Phase fractionation and the fate of bioessential elements during freezing of simulated Enceladus ocean fluids
Abstract
Saturn's ice-covered moon Enceladus exhibits large cryovolcanic plumes sourced from a global subsurface ocean that may contain the requisite conditions for life. Salt-rich ice particles encountered in the plumes by Cassini are thought to originate as frozen ocean droplets, thus capturing a snapshot of ocean chemistry. However, little is known about how Enceladus ocean fluids evolve as they freeze, and thus how key indicators of habitability, such as redox-sensitive metals and organic compounds, might be expressed within these salt-rich particles. We investigated fluid evolution and solid phase formation from simulated Enceladus ocean fluids during freezing at endmember cooling rates, using parallel thermodynamic modelling and experimental approaches. Cryo-imaging techniques showed that even at flash-freezing conditions (>10 K s-1), Enceladus-like fluids undergo segregation, whereby the crystallization of ice templates the formation of brine vein networks. The high solute concentrations and confined nature of these brine veins means that vitrification can occur at slower cooling rates than those typically required for vitrification of a bulk solution. At more gradual cooling rates (~1 K min-1), or if flash-frozen samples are re-warmed, crystalline salts precipitate at ice grain boundaries. Resulting crystallization textures differ markedly between cooling regimes, showing that they inherit a signature of their formation conditions, and can result in compositional heterogeneity on a sub-10 μm scale. Moreover, the mineralogy of carbonate phases can act as a probe for parent fluid pH. The fate of trace bioessential elements such as iron, which are expected in the ocean but have thus far evaded detection in Cassini data, are explored, alongside the implications for how organic material, including biological structures, may be incorporated and delivered to space. Our findings describe endmember possibilities for solid phase production from Enceladus-relevant fluids that add crucial context to the findings of Cassini and provide a strong rationale for non-destructive plume measurements by future missions.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMP003.0008F
- Keywords:
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- 4850 Marine organic chemistry;
- OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL;
- 5215 Origin of life;
- PLANETARY SCIENCES: ASTROBIOLOGY;
- 6282 Enceladus;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 8450 Planetary volcanism;
- VOLCANOLOGY