A mushy source for the geysers of Enceladus

Abstract

Enceladus is a primary target for astrobiology due to the $\rm H_2O$ plume ejecta measured by the Cassini spacecraft and the inferred subsurface ocean sustained by tidal heating. Sourcing the plumes via a direct connection from the ocean to the surface requires a fracture through the entire ice shell ($\sim$10 km). Here we explore an alternative mechanism in which shear heating within shallower tiger stripe fractures produces partial melting in the ice shell and interstitial convection allows fluid to be ejected as geysers. We use an idealized two-dimensional multiphase reactive transport model to simulate the thermomechanics of a mushy region generated by an upper bound estimate for the localized shear heating rate in a salty ice shell. From our simulations, we predict the temperature, porosity, salt content, melting rate, and liquid volume of an intrashell mushy zone surrounding a fracture. We find that the rate of internal melting can match the observed $\rm H_2O$ eruption rate and that there is sufficient brine volume within the mushy zone to sustain the geysers for $\sim350$ kyr without additional melting. The composition of the liquid brine is, however, distinct from that of the ocean, due to partial melting. This shear heating mechanism for geyser formation applies to Enceladus and other icy moons and has implications for our understanding of the geophysical processes and astrobiological potential of icy satellites.