Hydrous Silicates and Water on Venus

We used thermochemical equilibrium calculations to predict Water vapor, present at 30–45 parts per million by volstabilities of pure rock-forming hydrous silicates on Venus’ ume (ppmv) in Venus’ subcloud atmosphere (Drossart surface as a function of elevation, atmospheric H2O and SO2 concentrations, and oxygen fugacity (fO2). About 50 different et al. 1993; Pollack et al. 1993; DeBergh et al. 1995; Meadhydrous silicates were included in our calculations. We find ows and Crisp 1996; Ignatiev et al. 1997), is one of the three that many of these are unstable on Venus’s surface because of most important greenhouse gases (CO2, H2O, SO2) in the the low atmospheric H2O content of 30–45 parts per million present-day atmosphere of Venus. Water vapor is also the by volume (ppmv) and the high surface temperatures (660 K major reservoir of hydrogen in the lower atmosphere, and on Maxwell Montes to 740 K in the plains). Hydrous Feis an important reactant in chemical weathering reactions bearing silicates are unstable due to oxidation to magnetite that probably control the atmospheric abundances of HCl and/or hematite at the fO2 of the near-surface atmosphere. Caand HF on Venus. At present and during the past, loss of bearing hydrous silicates are unstable because of sulfatization to anhydrite. Some Fe-free micas (e.g., eastonite, eastonite– water from Venus, via oxidation of the surface and hydrophlogopite micas), and some alkali amphiboles might be stable gen escape to space, has been important in influencing the on Venus’ surface, especially in the lower temperature highoxidation state of the atmosphere and surface. At suffilands. We discuss hydrous mineral formation in the interior ciently high abundances, water vapor also participates in and on the surface of Venus. We review the literature on mica chemical weathering reactions leading to the formation of and amphibole thermal decomposition and find that dehydrahydrous minerals on Venus’s surface. tion of phlogopitic micas and fibrous amphiboles produces Information about the presence (or absence) of hydrous (metastable) dehydroxylated anhydrides that decompose to more stable minerals at temperatures hundreds of degrees minerals and OH-bearing nominally anhydrous minerals higher than the onset of dehydroxylation. These observations on Venus’s surface is important to understanding the presraise the possibility that anhydrides formed from hydrous silient and past atmospheric–lithospheric water cycle and wacates, which may have been present during a wetter period ter inventory on Venus, the petrology of convergent marin Venus’ history, may persist somewhere on Venus’ present gins of lithospheric blocks, the generation and physical surface. We discuss experiments that could be used on future properties of magmas, the atmospheric D/H ratio, the staspacecraft missions to detect hydroxyl in rocks and hydrous bility of Venus’s current climate and its evolution, the rate silicates on Venus. Finally, we review estimates of the amount of water outgassing from volcanism, the supply of water of water and OH (hydroxyl) in the Earth’s mantle. Based on this review, we suggest that even if no hydrous silicates are from cometary and asteroidal impacts, and the physical stable on Venus, significant amounts of water are plausibly properties (e.g., visible and infrared (IR) reflectance, elecpresent in surface rocks as OH in nominally anhydrous trical and thermal conductivity, dielectric properties, meminerals.  1997 Academic Press chanical strength) of Venus’s lithosphere, surface rocks,