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Cassini RADAR observations of Enceladus, Tethys, Dione, Rhea, Iapetus, Hyperion, and Phoebe

Ostro, Steven J. and West, Richard D. and Janssen, Michael A. and Lorenz, Ralph D. and Zebker, Howard A. and Black, Gregory J. and Lunine, Jonathan I. and Wye, Lauren C. and Lopes, Rosaly M. and Wall, Stephen D. and Elachi, Charles and Roth, Laci and Hensley, Scott and Kelleher, Kathleen and Hamilton, Gary A. and Gim, Yonggyu and Anderson, Yanhua Z. and Boehmer, Rudy A. and Johnson, William T. K. (2006) Cassini RADAR observations of Enceladus, Tethys, Dione, Rhea, Iapetus, Hyperion, and Phoebe. Icarus, 183 (2). pp. 479-490. ISSN 0019-1035. http://resolver.caltech.edu/CaltechAUTHORS:20121120-153035899

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Abstract

Cassini 2.2-cm radar and radiometric observations of seven of Saturn's icy satellites yield properties that apparently are dominated by subsurface volume scattering and are similar to those of the icy Galilean satellites. Average radar albedos decrease in the order Enceladus/Tethys, Hyperion, Rhea, Dione, Iapetus, and Phoebe. This sequence most likely corresponds to increasing contamination of near-surface water ice, which is intrinsically very transparent at radio wavelengths. Plausible candidates for contaminants include ammonia, silicates, metallic oxides, and polar organics (ranging from nitriles like HCN to complex tholins). There is correlation of our targets' radar and optical albedos, probably due to variations in the concentration of optically dark contaminants in near-surface water ice and the resulting variable attenuation of the high-order multiple scattering responsible for high radar albedos. Our highest radar albedos, for Enceladus and Tethys, probably require that at least the uppermost one to several decimeters of the surface be extremely clean water ice regolith that is structurally complex (i.e., mature) enough for there to be high-order multiple scattering within it. At the other extreme, Phoebe has an asteroidal radar reflectivity that may be due to a combination of single and volume scattering. Iapetus' 2.2-cm radar albedo is dramatically higher on the optically bright trailing side than the optically dark leading side, whereas 13-cm results reported by Black et al. [Black, G.J., Campbell, D.B., Carter, L.M., Ostro, S.J., 2004. Science 304, 553] show hardly any hemispheric asymmetry and give a mean radar reflectivity several times lower than the reflectivity measured at 2.2 cm. These Iapetus results are understandable if ammonia is much less abundant on both sides within the upper one to several decimeters than at greater depths, and if the leading side's optically dark contaminant is present to depths of at least one to several decimeters. As argued by Lanzerotti et al. [Lanzerotti, L.J., Brown, W.L., Marcantonio, K.J., Johnson, R.E., 1984. Nature 312, 139–140], a combination of ion erosion and micrometeoroid gardening may have depleted ammonia from the surfaces of Saturn's icy satellites. Given the hypersensitivity of water ice's absorption length to ammonia concentration, an increase in ammonia with depth could allow efficient 2.2-cm scattering from within the top one to several decimeters while attenuating 13-cm echoes, which would require a six-fold thicker scattering layer. If so, we would expect each of the icy satellites' average radar albedos to be higher at 2.2 cm than at 13 cm, as is the case so far with Rhea [Black, G., Campbell, D., 2004. Bull. Am. Astron. Soc. 36, 1123] as well as Iapetus.


Item Type:Article
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URLURL TypeDescription
http://dx.doi.org/10.1016/j.icarus.2006.02.019DOIArticle
http://www.sciencedirect.com/science/article/pii/S0019103506000790PublisherArticle
ORCID:
AuthorORCID
Janssen, Michael A.0000-0001-5476-731X
Additional Information:© 2006 Elsevier Inc. Received 19 November 2005; revised 24 February 2006. Available online 27 April 2006. Part of this research was conducted at the Jet Propulsion Laboratory (JPL), California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA). We thank two anonymous referees for their careful critiques of our paper, Peter Thomas and the ISS Team for the use of their Phoebe shape model, Andre Yavrouian for discussions of organic chemistry, Essam Heggy for discussions of electrical properties, Bonnie Buratti for discussions of optical properties, and scientists on the other Cassini teams for valuable interactions. We gratefully acknowledge those who designed, developed and operate the Cassini/Huygens mission, which is a joint endeavor of NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) and is managed by JPL/Caltech under a contract with NASA.
Funders:
Funding AgencyGrant Number
NASA/JPL/CaltechUNSPECIFIED
Subject Keywords:Satellites of Saturn; Surfacessatellite; Radar
Record Number:CaltechAUTHORS:20121120-153035899
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20121120-153035899
Official Citation:Steven J. Ostro, Richard D. West, Michael A. Janssen, Ralph D. Lorenz, Howard A. Zebker, Gregory J. Black, Jonathan I. Lunine, Lauren C. Wye, Rosaly M. Lopes, Stephen D. Wall, Charles Elachi, Laci Roth, Scott Hensley, Kathleen Kelleher, Gary A. Hamilton, Yonggyu Gim, Yanhua Z. Anderson, Rudy A. Boehmer, William T.K. Johnson, the Cassini RADAR Team, Cassini RADAR observations of Enceladus, Tethys, Dione, Rhea, Iapetus, Hyperion, and Phoebe, Icarus, Volume 183, Issue 2, August 2006, Pages 479-490, ISSN 0019-1035, 10.1016/j.icarus.2006.02.019. (http://www.sciencedirect.com/science/article/pii/S0019103506000790)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35590
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:20 Nov 2012 23:49
Last Modified:27 Nov 2017 22:09

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