MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109 TELEPHONE 818-354-5011
http://www.jpl.nasa.gov
Jia-Rui Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jccook@jpl.nasa.gov
Feature: 2010-399 Nov. 29, 2010
Thin Air: Cassini Finds Ethereal Atmosphere at Rhea
The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2010-399&cid=release_2010-399
NASA's Cassini spacecraft has detected a very tenuous atmosphere known as an
exosphere, infused with oxygen and carbon dioxide around Saturn's icy moon
Rhea. This is the first time a spacecraft has directly captured molecules of an
oxygen atmosphere – albeit a very thin one -- at a world other than Earth.
The oxygen appears to arise when Saturn's magnetic field rotates over Rhea.
Energetic particles trapped in the planet's magnetic field pepper the moon's water-
ice surface. They cause chemical reactions that decompose the surface and release
oxygen. The source of the carbon dioxide is less certain.
Oxygen at Rhea's surface is estimated to be about 5 trillion times less dense than
what we have at Earth. But the new results show that surface decomposition could
contribute abundant molecules of oxygen, leading to surface densities roughly 100
times greater than the exospheres of either Earth's moon or Mercury. 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 lead
author Ben Teolis, a Cassini team scientist based at Southwest Research Institute in
San Antonio. "Such chemistry could be a prerequisite for life. All evidence from
Cassini indicates that Rhea is too cold and devoid of the liquid water necessary for
life as we know it."
Releasing oxygen through surface irradiation could help generate conditions
favorable for life at an icy body other than Rhea that has liquid water under the
surface, Teolis said. If the oxygen and carbon dioxide from the surface could
somehow get transported down to a sub-surface ocean, that would provide a much
more hospitable environment for more complex compounds and life to form.
Scientists are keen to investigate whether life on icy moons with an ocean is
possible, though they have not yet detected it.
The tenuous atmosphere with oxygen and carbon dioxide makes Rhea, Saturn's
second largest moon, unique in the Saturnian system. Titan has a thick nitrogen-
methane atmosphere, but very little carbon dioxide and oxygen.
"Rhea is turning out to be much more interesting than we had imagined," said
Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory,
Pasadena, Calif. "The Cassini finding highlights the rich diversity of Saturn's moons
and gives us clues on how they formed and evolved."
Scientists had suspected Rhea could have a thin atmosphere with oxygen and
carbon dioxide, based on remote observations of Jupiter's icy moons by NASA's
Galileo spacecraft and Hubble Space Telescope. Other Cassini observations
detected oxygen escaping from icy Saturn ring particles after ultraviolet
bombardment. But Cassini was able to detect oxygen and carbon dioxide in the
exosphere directly because of how close it flew to Rhea – 101 kilometers, or 63
miles – and its special suite of instruments.
In the new study, scientists combined data from Cassini's ion and neutral mass
spectrometer and the Cassini plasma spectrometer during flybys on Nov. 26, 2005,
Aug. 30, 2007, and March 2, 2010. 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 of around
20 billion molecules per cubic meter (about 600 million molecules per cubic foot).
The plasma spectrometer saw clear signatures of flowing streams of positive and
negative ions, with masses that corresponded to ions of oxygen and carbon
dioxide.
"How exactly the carbon dioxide is released is still a puzzle," said co-author Geraint
Jones, a Cassini team scientist based at University College London in the U.K. "But
with Cassini's diverse suite of instruments observing Rhea from afar, as well as
sniffing the gas surrounding it, we hope to solve the puzzle."
The carbon dioxide may be the result of "dry ice" trapped from the primordial
solar nebula, as is the case with 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 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 ion and neutral mass
spectrometer team and the Cassini plasma 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 .
-end-
To remove yourself from this mailing, please go to http://www.kintera.org/TR.asp?a=mlLVL7MWLlKZLrI&s=lvL4JaORJhIQK9PXJvG&m=8eKFLMOqEhJOL1L
To remove yourself from all mailings from NASA Jet Propulsion Laboratory, please go to http://www.kintera.org/TR.asp?a=ihKNJVOGIiLJI6I&s=lvL4JaORJhIQK9PXJvG&m=8eKFLMOqEhJOL1L
No comments:
Post a Comment