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L. Schaefer, B. Fegley (Planetary Chemistry Laboratory, Washington University)
We modeled chemistry of elements and compounds proposed as radar reflective metallic frosts in the Venusian highlands (above 6054 km radius = 2.6 km altitude). Equilibrium calculations were done as a function of T and P from -2.6 to 50.6 km altitude. Several hundred compounds of the elements C, O, N, H, S, Cl, F (abundances from atmospheric gas abundances), and Cu, Zn, Ge, As, Sn, Pb, Se, Br, Cd, In, Ag, Sb, Hg, I, Tl, Bi, Te, and Au were considered. Abundances for Br, I, and the trace metals were assumed equal to those in Earth's basaltic oceanic crust, taken as an analog to Venus' basaltic crust. Complete degassing was assumed. We find that tellurium remains in the gas as TeS and TeSe, and it does not condense until 38.6 km altitude in the atmosphere. Tellurium condenses at 2.6 km if its abundance is increased ~60,000 times to 182 ppm. This is larger than the Ni abundance in Earth's oceanic crust and is geochemically unreasonable. Tellurium is not the metallic snow on Venus. However, high dielectric constant compounds of Pb and/or Bi do condense at the appropriate altitude. Bismuthite (Bi2S3) condenses at 1.6 km (terrestrial abundance) and has a dielectric constant of ~108. Bismuthite condenses at 2.6 km if about half the terrestrial abundance (~4 ppb) is used. Galena (PbS) is stable at 0 km, but is less stable and condenses at 2.6 km if the Pb abundance is ~0.1 times that in Earth's oceanic crust. Galenobismutite (PbBiS4), lillianite (Pb3BiS6), cannizarite (Pb4Bi5S11), and cosalite (Pb2Bi2S5) condense around terrestrial fumaroles. The estimated thermodynamic data that exist for these predict their condensation in the Venusian highlands. A lander equipped with laser induced breakdown spectroscopy (LIBS) can determine if these compounds are present. Supported by NASA NAG5-11037.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.