The Effects of a Porous Layer on the Dynamics and Two-way Radar Attenuation of Enceladus’ Ice Shell

Kurzfassung

The presence of an ocean beneath the Enceladus’ ice shell makes this Saturnian moon a high priority target for future planetary exploration [1]. Water jets that have been observed at the south pole by NASA’s Cassini mission [2] are thought to originate from the ocean and provide a direct window into the subsurface composition [3]. These jets generate a highly porous material that, due to its low thermal conductivity, affects the thermal state of the ice shell. The analysis of pit chains on the surface of Enceladus indicates that locally the porous layer can be as thick as 700 m [4]. Such a thick porous layer can locally increase the temperature of the ice shell, leading to a low viscosity. This may promote solid-state convection in regions where the ice shell is covered by such a layer, whereas regions with thin porous layers could be characterized by conductive heat transport. Moreover, due to its effect on the ice shell temperature, the porous layer can strongly attenuate the signal of radar sounders that have been proposed to investigate the Enceladus’ subsurface [5, 6]. Here, we use the geodynamical code GAIA [7] to investigate the effects of a porous layer on the thermal state and dynamics of Enceladus’ ice shell. Using the resulting thermal state we calculate the associated two-way radar attenuation at each location within the ice shell. We test different values of the ice shell thickness (5 – 35 km, [8]), porous layer thickness (d = 0 – 750 m), and its thermal conductivities (k = 0.1 – 0.001 Wm-1K-1 [9,10]). To account for chemical impurities within the ice shell we test a “low” loss scenario that considers a pure water ice shell and a “high” loss case that assumes a homogeneous mixture of water ice and chlorides in concentrations extrapolated from the particle composition of Enceladus’ plume [5]. Our results show that the porous layer thickness and its distribution have a first order effect on the thermal state and dynamics of the ice shell. Regions covered by a thick porous layer are characterized by a warm ice shell temperature and thus a lower viscosity, becoming more prone to convect. The vigor of convection depends on both the temperature-dependent ice shell viscosity and the temperature difference across the ice shell. While a thick porous layer would result in a low ice shell viscosity, thus increasing the convection vigor, such thick porous layers lead to an almost isothermal ice shell, due to their strong insulation, which, in turn, decreases the convection vigor. As discussed in a recent study that only investigated a purely conductive ice shell [6], the high temperatures may lead to the formation of shallow brines detectable by radar measurements.