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Last Friday, NASA’s Cassini mission detected molecular oxygen ions on Dione- one of Saturn’s moons- indicating that the moon has an atmosphere. The team involved with the mission includes researchers from Los Alamos National Laboratory in New Mexico, the European Space Agency, and the Italian Space Agency, all of which are a part of collaboration with NASA’s Cassini-Huygens mission.
“We now know that Dione, in addition to Saturn’s rings and the moon Rhea, is a source of oxygen molecules,” Robert Tokar, says to NASA. Tokar, the head author of the team’s paper, is a researcher at Los Alamos National Laboratory. “This shows that molecular oxygen is actually common in the Saturn system and reinforces that it can come from a process that doesn’t involve life.”
Cassini, launched in 1997 and arriving on Saturn in 2004, spotted the molecular oxygen ions in a flyby with one of its active sensors, the Cassini Plasma Spectrometer (CAPS) in 2010, when the researchers at Los Alamos were able to first notice them. Prior, the existence of the ions was postulated after NASA’s Hubble Space Telescope detected ozone. Only after Cassini studied Dione during its flyby, was their postulation confirmed.
Dione was discovered by Giovannia Cassini (after which the titular spacecraft was named) in 1684. As one of the 62 moons revolving around Saturn, it is the tiniest, having a diameter of around 1130km (700 miles). Dione is best known for its pockmarked surface, which is composed of a thick layer of solid water ice. Underneath the surface lies a possible layer of liquid water and a small rocky core.
The distance at which Dione orbits Saturn is the same distance as the Earth from the Sun. The tiny moon’s orbital period lasts every 2.7 days. Because Dione is well within Saturn’s magnetosphere, the ions from the magnetosphere bombard Dione’s surface, and molecular oxygen ions are then created.
These ions bounce off and are dispersed around the planet, creating an atmosphere, albeit a very thin one. According to NASA, there is “one [ion] for every 0.67 cubic inches of space (one for every 11 cubic centimeters of space) or about 2,550 per cubic foot (90,000 per cubic meter).”
“The concentration of oxygen in Dione’s atmosphere is roughly similar to what you would find in Earth’s atmosphere at an altitude of about 300 miles,” Tokar states in Los Alamos National Laboratory’s press release. “It’s not enough to sustain life, but—together with similar observations of other moons around Saturn and Jupiter—these are definitive examples of a process by which a lot of oxygen can be produced in icy celestial bodies that are bombarded by charged particles or photons from the Sun or whatever light source happens to be nearby.”