Cohesive properties of ice powders analogous to fresh plume deposits on Enceladus and Europa

Abstract

Active jets of water vapour and ice grains discovered by the Cassini spacecraft at the South Pole of Enceladus result in the deposit of freshly erupted materials on the icy surface of the moon, potentially coming directly from its subsurface water ocean. As suggested by sporadic detection of plume activity, similar processes may be at work on Europa as well. Such fresh ice deposit areas are primary targets for future landing and sampling missions for the search of potential bio-signatures. Determining the mechanical properties of such fresh icy powder-like materials is essential to identify safe landing sites and appropriate sampling techniques. Ice surface energy or cohesion is a key parameter driving the porosity of the material, its strength, stickiness and flowability, which determine the stability of fresh surface deposits. In this work we performed an experimental study on laboratory analogues of fresh icy regolith, consisting in investigating the flowability of ice powders through a rotating drum device at several temperatures. Our experimental results show that the flowability of fresh icy deposits significantly decreases with increasing temperatures over the range of 90 to 150 K. Using a simple scaling analysis, we derive the surface energy of ice grains from the observed angle of the flowing surface inside the drum and we show that the surface energy increase by a factor of 5 to 10 between 90 K to 150 K, a behaviour not observed in any other powder material. This result indicates that the flowability of fresh icy deposits at the surface of Enceladus and Europa should significantly change depending on the local surface temperature, which can vary spatially and temporally on the same range of temperature explored here. Due to lower gravity, ice powder will behave as a more cohesive powder on Enceladus than on Europa. Higher cohesion on Enceladus may result in the formation of a very loose regolith with high porosity, making stable landing more challenging. On Europa, the reduction of ice cohesiveness with decreasing temperature should help the flow of freshly eruptive granular ice materials, potentially explaining numerous flow features observed throughout the surface.