This is a color composite image of Saturn’s moon Rhea (pronounced REE-ah) made from raw images acquired by Cassini this past Saturday, March 9, during its most recent — and last — close pass of the moon. The visible-light colors of Rhea’s frozen surface have been boosted to make them more apparent — even so, it’s still a very monochromatic place!
The purpose of Saturday’s flyby was to probe the internal structure of Rhea by measuring its gravitational pull via the spacecraft’s radio link to NASA’s Deep Space Network here on Earth. The results will help scientists understand whether the 950-mile-wide Rhea is homogeneous all the way through or whether it has differentiated into separate internal layers of core, mantle and crust.
During the flyby Cassini came as close as 620 miles (997 kilometers) from Rhea’s rugged surface, capturing images of the moon’s terrain as it went.
The second-largest of Saturn’s 62 moons (Titan is 3 1/2 times bigger) Rhea is a very reflective world, indicating that its surface is made up of a lot of ice. It is also extremely cratered. In fact, Rhea is one of the most heavily-cratered worlds in the entire solar system.
Rhea is composed almost entirely of rock and water ice, which, at the low temperatures (-280 to -360 degrees F) found at that distance from the sun, is harder than rock is here on Earth.
In addition to measuring Rhea’s gravity field, Cassini also searched for evidence of dust flying off the moon in order to determine the rate of micrometeorite impacts on its surface.