Return to Newsletter Index

The Battered Moon

by Perry Pezzolanella

Some moons may seem like boring, cratered worlds, but a closer look at most of them often reveals a history rich in geological activity. Tethys is a moon of Saturn that looks dull from a distance, but close up there is evidence of a warm, active geological past.

Tethys is a fairly moderate size moon among the Saturn family of moons with a diameter of 662 miles and is quite massive for its size. It is composed almost entirely of water-ice and is very reflective with nearly 80% of the sunlight being reflected from its icy surface. The surface is bright because of the sandblasting of particles from Saturn’s E-ring, a faint ring composed of small, water-ice particles generated from the geysers of Enceladus. Tethys is heavily cratered and has huge cracks caused by faults in the ice. There are two different types of terrain on Tethys, one heavily cratered and the other a dark belt that is lightly cratered. The dark belt may originate from the migration of ices from the warmer equatorial region towards the poles, although the surface is a numbing -300ºF.

The Cassini spacecraft, which in orbit around Saturn, flew within 930 miles of Tethys on September 23, 2005 and made a crucial discovery. A huge fracture, over 1000 miles long, 60 miles wide, and up to 6 miles deep stretches almost half way around the circumference of Tethys. Known as Ithaca Chasma, it is thought to have formed when the internal liquid water solidified, causing the moon to expand and crack to accommodate the extra volume within. There had to be a lot of energy to create such a rift and since Tethys has very little rocky material to generate heat from radioactive decay, the heat source had to come from the tidal forces of Saturn. An underground ocean would allow enough heating of Tethys’ crust to flex it more strongly than if the moon was solid all the way to its core. Tidal flexing and heating are responsible for the geysers of Enceladus and volcanoes of Jupiter’s Io.

Further proof that Tethys was warmer and softer in its youth comes from a huge crater called Odysseus. The crater is 250 miles in diameter and is nearly 2/5 the size of Tethys itself. This crater is now quite flat as it has relaxed to nearly the same level as the rest of the surface. It has no high ring mountains or high central peaks, unlike the Moon and Mercury. In spite of the severe cold, water can remain a liquid when mixed with ammonia. As a moon freezes, the ammonia in the water becomes more concentrated and lowers the freezing point. This is because ammonia has a much lower freezing point than water and does not separate or freeze out as the temperature drops; therefore, it becomes more concentrated as the amount of liquid water decreases. It is possible that Tethys might still have some liquid water below the surface depending on how much ammonia its ocean once contained.

It is interesting that the more places that are being explored, the more places that are being found where subsurface oceans exist or have existed. This is exciting because where there is water, life may have existed or could exist today. The once frozen and desolate outer Solar System has become an exciting and worthwhile place to explore.