Rhea’s Oxygen Atmosphere

Categories: Saturn system

Cassini reveals oxygen atmosphere of Saturn’s moon Rhea

Saturn’s moons Iapetus, Mimas, Rhea and Tethys.
Image Credit: NASA/JPL

A tenuous atmosphere infused with oxygen and carbon dioxide has been
discovered at Saturn´s moon Rhea by the Cassini-Huygens mission — the
first time a spacecraft has captured direct evidence of an oxygen
atmosphere at a world other than Earth.

The NASA-led international mission made the discovery using combined
data from Cassini´s instruments, which includes a sensor designed and
built at UCL´s (University College London) Mullard Space Science

Published in Science Express, results from the mission reveal
that the atmosphere of Rhea, Saturn´s second largest moon at 1500 km
wide, is extremely thin and is sustained by high energy particles
bombarding its icy surface and kicking up atoms, molecules and ions
into the atmosphere.

The density of oxygen is probably about 5 trillion times less dense
than in Earth´s atmosphere. However, the formation of oxygen and
carbon dioxide could possibly drive complex chemistry on the surfaces
of many icy bodies in the universe.

"The new results suggest that active, complex chemistry involving
oxygen may be quite common throughout the solar system and even our
universe," said Dr. Ben Teolis, a Cassini team scientist based at
Southwest Research Institute in San Antonio and lead author. "Such
chemistry could be a prerequisite for life. All evidence from Cassini
indicates Rhea is too cold and devoid of the liquid water necessary
for life as we know it."

Oxygen has been detected in the thin atmosphere of Saturn’s moon Rhea. Image Credit: UCL

Dr. Geraint Jones, from the UCL Mullard Space Science Laboratory and a
co-author of the paper said: "The discovery of this tenuous atmosphere
provides key information on how radiation can drive chemistry on icy
surfaces throughout the universe."

Rhea´s tenuous atmosphere makes it unique in the Saturn system. Titan
has a very thick nitrogen-methane atmosphere, with very little carbon
dioxide and oxygen.

UCL´s Mullard Space Science Laboratory, supported by the Science and
Technology Facilities Council, led the design and building of the
electron spectrometer of the Cassini plasma spectrometer (CAPS), which
detected negative ions streaming off Rhea´s surface in 2005. Another
part of CAPS detected positive ions on the opposite side of Rhea in
2005 and 2007. Completing the picture of Rhea´s atmosphere, Cassini´s
ion and neutral mass spectrometer detected neutral particles when
Cassini swept within 100 km of the moon´s surface in March 2010.

Professor Andrew Coates, also from the UCL Mullard Space Science
Laboratory and co-author of the paper, said: "Our instrument turns out
to be a fabulous detector of negative ions as well as electrons. We´ve
already found negative ions are important at Titan and Enceladus —
and now, tracing back the trajectory of these ions really pinpoints
the source of the atmosphere near Rhea´s surface."

Cassini is one of the most scientifically capable spacecraft ever launched.
Credit: NASA

The ion and neutral mass spectrometer "tasted" peak densities of
oxygen of around 50 billion molecules per cubic meter (1 billion
molecules per cubic foot). It detected peak densities of carbon
dioxide around 20 billion molecules per cubic meter (about 600 million
molecules per cubic foot). The plasma spectrometer also saw clear
signatures of flowing streams of positive and negative ions, with
masses that corresponded to ions of oxygen and carbon dioxide.

"Rhea´s oxygen appears to come from water ice on Rhea´s surface when
Saturn´s magnetic field rotates over the moon and showers it with
energetic particles trapped in the magnetic field," said Professor

The carbon dioxide may be the result of "dry ice" trapped from the
primordial solar nebula, similar to the case of comets, or it may be
due to similar irradiation processes operating on the organic
molecules trapped in the water ice of Rhea. The carbon dioxide could
also come from carbon-rich materials deposited by tiny meteors that
bombarded Rhea´s surface.

The finding is consistent with earlier Cassini results that show Rhea
to be a particularly dark-looking moon, sporting some carbon-based
coating on it’s surface.