Rhea (moon)

Rhea is the second-largest natural satellite of Saturn and the ninth-largest moon in the Solar System, with a diameter of 1,528 kilometres (949 mi). It is the smallest body in the Solar System that is confirmed to be in hydrostatic equilibrium. Rhea has a nearly circular orbit around Saturn and is tidally locked, meaning it rotates with the same period it takes to orbit Saturn. Because of this, one side of Rhea always faces toward Saturn.

The moon is made up of about three-quarters ice and one-quarter rock, giving it a low density. Its surface is heavily cratered and shows distinct differences between its leading and trailing sides. Like the moon Dione, Rhea has bright ice cliffs that look like wispy streaks from space. The temperature on Rhea's surface varies between −174 °C (−281.2 °F) and −220 °C (−364.0 °F).

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

Discovery

Rhea was found by Giovanni Domenico Cassini on December 23, 1672, using a large telescope made by Giuseppe Campani. Cassini called the four moons he found, including Rhea, Sidera Lodoicea, to honor King Louis XIV. Rhea was the second moon of Saturn that Cassini discovered, and the third moon found around Saturn.

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, who were sisters and brothers of Cronus, for these moons.

Orbit

Rhea's orbit around Saturn is almost a perfect circle, with very little stretching or squeezing. It stays very close to Saturn's middle plane, tilting only a little bit.

Rhea is tidally locked, which means it spins at the same speed it moves around Saturn. Because of this, one side of Rhea always faces Saturn, and one side always faces forward as it travels.

Physical characteristics

Size, mass, and internal structure

Rhea is the second largest moon of Saturn. It has a diameter of 1,528 kilometers (949 miles), which is less than a third the size of Saturn's largest moon, Titan. Rhea is made mostly of ice with a density of about 1.236 g/cm³. This low density shows that it is made of about 25% rock and 75% water ice. Scientists think there might be a layer of 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, with different features on its front and back sides. 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 the young planet Saturn grew, it was surrounded by a disk of material that came together to create moons. A different idea by scientists Erik Asphaug and Andreas Reufer suggests that Rhea, along with 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 found that Rhea has a very thin atmosphere, called an exosphere. This atmosphere is made mostly of oxygen and carbon dioxide. The oxygen comes from water ice on Rhea’s surface, broken down by radiation from Saturn’s magnetic field. We are still learning where the carbon dioxide comes from.

Possible ring system

Main article: Rings of Rhea

In 2008, NASA shared news 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 caught 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 spotted 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 before.

Exploration

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

The Cassini orbiter, part of the Cassini–Huygens mission launched in 1997, flew close to Rhea five times. It passed Rhea at distances of 500 km in 2005, 5,750 km in 2007, 100 km in 2010, 69 km in 2011, and 992 km in 2013. During its mission, Cassini took over 450,000 pictures of the Saturn system.