Today, it was announced that phosphine has been found in Venus' atmosphere, rekindling the discussion about the possibility of extraterrestrial life. Venus has been visited by space probes for about 50 years now; how come none of these detected the presence of this gas? Much data was gathered about the composition of Venus' atmosphere, but not the presence of this molecule. Were we simply not looking for it, or were the instruments back then not precise enough to detect the amount of phosphine?
From the recently published paper Phosphine gas in the cloud decks of Venus.
The presence of PH3 implies an atmospheric, surface or subsurface source of phosphorus, or delivery from interplanetary space. The only measured values of atmospheric phosphorus on Venus come from Vega descent probes, which were only sensitive to phosphorus as an element, so its chemical speciation is not known. No phosphorus species have been reported at the planetary surface.