A Rich Variety of Environments

The satellites of Jupiter and Saturn offer a rich variety of environments. The innermost of the four Galilean satellites of Jupiter is Io (Figure 5). It is so close to Jupiter that it is tortured by massive land tides, which heat the moon and keep it active. There are spectacular volcanos on Io that spew sulfur and sulfur dioxide. The surface of Io is truly a place of fire and brimstone, like the traditional hell that I learned about in Sunday school, not the modern hell of dreary monotony, with no MTV or remote controller, that I heard about yesterday. 

Figure 5. Io

The next Galilean satellite is Europa, a small rocky planet covered by frozen oceans and laced with networks of cracks and ridges in the ice. Some planetary scientists speculate that there may be liquid water under the surface ice, and that this environment might support simple life forms. 

Figure 6. Europa

Next is Ganymede, the largest of the Galilean satellites, but still only twice the mass of the moon, one fortieth the mass of the earth. Ganymede is a rocky moon, and it has tectonic activity that has created a series of mountain ridges on its surface.

Figure 7. Ganymede

Callisto is a quieter world, made of more icy materials. It is less affected by the tidal forces from Jupiter, and some scientists consider it to be the most promising remaining site in the solar system for the evolution of simple life.

Figure 8. Callisto

Titan, the largest satellite of Saturn, is also very interesting, because it has managed to retain a substantial atmosphere.

Figure 9. Titan

The wide variety of environments that we find on the satellites of the giant planets in our own solar system suggests that the satellites of extrasolar giant planets may be the best hope for finding habitable abodes elsewhere. In the case of upsilon Andromedae I am not very optimistic that habitable satellites will be available, because the orbits of the outer two planets are too eccentric and therefore are not able to provide the thermal stability that we think is a prerequisite for the rise of life.

Now that we have a second example of a planetary system orbiting a solar-type star (actually a third system, if you include the planets orbiting the pulsar PSR1257+12),it is much easier to imagine that planets like to form in systems. But, we need to show that this is actually the case, by finding multiple planets in many more systems.

Contributed by: Dr. David Latham