Cassini Finds an Oxygen–Carbon Dioxide Atmosphere at Saturn’s Icy Moon Rhea

Extraterrestrial Atmosphere The detection of oxygen in the atmospheres of Jupiter's icy moons, Europa and Ganymede, and the presence of this gas as the main constituent of the atmosphere that surrounds Saturn's rings, has suggested the possibility of oxygen atmospheres around the icy moons that orbit inside Saturn's magnetosphere. Using the Ion Neutral Mass Spectrometer onboard the Cassini spacecraft, Teolis et al. (p. 1813, published online 25 November; see the Perspective by Cruikshank) report the detection of a very tenuous oxygen and carbon dioxide atmosphere around Saturn's icy moon Rhea. As with other icy satellites, this atmosphere is maintained through the dissociation of surface molecules and ejection into the atmosphere as a result of Saturn's magnetospheric radiation. Rhea’s atmosphere is maintained by chemical decomposition of surface water ice under irradiation from Saturn’s magnetosphere. The flyby measurements of the Cassini spacecraft at Saturn’s moon Rhea reveal a tenuous oxygen (O2)–carbon dioxide (CO2) atmosphere. The atmosphere appears to be sustained by chemical decomposition of the surface water ice under irradiation from Saturn’s magnetospheric plasma. This in situ detection of an oxidizing atmosphere is consistent with remote observations of other icy bodies, such as Jupiter’s moons Europa and Ganymede, and suggestive of a reservoir of radiolytic O2 locked within Rhea’s ice. The presence of CO2 suggests radiolysis reactions between surface oxidants and organics or sputtering and/or outgassing of CO2 endogenic to Rhea’s ice. Observations of outflowing positive and negative ions give evidence for pickup ionization as a major atmospheric loss mechanism.

[1]  Robert L. Tokar,et al.  The Dust Halo of Saturn's Largest Icy Moon, Rhea , 2008, Science.

[2]  W. Calvin,et al.  Condensed O2 on Europa and Callisto , 2002 .

[3]  Christopher T. Russell,et al.  Magnetic portraits of Tethys and Rhea , 2008 .

[4]  J. E. Richards,et al.  The Cassini Ion and Neutral Mass Spectrometer (INMS) Investigation , 2004 .

[5]  D. Strobel,et al.  Atmospheres and Plasma Interactions at Saturn’s Largest Inner Icy Satellites , 2005 .

[6]  John R. Spencer,et al.  Charge‐coupled device spectra of the Galilean satellites: Molecular oxygen on Ganymede , 1995 .

[7]  Lori M. Feaga,et al.  Temporal and Spatial Variability of Lunar Hydration As Observed by the Deep Impact Spacecraft , 2009, Science.

[8]  Bonnie J. Buratti,et al.  Carbon dioxide on the satellites of Saturn: Results from the Cassini VIMS investigation and revisions to the VIMS wavelength scale , 2010 .

[9]  B. Teolis,et al.  Formation, trapping, and ejection of radiolytic O2 from ion-irradiated water ice studied by sputter depth profiling. , 2009, The Journal of chemical physics.

[10]  R. Carlson,et al.  A tenuous carbon dioxide atmosphere on Jupiter's moon Callisto. , 1999, Science.

[11]  T. Owen,et al.  A spectroscopic study of the surfaces of Saturn's large satellites: H 2 O ice, tholins, and minor constituents , 2005 .

[12]  Remo Guidieri.  Res , 1995, RES: Anthropology and Aesthetics.

[13]  W. Ip,et al.  The structure and time variability of the ring atmosphere and ionosphere , 2010 .

[14]  Robert L. Tokar,et al.  Properties of the thermal ion plasma near Rhea as measured by the Cassini plasma spectrometer , 2009 .

[15]  R E Johnson,et al.  Detection of Ozone on Ganymede , 1996, Science.

[16]  M. Grande,et al.  Cassini Plasma Spectrometer Investigation , 2004 .

[17]  Paul D. Feldman,et al.  The Far-Ultraviolet Oxygen Airglow of Europa and Ganymede , 1998 .

[18]  J. Spencer.  Upper Limits for Condensed O2on Saturn's Icy Satellites and Rings , 1998 .

[19]  R. Clark,et al.  Compositional mapping of Saturn's satellite Dione with Cassini VIMS and implications of dark material in the Saturn system , 2007 .

[20]  R. E. Johnson,et al.  Detection of ozone on Saturn's satellites Rhea and Dione , 1997, Nature.

[21]  T. Hill,et al.  Negative ions in the Enceladus plume , 2010 .

[22]  B. Teolis,et al.  Ozone Synthesis on the Icy Satellites , 2006 .

[23]  Everett L. Shock,et al.  The oxidation state of hydrothermal systems on early Enceladus , 2008 .