Clathration as a Process for the Cryomagmatic Differentiation of Icy Satellites. Application to Enceladus and Europa

Introduction: Evidence of cryomagmatism has been detected in some icy satellites such as Europa and Enceladus. Examples of cryomagmatic structures on Europa are domes of various sizes, flat materials covering older features and low albedo deposits around fractures. In Enceladus, the features are less obvious because the composition of the magmatic materials and the surrounding surface seems to be very similar, since the albedo is extremely homogenous. Nevertheless, the cryomagmatic activity of Enceladus has been revealed recently by the Cassini sensors, showing ejections of bright materials from some of the fractures of its surface. A classic problem for explaining cryomagmatism is that pure aqueous cryomagmas have negative bouyancy in the usual concept of a pure water ice crust of an icy body like those sometimes assumed for Europa or Enceladus. However, some geochemical models show that crusts of these satellites include other components in water [1, 2]. Segregated cryomagmatic liquids should have lower compositional or temperature driven densities than the host materials in order to ascend in the crust. In the case of Europa, salt and sulfuric acid hydrates have been nominated as part of the composition of the surface and the crust suported by spectroscopic observations [3, 4, 5]. The main observed constituent of Enceladus' surface is water ice (both crystalline and amorphous); traces of short-chain hydrocarbons, CO2 ice, and isolated or complexed molecules of CO2 have been observed [7], and a waterdominated volatile plume includes one to several percent each of CH4, N2, and CO2 [8]. Other ices such as ammonia hydrates have been proposed to be present [2, 6, 9] but have not been detected by Cassini VIMS [7]. We suggest that the formation of clathrate hydrates from an aqueous magmatic chamber enriched in gasses and dissolved ions will result in the differentiation of the cryomagmas into the icy satellites. Then, distilled aqueous cryomagmas could ascend from the more dense residue, decompress, expand, and erupt explosively.