Of all the moons in the Solar System, Saturn’s moon Titan has by far the densest atmosphere. The atmospheric pressure on the surface of Titan is 1.4 times greater than at sea-level on Earth. Titan’s atmosphere is so thick that it obscures its entire surface. From space, Titan appears as a fuzzy orange orb with no visible indication of any surface features. After Titan, the moon with the next thickest atmosphere is Neptune’s moon Triton. Triton’s atmosphere is so rarefied that it is only ~1/20,000th the density of Earth’s atmosphere. Nevertheless, its atmosphere is still thick enough to have winds, clouds and weather.
Like Earth, the atmospheres of Titan and Triton are thick enough that their gas molecules collide with one another before travelling any appreciable distance. Such atmospheres are known as collisional atmospheres. In contrast, some other moons in the Solar System, including Earth’s Moon, have extremely tenuous atmospheres known as exospheres. The gas particles in an exosphere are spaced so far apart that they rarely collide with each other. An exosphere is basically a non-collisional atmosphere.
Besides Titan and Triton, Jupiter’s moon Io is the third moon in the Solar System with a collisional atmosphere. The atmospheric pressure on Io is roughly a billion times less than at sea-level on Earth. Most of Io’s atmosphere is comprised of sulphur dioxide. Some of the sulphur dioxide comes directly from its constantly erupting volcanoes and the rest comes from sublimating sulphur dioxide frost on its dayside.
Figure 1: Artist’s impression of Jupiter’s moon Callisto.
A recent paper by Cunningham et al. (2015) reports the detection of an oxygen-dominated atmosphere around Jupiter’s moon Callisto. The detection was made using the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST) and the observations were directed towards Callisto’s leading/Jupiter-facing hemisphere. Like the three other large Galilean moons, Callisto is also tidally-locked to Jupiter. This means the same hemisphere of Callisto is always oriented towards Jupiter. Furthermore, Callisto’s orbital motion around Jupiter means it has a leading hemisphere (forward-facing hemisphere) and a trailing hemisphere (aft-facing hemisphere).
Jupiter has four large Galilean satellites - Io, Europe, Ganymede and Callisto. Of the four Galilean satellites, Callisto is the outermost. Measurements by the COS instrument indicate that Callisto’s atmosphere at its leading/Jupiter-facing hemisphere has a column density of ~4×10^15 oxygen molecules per cm². This is dense enough for Callisto’s atmosphere to be collisional, making Callisto the fourth moon known in the Solar System that has a collisional atmosphere.
While the measurements are of Callisto’s leading/Jupiter-facing hemisphere, the column density over its trailing hemisphere may be ~10 times denser. In fact, Callisto has the second densest oxygen-rich collisional atmosphere in the Solar System, the densest one being Earth’s atmosphere. Since it is collisional, the atmosphere of Callisto should be able to support winds and other weather phenomena.
The oxygen molecules in Callisto’s atmosphere are produced when water molecules on Callisto’s icy surface dissociate into hydrogen and oxygen. The light hydrogen atoms escape into space while the heavy oxygen atoms remain behind, resulting in Callisto’s oxygen-dominated atmosphere. Europa and Ganymede also have oxygen-dominated atmosphere derived from the same processes as on Callisto. However, their atmospheres are several times more tenenuous Callisto’s. As a result, the atmospheres of Europa and Ganymede are non-collisional; hence they are exospheres, with no winds and no weather.
Figure 2: The four moons in the Solar System that are known to have collisional atmospheres. In order of decreasing atmospheric column density, they are: Saturn’s Titan, Neptune’s Triton, and Jupiter’s Io and Callisto. Image Credit: NASA / JPL / SSI / Ted Stryk / Jason Perry / Emily Lakdawalla.
Reference:Cunningham et al. (2015), “Detection of Callisto’s oxygen atmosphere with the Hubble Space Telescope”, Icarus 254, 178-189