NASA’s Cassini spacecraft revealed Dione’s atmosphere during a close flyby of the Saturnian satellite. Cassini’s data showed that Dione leaves behind “fingerprints” as it sweeps through Saturn’s huge magnetic field.
The moon is about 1.5 times as dense as liquid water, leading scientists to surmise that it’s made mostly of water ice with a rocky core.
However, Dione isn’t massive enough to hold on to a substantial atmosphere in the same way Earth does. Earth and other large bodies boast strong gravitational fields, which prevent atmospheric particles from escaping into space. Dione’s atmosphere lacks this gravitational aid—the moon’s thin layer of air exists only because it’s constantly being recharged.
Saturn is surrounded by a belt of highly energetic particles, akin to the Van Allen belts around Earth. Dione is located in this belt, and the reason it possesses an atmosphere is that these hot and very fast particles continuously splatter on the moon’s surface. When the particles hit Dione, they cause the moon’s surface ice to break apart chemically, releasing molecules that become the moon’s atmosphere.
Image credit: NASA/JPL/Space Science Institute