Maybe Venus’ clouds are sown with airborne microbes resembling sulfur-reducing autotrophs on Earth, which are able to reduce elemental sulfur to hydrogen sulfide, and therefore thrive in the absence of oxygen. Or perhaps they resemble some types of photosynthesizing organisms, whose peak absorption is of blue-violet or ultraviolet light instead of the blue, yellow, and red light that terrestrial analogues like algae and plants optimally absorb. As discussed in a paper by Sanjay Limaye of the University of Wisconsin-Madison and his colleagues in 2018, there are many compounds that absorb the same wavelengths of light found in the absorption bands of Venus’ spectrum. These include iron-containing proteins like heme (a precursor to hemoglobin), iron sulfide (the most common sulfide mineral in the Earth’s crust, which is found in hydrothermal deposits like those at Yellowstone), and photosynthetic pigments like chlorophylls. Indeed, the absorption spectrum of Thiobacillus ferrosxidans, a highly acidophilic (pH 1.5 to 2.0) bacterium that obtains its energy through the oxidation of ferrous iron or reduced inorganic sulfur compounds, is markedly similar to the spectrum of Venus’ clouds.
There are also the recent claims of the detection of phosphine in Venus’ clouds, which has spurred significant debate in the scientific community. That’s because microorganisms produce the gas on Earth, and phosphine production requires a reducing atmosphere — one that removes oxygen. Reducing compounds such as methane, ammonia, amino acids, and the like are not stable in an oxidized atmosphere because they oxidize. So for a gas like phosphine to be in Venus’ clouds, it is necessary for it to be replenished somehow. But exactly how has yet to be determined. A recent analysis (published in March in Geophysical Research Letters) of data obtained for Venus’ middle cloud layers by the Pioneer Venus mission in 1978 supports the potential existence of phosphine, as well as traces of several other compounds consistent with the presence of a reducing atmosphere, which venusian microorganisms could use to support metabolic processes. On the other hand, some researchers dispute this, arguing that volcanic eruptions on the surface or lightning strikes in the clouds could explain the phosphine surplus.