Chemical Weathering on Venus
Authors: Zolotov Mikhail 
Published: January 2018

Chemical and phase compositions of the surface of Venus could reflect a history of gas–rock and fluid–rock interactions, recent and past climate changes, and a loss of water from the Earth’s sister planet. The concept of chemical weathering on Venus through gas–solid type reactions was established in the early 1960s after the discovery of the hot and dense CO2-rich atmosphere of the planet, inferred from Earth-based and Mariner 2 radio emission data. Initial models suggested carbonation, hydration, and oxidation of exposed igneous rocks and a control (buffering) of atmospheric gases by solid–gas type chemical equilibria in the near-surface rocks. Carbonates, phyllosilicates and Fe oxides were considered likely secondary minerals. From the late 1970s onward, measurements of trace gases in the sub-cloud atmosphere by the Pioneer Venus and Venera entry probes and by Earth-based infrared spectroscopy challenged the likelihood of hydration and carbonation. The atmospheric H2O gas content appeared to be low enough to allow the stable existence of H2O-bearing and a majority of OH-bearing minerals. The concentration of SO2 gas was too high to allow the stability of Ca-rich carbonates and silicates with respect to sulfatization to CaSO4. In the 1980s, the detection of an elevated bulk S content at the Venera and Vega landing sites suggested ongoing consumption of atmospheric SO2 to surface sulfates. The supposed composition of the near-surface atmosphere implied oxidation of ferrous minerals to Fe oxides, magnetite and hematite, consistent with the infrared reflectance of surface materials. The likelihood of sulfatization and oxidation has been illustrated in modeling experiments in simulated Venus’ conditions. The morphology of Venus’ surface suggests contact of atmospheric gases with hot surface materials of mainly basaltic composition during the several hundreds of millions years since a global volcanic/tectonic resurfacing. Some exposed materials could have reacted at higher and lower temperatures in a presence of diverse gases at different altitudinal, volcanic, impact, and atmospheric settings. On highly deformed tessera terrains, more ancient rocks of unknown composition may reflect interactions with putative water-rich atmospheres and even aqueous solutions. Geological formations rich in salt, carbonate, Fe oxide, or silica will indicate past aqueous processes. The apparent diversity of affected solids, surface temperatures, pressures, and gas/fluid compositions throughout Venus’ history implies multiple signs of chemical alterations that remain to be investigated. The current understanding of chemical weathering is limited by the uncertain composition of the deep atmosphere, by the lack of direct data on the phase and chemical composition of surface materials, and by the uncertain data on thermodynamics of minerals and their solid solutions. In preparation for further atmospheric entry probe and lander missions, rock alteration could be investigated through chemical kinetic experiments and calculations of solid-gas/fluid equilibria to constrain past and present processes.