Cassini Detects Hint of Fresh Air at Dione

MEDIA RELATIONS OFFICE
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Jia-Rui C. Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jia-rui.c.cook@jpl.nasa.gov

News feature: 2012-056 March 2, 2012

Cassini Detects Hint of Fresh Air at Dione

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2012-056&cid=release_2012-056

NASA's Cassini spacecraft has "sniffed" molecular oxygen ions around Saturn's icy
moon Dione for the first time, confirming the presence of a very tenuous
atmosphere. The oxygen ions are quite sparse – one 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) – show that Dione has an extremely thin neutral
atmosphere.

At the Dione surface, this atmosphere would only be as dense as Earth's atmosphere
300 miles (480 kilometers) above the surface. The detection of this faint
atmosphere, known as an exosphere, is described in a recent issue of the journal
Geophysical Research Letters.

"We now know that Dione, in addition to Saturn's rings and the moon Rhea, is a
source of oxygen molecules," said Robert Tokar, a Cassini team member based at
Los Alamos National Laboratory, Los Alamos, N.M., and the lead author of the paper.
"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."

Dione's oxygen appears to derive from either solar photons or energetic particles
from space bombarding the moon's water ice surface and liberating oxygen
molecules, Tokar said. But scientists will be looking for other processes, including
geological ones, that could also explain the oxygen.

"Scientists weren't even sure Dione would be big enough to hang on to an
exosphere, but this new research shows that Dione is even more interesting than we
previously thought," said Amanda Hendrix, Cassini deputy project scientist at
NASA's Jet Propulsion Laboratory, Pasadena, Calif., who was not directly involved in
the study. "Scientists are now digging through Cassini data on Dione to look at this
moon in more detail."

Several solid solar system bodies – including Earth, Venus, Mars and Saturn's largest
moon Titan – have atmospheres. But they tend to be typically much denser than
what has been found around Dione. However, Cassini scientists did detect a thin
exosphere around Saturn's moon Rhea in 2010, very similar to Dione. The density of
oxygen at the surfaces of Dione and Rhea is around 5 trillion times less dense than
that at Earth's surface.

Tokar said scientists suspected molecular oxygen would exist at Dione because
NASA's Hubble Space Telescope detected ozone. But they didn't know for sure until
Cassini was able to measure ionized molecular oxygen on its second flyby of Dione
on April 7, 2010 with the Cassini plasma spectrometer. On that flyby, the spacecraft
flew within about 313 miles (503 kilometers) of the moon's surface.

Cassini scientists are also analyzing data from Cassini's ion and neutral mass
spectrometer from a very close flyby on Dec. 12, 2011. The ion and neutral mass
spectrometer made the detection of Rhea's thin atmosphere, so scientists will be
able to compare Cassini data from the two moons and see if there are other
molecules in Dione's exosphere.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space
Agency, and the Italian Space Agency. NASA's Jet Propulsion Laboratory, Pasadena,
Calif., a division of the California Institute of Technology in Pasadena, manages the
mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini
orbiter was designed, developed and assembled at JPL. The Cassini plasma
spectrometer team and the ion and neutral mass spectrometer team are based at
Southwest Research Institute, San Antonio.

For more information about the Cassini mission, visit: http://www.nasa.gov/cassini
and http://saturn.jpl.nasa.gov .

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