Thermochemical modelling of the ocean composition of Enceladus from ocean floor to outer space

Rachael E Hamp,Karen Olson Francis,Victoria K Pearson,Susanne P Schwenzer

doi:10.5194/epsc2022-1235

Abstract

&lt;p&gt;Enceladus, an icy moon of Saturn, is a potentially habitable environment. Its South Polar Region hosts active plumes that eject material from the subsurface into space. Plume material was analysed by the Cassini spacecraft, which confirmed the presence of a global subsurface ocean, active hydrothermal activity, and the presence of bio-essential elements (carbon, nitrogen, and hydrogen) [1-4]. Data from the plumes provided a snapshot of the subsurface chemical environment, however, it could not fully constrain the composition of the silicate interior or specific ocean chemistry. We present work constraining a modern-day Enceladus ocean composition utilising theoretical compositions for the icy exterior (and subsurface ocean) and silicate interior of an early Enceladus and studying the evolution of the ocean chemistry over time.&lt;/p&gt; &lt;p&gt;The modern-day ocean composition was determined by thermochemical modelling using CHIM-XPT [5], where hypothetical starting compositions for the ocean interact with the silicate interior. A CI carbonaceous chondrite was used as an analogue to represent the silicate interior of Enceladus [6] and a fluid of cometary ice composition was used as an analogue for the moon&amp;#8217;s early ocean [7]. The water-rock interactions of these two analogues were modelled and used to investigate the evolution of the ocean chemistry of Enceladus over time. The models were run over a range of temperature and pressure conditions representative of the ocean floor conditions of Enceladus. For example, it has been determined [2] that a minimum temperature of 90 &amp;#8451; is required to form the SiO&lt;sub&gt;2&lt;/sub&gt; particles that have been measured in the stream believed to originate from Enceladus and the pressure hypothesized for the water-rock interface is 80 bar, based upon the combined depth of the subsurface ocean and ice crust, along with gravitational data measurements [2], so these conditions were modelled. Further modelling was carried out at higher temperatures (up to 250 &amp;#8451;) and higher pressures (up to 150 bar), to explore reactions that may occur deeper within the porous silicate interior. The resultant modern-day ocean fluid was then both cooled and depressurized (using CHIM-XPT) to study changes in the chemical composition as water ascends from the water-rock interface to the ice-water interface on Enceladus.&lt;/p&gt; &lt;p&gt;Finally, the ocean composition at the ice-ocean interface was frozen using FREZCHEM to replicate the freezing process in the plumes of Enceladus. The ice grain composition generated through thermochemical modelling was then critically compared to Cassini&amp;#8217;s plume data. The ice grains generated through this study were dominated by water ice, with NaCl, NaHCO&lt;sub&gt;3&lt;/sub&gt;, Na&lt;sub&gt;2&lt;/sub&gt;CO&lt;sub&gt;3&lt;/sub&gt;, KCl and a low concentration of Na&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt;, all of which have been detected in the plumes or E-ring, with the exception of Na&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt;, where the concentration may have been below the limit of detection of the instruments onboard Cassini [3].&lt;/p&gt; &lt;p&gt;These results suggests that a CI chondrite and a cometary ice composition may be suitable analogues for the silicate interior and the icy exterior of a proto-Enceladus. A complex modern-day ocean composition has been generated that is more complex that has been indicated by analysed ice grains. Additionally, this work may be able to constrain potential concentrations of aqueous species that have not been observed by Cassini, in particular trace species such as sulfates, transition metals and phosphates, that could be used to define the detection limits required to identify target chemical species on instruments onboard future missions. We will present the outcomes from this modelling, which includes a modern-day ocean composition and our findings from modelling of the cooling and freezing of ocean fluids as they are transported from the ocean floor to outer space.&lt;/p&gt; &lt;p&gt;[1] Thomas P. C. et al., (2016), &lt;em&gt;Icarus&lt;/em&gt;, &lt;strong&gt;264&lt;/strong&gt;, 37-47&lt;/p&gt; &lt;p&gt;[2] Hsu H. W. et al., (2015), &lt;em&gt;Nature, &lt;/em&gt;&lt;strong&gt;519&lt;/strong&gt;, 207-210&lt;/p&gt; &lt;p&gt;[3] Waite, J. H., et al., &lt;em&gt;Nature&lt;/em&gt;, &lt;strong&gt;460&lt;/strong&gt;, 487-490, 2009&lt;/p&gt; &lt;p&gt;[4] Postberg F. et al., (2018) &lt;em&gt;Nature&lt;/em&gt;, &lt;strong&gt;558&lt;/strong&gt;, 564-568&lt;/p&gt; &lt;p&gt;[5] Reed, M. H., Spycher, N. F., Palandri, J., (2010) User guide for CHIM-XPT, University of Oregon, Oregon&lt;/p&gt; &lt;p&gt;[6] Hamp R. E. et al., (2019), 50th LPSC 2019, Abstract &lt;strong&gt;1091&lt;/strong&gt;&lt;/p&gt; &lt;p&gt;[7] Hamp R.E et al., (2021), 52&lt;sup&gt;nd&lt;/sup&gt; LPSC 2021, Abstract &lt;strong&gt;2548 &lt;/strong&gt;&lt;/p&gt;

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