Saturn's frozen snowball hides a secret of internal homogeneity

Space · Published 26 October 2026 Swipe more like this →

In short: Rhea is often dismissed as a simple, cratered ice ball orbiting Saturn. However, data from the Cassini spacecraft suggests its interior might be far more uniform than expected. By looking at how Rhea rotates, scientists have uncovered clues about its composition and why it lacks the smooth plains of its sister moons.

Discovered by Giovanni Cassini on December 23, 1672, Rhea is Saturn's second-largest moon. With a mean radius of 475 miles (764 kilometers), this body is notably small, frigid, and lacks an atmosphere. Completing a revolution every 4.5 Earth days, it stays tidally locked so one side constantly faces Saturn. Its surface temperature fluctuates significantly based on sunlight, ranging from -281 degrees Fahrenheit (-174 degrees Celsius) in lit areas to as low as -364 degrees Fahrenheit (-220 degrees Celsius) in the shadows.

The moon's physical properties provide a window into its history. With a density of 1.233 times that of liquid water, Rhea is estimated to be composed of three-quarters ice and one-quarter rock. Its high reflectivity suggests a surface dominated by water ice, which acts like solid rock at these extreme temperatures. However, Cassini measurements regarding the moon's moment of inertia—a measure of its rotational resistance—revealed a value higher than what would be expected for a body with a distinct rocky core. This suggests Rhea is likely a homogenous mixture of ice and rock, often described as a 'frozen dirty snowball.'

Rhea’s geography stands in stark contrast to its neighbors, Dione and Tethys. Because Rhea orbits further away from Saturn at 327,500 miles (527,000 kilometers), it lacks the intense tidal heating that influences those moons. This lack of internal warmth means liquid water likely never reached Rhea's surface to pond in craters and create smooth plains. Consequently, while Dione and Tethys feature erased landscapes, Rhea remains heavily cratered, preserving a more violent record of ancient impacts.

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