The speciation of carbon in subseafloor hydrothermal systems has direct implications for the maintenance of life in present day vent ecosystems and possibly the origin of life on early Earth. Carbon monoxide is of particular interest because it represents a key reactant during the abiotic synthesis of reduced carbon compounds via Fischer-Tropsch-type processes. Laboratory experiments were conducted to constrain reactions that regulate the speciation of aqueous single carbon species under hydrothermal conditions and determine kinetic parameters for the oxidation of CO according to the water water-gas shift reaction (CO 2 + H 2 = CO + H 2 O). Aqueous fluids containing added CO 2 , CO, HCOOH, NaHCO 3 , NaHCOO, and H 2 were heated at 150, 200, and reduced carbon species such as HCOOH and CH 3 OH and the consumption of H 2 . The present study suggests that abiotic reactions involving aqueous carbon compounds in hydrothermal systems are sufficiently rapid to influence metabolic pathways utilized by organisms that inhabit vent environments.-2 -

Abstract. We present theoretical models of the composition, the relative abundances, and the stability of hydrated salts on the surface and in the icy shell of Jupiter's satellite Europa and discuss whether those salts have an oceanic origin. The evaluations were done with thermodynamic calculations of (1) salt dehydration equilibria at the conditions of the surface of Europa and its icy shell, (2) chemical equilibria involving solids and water vapor in the Na-K-Mg-Ca-S-C1-H20 system at surface temperatures and variable partial pressures of water vapor, and (3) changes in aquatic chemistry and sequences of salt precipitation from freezing oceanic water, using cosmochemical, mass balance, and physical-chemical constraints on the elemental and ionic composition of the ocean. Mass balance calculations of total or partial extraction of elements into an ocean from a carbonaceous chonddhte type mantle show that magnesium and sulfate rather than chloride and sodium could be the most abundant solutes in the ocean. Freezing oceanic water of this composition leads to brines that differ in composition from the original water and to deposition of ice and highly hydrated sulfates of Mg, Na, and Ca as well as alkali chlorides. After freezing is complete, highly hydrated salts remain stable in ice-bearing surface matedhals and throughout the icy shell. For hypothetical surface salt lag deposits, formed through sublimation/sputtedhng of ice and dehydration of salts, we predict hydration stratification with depth, approaching the highest hydration states in ice-beadhng matedhals in the lowest parts of the deposits. We discuss the effects of fast disequilibrium freezing and variable dehydration rates of salts on the predicted mineral assemblages at the surface. All of our models, which are independent of observations, predict the predominance of Mg and Na sulfates in surface salts, in agreement with spectroscopic models for the nonicy surface matedhal in the near infrared spectral region. IntroductionGalileo spacecraft reflectance spectroscopy in the near-infrared spectral region tentatively indicates the presence of hydrated saks in a non-H20 ice component on the surface of Jupiter's moon Europa [McCord et al., 1998a[McCord et al., , 1998b[McCord et al., , 1999. It was shown that magnesium and sodium sulfates (MgSO4.7H20, epsomite; MgSO4.Na2SO4.4H20, bloedite; Na2SO4.10H20, mirabilite), sodium carbonate (Na2CO3.10H20, natron), and mixtures of these salts provide good fits for the near-infrared spectra for the nonicy material Here we undertake evaluations of this type from a theoretical basis. In this paper we present internally consistent geochemical and thermodynamic models that describe leaching of aqueous components from a silicate mantle, aquatic chemistry of the ocean, freezing of oceanic water during upwelling in fractures in the icy crust and on the surface, and dehydration of salts on the surface of Europa. The results are compared with observational models for the composition and distribution of the nonicy surface mater...

[1] The oceanic composition on Saturn's moon Enceladus is evaluated through calculations of thermochemical equilibria at hydrothermal and freezing settings. Conditions of rock alteration are constrained from assumptions and models for the moon's interior composition and thermal evolution, and from the composition of Enceladus' plume. Results show that an early ocean was an alkaline NaUnderlying altered rocks consisted of Mg-phyllosilicates, magnetite, Fe and Ni sulfides, and carbonates. Subsequent freezing of oceanic water caused the deposition of a NaCl hydrate, Na, K and Ca carbonates, and the formation of a salt-free ice shell. If an aqueous phase exists on today's Enceladus, it could consist of eutectic Na-Cl-HCO 3 À brine that at least locally decouples the ice shell and facilitates tidal heating. A lack of firm detection of Na and Cl at Enceladus is consistent with the accumulation of salts at the ice-rock boundary and implies the plume formation via sublimation in the ice shell.

Samples of the carbonaceous asteroid Ryugu were brought to Earth by the Hayabusa2 spacecraft. We analyzed seventeen Ryugu samples measuring 1-8 mm. CO 2 -bearing water inclusions are present within a pyrrhotite crystal, indicating that Ryugu’s parent asteroid formed in the outer Solar System. The samples contain low abundances of materials that formed at high temperatures, such as chondrules and Ca, Al-rich inclusions. The samples are rich in phyllosilicates and carbonates, which formed by aqueous alteration reactions at low temperature, high pH, and water/rock ratios < 1 (by mass). Less altered fragments contain olivine, pyroxene, amorphous silicates, calcite, and phosphide. Numerical simulations, based on the mineralogical and physical properties of the samples, indicate Ryugu’s parent body formed ~ 2 million years after the beginning of Solar System formation.

[1] The discovery of sulfate-rich layered deposits with hematite spherules at the landing site of the Opportunity rover is consistent with mineral deposition in an aqueous environment. We evaluate conditions responsible for the formation of a jarosite-goethite-gypsum assemblage with speciation calculations. The results show that the assemblage could have precipitated from acidic solutions formed through near-surface aqueous oxidation of pyrite. Our hypothesis is that regional heating in the Meridiani Planum caused a release of sulfide-rich hydrothermal waters, leading to formation of pyrite-rich regional deposits in a depression. Aqueous oxidation of these deposits by atmospheric O 2 created an acidic environment that allowed formation of sulfates and goethite. Partial neutralization of the solution caused further goethite precipitation and conversion of jarosite to goethite, leading to formation of goethite concretions. Subsequent dehydration of goethite to coarse-grained hematite would also have been facilitated by regional heating.