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The surface of Ariel, an icy satellite orbiting Uranus, shows extensional tectonic features suggesting an episode of endogenic heating in the satellite's past. Using topography derived from stereo-photoclinometry, we identified flexural uplift at a rift zone suggesting elastic thickness values in the range 3.8-4.4 km. We estimate the temperature at the base of the lithosphere to be in the range 99 to 146 K, depending on the strain rate assumed, with corresponding heat fluxes of 28-92 mW/m2. Neither tidal heating, assuming Ariel's current eccentricity, nor radiogenic heat production from the silicate core are enough to cause the inferred heat fluxes. None of three proposed ancient mean-motion resonances produce equilibrium tidal heating values in excess of 4.3 mW/m2. Thus, the origin of the inferred high heat fluxes is currently mysterious.

The solidification of the Lunar Magma Ocean (LMO) and formation of impact basins are important events that took place on the early Moon. The relative timing of these events, however, is poorly constrained. The aim of this study is to constrain the formation ages of old impact basins based on inferences of their thermal state. Most proposed basins formed before Pre-Nectarian (PN) 5 stage do not exhibit clear concentric features in either topography or gravity, suggesting substantial viscous lateral flow in the crust. Recent geodetic measurements reveal that the lunar crust is thinner than previously estimated, indicating that an extremely high crustal temperature is required for lateral flow to occur. In this study, we calculate lunar thermal evolution and viscoelastic deformation of basins and investigate the thermal state at the time of basin formation using recent crustal thickness models. We find that a Moho temperature >1300–1400K at the time of basin formation is required for substantial viscous relaxation of topography to occur; the implied elastic thickness at the time of loading is

We investigate the origins of Triton’s deformed and young surface. Assuming Triton was captured early in solar system history, the bulk of the energy released during capture will have been lost, and cannot be responsible for its present-day activity. Radiogenic heating is sufficient to maintain a long-lived ocean beneath a conductive ice shell, but insufficient to cause convective deformation and yielding at the surface. However, Triton’s high inclination likely causes a significant ( 0.7) obliquity, resulting in large heat fluxes due to tidal dissipation in any subsurface ocean. For a 300 km thick ice shell, the estimated ocean heat production rate (0.3 TW) is capable of producing surface yielding and mobile-lid convection. Requiring convection places an upper bound on the ice shell viscosity, while the requirement for yielding imposes a lower bound. Both bounds can be satisfied with an ocean temperature 240 K for our nominal temperature-viscosity relationship, suggesting the presence of an antifreeze such as NH3. In our view, Triton’s geological activity is driven by obliquity tides, which arise because of its inclination. In contrast, Pluto is unlikely to be experiencing significant tidal heating. While Pluto may have experienced ancient tectonic deformation, we do not anticipate seeing the kind of young, deformed surfaces seen at Triton.

Tidal heating plays a significant role in the evolution of many satellites in the outer Solar System; however, it is unclear whether tidal dissipation in a global liquid ocean can represent a significant additional heat source. Tyler (Tyler, R.H. [2008]. Nature 456, 770-772; Tyler, R.H. [2009]. Geophys. Res. Lett. 36, doi:10.1029/2009GL038300) suggested that obliquity tides could drive large-scale flow in the oceans of Europa and Enceladus, leading to significant heating. A critical unknown in this previous work is what the tidal quality factor, Q, of such an ocean should be. The corresponding tidal dissipation spans orders of magnitude depending on the value of Q assumed. To address this issue we adopt an approach employed in terrestrial ocean modeling, where a significant portion of tidal dissipation arises due to bottom drag, with the drag coefficient O (0.001) being relatively well-established. From numerical solutions to the shallow-water equations including nonlinear bottom drag, we obtain scalings for the equivalent value of Q as a function of this drag coefficient. In addition, we provide new scaling relations appropriate for the inclusion of ocean tidal heating in thermal-orbital evolution models. Our approach is appropriate for situations in which the ocean bottom topography is much smaller than the ocean thickness. Using these novel scalings, we calculate the ocean contribution to the overall thermal energy budgets for many of the outer Solar System satellites. Although uncertainties such as ocean thickness and satellite obliquity remain, we find that for most satellites it is unlikely that ocean tidal dissipation is important when compared to either radiogenic or solid-body tidal heating. Of known satellites, Triton is the most likely icy satellite to have ocean tidal heating play a role in its present day thermal budget and long-term thermal evolution. © 2013 Elsevier Inc.

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

New Horizons images of Pluto's companion Charon show a variety of terrains that display extensional tectonic features, with relief surprising for this relatively small world. These features suggest a global extensional areal strain of order 1% early in Charon's history. Such extension is consistent with the presence of an ancient global ocean, now frozen.