EXPERIMENTAL ALTERATION OF VENUSIAN SURFACE BASALTS IN A HYBRID CO2-SO2 ATMOSPHERE

Venus’ surface and interior dynamics remain largely unconstrained, due in great part to the major obstacles imposed by its 470°C, 90 bars surface conditions and its thick, opaque atmosphere. Orbiter-based thermal emission data provide an opportunity to characterize the Venus surface. However, interpretations of such spectra critically depend on understanding interactions between the planet’s surface basaltic rocks and its caustic, SO₂-bearing CO₂ atmosphere. Several studies, using remote sensing, thermodynamic modeling, and laboratory experiments have placed constraints on alteration mineralogies and rates. Yet constraint with respect to SO₂-mediated reactions with basalts of contrasting compositions remains incomplete. Here, we present new data from a series of gas-solid reaction experiments, in which samples of two basalt compositions were reacted in an SO₂-bearing CO₂ atmosphere, at relevant Venus temperatures, pressure, and fO₂. We subjected reacted specimens to SEM analyses where their surface alteration products were characterized, and their abundances estimated. We demonstrate that metathenardite and anhydrite are the prevalent alteration products on alkaline and tholeiitic basalt samples, respectively, and Fe-oxide phases are minor. Moreover, alteration products may cover alkali samples within a few weeks and tholeiite specimens within ~60 years. We suggest that these rates indicate that some unweathered volcanic features observed on Venus, as determined from emissivity intensity contrasts, may have been emplaced within a few weeks to less than 100 years before they were detected.