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.
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
Figure 8. Callisto
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