NASA's Cassini spacecraft revealed Dione's atmosphere during a recent close flyby of the Saturnian satellite. (Related pictures: "New Views of Saturn's 'Sponge' Moon Hyperion.")
Cassini's data showed that Dione leaves behind "fingerprints" as it sweeps through Saturn's huge magnetic field.
"If a moon did not possess an atmosphere at all—if it was simply an ice ball—the magnetic field lines wouldn't be disturbed at all upstream of the moon, because you need conductivity to disturb the field," said study co-author Sven Simon.
"You only get that [disturbance] when you have charged particles, for instance, from a moon's atmosphere," explained Simon, of the Institute of Geophysics and Meteorology at the University of Cologne in Germany.
"Dione's atmosphere is strong enough to disturb [Saturn's] magnetic field."
Particle Splatter Recharges Atmosphere
At about 698 miles (1,123 kilometers) wide, Dione is the 15th largest moon in the solar system. (Related: "Saturn Moon Has Ice Volcano—And Maybe Life?")
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, Simon said. (Related: "Antimatter Found Orbiting Earth—A First.")
"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.
Last Chance for Close Looks?
Another Saturn moon, Rhea, is big enough to retain a dilute atmosphere due to gravity and was recently found to have oxygen-filled air. (See "Saturn Moon Has Oxygen Atmosphere.")
Because Dione's thin air was detected via magnetic field data, scientists don't yet know what the moon's newfound atmosphere is made of—though it may also be mostly oxygen, the team suspects, since it's created from water ice.
Simon hopes that existing data from other Cassini instruments may contain clues to the atmosphere's chemical composition—and perhaps will help direct upcoming spacecraft observations.
"Cassini is going back to Dione—the next flyby takes place on December 12," he said.
"Now that we know there is an atmosphere, we can adjust particle detectors and electron spectrometers to take advantage."
It's almost now or never, Simon added: "In addition to the December flyby, there are only two more close Cassini flybys of Dione and a single additional visit to Rhea" during the remainder of Cassini's current mission, which is slated to end in September 2017.
"It's the last chance to have a close look at this family of icy moons."
The discovery of Dione's atmosphere is described in the August 12 edition of Geophysical Research Letters.