Hamp, Rachael

Olsson-Francis, Karen

Pearson, Victoria

Ramkissoon, Nisha

Schwenzer, Susanne

English

Open Research Online

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsA New Simulant to Represent The Silicate Interior ofEnceladusConference or Workshop ItemHow to cite:Hamp, Rachael; Ramkissoon, Nisha; Olsson-Francis, Karen; Schwenzer, Susanne and Pearson, Victoria (2019).A New Simulant to Represent The Silicate Interior of Enceladus. In: 50th Lunar and Planetary Science Conference2019 Proceedings.For guidance on citations see FAQs.c© [not recorded]Version: Version of RecordLink(s) to article on publisher’s website:https://pdfs.semanticscholar.org/c6a8/52f8181b90c74d4052ae7ac42ce6fb599b39.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukA NEW SIMULANT TO REPRESENT THE SILICATE INTERIOR OF ENCELADUS.  R. E. Hamp1, N. K. Ramkissoon1, K. Olsson-Francis1, S.P. Schwenzer1 and V.K. Pearson1, 1Faculty of Science, Technology, Engineering and Mathematics, The Open University, Milton Keynes, UK. (rachael.hamp@open.ac.uk)    Introduction: Hypothetically, Enceladus is a hab-itable environment: there is a global sub-surface ocean [1], potential hydrothermal activity [2], and bio-essential 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 assump-tions 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 mate-rials. 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 com-position 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 hy-drogen 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 car-bonaceous chondrite-type meteorite [8-10].  The modelled core density is approximately 2.4 g km-3 [11], which has led to the assumption that the in-terior is porous, unconsolidated and has not experi-enced significant melting [12, 13]. Further to this, a porous interior increases the relative surface area ex-posed 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 hy-drated and that these processes continue today [7, 9]. Simulant design: To satisfy the compositional re-quirements, 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-3) [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 cal-culated (Table 1).   Table 1- Comparison between the calculated average CI composition and the chemistry expected for the fi-nal simulant based upon the ratio of minerals selected.   To replicate the composition of a CI chondrite as accurately as possible, minerals were used (as far as possible) that have previously been detected in CI chondrites [16, 17] (Table 2). The chemistry expected, based upon average mineral compositions, was com-pared to the calculated average CI composition (Table 1).  The maximum variation between the calculated composition and the expected composition is 1.5 wt%.        Oxide Average CI composition (wt %) Expected analogue composition (wt %) SiO2 27.96 27.62 FeO 10.47 11.90 Fe2O3 11.63 10.52 SO3 5.23 5.76 MgO 17.83 17.45 Al2O3 2.15 2.12 Cr2O3 0.60 0 Na2O 0.43 0.11 K2O 0.20 0 P2O5 0.09 0.42 TiO2 0.07 0.003 CaO 0.41 0.73 MnO 0.17 0.01 NiO 2.00 2.17  Table 2- The proposed mineralogy for the silicate sim-ulant based upon the chemistry for a CI chondrite.   Carbon chemistry: In line with the chemistry of CI chondrites, our simulant needs to contain approximate-ly 3 wt% carbon [18], in both inorganic and organic form. Dolomite, bruennerite and calcite (Table 1) have all been detected in CI chondrites [19] and can be used to represent inorganic carbon. Other forms of carbon, nanodiamonds, silicon carbide and graphite are present in very small concentrations in CI chondrites, but be-cause of the conditions that are expected to occur on Enceladus at the rock-water interface, these compo-nents are not expected to react.  Therefore these will be omitted from the simulant. Organic molecules in CI chondrites are predomi-nantly present as the ‘macromolecular material’, which predominantly consists of a high aromatic structure.  This constitutes 70% of their organic fraction [see 20 for a review].  Until recently, only short chained hy-drocarbons had been detected in Enceladus’ plumes, however data from Cassini’s CDA and INMS instru-ments have indicated organics with masses of up to 200 au (the limit of the CDA) [21]. This has led to the as-sumption that the silicate interior likely contains large macromolecular organics, predominantly composed of discrete benzene-like sub-structures connected through non-carbon functional groups or dehydrogenated car-bons [21]. Although similar to chondritic macromolec-ular material, its structure is less aromatic and  contains shorter bridges between the aromatic structures. Ter-restrial kerogen has been used as an analogue for chondritic organic material [e.g., 22] but also has a greater predominance of fused aromatic rings bridged by long carbon chains, and so does not represent a good analogue for Enceladus’ organic matter.  Enceladus’ organic fraction will therefore be repre-sented in the simulant by humic acid.  Its structure is predominantly discrete benzene subunits connected through dehydrogenated carbon bonds or through func-tional groups.  It is also more readily available. Production: The simulant is in the process of being finalised and produced. Currently, test samples of each mineral have been characterised; these results are cur-rently being analysed to recalculate the ratio of miner-als required based upon specific mineral samples. Bulk samples will be purchased, prepared and mixed, and then further characterisation of the final simulant can be conducted. This final chemistry will be presented.  Uses: The simulant will be used to replicate the sub-surface environment of Enceladus in simulations of the reaction cycles occurring at the rock-water inter-face.  References: [1] Thomas P. C. et al., (2016), Ica-rus, 264, 37-47 [2] Hsu H. W. et al., (2015), Nature, 519, 207-210 [3] McKay, C. P. et al., (2014) Astro-biology, 14, 352-355 [4] Postberg F. et al., (2009), Nature, 459, 1098-1101 [5] Waite J. H. et al., (2009) Nature, 460, 487-490 [6] Bouquet A. et al., (2015), GRL, 42, 1334-1339 [7] Waite J. H. et al., (2017) Sci-ence, 356, 155-159 [8] Zolotov M. Y., (2007), Ge-ophys Res Lett, 34, L23203 [9] Sekine Y. et al., (2015) Nature Comm., 8604 [10] Weisberg M. et al., (2006), Meteorites and the early solar system II, 19-52 [11] Iess L. et al., (2014), Science, 344, 77-80 [12] Roberts J., (2015) Icarus, 258, 54-66 [13] Glein C., (2015), Geochimica et Cosmochimica, 162, 202-219 [14] Guaita, C., (2012), Mem. S.A.It. Suppl., 20, 74-83 [15] Macke R. J., et al.,  (2011), Meteorit. Planet. Sci., 46, 1842-1842 [16] Scott E. & Krot A., (2013) Treatise on Geochemistry, 1, 65-137 [17] King A. et al., (2015), Geochimica et Cosmochimica Acta, 165, 148-160 [18] Kerridge J., (1985), Geochemica et Cosmochimica, 49, 1707-1714 [19] Endreβ M. & Bischoff A., (1996), Geochimicia et Cosmochimicia Acta, 60, 489-507  [20] Pearson et al., (2010), Astrobiology, Emergence, Search and Detection of Life, American Scientific Pub-lishers, pp155-173 [21] Postberg F. et al.,  (2018) Na-ture, 558, 564-568 [22] Matthewman R. et al., (2013), Astrobiology, 13, 324-333    Mineral Weight % Olivine (9:1 Foster-ite:fayalite) 15 Magnetite 15 Serpentine 15 Almandine 10 Diopside 5 Talc 5 Quartz 5 Pyrite 3 Anhydrite 1 Apatite 1 Dolomite 1 Bruennerite 0.15 Calcite 0.02  

A New Simulant to Represent The Silicate Interior of Enceladus

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A New Simulant to Represent The Silicate Interior of Enceladus

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