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The Ultraviolet Absorber, An ongoing mystery
post Aug 23 2016, 08:29 PM
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A few months back, Vladimir Krasnopolsky published an interesting result, purporting to exclude sulfur as a candidate for the atmospheric agent that creates dark/light patterns in Venus' clouds as seen in ultraviolet light. This analysis was based on data from Venera 14!


It was published in Icarus; I'm keen to see if his peers feel that Krasnopolsky has really closed the book on sulfur as a possibility or if it's just a suggestive argument.

Previous analysis, including Venus Express observations of a "glory" seen in the clouds, suggested that the two main candidates for the UV absorber were sulfur and ferric chloride (FeCl3). By eliminating sulfur, Krasnopolsky's work would seem to elevate the probability of FeCl3 as a match. Note: Krasnopolsky has argued for this as far back as 1985, looking at Venera 12 data.

Those of us used to thinking about Earth's and/or Mars's atmospheres probably don't think often of iron as an element particularly likely to occur in air. However, FeCl3 is certainly a gas at Venus's surface temperature, but it would liquify at an altitude near the bottom of the cloud deck and freeze a bit higher than that. Venus Express data has also shown that the UV absorber seems to be found in air that is rising from below. Perhaps what is happening is that FeCl3 is condensing on rising plumes, then solidifying, with the crystals serving as nucleation points for droplets of sulfuric acid.

Akatsuki may help clarify the mid-atmosphere dynamics that could argue for/against this model. And we'll know in a few months if DAVINCI will be a winner in the upcoming Discovery mission, which could perhaps answer the question for once and for all.
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