Michael T. Bland

Enceladus' extreme heat flux as revealed by its relaxed craters

[1]xa0Enceladus cratered terrains contain large numbers of unusually shallow craters consistent with deformation by viscous relaxation of water ice under conditions of elevated heat flow. Here we use high-resolution topography to measure the relaxation fraction of craters on Enceladus far from the active South Pole. We find that many craters are shallower than expected, with craters as small as 2xa0km in diameter having relaxation fractions in excess of 90%. These measurements are compared with numerical simulations of crater relaxation to constrain the minimum heat flux required to reproduce these observations. We find that Enceladus nominal cold surface temperature (70xa0K) and low surface gravity strongly inhibit viscous relaxation. Under such conditions less than 3% relaxation occurs over 2xa0Ga even for relatively large craters (diameter 24xa0km) and high, constant heat fluxes (150xa0mW m−2). Greater viscous relaxation occurs if the effective temperature at the top of the lithosphere is greater than the surface temperature due to insulating regolith and/or plume material. Even for an effective temperature of 120xa0K, however, heat fluxes in excess of 150xa0mW m−2are required to produce the degree of relaxation observed. Simulations of viscous relaxation of Enceladus largest craters suggest that relaxation is best explained by a relatively short-lived period of intense heating that decayed quickly. We show that infilling of craters by plume material cannot explain the extremely shallow craters at equatorial and higher northern latitudes. Thus, like Enceladus tectonic terrains, the cratered regions of Enceladus have experienced periods of extreme heat flux.

Mountains on Titan: Modeling and observations

[1]xa0We have developed a thermal model of Titans interior to study changes in volume during partial freezing or melting of a subsurface ocean due to heat flux variations from the interior. We find that the long-term cooling of Titan can cause global volume contraction ΔV/V ∼0.01. We then simulate two-dimensional contractional deformation of Titans icy lithosphere, finding that contractional deformation can produce tectonic activity and fold formation. Folds could potentially achieve a topographic height of several kilometers for high local strain (∼0.16), and for high temperature gradients in the ice I shell (order of 10 K km−1), corresponding to an ancient high heat flux from the interior (order of 0.02–0.06 W m−2). Examination of Synthetic Aperture Radar (SAR) imagery obtained by Cassini Radar shows possible evidence of contractional tectonism in the equatorial regions of Titan, although the moderate resolution of the Cassini SAR imagery does not permit an unambiguous geological interpretation.