both the Voyager and Galileo spacecraft numerous active volcanic plumes were observed, some extending to heights of hundreds of kilometers above the
surface. Io (Figures 1–77 and 1–78) displays a great diversity of color and albedo; spectral reflectance data suggest that its surface is dominated by sulfur-rich materials that account for the variety of colors – orange, red,
white, black, and brown. Io’s volcanism is predominantly silicate-based as on the terrestrial planets though sulfur-based volcanism also occurs. The circular features on Io are caldera-like depressions; some have diameters as large as 200 km. There are no recognizable impact craters on the satellite, although the flux of impacting objects in the
early Jovian system is believed to be as large as it was around the terrestrial planets. Io’s surface is geologically very young, the silicate and sulfur lavas having only recently resurfaced the planet. Relatively few of Io’s calderas
are associated with structures of significant positive relief. Thus they are quite unlike the calderas of the Hawaiian volcanoes or the Tharsis volcanoes on Mars. There are isolated mountains with considerable relief on Io (∼10
km), but their exact height as well as their mode of origin is uncertain. The source of heat for Io’s volcanism is tidal dissipation. The gravitational interaction of Io with Europa and Ganymede forces Io into an orbit with
higher eccentricity than it would have if it were circling Jupiter by itself. The resulting tidal flexing of Io in Jupiter’s gravity field dissipates very large quantities of heat; Io’s surface heat flow exceeds the global heat loss
from the Earth by a factor of 3 or more. Tidal dissipation is insignificant as a heat source for the terrestrial planets that are heated mainly by the decay of radioactive elements. However, the special circumstances of Io’s
orbit around a massive planetmakes tidal heating an unusually effective heat source for Io.