Mikhail Yu. Zolotov

Composition and stability of salts on the surface of Europa and their oceanic origin

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 Jupiters 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-Cl-H2O 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 chondrite 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 materials and throughout the icy shell. For hypothetical surface salt lag deposits, formed through sublimation/sputtering of ice and dehydration of salts, we predict hydration stratification with depth, approaching the highest hydration states in ice-bearing materials 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 material in the near infrared spectral region.

A thermodynamic assessment of the potential synthesis of condensed hydrocarbons during cooling and dilution of volcanic gases.

The possibility for abiotic synthesis of condensed hydrocarbons in cooling/diluting terrestrial volcanic gases has been evaluated on the basis of the consideration of metastable chemical equilibria involving gaseous CO, CO2, H2 and H2O. The stabilities of n-alkanes and polycyclic aromatic hydrocarbons (PAHS) have been evaluated for several typical volcanic gas compositions under various conditions for cooling/diluting of quenched volcanic gas. The modeling shows that n-alkanes and PAHs have a thermodynamic potential to form metastably from H2 and CO below approximately 250 degrees C within the stability field of graphite. Despite the predominance of CO2 in volcanic gases, synthesis of hydrocarbons from CO2 and H2 is less favored energetically than from CO and H2. Both low temperature and a high H/C atomic ratio in volcanic gas generally favor stability of hydrocarbons with higher H/C ratios. PAHs are thermodynamically stable at temperatures approximately 10 degrees -50 degrees C higher than large n-alkanes; however, at lower temperatures, PAHs and n-alkanes have similar stabilities and are likely to form metastable mixtures. Both the energetic drive to form hydrocarbons and possible temperatures of formation increase as the oxidation state (fO2) of the volcanic gases decreases and as the cooling/dilution ratios of volcanic gases increase. Synthesis of hydrocarbons is energetically more likely in cooling trapped gases than in ashcloud eruptive columns. Mechanisms for hydrocarbon formation may include Fischer-Tropsch-type synthesis catalyzed by magnetite from solid volcanic products. On the early Earth, Mars, and Jupiters satellite Europa, several factors would have provided more favorable conditions for hydrocarbon synthesis in volcanic gases than under current terrestrial conditions and might have contributed to the production of organic compounds required for the emergence of life.

Eruption conditions of Pele volcano on Io inferred from chemistry of its volcanic plume

We use thermodynamic models and published 1996 Hubble Space Telescope (HST) observations of SO2, SO, and monatomic sulfur gas in the Pele volcanic plume on Jupiters moon Io to evaluate the temperature (1440 K) and oxidation state (3.3 log fO2 units below the Ni-NiO buffer) of Peles magma and exsolved volcanic gas. Combination of these results with 1999 HST data on the SO2/S2 ratio in the Pele plume allows us to calculate pressures (10−4.7–10−5.4 bar) in the vicinity of volcanic vent and to present a detailed chemical model for the plume. Our model indicates that the Pele plume represents volcanic gas, which last equilibrated at magmatic temperature and was not significantly altered in the plume. Finally, the redox state of Pele plume indicates that Io is differentiated and has an oxidized, Fe-metal free mantle.

Abiotic synthesis of polycyclic aromatic hydrocarbons on Mars

Thermochemical calculations of metastable equilibria are used to evaluate the stability of condensed polycyclic aromatic hydrocarbons (PAHs) in cooling thermal gases and hydrothermal fluids on ancient Mars, which are roughly similar to their terrestrial counterparts. The effects of temperature, pressure, the extent of PAH alkylation, and the relative stability of PAHs and alkanes are considered. Inhibition of methane and graphite formation favors synthesis of metastable mixtures of hydrocarbons from aqueous or gaseous CO, CO2, and H2 below 200°–300°C. High-temperature quenching of H2 and CO in volcanic and impact gases and dynamic hydrothermal fluids also favor the synthesis of hydrocarbons. In addition, an excess of CO in cooling systems relative to equilibrium makes the synthesis from CO and H2 more favorable energetically than from CO2 and H2. Both the CO-H2 reactions through Fischer-Tropsch (FT) type processes and the CO2-H2 reactions could be catalyzed by magnetite. Volcanic gases and hydrothermal fluids related to mafic and ultramafic magmas and rocks are more favorable for FT type synthesis than those associated with oxidized Fe2O3-bearing rocks and regolith. We conclude that PAHs and aliphatic hydrocarbons on Mars and Earth could be formed without the contribution of biogenic carbon. Some PAHs could be formed because of pyrolysis of other hydrocarbons formed earlier by the FT type synthesis or other processes. If the PAHs found in the ALH 84001 martian meteorite formed together with other hydrocarbons through FT type synthesis, it may be possible to bracket the temperature of the synthesis. The approach presented here can be generalized to study the synthesis of hydrocarbons in terrestrial volcanic and hydrothermal processes.

Extreme volcanism on Io: Latest insights at the end of Galileo era

Galileo has now completed 7 years exploring Jupiter. The spacecraft obtained breathtaking views of the four major satellites, and studied Jupiters clouds and atmospheric composition, rings, small satellites, and magnetic field. It had five successful close flybys and many distant observations of Io. Scientists already knew from Voyager and Earth-based astronomy that Io is by far the most volcanically active object in the solar system. Galileo has given us stunning color panoramas of Ios surface and unprecedented close views of erupting volcanoes (Figure 1) and the largest active flows observed anywhere. Among recent discoveries about Io, perhaps most astonishing since Voyager, is that some lavas possess emission temperatures greater than any lavas erupted on Earth today and possibly since the start of Earths geologic history. The Io science community has identified three alternative interpretations of Ios hottest lavas: (1) ultramafic material similar to komatiite; (2) superheated lava; or (3) an ultra-refractory substance deficient in silica and rich in Ca-Al oxides.