The Great Basin on Saturn's Tethys  (Credit: Cassini Imaging Team)

The Great Basin on Saturn’s Tethys (Credit: Cassini Imaging Team)

It’s been too long since we headed out to space.  This is true of humankind, but it is also true of this blog…so today we are going to cast our eyes across the solar system to Tethys a mid-sized moon of Saturn. In 1684 the Italian astronomer Giovanni Domenico Cassini discovered Tethys. He initially named the moon in honor of Louis XIV, but his choice was later changed so that the moon is named for a first generation Greek titan-goddess.  The moon has been approached by several human spacecraft, most notably…Cassini, which has dropped by several times (the robot space probe is named after the astronomer—the poor fellow has not been drifting in space since the 17th century).

Giovanni Domenico Cassini

Giovanni Domenico Cassini

Of all the major moons in the solar system, Tethys has the lowest density: 0.98 g/cm3 ! This means that almost the entire moon is made of frozen water—it is essentially a huge round ice cube floating around Saturn. Tethys has two extremely prominent features—a giant crater 450 kilometers (280 miles) across (named Odysseus) and a huge ice canyon 2000 kilometers (1200 miles) long, 100 km (62 miles) wide, and 3 km (1.8 miles) deep, which stretches most of the way across the moon.  Unsurprisingly astronomers speculate that the two features are related and the massive impact which created Odysseus melted a chasm along the entire side of the planetoid.


Although you might be inclined not to expect much activity from a ball of ice in the depths of space, Tethys seems like it may be geologically active, or, at least, it may have been once.  The area around the hemisphere is comparatively flat and free of craters—which suggests that tidal flux from Saturn causes some melting—and possibly cryovolcanoes.

Ithaca Chasma: The Great Rift on Saturn's Tethys  (Credit: Cassini Imaging Team)

Ithaca Chasma: The Great Rift on Saturn’s Tethys
(Credit: Cassini Imaging Team)

Paleontologists and sharp-eyed readers already know the name Tethys.  During the age of Pangaea (when all of the world’s continents joined to form a single land mass), the great ocean in the midst was named the Tethys Ocean. In Greek mythology, Tethys was the daughter of Gaea (the mother earth) and Uranus (the heavens).  She was regarded as the mother of all waters and was married to her brother Oceanus, the first lord of the seas.  The astronomers of the age of enlightenment who renamed the moon, could not have known it was composed mostly of water, but they chose well.