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David Bradley ISSUE #36
February 2004

The metallic mounds of Venus

  Credit: David Bradley/imagingstorm.co.uk

If you think winters on earth can be challenging, spare a thought for the highlands of Venus where heavy metal frost coats the ground and toxic lead compounds fall as snow!

It is hot enough on the planet's surface to melt lead and other materials, which can vaporize into the atmosphere and condense at high altitudes. Now, Laura Schaefer and Bruce Fegley, Jr of the Planetary Chemistry Laboratory at Washington University, St. Louis, have modeled the chemistry of numerous elements and compounds that have been proposed as radar reflective metallic frosts in the Venusian highlands (above the 6054 km radius, at 2.6 km altitude).

The team's equilibrium calculations took into account the variation in temperature and pressure in Venus' lower atmosphere and looked at several hundred possible compounds formed from combinations of C, O, N, H, S, Cl, and F (found in atmospheric gases) and Cu, Zn, Ge, As, Sn, Pb, Se, Br, Cd, In, Ag, Sb, Hg, I, Tl, Bi, Te, and Au (which are typically found in sublimates around terrestrial volcanoes).

Credit: NASA  
Many of the compounds tested in the calculations revealed themselves to be very unlikely to form but lead and/or bismuth sulfides do condense at the appropriate altitude. The team concludes that bismuthite (Bi2S3) condenses at 1.6 km and has a dielectric constant of about 108, which is important in explaining the brightness, the reflectivity of Venus, in the sky. This mineral condenses at 2.6 km if half the terrestrial abundances of the elements are used in the calculations. The argon-40 abundance and an upper limit on the mercury abundance on Venus suggest that Venus has less volatiles than Earth, or has degassed less efficiently, or both.
Credit: NASA
One of the highest mountains on Venus - Maats Mons - reaches 5 miles above the mean venusian plain.

Galena (PbS), in contrast, is stable at 0 km, but is less stable and condenses at 2.6 km if the lead abundance is about a tenth of that found in the Earth's oceanic crust. Other minerals, such as galenobismutite (PbBiS4), lillianite (Pb3BiS6), cannizarite (Pb4Bi5S11), and cosalite (Pb2Bi2S5) also condense.

What their calculations boil down to is that the frigid mountains of Venus are likely to be dusted with a heavy metal frost and continually dusted by a metallic snow. "A lander equipped with laser induced breakdown spectroscopy (LIBS) could determine if these compounds are present," the researchers suggest.

Related work is published in Icarus, 2004, in press; http://dx.doi.org/10.1016/j.icarus.2003.08.023