Cosmorphology: Creating forms thru geologic and artistic processes
Mayterm 2009 - May 1

Goals for today:



There are at least 125 satellites (moons) in the outer solar system. While the major planets of the outer solar system (Jupiter, Saturn, Uranus, and Neptune) are very different places from the inner, terrestrial planets (Mercury, Venus, Earth, and Mars), several of the moons of those planets have solid surfaces and geology not unlike that on terrestrial worlds. The major processes of impact cratering, volcanism, tectonics, and erosion occur on these moons. However, the surface materials involved in these processes can be completely different. In the cold of the outer solar system, substances that on Earth are liquids or gases, are as hard a rocks and can make up the bedrock of these moons.

Small, Medium, and Large Moons

Generally speaking, moons of the outer solar system can be dived into three groups: small moons are those less than about 300 km in diameter, medium moons range from 300 to 1,500 km in diameter, and large moons are greater than 1,500 km in diameter. Most of the small moon are dead worlds with little to no evidence of internal geologic activity. They do not have enough mass for gravity to force them into a spherical shape, so they generally resemble potatoes. They are so small and difficult to find that many have only recently been discovered and there are likely many more yet to be found.

The medium and large moons (see figure) are planetlike in almost all ways. They are generally spherical and have features on their surfaces created by both active and external geologic processes. Some have atmospheres and some have hot interiors that cause ongoing geologic activity. If you think about the role of planetary size in the geology of the terrestrial worlds, you might expect all of the outer planet moons to be geologically dead. After all, only two of the moons are even as large as Mercury, and Mercury has been geologically dead for more than 3 billion years. There are two major factors that can explain how the moons can be more geologically active than larger bodies. First, tidal heating is an additional form of internal energy that we have not yet discussed. Secondly, the outer planet moons tend to be made from more volatile substances (those that are normally liquids or gases on earth) that can undergo geological change at much lower temperatures than rock.

A tour of some moons

Io is the most unique of the outer solar system moons. it is the most similar to the terrestrial planets because it is composed of silicate-based rock. The other three Galilean satellites of Jupiter, Europa, Ganymede, and Callisto, are composed mostly of water ice. The four Galileo satellites are large and the remainder of Jupiter's moons are small, with little interesting geology. Saturn has one large moon, Titan, the only moon with a substantial atmosphere. Many other interesting, medium-sized, moons orbit Saturn, including Enceladus, Rhea, Dione, Tethys, Mimas, and Iapetus. All moons of Saturn, Uranus, and Neptune, are made primarily of ices like water ice and carbon dioxide ice (dry ice). Miranda is the most interesting moon of Uranus, while Triton is the largest and most geologically active moon of Neptune.