Convection in Enceladus' ice shell: Conditions for initiation

Observations of Enceladus by the Cassini spacecraft indicate that this tiny Saturnian moon is geologically active, with plumes of water vapor and ice particles erupting from its southern polar region. This activity suggests that tidal dissipation has become spatially localized, perhaps due to a compositional, rheological, and/or thermal anomaly in its ice shell. Here we examine the role that solid‐state convection may have played in Enceladus' prolific activity by creating a suitable rheological and thermal anomaly. We find convection can only initiate in the pure water ice I shell of a differentiated Enceladus if the ice grain size is less than 0.3 mm, which is quite small, but may be realistic if non‐water‐ice impurities (and/or tidal stresses) keep grains from growing. This grain‐size restriction becomes more severe for lower basal ice temperatures, which implies that any ammonia present has not become strongly concentrated in a thin basal ocean (while convection occurs). For a maximally thick pure ice shell and underlying ocean, convective heat flows are ∼7–11 mW m −2 for ice grain sizes of 0.1–0.3 mm, compared with the ∼100 mW m −2 measured for Enceladus' south polar terrain from Cassini CIRS observations. Thus whereas solid‐state convection may be a prerequisite for Enceladus' geological activity, the observed heat flow requires strong tidal dissipation within the convecting region, and possibly, that convection reaches the surface.