Rhea: Saturn's Dirty Snowball Moon

NASA's Cassini spacecraft has taken its last closeup photos of the icy Saturn moon Rhea.
This raw image of Saturn's icy moon Rhea was taken on March 10, 2013 by NASA's Cassini spacecraft, and received on Earth March 10, 2013. The camera was pointing toward Rhea at approximately 174,181 miles (280,317 kilometers) away. (Image credit: NASA/JPL/Space Science Institute)

Rhea, the second largest moon of Saturn, is a dirty snowball of rock and ice. The only moon with an oxygen atmosphere, thin though it may be, Rhea is one of the most heavily cratered satellites in the solar system.

Discovery and naming

Rhea is one of the four moons discovered by Italian astronomer Giovanni Cassini. He spotted the icy moon on Dec. 23, 1672. It was the second moon he observed, and the third to be found in orbit around the ringed planet. At 949 miles (1,528 kilometers) in diameter, it is the second largest moon of Saturn, but less than a third the size of the largest contender, Titan.

Cassini named the four moons he discovered the Sidera Lodoicea — the Stars of Louis — after King Louis IV. The moon was first referred to numerically as Saturn V, based on its distance from the planet. It was not formally named until 1847, when John Herschel (son of astronomer William Herschel) suggested that the satellites of Saturn be named for the Titans, the brothers and sisters of the Greek god Cronus (known to the Romans as Saturn).

Rhea, the daughter of Uranus and Gaea, was the wife of Cronus. Known as the mother of the gods, she gave birth to several of the Olympian gods and goddesses, including Zeus and Hera. She was also mother to Hestia, Hades, Demeter, and Poseidon.

The features on the moon are named for people and places from various creation myths. [Photo Gallery: Icy Rhea: Photos of Saturn's Second-Largest Moon]

On the surface

Rhea is an icy body with a density 1.233 times that of liquid water, which implies that it is three-quarters ice and one-quarter rock. The highly reflective surface provides further evidence. When NASA's Cassini-Huygens spacecraft examined Rhea, measurements based on its angular motion led scientists to conclude the moon lacked an icy core. Instead, it resembles a frozen dirty snowball, with ice and rock mixed together.

Temperatures on Rhea range from minus 174 degrees C (minus 281 degrees F) in the warmer, sunlit areas to minus 220 C (minus 364 F) in the shade. At these frigid temperatures, the ice on the moon behaves like rock.

Rhea is one of the most heavily cratered satellites in the solar system. Regions on the moon are classified based on their cratering. Heavily cratered, brighter terrain contains impacts larger than 25 miles (40 km), while more lightly cratered regions near the poles and equatorial regions have smaller impacts.

Rhea is more heavily cratered than Saturn's other icy moons, Dione and Tethys. It also orbits farther from the planet, averaging 327,490 miles (527,040 km) away. As a result, Saturn's gravitational tug has less of an effect on the moon. Dione and Tethys both show signs of smooth plains, which may have been created when the internal warmth caused by Saturn's gravity melted and re-iced cratered regions. Another explanation is that the distant moon could simply have received more impacts.

The March 2010 Rhea flyby trajectory and oxygen atmosphere distribution (as simulated by computer models). Inset: Predicted oxygen density (yellow), compared to Cassini's measurements (white) taken during the flyby. (Image credit: Science/AAAS)

Rhea also contains wispy lines, running tens to hundreds of kilometers across the planet. The Cassini craft revealed that these wisps were fractures in the surface that created canyons, some of which reached several hundred meters in height. As dark material falls from the walls, bright icy material beneath is revealed. Dione and Tethys also house similar wisps. [Photo Gallery: Icy Rhea: Photos of Saturn's Second-Largest Moon]

A thin atmosphere and rings that weren't

A very faint oxygen atmosphere exists around Rhea, the first direct evidence of an oxygen atmosphere on a body other than Earth. The atmosphere is thin, with oxygen measuring about 5 trillion times less dense than that found on Earth. Oxygen could be released as the surface is irradiated by ions from Saturn's magnetosphere. The source of the carbon dioxide is less clear, but could be the result of similar irradiation, or from dry ice much like comets.

At one point, scientists announced the possibility of three narrow bands filled with material ranging in size from pebbles to boulders. The original detection came from plasma measurements taken by Cassini, but later optical observations revealed no rings. The source of the plasma measurements remains a mystery.

From maps and data Cassini obtained, scientists are trying to figure out how old features on the surface are, as well as the nature of these features. A 2014 study in the journal Icarusshowed that Rhea's Inktomi crater and the blanket of debris surrounding it have more thermal inertia (resistance to being the same temperature as the surroundings) than other areas of the moon. A separate presentation at the American Astronomical Society in 2016suggests that Rhea (as well as several other moons) would have pelted from the debris of a generation of older moons, in its distant past.

A 2015 study in Icaruslooked at the relative amounts of crystalline and amorphous (less distinct) water ice, which is useful for worlds that are predominantly icy. The team studied two craters, Inktomi and Oblata, and found that most of the crystalline ice was in the center of these craters, which was expected because that's where the small body impacted the surface and caused the most disruption. Over time, crystalline ice is exposed to charged particles and it becomes less pristine. Using a known rate of ice transformation, the team estimated the Oblata crater was about 450 million years old.

More facts about Rhea

  • Diameter: 949 miles (1,528 km)
  • Equatorial circumference: 2,057,770 miles (3,311,665 km)
  • Surface area: 2,834,260 square miles (7,340,701 square miles)
  • Mass: 2.306 x 1021 kilograms
  • Density: 1.233 grams per cubic centimeter
  • Average orbit around Saturn: 327,505 miles (527,068 km)
  • Closest approach: 327,177 miles (526,541 km)
  • Farthest approach: 327,832 (527,595 km)
  • Orbit eccentricity: 0.001
  • Orbit period: 4.5 Earth days
  • Escape velocity: 1,420 mph (2,285 kph)

Additional reporting by contributor Elizabeth Howell.

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Nola Taylor Tillman
Contributing Writer

Nola Taylor Tillman is a contributing writer for Space.com. She loves all things space and astronomy-related, and enjoys the opportunity to learn more. She has a Bachelor’s degree in English and Astrophysics from Agnes Scott college and served as an intern at Sky & Telescope magazine. In her free time, she homeschools her four children. Follow her on Twitter at @NolaTRedd