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A New Simulant to Represent The Silicate Interior of Enceladus

Introduction: Hypothetically, Enceladus is a habitable environment: there is a global sub-surface ocean [1], potential hydrothermal activity [2], and bioessential elements have been detected within the plumes emanating from the South Polar Region (SPR) [3]. Understanding this environment, and the reactions occurring within it, will further our understanding of its potential habitability. However, this relies on assumptions about the physical and environmental conditions, including the composition of the silicate postulated to be at its core. Here, we present an evaluation of the possible silicate composition, including organic materials. From this evaluation, a new simulant to represent this silicate has been designed, which can be used in simulation experiments and modelling of the Enceladus interior. Current understanding of the silicate interior: Evaluation of the interior of Enceladus can only be achieved through assumptions and from the indirect measurements of the plumes made by the instruments onboard Cassini, predominantly INMS and CDA [4-6]. The detection of SiO2 nanoparticles within the plumes infers a silicate interior [2], however the precise composition of the silicate is not confirmed. Analysis of particles within Saturn’s E-ring and the SPR plumes suggests that the silicate contains Mg-rich, Al-poor minerals [4] and organic species [5,6]. Molecular hydrogen has also been identified within the plumes [7], which may infer that serpentinisation reactions occur at the rock-water interface, for example between olivine and the sub-surface ocean. The extrapolated silicate composition from these data is one equivalent to a carbonaceous chondrite-type meteorite [8-10]. The modelled core density is approximately 2.4 g km [11], which has led to the assumption that the interior is porous, unconsolidated and has not experienced significant melting [12, 13]. Further to this, a porous interior increases the relative surface area exposed to the sub-surface ocean, increasing the rate of water-rock interactions. This supports the likelihood of long term water-rock interactions that will have altered the silicate chemistry and mineralogy [8, 9]. However, the detection of both the SiO2 particles and hydrogen within the plumes suggests the silicate is not fully hydrated and that these processes continue today [7, 9]. Simulant design: To satisfy the compositional requirements, we have based the silicate composition on the chemistry of CI carbonaceous chondrites, which display extensive aqueous alteration and contain up to 20 % water [14]. Further to this, the grain density of a CI chondrite (2.42 ± 0.060 g km) [15] is comparable to the modelled silicate density of Enceladus. Inorganic: To acquire the best representation of a CI chondrite, the average chemistry of the three most widely studied CIs (Ivuna, Orgueil and Alais) was calculated (Table 1).