Enceladus and the Icy Moons of Saturn (2016) 3042.pdf

THE GEOCHEMISTRY OF ENCELADUS’ OCEAN TOWARD THE END OF THE CASSINI MISSION. C. R. Glein1, M. Y. Zolotov2, S. D. Vance3, E. L. Shock2,4, and F. Postberg5,6. 1Southwest Research Institute ([email protected]), 2School of Earth and Space Exploration, Arizona State University, 3Jet Propulsion Laboratory, California Institute of Technology, 4Department of Chemistry and Biochemistry, Arizona State University, 5Institut für Geowissenschaften, Universität Heidelberg, 6Institut für Raumfahrtsysteme, Universität Stuttgart.

Introduction: Enceladus has put on a great perfor- example, the low pH endmember would be consistent with mance. By erupting gases and solids in a cryovolcanic plume fluid buffering by an ocean floor mineral assemblage of

that emanates from an ice-covered ocean, we are able to quartz-talc-magnesite-dolomite (high activities of SiO2 and obtain constraints that allow us to assemble the geochemical CO2), while the high pH endmember can be reproduced by a story of this world. Here, we focus on the chemistry of the model assemblage of chrysotile-talc-calcite-dolomite. subsurface ocean, and associated geochemical processes. Geochemical processes: Four processes have emerged Observations: Plume compositionl constraints are pro- as plausible “forces of geochemistry” on Enceladus: (1) vided by the Ion and Neutral Mass Spectrometer (INMS), the aqueous alteration of ultramafic rocks [9,11], (2) hydrother- Ultraviolet Imaging Spectrograph (UVIS), and the Cosmic mal (>0°C) mass transfer [8], (3) formation of gas hydrates Dust Analyzer (CDA). INMS and UVIS measured the gase- (clathrates) [e.g., 12], and (4) organic compound transfor- ous component of the plume, while CDA characterized mations [13]. These processes may have been active during

plume particles. All three instruments showed that H2O is the the history of Enceladus, and today if there are sources of dominant constituent. INMS determined that CO2, CH4, thermal or chemical disequilibria that have not been dissipat- NH3, and possibly H2 are key minor constituents [1,2]. Nu- ed or are replenished by a geophysically active interior. merous trace organic species were also detected [1,2]. UVIS Aqueous alteration of ultramafic rocks would produce an

has helped to constrain the abundances of CO, N2, and H2 alkaline pH and abundant H2 that may be present in the [3,4]. CDA data showed that Enceladus’ salty ocean is a plume [2]. In low CO2 cases, the formation of serpentine source of the plume, and the dominant salts are NaCl and (serpentinization) would lead to a pH of ~11-12. The pH

NaHCO3 or Na2CO3 [5,6]. Potassium is a minor species, and would not be as high (~8-9) in high CO2 cases because of sulfates were not observed. In addition, CDA detected organ- carbonation reactions [10]. The latter scenario would be ic mass fragments [7] complementary to those observed by more consistent with a hydrothermal model of nanosilica

INMS [1,2]. Recent CDA observations indicate that SiO2 formation [8]. However, a high pH ocean would be rich in nanoparticles are sourced from Enceladus’ plume [8]. dissolved Si, which could facilitate alternative mechanisms

Ocean chemistry: Initial chemical models of ocean wa- of forming SiO2 nanoparticles [14]. If hydrothermal fluids ter on Enceladus [9-11] are generally consistent with the are present today, they may provide volatile species (e.g.,

observational constraints and with each other. The most CH4) that can be outgassed or sequestered into clathrates striking similarity between Enceladus’ ocean and terrestrial [15]. In addition to CH4, there are other more complex but seawater (as a reference) is the dominance of NaCl, which less abundant organic compounds inside Enceladus, which stems from its high solubility, which allows efficient extrac- may be dissolved in ocean water, or present as oil droplets or tion from rocky sources. However, Enceladus’ ocean may be suspended particulates [2,7]. Hypothesized sources of these less concentrated in NaCl than terrestrial seawater because of organic compounds include leached primordial species, the much greater abundance of water relative to rock on thermally processed organic materials, abiotic (e.g., Fischer- Enceladus. Enceladus’ ocean also appears to differ from Tropsch-type) synthesis, and biological carbon fixation. terrestrial seawater in being relatively rich in dissolved inor- References: [1] Waite J. H. et al. (2009) Nature, 460,

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