Jupiter

Jupiter has 17 moons (a tiny one was just recently discovered). Additionally, closer to the planet than the inner moons are faint dust rings formed by debris from the original solar nebula,

Many of Jupiter's moons are small in size, and may have been asteroids captured by the giant planet's large gravitational pull. The four galilean moons, however, have a relatively large mass. None of them is massive enough to keep an atmosphere, but two of them, Io and Europa, are geologically active. Io and Europa, are decidedly rocky, with a density similar to that of Mars. The outer moons, Ganimede and Callisto, are made of water ice mixed with rock.

It is probable that Jupiter formed as a miniature solar system. Jupiter was once quite warm, with a surface temperature reaching about 1,000 K. The temperature in the sub-nebula from which the moons formed was high enough for the ice to sublimate, leaving the inner moons richer in rocky materials. The geological activity of the inner moons later left them even more depleted of volatile compounds, such as water.

Io is the nearest to Jupiter of the four major moons. It is also the most geologically active, as was observed by the two Voyager missions in 1979. There are eight currently active volcanoes on Io, erupting both lava and gas. The gases from the volcanoes are quickly stripped from the moon by the interaction with a particle wind coming from Jupiter. Io is also quite a small body, with a relatively small surface gravity: its mass is just 1.22 times that of the Moon. Like our Moon and Mercury, it would be too small to have an atmosphere even if Jupiter were at a safer distance.

The volcanoes on Io are quite different from those in the inner solar system. They do not erupt lava made up largely of silicates, like Earth's volcanoes, but rather lava that is mostly made of fluid sulfur mixed with other elements. The large lava flows we see covering vast areas of the moon are yellow and red in color. Volcanoes also erupt large gas plumes, mostly made of sulfur and sulfur oxide (SO₂), and smaller quantities of hydrogen sulfide (H₂S). These plumes rise up for hundreds of kilometers above Io. Sulfur oxide condenses into ice and partially rains back to the surface, together with sulfur. The moon is covered by this dust, and that is the reason why we do not see meteoric impacts on its surface.

Io is a rocky world. Its internal heat is the engine of the volcanoes, and made it impossible for Io to retain water or any other volatile compound. Its core is probably molten. The source of the internal heat is subject of educated speculations, but it is probable that the heat derives from the gravitational interaction of the moon with Jupiter.

Just like our Moon, tidal forces with a major body in the vicinity forced a change in the period of rotation of Io, so that Io ended up have a period or rotation equal to its orbital period (see the SparkNote on Mercury for more detail on this mechanism). The eccentricity of Io's orbit, however, is such that tidal stresses are still present on Io. The same does not happen to our Moon because its orbit is almost circular. The material in the interior of Io undergoes periodic deformations leading to friction and therefore heat. The heat escapes the moon by means of volcanic activity.

One of the most exciting findings of the Voyager missions is that Europa, the second closest moon of Jupiter, is covered by a layer of water ice so young that it may hide a vast ocean. The initial conjecture was greatly strengthened by the flood of information from the Galileo orbiter that took many detailed images of this interesting moon.

The first thing one notices on Europa is the near absence of meteoric craters, a feature that Europa shares with Io (which is volcanically active) but not with the other moons of Jupiter. The ice on the surface of Europa is white, similar to the ice at the North Pole of our planet. It also shows many cracks and ridges, on scales that range from a large fraction of the size of the moon to hundreds of meters.

The criss-crossing pattern of the ridges is very similar to what we observe when watching Earth's North Pole from space. This is a clear indication that the Europa's ice may be quite thin, and floating on a liquid ocean. The ocean does not wholly freeze due to the moon's internal heat, generated by a mechanism probably similar to that working on Io.

This is a major discovery because we think that life needs liquid water to develop. So far, no place we've explored in the solar system has any liquid water on its surface, but Europa seems to have plenty in its interior. Recent research shows that, even in the absence of light, life thrives deep in Earth's oceans by using the chemicals in hot vents as a source of energy. Indeed, the bacteria found around such vents are so archaic that they could be the progenitors of all other forms of life on our planet. It is not a large leap of imagination to theorize that Europa's underground oceans may harbor primitive life forms.

Although Ganimede and Callisto are larger than Io and Europa, they are not geologically active. Jupiter's tidal action is not as effective at the greater distance where these two moons orbit. Hence, these two moons do not have good sources of internal heat, and they may be completely frozen inside.

Callisto does not seem to have changed much during the last 4 billion years. Its surface is dotted by a myriad of craters, indicating that it underwent extensive meteoric bombardment that was never erased by any geological forces reshaping the moon (in contrast to Europa and Io). The relatively dark color of the water ice covering Callisto may be an indication that Callisto's interior never differentiated. In other words it never achieved a temperature high enough to melt the ice and let the rocky (heavier) materials sink toward its center.

Ganimede seems to have had some geological past, since a portion of its surface is dotted by fewer meteoric craters. Those regions of the moon could be about 3 billion years old, and they may have been created by some event that let liquid water closer to its surface. In many respects, Ganimede is a moon with characteristics intermediate between Europa and Callisto.