Saturn's Moon Rhea

The NASA Cassini spacecraft flew only 97 kilometers (60 miles) above the surface of Saturn’s moon Rhea and found that it has an atmosphere composed of oxygen and carbon dioxide. This is the first time a moon of Saturn has been discovered to have an atmosphere.

The spacecraft actually flew (in March 2010) through the “tenuous” atmosphere held together by high-energy particles from Saturn that constantly hit the surface of Rhea.
The NASA spacecraft found that the atmosphere is very thin, with the oxygen in the atmosphere reported to be about five trillion times less than the oxygen within Earth’s atmosphere.

They report in the abstract to the paper the finding of a“tenuous oxygen-carbon dioxide atmosphere” that is “sustained by chemical decomposition of the surface water ice under irradiation from Saturn’s magnetospheric plasma.” Dr. Ben Teolis of the Southwest Research Institute, Space Science & Engineering Division, who led the study says “As the magnetic field rotates around Saturn, particles carried in the field slam into the hemisphere of Rhea that’s facing their flow. They hit that hemisphere and break water molecules on the surface. The atoms are then rearranging themselves to make O2 molecules, which are sputtered from the surface by additional impacting particles.”

The Cassini researchers added that “The presence of CO2 suggests radiolysis reactions between surface oxidants and organics, or sputtering and/or outgassing of CO2 endogenic to Rhea’s ice. Observations of outflowing positive and negative ions give evidence for pickup ionization as a major atmospheric loss mechanism.”

The research team state that such very thin (tenuous) atmospheres on such moons (and planets) could be very common in the Milky Way galaxy and other such galaxies within the universe.

After completing its mission in June 2008, the mission was renamed Cassini Equinox.
When it ended in September 2010, the spacecraft was renamed a second time to Cassini Solstice. The mission’s extension, which goes through September 2017, is named for the Saturnian summer solstice occurring in May 2017. The northern summer solstice marks the beginning of summer in the northern hemisphere and winter in the southern hemisphere. Since Cassini arrived at Saturn just after the planet’s northern winter solstice, the extension will allow for the first study of a complete seasonal period.

via jpl.nasa