Search

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.12.012 Byline: Gerald Schubert (a)(b), John D. Anderson (c), Bryan J. Travis (d), Jennifer Palguta (a) Keywords: Enceladus; Interiors; Saturn; satellites Abstract: Pre-Cassini images of Saturn's small icy moon Enceladus provided the first indication that this satellite has undergone extensive resurfacing and tectonism. Data returned by the Cassini spacecraft have proven Enceladus to be one of the most geologically dynamic bodies in the Solar System. Given that the diameter of Enceladus is only about 500 km, this is a surprising discovery and has made Enceladus an object of much interest. Determining Enceladus' interior structure is key to understanding its current activity. Here we use the mean density of Enceladus (as determined by the Cassini mission to Saturn), Cassini observations of endogenic activity on Enceladus, and numerical simulations of Enceladus' thermal evolution to infer that this satellite is most likely a differentiated body with a large rock-metal core of radius about 150 to 170 km surrounded by a liquid water-ice shell. With a silicate mass fraction of 50% or more, long-term radiogenic heating alone might melt most of the ice in a homogeneous Enceladus after about 500 Myr assuming an initial accretion temperature of about 200 K, no subsolidus convection of the ice, and either a surface temperature higher than at present or a porous, insulating surface. Short-lived radioactivity, e.g., the decay of.sup.26Al, would melt all of the ice and differentiate Enceladus within a few million years of accretion assuming formation of Enceladus at a propitious time prior to the decay of.sup.26Al. Long-lived radioactivity facilitates tidal heating as a source of energy for differentiation by warming the ice in Enceladus so that tidal deformation can become effective. This could explain the difference between Enceladus and Mimas. Mimas, with only a small rock fraction, has experienced relatively little long-term radiogenic heating; it has remained cold and stiff and less susceptible to tidal heating despite its proximity to Saturn and larger eccentricity than Enceladus. It is shown that the shape of Enceladus is not that of a body in hydrostatic equilibrium at its present orbital location and rotation rate. The present shape could be an equilibrium shape corresponding to a time when Enceladus was closer to Saturn and spinning more rapidly, or more likely, to a time when Enceladus was spinning more rapidly at its present orbital location. A liquid water layer on Enceladus is a possible source for the plume in the south polar region assuming the survivability of such a layer to the present. These results could place Enceladus in a category similar to the large satellites of Jupiter, with the core having a rock-metal composition similar to Io, and with a deep overlying ice shell similar to Europa and Ganymede. Indeed, the moment of inertia factor of a differentiated Enceladus, C/MR.sup.2, could be as small as that of Ganymede, about 0.31. Author Affiliation: (a) Department of Earth and Space Sciences, University of California, 595 Charles E. Young Drive East, Los Angeles, CA 90095-1567, USA (b) Institute of Geophysics and Planetary Physics, University of California, 603 Charles E. Young Drive East, Los Angeles, CA 90095-1567, USA (c) Global Aerospace Corporation, 711 West Woodbury Road, Suite H, Altadena, CA 91001-5327, USA (d) Earth & Environmental Sciences Division, EES-2/MS-F665, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Article History: Received 30 May 2006; Revised 27 November 2006

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.08.020 Byline: Jennifer Palguta (a), John D. Anderson (b), Gerald Schubert (a)(c), William B. Moore (a) Keywords: Satellites of Jupiter; Ganymede; Surfaces; satellite Abstract: Radio Doppler data, generated with NASA's Galileo spacecraft during its second encounter with Jupiter's moon Ganymede, are used to infer the locations and magnitudes of mass anomalies on Ganymede. We construct models for both surface and buried anomalies. With only one flyby and no global coverage, a solution for mass anomalies cannot be uniquely determined. However, we are able to constrain acceptable solutions for mass anomalies to four broad regions -- a near polar region and three that are roughly equatorial. If the mass anomalies are constrained to lie at the surface, the centers of the regions are located near the coordinates (77[degrees] N, 333[degrees] W), (36[degrees] N, 0[degrees] W), (33[degrees] N, 130[degrees] W), and (7[degrees] N, 194[degrees] W). If the mass anomalies are located at the deep ice-rock interface 800 km below the surface, the regions' centers are approximately (65[degrees] N, 17[degrees] W), (32[degrees] N, 30[degrees] W), (37[degrees] N, 175[degrees] W), and (15[degrees] N, 211[degrees] W). For both models, the regions are up to a few thousand kilometers across. The magnitude of mass anomalies on the surface is on the order of 10.sup.17 kg. Mass anomalies at the ice-rock interface are on average no more than an order of magnitude larger (10.sup.18 kg). There are two positive and two negative mass anomalies in both the surface and ice-rock interface models. One of the positive mass anomalies at the surface is associated with Galileo Regio. The other positive surface mass anomaly is located at high northern latitudes with no obvious geological association. Negative surface mass anomalies lie near Uruk Sulcus and between Perrine Regio and Barnard Regio near Sicyan Sulcus and Phrygia Sulcus. The locations of the ice-rock interface mass anomalies lie approximately radially below the surface anomalies. Positive mass anomalies at the surface could be associated with the silicate-rich ice or accumulated silicate layers of the dark regions. Negative mass anomalies at the surface could be associated with the relatively clean, low-lying ice of sulci. Alternatively, Ganymede's mass anomalies could be associated with the topography or other mass concentrations at the deep ice-rock interface. Author Affiliation: (a) Department of Earth and Space Sciences, University of California, 3806 Geology, Los Angeles, CA 90095-1567, USA (b) Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099, USA (c) Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA Article History: Received 19 April 2005; Revised 2 August 2005