Mobile lid convection beneath Enceladus' south polar terrain

摘要

Enceladus' south polar region has a large heat flux, 55–110 mW m?2, that is spatially associated with cryovolcanic and tectonic activity. Tidal dissipation and vigorous convection in the underlying ice shell are possible sources of heat, however, prior predictions of the heat flux carried by stagnant lid convection range from F conv ～ 15 to 30 mW m?2, too low to explain the observed heat flux. The high heat flux and increased cryovolcanic and tectonic activity suggest that near-surface ice in the region has become rheologically and mechanically weakened enough to permit convective plumes to reach close to the surface. If the yield strength of Enceladus' lithosphere is less than ～1–10 kPa, convection may instead occur in the “mobile lid” regime, which is characterized by large heat fluxes and large horizontal velocities in the near-surface ice. I show that model ice shells with effective surface viscosities between 1016 and 1017 Pa s and basal viscosities between 1013 and 1015 Pa s have convective heat fluxes comparable to that observed by Composite Infrared Spectrometer. If this style of convection is occurring, the south polar terrain should be spreading horizontally with v ～1–10 mm a?1 (where a is years) and should be resurfaced in ～0.1–10 Ma. On the basis of Cassini imaging data, the south polar terrain is ～0.5 Ma old, consistent with the mobile lid hypothesis. Maxwell viscoelastic tidal dissipation in such ice shells is not capable of generating enough heat to balance convective heat transport. However, tidal heat may also be generated in the near-surface along faults as suggested by Nimmo et al. (2007) and/or viscous dissipation within the ice shell may occur by other processes not accounted for by the canonical Maxwell dissipation model.