Venus as seen in ultraviolet by Mariner 10,
February 5, 1974

The uncertainty of science: Since the mid-1970s scientists have been trying to figure out the cause of the dark streaks seen in the atmosphere of Venus when photographed in ultraviolet. The image to the right was the first such UV image, taken by Mariner 10 during its fly-by on its way to Mercury. These streaks are caused by some unknown material that absorbs the UV light.

Researchers have now proposed that two different iron-bearing minerals, floating in Venus’s atmosphere, are what cause these dark UV streaks. From the absract:

Our results demonstrate that ferric iron can react with sulfuric acid to form two mineral phases: rhomboclase [(H5O2)Fe(SO4)2·3H2O] and acid ferric sulfate [(H3O)Fe(SO4)2]. A combination of these two mineral phases and dissolved Fe3+ in varying concentrations of sulfuric acid are shown to be good candidates for explaining the 200- to 300-nm and 300- to 500-nm features of the reported unknown UV absorber.

Iron has been detected in the atmosphere previously, and is theorized could come from volcanic eruptions on the planet’s million-plus volcanoes.

This new hypothesis explaining the dark UV absorber however remains unconfirmed, and joins a list of other candidates for that absorber, many of which propose either iron or sulfur in different combinations. Because so little data really exists about the atmosphere of Venus, with what is known also uncertain, it is presently impossible to solve this mystery with confidence. The problem is made even more difficult in that scientists understand almost nothing about the chemistry of materials under the conditions seen in Venus’s atmosphere, which is extremely hot (800 degrees Fahrenheit) and very thick and dense (100 times that of Earth).