Rhea (moon)

Rhea is the second-largest natural satellite of Saturn. It is also the ninth-largest moon in our Solar System. With a diameter of 1,528 kilometers, it is a massive icy world. Rhea is unique because it is the smallest body in the Solar System confirmed to be in hydrostatic equilibrium. This means its own gravity is strong enough to pull it into a nearly round shape.

The moon follows a nearly circular orbit around Saturn. It has a very low eccentricity of 0.001. This means its path is almost a perfect circle. Rhea is also tidally locked to Saturn. This means it rotates at the same speed it revolves around the planet. Because of this synchronous rotation, one hemisphere always faces Saturn. This side is known as the near pole.

We can also divide Rhea into two other distinct sides. One side is the leading hemisphere, which always faces the direction of its movement. The other side is the trailing hemisphere, which faces backward relative to its motion. These sides look very different from one another. The leading hemisphere is heavily cratered and uniformly bright. The trailing hemisphere has fewer craters but features bright, wispy streaks.

Rhea's composition explains its low density of about 1.236 g/cm3. It is made of roughly 75% water ice and 25% rock. This makes it much less dense than the moon Oberon. While Rhea is slightly larger by volume, Oberon is more massive because it is much denser. Some scientists believe a layer of Ice II exists beneath the surface. Ice II is a special form of ice that forms under high pressure and low temperatures. Recent data suggests Rhea might have a homogeneous interior. This means its materials are spread fairly evenly rather than being layered into a rocky core.

Human discovery of Rhea began in 1672. Giovanni Domenico Cassini discovered the moon using a telescope made by Giuseppe Campani. He originally named his discoveries Sidera Lodoicea to honor King Louis XIV. The name Rhea comes from Greek mythology. Rhea was a Titan and the mother of the first Olympian gods. In 1847, John Herschel suggested using these mythological names for the moons. Since then, we have sent many machines to study it. The Voyager probes visited Rhea, and the Cassini orbiter performed close flybys in 2005, 2007, 2010, 2011, and 2013.

The surface of Rhea is a landscape of extreme cold and impact scars. Temperatures range from -174 °C in sunlight to as low as -220 °C in the shade. Large impact basins like Tirawa, which is 400 to 500 kilometers wide, mark the surface. There is also a prominent crater called Inktomi, nicknamed "The Splat." It features bright rays that stretch across the hemisphere. The trailing hemisphere contains bright streaks called chasmata. These are deep fractures or troughs. The whiteness comes from the high albedo, or reflectivity, of the ice on the cliff sides.

Rhea also possesses a very thin atmosphere called an exosphere. This atmosphere consists of oxygen and carbon dioxide. The oxygen comes from radiolysis, which is when radiation breaks down water ice. The carbon dioxide might come from the moon's interior or from organic compounds. Scientists also believe Rhea might have a weak ring system. This was inferred from how electrons flowed around the moon during Cassini's flybys.

Understanding Rhea helps us learn about how moons form. Some models suggest Rhea and Iapetus formed from the debris of giant collisions. These collisions may have helped create the moon Titan. By studying Rhea's gravity and surface, scientists can better understand the history of the Saturnian system. It remains a vital piece of the puzzle in our study of the outer Solar System.