CARBONATE SULFATE VOLCANISM ON VENUS

Venusian canali, outflow channels, and associated volcanic deposits re semble fluvial landforms more than they resemble volcanic features on Earth and Mars. Some canali have meandering habits and features indica tive of channel migration that are very similar to meandering river ch annels and flood plains on Earth, venusian outflow channels closely re semble water-carved outflow channels on Mars and the Channeled Scablan d in Washington, collapsed terrains at the sources of some venusian ch annels resemble chaotic terrains at the sources of martian outflow cha nnels, venusian lava deltas are similar to bird's-foot deltas such as the Mississippi delta, and venusian valley networks indicate sapping. The depositional fluvial-type features (deltas, braided bars, and chan neled plains) are generally among the smoothest terrains at the Magell an radar wavelength (12.6 cm) on Venus. These features suggest the inv olvement of an unusual lava, unexpected processes, and/or extraordinar y eruption conditions. Possibly the lava was an ordinary silicate lava such as basalt or a less common type of silicate lava, and conditions unique to Venus or to those particular eruptions may have caused an u nusual volcanological behavior. We have developed the alternative poss ibility that the lava had a water-like theology and a melting point sl ightly greater than Venus' surface temperature, thus accounting for th e unusual behavior of the lava. Unlike silicate lavas, some carbonatit es (including carbonate-sulfate-rich liquids) have these properties; t hus they can flow great distances while retaining a high fluidity, sig nificant mechanical erosiveness, and substantial capacity to transport and deposit sediment. Venusian geochemistry and petrology are consist ent with extensive eruptions of carbonatite lavas, which could have cr ustal and/or mantle origins. Venus' atmosphere (especially CO2, HCl, a nd HF abundances) and rocks may be in local chemical equilibrium, whic h suggests that the upper crust contains large amounts of calcite, anh ydrite, and other salts. Chemical analyses indicate, according to some models, that Venusian rocks may contain 4-19% calcite and anhydrite. Mixtures of crustal salts could melt at temperatures a few tens to a f ew hundred Kelvins higher than Venus' surface temperature; hence, melt ing may be induced by modest endogenetic or impact heating. Salts may have many of the same geologic roles on Venus as water and ice have on Mars. A molten salt (carbonatite) ''aquifer'' may exist beneath a few hundred meters to several kilometers of solidified salt-rich ''permaf rost.'' Many geologic features can be explained by carbonatite magmati sm: (1) impact melting of crustal salts can explain crater outflows, ( 2) small, sustained eruptions from molten salt aquifers can explain sa pping valleys, (3) large, sustained eruptions may explain canali and t heir flood plans, and (4) catastrophic outbursts may have formed outfl ow channels and chaotic terrain. Landforms created by carbonate-rich l avas would be thermally stable on Venus' surface, though some minerals may weather to other solid substances. (C) 1994 Academic Press, Inc.