M. Yu. Zolotov

Equilibrium-kinetic model of water-rock interaction

A computer model was developed for chemical interaction in water-rock systems. The model is based on the concept of partial equilibrium [1] and combines the calculation of chemical equilibria in multicomponent systems with accounting for the kinetics of the congruent dissolution of minerals as a function of pH (zeroth order kinetic reactions). The development of the process in time is simulated as a series of sequential partial equilibria, and the bulk chemical composition of the system is calculated at each time step from the chemical composition of aqueous solution at the beginning of the step and masses of minerals dissolved during time Δt. The dissolution rates of individual minerals are calculated at each time step for the given temperature, current pH value, and the degree of solution saturation with respect to minerals. Variations in the surface area of minerals due to precipitation and dissolution are accounted for. Model application is exemplified by the calculation of chemical equilibria in the water-granite system. The model may be useful for understanding the character of low-temperature interactions in water-rock systems under stagnant conditions, in particular, the multistage development of groundwater chemistry, interaction of liquid radioactive waste injected into underground repositories, etc.

Lithospheric-atmospheric interaction on Venus.

Physicochemical methods, especially equilibrium calculations of the phase relations in multicomponent natural systems, inevitably are applied to the investigation of objects that are impractical for experimental treatment.

Weathering of Martian surface rocks

The early 1960s marked the beginning of an extremely important trend in Martian investigation, i.e., exploration by space missions. From 1962 to 1976 more than a dozen spacecraft in the framework Mars, Mariner, and Viking space missions were launched, providing new and important data on the composition and structure of the Martian atmosphere and soil, as well as data on the planet’s physical characteristics.

Solid Planet–Atmosphere Interactions

Even without an abundant aqueous phase, planetary surfaces are coupled with planetary atmospheres through complex physical and chemical processes. On Mars and Venus, dissociation of H 2 O and hydrogen escape from early atmospheres have caused partial oxidation of crustal materials. Although early atmospheric conditions could have favored the formation of carbonates and hydrated minerals, current environments may not be suitable for carbonate formation. Throughout history, volcanic/impact degassing was followed by consumption of sulfur and halogens into surface materials through weathering reactions. Atmosphere–surface interactions could have been affected by changing solar luminosity, orbital parameters, the greenhouse heating, internal processes, and impact cratering.

Stability of micas on the surface of Venus

Abstract Recent thermodynamic modeling shows that some micas might be stable on Venussurface. However, prior studies considered only pure micas and did not consider mica solidsolutions, which are commonly observed on Earth. Here we use chemical equilibriumcalculations to evaluate the stability of mica solid solutions on Venus surface as a function ofatmospheric chemistry (H 2 O and HF abundances, and redox state), and surface elevation. Ourprior calculations show that the end-member micas eastonite (KMg 2 Al 3 Si 2 O 10 (OH) 2 ) andfluorphlogopite (KMg 3 AlSi 3 O 10 F 2 ) are stable on Venus surface, while the end-member micasphlogopite (KMg 3 AlSi 3 O 10 (OH) 2 ), annite (KFe 3 AlSi 3 O 10 (OH) 2 ), and siderophyllite ( KFe 2+ 2 Al 3 Si 2 O 10 ( OH ) 2 ) are unstable. Based on these results and known petrologic phase relationships, weconsider binary solutions of eastonite with either phlogopite or siderophyllite, andfluorphlogopite with phlogopite. We calculate that micas along all three binaries are stable onVenus. Micas containing ∼20 mole% eastonite and ∼80% phlogopite are stable in the lowertemperature highlands, and very eastonite-rich micas are stable over Venus entire surface.Fluorphlogopite-rich micas are also stable over Venus surface, while fluorphlogopite-poor micasare stable at higher elevations. Iron-poor micas along the eastonite-siderophyllite join, containing>80 mole% eastonite, are stable in both the highlands and lowlands. Finally, we use thethermodynamic calculations, terrestrial geology, and petrologic phase equilibria to discussplausible geological settings where micas may be present on Venus. These suggestions areimportant for the design of geochemical experiments on future lander and automated balloonmissions to Venus.

10.10 – Solid Planet–Atmosphere Interactions

Planetary surfaces are coupled with planetary atmospheres through complex physical and chemical processes. On Mars and Venus, surface composition and climatic evolution are affected by volcanism, impacts, and atmosphere–surface interactions with limited and/or temporal influences from aqueous processes. Despite significant differences in environmental conditions on Mars and Venus, atmosphere–surface interactions reveal oxidation of surface materials by atmospheric gases and consumption of sulfur and halogens through weathering reactions. Atmosphere–surface interactions are altitude and latitude dependent owing to variations in surface temperature, pressure, and concentrations of atmospheric gases. On both planets, the composition of atmosphere and surface materials reveal past geological events, including intensive volcanic/impact degassing and resurfacing, as well as processes that occurred under wetter conditions. Although early atmospheric conditions could have favored formation of carbonates and hydrated minerals, current environments may not be feasible for carbonate formation.