A Whack and a Crack Characterizes Saturn’s Moon Tethys

Saturn’s ninth moon, Tethys, is just 600 miles across. Spherical in shape, its density is .97 percent of water’s density: it’s made of ice. There are probably some rocks mixed into that ice – the Saturnian system is a rocky place, after all – but Tethys is more of a hard-packed snowball than a mudball.

Her surface, shown to us first by the Voyager spacecraft and more recently by the Cassini probe, shows a wilderness of impact craters. The craters are heaped upon each other in the polar regions – ancient craters that have been pummeled and pelted by more recent meteorites so that their ridges and rings are broken down by newer craters.

The moon’s equator, however, shows surprisingly fewer impact craters. This leads scientists to believe that some degree of “resurfacing” has taken place. Perhaps there is a degree of geological activity that melts some of that tightly packed ice into liquid water which flows to the surface and fills in the craters. The source of that internal heating would probably be gravitational friction from Saturn and the pull of other moons. Perhaps the ice on Tethys isn’t all that hard-packed after all.

The release of liquid water and its subsequent freezing might explain one of Tethys’ two most dramatic features: Ithaca Chasma.

This twelve hundred mile long grand canyon runs from Tethys’ north pole to the south. At roughly sixty miles wide and two to three miles deep, this startling features raises more questions than it answers. Where did it come from? The moon’s internal heating and surface cooling may have created the fracture – a deep crack developed in the ice as the warm internal liquid water expanded against the hard frozen ice shell. Water emerged inside the crack and was frozen in place. That’s one theory.

The other theory is tied directly to the other major feature: Odysseus Crater. Where Ithaca Chasma runs down the moon’s eastern hemisphere, this enormous crater, roughly 2/5 the size of the moon itself, sits firmly in the western hemisphere. It lies directly on the other side of the moon.

An impact of that size would destroy a solid rock moon.  Tethys’ water/ice mix, howeer, was slushy enough to absorb the whack from another body. This is the other theory about Ithaca Chasma: another celestial body, perhaps one of Saturn’s small moons, whacked into Tethys so hard that it almost blew her apart. An enormous crack opened on the other side of the moon as it absorbed the impact energy. Again, water seeped out from the moon’s interior and filled the crack.

No one knows the origin of the Ithaca Chasma, although we’re fairly certain Odysseus Crater is a result of an impact. We’ll learn more about the tiny moon as the Cassini mission continues.

The Cassini spacecraft, exploring the Saturnian system, has taught us more about Tethys in the last twelve months than we have known about her since her discovery in 1684. Her first discoverer, by the way, was an astronomer named Giovanni Cassini.

You can learn more about Tethys, with that enormous whack crater on one side and the enormous crack on the other, at NASA’s Solar System website, and at Nine Planets.