Planet Saturn with Moons Tethys, Mimas, & Janus | NASA Cassini Mission

Planet Saturn with Moons Tethys, Mimas, & Janus | NASA Cassini Mission

Image of planet Saturn with moons Tethys, Mimas, and Janus captured by NASA's Cassini spacecraft on March 13, 2006.

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. This suggests that Tethys is composed almost entirely of water ice plus a small amount of rock.

Less than 123 miles (198 kilometers) in mean radius, crater-covered Mimas is the smallest and innermost of Saturn’s major moons. It is not quite large enough to hold a round shape, so it is somewhat ovoid with dimensions of 129 x 122 x 119 miles (207 x 197 x 191 kilometers, respectively). Its low density suggests that it consists almost entirely of water ice. It is the only substance detected on Mimas.

At a mean distance just over 115,000 miles (186,000 kilometers) from the massive planet, Mimas takes only 22 hours and 36 minutes to complete an orbit. Mimas is tidally locked: it keeps the same face toward Saturn as it flies around the planet, just as our Moon does with Earth.

Janus is an inner satellite of Saturn. It is also known as Saturn X. It is named after the mythological Roman god, Janus. This natural satellite was first identified by French astronomer Audouin Dollfus on December 15, 1966, although it had been unknowingly photographed earlier by French optical engineer, Jean Texereau. 

NASA's Cassini spacecraft arrived in the Saturn system in 2004 and ended its mission in 2017 by deliberately plunging into Saturn's atmosphere. This method was chosen because it is necessary to ensure protection and prevent biological contamination to any of the moons of Saturn thought to offer potential habitability.

The Cassini-Huygens mission was a cooperative project of NASA, European Space Agency (ESA) and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, manages the mission for NASA's Science Mission Directorate, Washington. JPL designed, developed and assembled the Cassini orbiter. The Cassini radar instrument was built by JPL and the Italian Space Agency, working with team members from the U.S. and several European countries.

NASA's Cassini Mission: 
https://solarsystem.nasa.gov/cassini

Image Credit: NASA/JPL-Caltech/SSI/CICLOPS

Processing: Kevin M. Gill
Image Date: March 13, 2006
Release Date: Jan. 14, 2025

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