Around 30 miles (48 km) above the planet’s surface is a lower cloud layer with temperatures around 60° C (140° F) and pressures similar to that of Earth (unlike the planet’s surface with a pressure of 90 atmospheres, equivalent to the pressure felt over a half a mile beneath the ocean). It’s not impossible to think that microbial life could exist in this atmospheric Goldilocks zone, after all, here on Earth microorganisms have been found alive at altitudes as high as 25 miles (41 km).
In addition to these incredibly suitable environmental conditions, it’s a strange and inexplicable atmospheric phenomenon that is really driving the hypothesis. As long as we have had telescopes good enough to observe Venus in detail, we have witnessed mysterious dark patches moving through its atmosphere. These dark patches seem to be composed of sulfuric acid, alongside unknown particles that absorb ultraviolet light.
“Venus shows some episodic dark, sulfuric rich patches, with contrasts up to 30–40 percent in the ultraviolet, and muted in longer wavelengths,” says planetary scientist Sanjay Limaye. “These patches persist for days, changing their shape and contrasts continuously and appear to be scale dependent.”