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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.02.020 Byline: F. Nimmo (a), B.G. Bills (b) Keywords: Titan; Tides, Solid body; Geophysics Abstract: The long-wavelength topography of Titan has an amplitude larger than that expected from tidal and rotational distortions at its current distance from Saturn. This topography is associated with small gravity anomalies, indicating a high degree of compensation. Both observations can be explained if Titan has a floating, isostatically-compensated ice shell with a spatially-varying thickness. The spatial variations arise because of laterally-variable tidal heating within the ice shell. Models incorporating shell thickness variations result in an improved fit to the observations and a degree-two tidal Love number h.sub.2t consistent with expectations, without requiring Titan to have moved away from Saturn. Our preferred models have a mean shell thickness of [approximately equal to]100km in agreement with the observed gravity anomalies, and a heat flux appropriate to a chondritic Titan. Shell thickness variations are eliminated by convection; we therefore conclude that Titan's ice shell is not convecting at the present day. Author Affiliation: (a) Dept. Earth and Planetary Sciences, University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064, USA (b) Jet Propulsion Laboratory, MS 183-301, 4800 Oak Grove Drive, Pasadena, CA 91109, USA Article History: Received 27 October 2009; Revised 3 February 2010; Accepted 26 February 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.04.006 Byline: P.M. Grindrod (a), A.D. Fortes (a), F. Nimmo (b), D.L. Feltham (c), J.P. Brodholt (a), L. VoAadlo (a) Keywords: Titan; Ices; Interiors; Thermal histories; Astrobiology Abstract: We model the thermal evolution of a subsurface ocean of aqueous ammonium sulfate inside Titan using a parameterized convection scheme. The cooling and crystallization of such an ocean depends on its heat flux balance, and is governed by the pressure-dependent melting temperatures at the top and bottom of the ocean. Using recent observations and previous experimental data, we present a nominal model which predicts the thickness of the ocean throughout the evolution of Titan; after 4.5 Ga we expect an aqueous ammonium sulfate ocean 56 km thick, overlain by a thick (176 km) heterogeneous crust of methane clathrate, ice I and ammonium sulfate. Underplating of the crust by ice I will give rise to compositional diapirs that are capable of rising through the crust and providing a mechanism for cryovolcanism at the surface. We have conducted a parameter space survey to account for possible variations in the nominal model, and find that for a wide range of plausible conditions, an ocean of aqueous ammonium sulfate can survive to the present day, which is consistent with the recent observations of Titan's spin state from Cassini radar data [Lorenz, R.D., Stiles, B.W., Kirk, R.L., Allison, M.D., del Marmo, P.P., Iess, L., Lunine, J.I., Ostro, S.J., Hensley, S., 2008. Science 319, 1649-1651]. Author Affiliation: (a) Centre for Planetary Sciences, Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, UK (b) Department of Earth and Planetary Sciences, University of California at Santa Cruz, 1156 High Street, Santa Cruz, CA 95064-1077, USA (c) Centre for Polar Observation and Modelling, Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, UK Article History: Received 1 October 2007; Revised 3 April 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.05.002 Byline: F. Nimmo (a), T. Kleine (b) Keywords: Mars; Accretion; Planetary formation; Impact processes; Interiors Abstract: Estimates for the martian core formation timescale based on the hafnium-tungsten (Hf-W) isotopic system have varied by almost an order of magnitude, because of uncertainties in the martian mantle Hf/W ratio. Here we argue that the Hf/W ratio is [approximately equal to]4 but is uncertain by [approximately equal to]25%, resulting in (instantaneous) martian core formation timescales ranging from 0 to 10 Myr; accordingly, Hf-W isotope observations currently have limited utility in distinguishing between scenarios in which Mars formed as a stranded embryo and scenarios in which Mars suffered a prolonged accretion history. Author Affiliation: (a) Department of Earth and Planetary Sciences, University of California Santa Cruz, 1156 High St., Santa Cruz, CA 95064, USA (b) Institut fur Isotopengeologie und Mineralische Rohstoffe, ETH Zurich, CH-8092 Zurich, Switzerland Article History: Received 14 September 2006; Revised 24 April 2007

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.04.021 Byline: F. Nimmo (a), P.C. Thomas (b), R.T. Pappalardo (c), W.B. Moore (d) Keywords: Jupiter; satellites; Satellites; shapes; Interiors; Tides; solid body Abstract: The global shape of Europa is controlled by tidal and rotational potentials and possibly by lateral variations in ice shell thickness. We use limb profiles from four Galileo images to determine the best-fit hydrostatic shape, yielding a mean radius of 1560.8[+ or -]0.3km and a radius difference a-c of 3.0[+ or -]0.9km, consistent with previous determinations and inferences from gravity observations. Adding long-wavelength topography due to proposed lateral variations in shell thickness results in poorer fits to the limb profiles. We conclude that lateral shell thickness variations and long-wavelength isostatically supported topographic variations do not exceed 7 and 0.7 km, respectively. For the range of rheologies investigated (basal viscosities from 10.sup.14 to 10.sup.15Pas) the maximum permissible (conductive) shell thickness is 35 km. The relative uniformity of Europa's shell thickness is due to either a heat flux [greater than or equal to]7mWm.sup.-2 from the silicate interior, lateral ice flow at the base of the shell, or convection within the shell. Author Affiliation: (a) Department Earth and Planetary Sciences, University of California Santa Cruz, 1156 High St, Santa Cruz, CA 95064, USA (b) Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853, USA (c) Geophysics and Planetary Geosciences Group, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA (d) Department of Earth and Space Sciences, University of California Los Angeles, 595 Charles Young Drive E, Los Angeles, CA 90095, USA Article History: Received 17 November 2006; Revised 9 April 2007

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2004.10.034 Byline: F. Nimmo (a), B. Giese (b) Keywords: Europa; Satellites of Jupiter; Ices; Geophysics; Tectonics Abstract: Stereo topography of an area near Tyre impact crater, Europa, reveals chaos regions characterised by marginal cliffs and domical topography, rising to 100-200 m above the background plains. The regions contain blocks which have both rotated and tilted. We tested two models of chaos formation: a hybrid diapir model, in which chaos topography is caused by thermal or compositional buoyancy, and block motion occurs due to the presence of near-surface (1-3 km) melt; and a melt-through model, in which chaos regions are caused by melting and refreezing of the ice shell. None of the hybrid diapir models tested generate any melt within 1-3 km of the surface, owing to the low surface temperature. A model of ocean refreezing following melt-through gives effective elastic thicknesses and ice shell thicknesses of 0.1-0.3 and 0.5-2 km, respectively. However, for such low shell thicknesses the refreezing model requires implausibly large lateral density contrasts (50-100 kgam.sup.-3) to explain the elevation of the centres of the chaos regions. Although a global equilibrium ice shell thickness of [approximately equal to]2 km is possible if Europa's mantle resembles that of Io, it is unclear whether local melt-through events are energetically possible. Thus, neither of the models tested here gives a completely satisfactory explanation for the formation of chaos regions. We suggest that surface extrusion of warm ice may be an important component of chaos terrain formation, and demonstrate that such extrusion is possible for likely ice parameters. Author Affiliation: (a) Department of Earth and Space Sciences, UCLA, 595 Charles Young Drive E, Los Angeles, CA 90095-1567, USA (b) DLR, Rutherfordstrasse 2, 12489 Berlin, Germany Article History: Received 9 July 2004; Revised 12 October 2004

Models of topographic support on Europa by lateral shell thickness variations have previously assumed a Newtonian ice viscosity. Here I show that using a more realistic stress-dependent viscosity gives relaxation times which can be significantly different. Topography of wavelength 100 km cannot be supported by lateral shell thickness variations for ~ 50 Myr, unless the shell thickness is < 10 km or the ice grain size > 10 mm. Shorter wavelength topography would require even thinner shells, but may be supported elastically. Global-scale variations in shell thickness, however, can be supported for geological timescales if the shell thickness is O(10 km). Keywords: Ice; Rheology; Shell thickness; Grain size

We use stereo-derived topography of extensional bands on Europa to show that these features can be elevated by 100-150 m with respect to the surroundings, and that the positive topography sometimes extends beyond the band margins. Lateral variations in shell thickness cannot maintain the observed topography for timescales greater than ~ 0.1 Myr. Lateral density variations can maintain the observed topography indefinitely; mean density contrasts of 5 and 50 kg [m.sup.-3] are required for shell thicknesses of 20 and 2 km, respectively. Density variations caused by temperature contrasts require either present-day heating or that bands are young features (< 1 Myr old). Stratigraphic analyses suggest that these mechanisms are unlikely. The observation that bands form from ridges may be explained by an episode of shear-heating on ridges weakening the ridge area, and leading to strain localization during extension. Fracture porosity is likely to persist over Myr timescales in the top one-third to one-quarter of the conductive part of the ice shell. Lateral variations in this porosity (of order 20%) are the most likely mechanism for producing band topography if the ice shell is thin [approximately equal to] 2 km); porosity variations of 2% or less are required if the shell is thicker [approximately equal to] 20 km). If the ice shell is thick, lateral variations in salt content are a more likely mechanism. Warm ice will tend to lose dense, low-melting temperature phases and be buoyant relative to colder, salt-rich ice. Thus, lateral density variations will arise naturally if bands have been the sites of either localized heating or upwelling of warm ice during extension. Keywords: Satellites of Jupiter; Tectonics; Ices; Geological processes

Abstract: We identify two likely normal faults on Europa, of lengths ≈30 and 11km. A simple flexural model of fault-related topography gives effective elastic thicknesses of 1.2 and 0.15km, respectively, and the resulting inferred fault strength is of order 1MPa. The maximum fault displacement: length ratio for each fault is ≈0.02, comparable with values on silicate planets. We combine this observation with a modified linear elastic fracture mechanics model to conclude that the shear modulus of the Europan surface must be significantly less than that for unfractured ice. The low value of the modulus is probably due to near-surface fracturing or porosity, which will affect the material''s radar properties and seismic velocities. For a likely reduction in shear modulus of an order of magnitude, the driving stresses inferred are about 6–8MPa, much higher than present-day diurnal tidal stresses. However, stresses approaching these values can be generated by non-synchronous rotation or polar wander, while stresses exceeding these values arise during ice shell freezing. If the entire larger fault breaks, it will generate an event of seismic magnitude M s≈5.3. [Copyright &y& Elsevier]

Saturn׳s moons, Titan and Enceladus, are two of the Solar System׳s most enigmatic bodies and are prime targets for future space exploration. Titan provides an analogue for many processes relevant to the Earth, more generally to outer Solar System bodies, and a growing host of newly discovered icy exoplanets. Processes represented include atmospheric dynamics, complex organic chemistry, meteorological cycles (with methane as a working fluid), astrobiology, surface liquids and lakes, geology, fluvial and aeolian erosion, and interactions with an external plasma environment. In addition, exploring Enceladus over multiple targeted flybys will give us a unique opportunity to further study the most active icy moon in our Solar System as revealed by Cassini and to analyse in situ its active plume with highly capable instrumentation addressing its complex chemistry and dynamics. Enceladus׳ plume likely represents the most accessible samples from an extra-terrestrial liquid water environment in the Solar system, which has far reaching implications for many areas of planetary and biological science. Titan with its massive atmosphere and Enceladus with its active plume are prime planetary objects in the Outer Solar System to perform in situ investigations. In the present paper, we describe the science goals and key measurements to be performed by a future exploration mission involving a Saturn–Titan orbiter and a Titan balloon, which was proposed to ESA in response to the call for definition of the science themes of the next Large-class mission in 2013. The mission scenario is built around three complementary science goals: (A) Titan as an Earth-like system; (B) Enceladus as an active cryovolcanic moon; and (C) Chemistry of Titan and Enceladus – clues for the origin of life. The proposed measurements would provide a step change in our understanding of planetary processes and evolution, with many orders of magnitude improvement in temporal, spatial, and chemical resolution over that which is possible with Cassini–Huygens. This mission concept builds upon the successes of Cassini–Huygens and takes advantage of previous mission heritage in both remote sensing and in situ measurement technologies. [ABSTRACT FROM AUTHOR]