Image Credit: NASA/JPL/Space Science Institute
Rhea is, at 1,528 kilometers (949 miles) across, the second-largest moon of Saturn and the ninth largest moon in the Solar System. At a distance of 527,040 km (327,490 miles), Rhea orbits Saturn in a nearly perfect circle in 4 days 12 hours and 25 minutes, turning once on its axis during that time. The moon is tidally locked with its parent, what means that one side always faces toward Saturn.
It is an icy body with a density of 1.233 times that of liquid water (it is only the tenth-most-massive moon in the Solar System) what indicates that it is made of about 25% rock and 75% water ice. Probably Rhea lacks a rocky core. Thus, it is thought that this moon is composed of a homogenous mixture of ice and rock — a frozen dirty snowball. The temperature on Rhea is 99 K (−174 °C) in direct sunlight and between 73 K (−200 °C) and 53 K (−220 °C) in the shade. Rhea could sustain an internal liquid water ocean through heating by radioactive decay.
Rhea has a rather typical heavily cratered surface, with the exceptions of a few large fractures, caused by extensive tectonism, on the trailing hemisphere (the side facing away from the direction of motion along Rhea’s orbit) and a very faint “line” of material at Rhea’s equator that may have been deposited by material deorbiting from its rings (yes, Rhea might have a tenuous ring system).
The moon has two very large impact basins, which are about 400 and 500 km across. They are both covered in impact craters, indicating they are quite ancient. The more northerly and less degraded of the two is called Tirawa; the other one Mamaldi. There is also a 48 km-diameter impact crater that is prominent because of an extended system of bright rays. This crater, called Inktomi, is nicknamed “The Splat”, and may be one of the youngest craters on the inner moons of Saturn.
Its surface can be divided into two geologically different areas based on crater density; the first area contains craters which are larger than 40 km in diameter, whereas the second area, in parts of the polar and equatorial regions, has only craters under that size. This difference may indicate there was a major resurfacing event some time in Rhea’s history. However, it would have been long ago because there are few young craters with rays extending away from them (as on Earth’s Moon), and the average age of the plains is thought to be around four billion years old.
The leading hemisphere is heavily cratered and uniformly bright. The craters lack the high relief features seen on the Moon and Mercury. On the trailing hemisphere there is a network of bright streaks on a dark background with lengths of tens to hundreds of kilometers, often cutting through plains and craters. These wispy areas are subsidence fractures that make canyons (some of them several hundred meters high). The walls of those canyons are bright because darker material falls off them, exposing fresh bright water ice. These fracture cliffs show Rhea may have been tectonically active in its past, thus, in fact the streaks are tectonically formed ice cliffs.
Rhea has a thin atmosphere, more properly called an exosphere (where the atmosphere merges into space), consisting of oxygen and carbon dioxide in proportion of roughly 5 to 2. The main source of oxygen is water ice at its surface. The source of the carbon dioxide is less clear, but it may be related to oxidation of the organics present in ice or to outgassing of the moon’s interior.
On March 6, 2008, NASA announced that as Cassini passed by Rhea it discovered a tenuous ring system. This would mark the first discovery of rings about a moon. Unfortunately, a careful search later on failed to find any evidence of rings. Astronomers still don’t agree about how to explain the data which suggested their existence.
The first images of Rhea were obtained by Voyager 1 & 2 spacecraft in 1980–1981. After that, there were four close targeted fly-bys by the Cassini orbiter: at a distance of 500 km in November 2005, at 5,750 km in August 2007, at 100 km in March 2010 and at 69 km in January 2011. Rhea has been also imaged many times from long to moderate distances by the orbiter.
This image is Cassini view of Rhea’s anti-Saturnian hemisphere, showing the moon’s two largest impact basins (Mamaldi above and left of center, and adjacent Tirawa to its upper right).