Meanwhile, Venus, hot and poisonous, was long considered too inhospitable for anything to survive. But now, digging through archival NASA data, Rakesh Mogul, a biochemist at Cal Poly Pomona in California, and colleagues have found a hint of phosphine picked up by Pioneer 13 — a probe that reached Venus in December 1978.

"When the [Nature Astronomy paper] came out, I immediately thought of the legacy mass spectra," Mogul told Live Science.

Pioneer 13 carried an instrument which dropped through Venus' atmosphere suspended from a parachute called the Large Probe Neutral Mass Spectrometer (LNMS). Here is what a fresh look at the data found:

LNMS wasn't built to hunt phosphine-like compounds, and would have had a hard time distinguishing the gas from other molecules that have similar masses. But Pioneer 13's sample did have evidence of some molecule present in the gas that had the same mass as phosphine — in amounts that match the levels described in the Nature Astronomy paper.

"I believe that evidence for [trace chemicals that could be signatures of life] in the legacy data were sort of discounted because it was thought that they could not exist in the atmosphere," Mogul said. "I think many people are now revisiting the notion of Venus as a fully oxidizing environment." (A "fully oxidizing environment" wouldn't include phosphine or most other chemicals seen as signs of life.)

Mogul and his colleagues also found hints of other chemicals that shouldn't arise naturally in Venus' clouds — substances like chlorine, oxygen and hydrogen peroxide.

We should be seeing some new atmospheric data on Venus soon as BepiColombo swings by this week for the first of two gravity assists from Venus on it's way to Mercury.