Rhea (moon)

Rhea is the second-largest natural satellite of Saturn and the ninth-largest moon in the Solar System. It is about 1,528 kilometres wide.

Rhea orbits Saturn in a nearly circular path and is tidally locked. This means it rotates at the same speed it takes to orbit Saturn, so one side of Rhea always faces Saturn.

The moon is made mostly of ice and some rock, which makes it have a low density. Its surface is covered in many craters and has bright ice cliffs that look like wispy lines from space. The temperature on Rhea's surface is very cold, ranging between −174 °C and −220 °C.

Rhea was discovered in 1672 by Giovanni Domenico Cassini. It has been studied by the Voyager probes and the Cassini orbiter during flybys in 2005, 2007, 2010, 2011, and 2013.

Discovery

Rhea was found by Giovanni Domenico Cassini on December 23, 1672. He used a big telescope made by Giuseppe Campani. Cassini called the moons he found Sidera Lodoicea, to honor King Louis XIV. Rhea was the second moon of Saturn that Cassini discovered.

Name

Rhea is named after the Titan Rhea from Greek mythology. She was the mother of the first group of Olympian gods and the wife of Cronus, who is the Greek version of the god Saturn.

Rhea was not given its name until 1847. An astronomer named John Herschel suggested using the names of Titans for these moons.

Orbit

Rhea's orbit around Saturn is almost a perfect circle. It stays very close to Saturn's middle plane and tilts only a little.

Rhea is tidally locked. This means it spins at the same speed it moves around Saturn. So, one side of Rhea always faces Saturn, and one side always faces forward.

Physical characteristics

Size, mass, and internal structure

Rhea is the second largest moon of Saturn. It is 1,528 kilometers (949 miles) wide, which is smaller than Saturn's largest moon, Titan. Rhea is mostly made of ice and has a density of about 1.236 g/cm³. This low density means it is about 25% rock and 75% water ice. Scientists think there might be a special kind of ice, called Ice II, starting about 350 to 450 kilometers beneath the surface.

Before the Cassini–Huygens mission, scientists thought Rhea had a rocky center. But close flybys by the Cassini orbiter in 2005 suggested that Rhea might have a more even mix inside, without a separate rocky core. This idea came from measuring how Rhea spins, which can tell us about its inside structure.

Rhea's shape fits what we would expect for a body that spins steadily, like many of Saturn's moons do. Some models suggest Rhea might once have had a warm layer of water deep inside, but newer data suggest this might not be the case anymore.

Surface features

Main article: List of geological features on Rhea

Rhea's surface looks similar to another moon of Saturn called Dione. The temperature on Rhea is very cold, about 99 K (−174 °C) in sunlight and between 73 K (−200 °C) and 53 K (−220 °C) in shadow.

Rhea has many craters, but it also has some special features. On the side that faces away from Saturn, there are large cracks and a faint line of material along the equator. Rhea has two very large impact basins on its far side, each about 400 to 500 kilometers across. One of these, called Tirawa, is similar in size to a big basin on another moon, Tethys. There is also a notable crater named Inktomi, known as "The Splat," which has bright streaks stretching far from it.

Rhea's craters are sharper than those on some other moons, possibly because of its lower gravity. The surface can be divided into two types: one with larger craters and another with smaller ones, suggesting that its surface changed a long time ago. The side facing forward has many craters and looks uniformly bright. The back side has fewer craters and shows bright lines on a darker background. These lines are thought to be cracks in the surface, with ice on the sides making them look bright.

Rhea shows some signs of activity from inside, like fault lines and craters with raised centers, especially in the larger craters.

Formation

The moons of Saturn likely formed through a process called co-accretion, similar to how planets formed. As Saturn grew, it was surrounded by a disk of material that came together to make moons. Scientists Erik Asphaug and Andreas Reufer have a different idea. They think Rhea and another moon named Iapetus might have come from pieces left over after big crashes between older moons. In this idea, a moon called Titan formed from these huge impacts, and Rhea and Iapetus were created from the debris.

Atmosphere

In 2010, NASA discovered that Rhea has a very thin atmosphere, called an exosphere. This atmosphere is mostly oxygen and carbon dioxide. The oxygen comes from water ice on Rhea’s surface, broken down by radiation from Saturn’s magnetic field. Scientists are still learning where the carbon dioxide comes from.

Possible ring system

Main article: Rings of Rhea

In 2008, NASA said that Rhea might have a very thin ring system. This would be the first time rings were found around a moon. Scientists thought they saw signs of rings when they noticed changes in tiny particles in Saturn's magnetic field as the Cassini spacecraft flew near Rhea. These particles might stretch far from Rhea but seem thicker closer to the moon. Later, they saw bright spots along Rhea's middle that could be where ring pieces fall back onto the moon. But when Cassini looked closely at where the rings should be, no rings were found, so scientists are still trying to understand what they saw.

Exploration

The first pictures of Rhea were taken by the Voyager 1 & 2 spacecraft in 1980–1981.

The Cassini orbiter flew close to Rhea five times. It passed Rhea at different distances during its mission. Cassini took many pictures of the Saturn system.