Saturn and Tethys | NASA Cassini Mission

Saturn and Tethys | NASA Cassini Mission

Italian mathematician, astronomer and engineer, Giovanni Cassini, discovered Tethys on March 21, 1684.

Overview

Tethys is Saturn's fifth largest moon. Its irregular shape is 331 miles (533 kilometers) in mean radius, with dimensions 669 x 657 x 654 miles (1076.8 x 1057.4 x 1052.6 kilometers). This cold, airless and heavily scarred body is very similar to sister moons Dione and Rhea except that Tethys is not as heavily cratered as the other two. This may be because its proximity to Saturn causes more tidal warming, and that warming kept Tethys partially molten longer, erasing or dulling more of the early terrain.

Tethys' density is 0.97 times that of liquid water, which suggests that Tethys is composed almost entirely of water ice plus a small amount of rock.

Tethys has a high reflectivity (or visual albedo) of 1.229 in the visual range, again suggesting a composition largely of water ice, which would behave like rock in the Tethyan average temperature of -305 degrees Fahrenheit (-187 degrees Celsius). Many of the crater floors on Tethys are bright, which also suggests an abundance of water ice. Also contributing to the high reflectivity is that Tethys is bombarded by Saturn E-ring water-ice particles generated by geysers on Enceladus.

Tethys appeared as a tiny dot to astronomers until the Voyager (1 and 2) encounters in 1980 and 1981. The Voyager images showed a major impact crater and a great chasm. The Cassini spacecraft has added details including a great variety of colors at small scales suggesting a variety of materials not seen elsewhere.

For more than a decade, NASA’s Cassini spacecraft shared the wonders of Saturn and its family of icy moons—taking us to astounding worlds where methane rivers run to a methane sea and where jets of ice and gas are blasting material into space from a liquid water ocean that might harbor the ingredients for life.

Cassini revealed in great detail the true wonders of Saturn, a giant world ruled by raging storms and delicate harmonies of gravity.

Cassini carried a passenger to the Saturn system, the European Huygens probe—the first human-made object to land on a world in the distant outer solar system.

After 20 years in space — 13 of those years exploring Saturn — Cassini exhausted its fuel supply. And so, to protect moons of Saturn that could have conditions suitable for life, Cassini was sent on a daring final mission that would seal its fate. After a series of nearly two dozen nail-biting dives between the planet and its icy rings, Cassini plunged into Saturn’s atmosphere on Sept. 15, 2017, returning science data to the very end.

Credit: NASA/JPL-Caltech/Space Science Institute (SSI)/CICLOPS/Kevin M. Gill

Image Date: August 19, 2012

Release Date: January 3, 2022

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