Presentation #100.01 in the session Fred Kavli Prize Plenary Lecture.
A few-hour search for phosphine in Venus’ atmosphere was made in 2017 using the James Clerk Maxwell Telescope, motivated by curiosity about PH3’s potential as a biosignature. After the astonishing indication that PH3 was present, there has been much follow-up. Further ground-based observations have confirmed the same PH3 line (with ALMA), and rule out contamination by other molecules known in the atmosphere. The altitudes where PH3 could be present are still enigmatic, complicating the search for sources. Phosphine is certainly unexpected from any surface or gaseous chemistry, including even stochastic effects like volcanism. I will discuss recent developments in interpretation and new observations. A new model proposes that acidity in cloud droplets may be reduced if a neutralising gas-species is present, and I will describe our search for ammonia (also a candidate biosignature gas) using the Green Bank Telescope. I will also compile new detections and limits on phosphine from various instruments, to see what distributions by altitude are possible and attempt to cast light on the origin problem. Ultimately, this can only be settled by instruments going to Venus, and an exciting decade of new missions is ahead.