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.
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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.
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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