Mercury's low‐degree geoid and topography controlled by insolation‐driven elastic deformation

©2015. American Geophysical UnionMercury experiences an uneven insolation that leads to significant latitudinal and longitudinal variations of its surface temperature. These variations, which are predominantly of spherical harmonic degrees 2 and 4, propagate to depth, imposing a long‐wavelength thermal perturbation throughout the mantle. We computed the accompanying density distribution and used it to calculate the mechanical and gravitational response of a spherical elastic shell overlying a quasi‐hydrostatic mantle. We then compared the resulting geoid and surface deformation at degrees 2 and 4 with Mercury's geoid and topography derived from the MErcury, Surface, Space ENvironment, GEochemistry, and Ranging spacecraft. More than 95% of the data can be accounted for if the thickness of the elastic lithosphere were between 110 and 180 km when the thermal anomaly was imposed. The obtained elastic thickness implies that Mercury became locked into its present 3:2 spin orbit resonance later than about 1 Gyr after planetary formation

Solid tides in Io’s partially molten interior: Contribution of bulk dissipation

International audienceContext. Io’s spectacular activity is driven by tidal dissipation within its interior, which may undergo a large amount of melting. While tidal dissipation models of planetary interiors classically assume that anelastic dissipation is associated only with shear deformation, seismological observation of the Earth has revealed that bulk dissipation might be important in the case of partial melting.Aims. Although tidal dissipation in a partially molten layer within Io’s mantle has been widely studied in order to explain its abnormally high heat flux, bulk dissipation has never been included. The aim of this study is to investigate the role of melt presence on both shear and bulk dissipation, and the consequences for the heat budget and spatial pattern of Io’s tidal heating.Methods. The solid tides of Io are computed using a viscoelastic compressible framework. The constitutive equation including bulk dissipation is derived and a synthetic rheological law for the dependence of the viscous and elastic parameters on the melt fraction is used to account for the softening of a partially molten silicate layer.Results. Bulk dissipation is found to be negligible for melt fraction below a critical value called rheological critical melt fraction. This corresponds to a sharp transition from the solid behavior to the liquid behavior, which typically occurs for melt fractions ranging between 25 and 40%. Above rheological critical melt fraction, bulk dissipation is found to enhance tidal heating up to a factor of ten. The thinner the partially molten layer, the greater the effect. The addition of bulk dissipation also drastically modifies the spatial pattern of tidal dissipation for partially molten layers.Conclusions. Bulk dissipation can significantly affect the heat budget of Io, possibly contributing from 50 to 90% of the global tidal heat power. More generally, bulk dissipation may play a key role in the tidally induced activity of extrasolar lava worlds

Timing of water plume eruptions on Enceladus explained by interior viscosity structure

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Mercury's low-degree geoid and topography controlled by insolation-driven elastic deformation

Mercury experiences an uneven insolation that leads to significant latitudinal and longitudinal variations of its surface temperature. These variations, which are predominantly of spherical harmonic degrees 2 and 4, propagate to depth, imposing a long-wavelength thermal perturbation throughout the mantle. We computed the accompanying density distribution and used it to calculate the mechanical and gravitational response of a spherical elastic shell overlying a quasi-hydrostatic mantle. We then compared the resulting geoid and surface deformation at degrees 2 and 4 with Mercury's geoid and topography derived from the MErcury, Surface, Space ENvironment, GEochemistry, and Ranging spacecraft. More than 95% of the data can be accounted for if the thickness of the elastic lithosphere were between 110 and 180 km when the thermal anomaly was imposed. The obtained elastic thickness implies that Mercury became locked into its present 3:2 spin orbit resonance later than about 1 Gyr after planetary formation

A rigid and weathered ice shell on Titan

Several lines of evidence suggest that Saturn's largest moon, Titan, has a global subsurface ocean beneath an outer ice shell 50 to 200 kilometres thick. If convection is occurring, the rigid portion of the shell is expected to be thin; similarly, a weak, isostatically compensated shell has been proposed to explain the observed topography. Here we report a strong inverse correlation between gravity and topography at long wavelengths that are not dominated by tides and rotation. We argue that negative gravity anomalies (mass deficits) produced by crustal thickening at the base of the ice shell overwhelm positive gravity anomalies (mass excesses) produced by the small surface topography, giving rise to this inverse correlation. We show that this situation requires a substantially rigid ice shell with an elastic thickness exceeding 40 kilometres, and hundreds of metres of surface erosion and deposition, consistent with recent estimates from local features. Our results are therefore not compatible with a geologically active, low-rigidity ice shell. After extrapolating to wavelengths that are controlled by tides and rotation, we suggest that Titan's moment of inertia may be even higher (that is, Titan may be even less centrally condensed) than is currently thought