Tethys, Saturn's moon of almost pure ice

Updated October 19, 2026 · 4 min read

Tethys is Saturn's fifth-largest moon, 1,062 km across, and one of the most icy bodies in the solar system: its density is barely above that of water, so it must be made almost entirely of water ice. Its surface carries two giant scars, the 445-km Odysseus crater and the Ithaca Chasma canyon system stretching about 2,000 km, as well as faint red arcs first seen by Cassini whose origin is still unexplained.

Discovery and name

Giovanni Domenico Cassini, working at the Paris Observatory, found Tethys in March 1684, together with Dione, using long aerial telescopes with very long focal lengths. He had already found Iapetus and Rhea and called his four moons Sidera Lodoicea, "the stars of Louis", in honor of Louis XIV. The modern name, given by John Herschel in 1847, refers to the Titaness Tethys, sister and wife of Oceanus and mother of the rivers.

A ball of ice

With a mean density of about 0.98 g/cm³, Tethys is the least dense of Saturn's round moons. That leaves room for only a small fraction of rock; the rest is water ice, possibly with some porosity. Like its neighbors, Tethys is extremely bright: it is constantly coated with ice grains from Saturn's E ring, which is fed by the plumes of Enceladus.

Its colors reveal its environment. The leading hemisphere, which plows into E ring material, is brighter; the trailing hemisphere is darker and slightly redder, because Saturn's magnetospheric plasma, which overtakes Tethys in its orbit, hits that side. Cassini also found a dark, lens-shaped band across the leading hemisphere's equator, caused by high-energy electrons, and a "Pac-Man" thermal pattern like the one on Mimas.

Odysseus crater

On the leading hemisphere lies Odysseus, an impact basin about 445 km wide, roughly two-fifths of the moon's diameter, among the largest craters in the solar system relative to the size of its host. Unlike the steep, deep Herschel crater on Mimas, Odysseus is relatively shallow: its floor has risen and flattened over time, a sign that the ice of the young Tethys was warm enough to flow slowly and relax the topography.

Ithaca Chasma

A huge system of valleys, Ithaca Chasma, runs for about 2,000 km, three quarters of the way around the moon. It is up to 100 km wide and several kilometers deep, with raised rims. Two main explanations are debated:

• Expansion from freezing. If Tethys once had a subsurface ocean that froze, the expansion of water as it turned to ice would have stretched and cracked the crust, opening the chasm.
• A consequence of Odysseus. The chasm lies roughly along a great circle around the crater, so shock waves or crustal readjustment after the impact could have fractured the surface. Crater counts, however, suggest Ithaca Chasma may be older than Odysseus, which weakens this idea.

The mysterious red arcs

In images released in 2015, Cassini scientists noticed narrow, curved streaks of reddish material on Tethys, visible only in enhanced-color views. They are among the few red features on Saturn's icy moons, and they cut across older terrain, so they must be geologically young. Their origin is unknown: proposed explanations include exposed ice with different chemistry, outgassing along fractures, or material deposited from outside. Higher-resolution observations would be needed to decide.

Companions and resonances

Tethys shares its orbit with two small moons: Telesto, about 60 degrees ahead, and Calypso, about 60 degrees behind, sitting at the stable L4 and L5 Lagrange points. Both were discovered in 1980 from Earth during a ring-plane crossing. They are described on the Trojan moons page. Tethys is also locked in a 4:2 orbital resonance with Mimas, which affects the tilt of both orbits.

Exploration

Voyager 1 and Voyager 2 photographed Tethys in 1980 and 1981; Voyager 2 discovered Ithaca Chasma. Cassini made its closest targeted flyby in September 2005, at about 1,500 km, and observed Tethys many times from a distance until 2017, producing complete maps. No dedicated mission is planned.

Sources

• NASA Science: Tethys
• JPL Solar System Dynamics: planetary satellite physical parameters
• Schenk, P., et al., "Plasma, plumes and rings: Saturn system dynamics as recorded in global color patterns on its midsize icy satellites", Icarus 211 (2011)
• JPL Photojournal (Cassini images of Tethys)