Ken Tapping, February 17, 2010

A few decades ago the only moon in the Solar System with a surface we could study was our own. All the others were either dots or blurred discs seen through telescopes. Not surprisingly, in science fiction and science fact we assumed other moons were more or less like ours. They could be smaller or larger, or colder, but otherwise the same. One exception was Titan, the largest moon of Saturn. That one looks brownish through telescopes and has an atmosphere. On the whole, moons did not seem to be that interesting. We were wrong!

Our Moon is an airless rockball, covered with impact craters and great lava flows that solidified billions of years ago. There are few signs of any volcanic or other activity now. Phobos and Deimos, the two moons of Mars, are smaller airless rockballs, shaped like potatoes because their gravity is not strong enough to pull them into spheres. They were no big surprises there.

Our surprises start at Jupiter. This planet's four largest moons: Io, Europa, Ganymede and Callisto were discovered by Galileo. However, it took close examination by space probes to show us how fascinating these bodies are. Io, Europa and Ganymede gravitationally interact with one another and Jupiter so that they are constantly flexed and pulled out of shape, which produces an enormous amount of heat. This "tidal heating" dramatically changes the situation. It is strongest in Io, the closest moon to Jupiter, and makes Io the most volcanic object in the Solar System. Its surface has numerous volcanoes venting yellow, orange, red and brown molten sulphur, which streams out over the landscape. The heating is rather less in Europa, the next moon out from Jupiter. Europa is covered with ice, under which is a dark ocean of liquid water. The heat and chemicals from volcanic vents on the seabed could very well support communities of bizarre life forms, as is the case with the hydrothermal vents at the bottom of our oceans.

Ganymede, further from Jupiter, is cooler, but is still volcanically active and possibly has a liquid ocean buried under a thick covering of ice. Callisto, further out, does not enjoy much tidal heating. There may be some liquid water inside, but Callisto is close to being the icy rockball we would have expected.

Even before spacecraft went for a closer look, we knew Titan, Saturn's largest moon, was different. It has an obvious atmosphere, and now, after observations from orbit and from its surface, we know it to be a world with an atmosphere of methane and other hydrocarbons, where it rains liquid methane and ethane, and has rivers and lakes of those substances. Is there life there?

The moons of Uranus are rockballs. Miranda, the closest, looks very much as though it was knocked to bits, with the bits falling back together again. Triton, the closest moon to Neptune, is another interesting one. Once again the cause is tidal heating. From space we see countless dark geysers of gas and dark material blowing high into the tenuous atmosphere, where they are grabbed by the winds and blown sideways.