Rhea Moon: Saturn's Second-Largest Satellite

The planet Saturn is orbited by at least 62 moons, some of which exist within the rings and others outside of the ring system. Rhea moon is the second-largest Saturnian satellite (only Titan is larger). It's made mostly of ice, with a small amount of rocky material inside. Among all the moons of the solar system, it's the ninth-largest, and if it weren't orbiting a larger planet, it might be considered a dwarf planet.

The History of Rhea Exploration

Although most of what scientists know about Rhea has come from recent spacecraft explorations, it was first discovered in 1672 by Giovanni Domenico Cassini, who found it as he was observing Jupiter. Rhea was the second moon he found. He also found Tethys, Dione, and Iapetus, and named the group of four moons Sidera Lodoicea in honor of King Louis XIV of France. The name Rhea was assigned 176 years later by English astronomer John Herschel (son of astronomer and musician Sir William Herschel). He suggested that the moons of Saturn and other outer planets be named from characters in mythology. Saturn's moon names came from the Titans in Greek and Roman mythology. Thus, Rhea orbits Saturn along with the moons Mimas, Enceladus, Tethys, and Dione. 

The best information and images about Rhea have come from the twin Voyager spacecraft and Cassini Missions. Voyager 1 swept past in 1980, followed by its twin in 1981. They provided the first "up-close" images of Rhea. Before that time, Rhea had been simply a small dot of light in Earth-bound telescopes. The Cassini mission followed up the exploration of Rhea beginning in 2005 and made five close flybys over the next few years.

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Rhea Moon's Surface

Rhea is small compared to Earth, only about 1500 kilometers across. It orbits Saturn once every 4.5 days. Data and images show many craters and icy scars stretching across its surface. Many of the craters are quite large (around 40 km across). The largest one is called Tirawa, and the impact that created it may have sent ice spraying across the surface. This crater is also covered with younger craters, confirming the theory that it's a very old one.

There are also scarps, jagged cliffs that turned out to be large fractures. These all imply that impacts have really battered Rhea over time. There are also some dark regions scattered around the surface. These are made of organic compounds created as ultraviolet light bombards the surface ice.

Rhea's Composition and Shape

This little moon is made mostly of water ice, with rock comprising at most 25 percent of its mass. Scientists once thought it might have a rocky core, as many other worlds of the outer solar system do. However, the Cassini mission produced data that suggests that Rhea may have some rocky material mixed throughout, rather than concentrated at the core. Rhea's shape, which planetary scientists refer to as "triaxial" (three axes), also gives important clues to the interior makeup of this moon. 

It's possible that Rhea could have a small ocean underneath its icy surface, but how that ocean is maintained by heat is still an open question. One possibility is a sort of "tug of war" between Rhea and the strong gravitational pull of Saturn. However, Rhea orbits far enough from Saturn, at a distance of 527,000 kilometers, that heating caused by this so-called "tidal heating" is not enough to warm up this world. 

Another possibility is a process called "radiogenic heating." That happens when radioactive materials decay and give off heat. If there's enough of them inside Rhea, that might provide enough warmth to partially melt the ice and create a slushy ocean. There's not enough data to prove either idea yet, but Rhea's mass and rotation on its three axes suggest that this moon is a ball of ice with some rock in it. That rock could have the radiogenic materials needed to warm an ocean.

Although Rhea is a frozen moon, it does seem to have a very thin atmosphere. That tenuous blanket of air is made of oxygen and carbon dioxide and was discovered in 2010. The atmosphere is created when Rhea passes through Saturn's magnetic field. There are energetic particles trapped along the magnetic field lines, and they blast into the surface. That action causes chemical reactions that release oxygen. 

The Birth of Rhea

The births of Saturn's moons, including Rhea, are thought to have happened when materials coalesced in orbit around the infant Saturn, billions of years ago. Planetary scientists suggest several models for this formation. One includes the idea that the materials were scattered in a disk around young Saturn and gradually clumped together to make moons. Another theory suggests that Rhea may have formed when two larger Titan-like moons collided. The leftover debris eventually clumped together to make Rhea and its sister moon Iapetus.

Sources

• “In Depth | Rhea – Solar System Exploration: NASA Science.” NASA, NASA, 5 Dec. 2017, solarsystem.nasa.gov/moons/saturn-moons/rhea/in-depth/.
• NASA, NASA, voyager.jpl.nasa.gov/mission/.
• “Overview | Cassini – Solar System Exploration: NASA Science.” NASA, NASA, 22 Dec. 2018, solarsystem.nasa.gov/missions/cassini/overview/.
• “Rhea.” NASA, NASA, www.nasa.gov/subject/3161/rhea.
• “Saturn's Moon Rhea.” Phys.org - News and Articles on Science and Technology, Phys.org, phys.org/news/2015-10-saturn-moon-rhea.html.