I’m pretty sure I’ve read a hard SF book where a diverse group of plucky scientists have a wacky idea to look for life on a nearby planet, put it into effect not really hoping to get anything positive, and then find that their results show something utterly surprising and unexpected. They then face the slings and arrows of more staid and establishment scientists, fighting them off with skill and ingenuity to finally win the day by showing that they were incontrovertibly right.
In fact, I think I’ve probably read that basic plot many times over the years in various different forms and by multiple different authors.
But this has also been a big part of my working life over the last 18 months.
I have become a character in that very science fiction story.
I’ve always been interested in the possibility of extraterrestrial life – what SF fan or writer isn’t? – but at the time I started working in astrophysics research we didn’t even know that planets existed around other stars. I got involved with astronomy on the largest scales, looking at distant galaxies and working on observations of the cosmic microwave background. I was thus a spectator when the exoplanet revolution arrived, and we found handfuls, then hundreds, and then thousands of planets orbiting other stars. This is now a huge industry, with space missions and huge amounts of time on ground-based telescopes dedicated to the study of exoplanets, their atmospheres and, eventually, the search for signs of life.
Life searches closer to home were dominated by large NASA and ESA missions to Mars which seemed to be the only hope for finding signs of life in our own Solar System. Not having any background in biology, geology, or chemistry, there wasn’t a lot I could contribute to such work.
Then things got more interesting.
The Cassini spacecraft orbiting Saturn discovered plumes of water vapour emerging from the interior of the moon Enceladus. The possibility of liquid water lying beneath the icy surface of gas giant moons was something for which science fiction had prepared me, most notably in 2010: Odyssey 2. The idea that some of this interior material might leak out, allowing it to be studied for signs of biological activity, was not something I, or I think anybody else, had considered. It also provided a way for me to get into this game.
My introduction to astrobiology in the Solar System took place in a bar in Hilo, the largest town on the Big Island of Hawaii, and a home from home for many observational astronomers on their way up to, or down from, the observatories on Maunakea. I was heading up the mountain when I bumped into a colleague, Prof Jane Greaves, in Uncle Billy’s hotel bar. Prof Greaves had just been observing at the James Clerk Maxwell telescope with Dr Helen Fraser. They had been observing Enceladus, looking to confirm earlier observations that had found methane in the plumes of material being emitted by the planet. We had a good discussion about this, and came up with the idea of using the yet-to-be-launched far-infrared space telescope Herschel, which I was working on at the time, to look for more molecular species in the Enceladus plumes, searching for signs of biological activity. To cut a long story short, we never got the Herschel time, and the methanol they had detected was actually the result of chemistry in the plumes driven by UV light from the Sun, rather than anything taking place inside Enceladus. Claims that biochemistry and even life is present inside Enceladus, though, persist, but remain controversial.
Time moves on, Herschel launches and is a great success. I move on to a permanent academic post and become the UK project scientist for SPICA, a proposed followup mission to Herschel. As part of this project I organise several meetings to get UK scientists involved, and to look at the scientific potential of the mission. At one of these I challenge Prof Greaves to come up with some interesting ideas for SPICA in our own Solar System. Most of what she came up with had to do with the outer Solar System, but the final slide of her talk raised the possibility of looking for phosphine, PH3, in the atmosphere of Venus.
Phosphine had been found in the atmospheres of the gas giant planets Jupiter and Saturn by ESA’s Infrared Space Observatory (ISO) back in the 1990s. In gas giants the compound is produced deep in their atmospheres, where it is very hot and atmospheric pressure is millions of times of that of Earth at sea level. Gas giant atmosphere are also full of hydrogen, so pretty much everything is combined with this element. If you apply that principle to phosphorous you get phosphine. What Prof Greaves was suggesting, though, was to look for a far-IR line of phosphine in the atmosphere of Venus, which is dominated by oxygen compounds, not hydrogen compounds. Phosphine would have no right to be there, leading to the possibility that, if detected, it might be a biomarker – a sign that life might exist on Venus.
Of course every SF fan knows that Venus is hell – Carl Sagan told us! It is the victim of a runaway greenhouse effect with a thick carbon dioxide dominated atmosphere that has a surface pressure 90 times that of the Earth at sea level, and surface temperatures that are high enough to melt lead. Why would anybody think there might be life on Venus?
But Carl Sagan also told us about the clouds. About 55 km above the surface of Venus the atmospheric pressure is comparable to sea level on Earth, and the temperatures have fallen to a balmy 40 C or so. Liquid water can thus persist, and exsts in droplet form in the permanent cloud layer at this height. It’s not a bed of roses though. The environment is still forty times drier than the driest place on Earth, and those droplets are only 10% water, with the remaining 90% acid. But there is liquid water and, to an astrobiologist, liquid water means there might be life. And, unlike other locations in the Solar System with liquid water, it isn’t hiding beneath many kilometres of ice.
So Prof Greaves presented the possibility of looking for phosphine on Venus with SPICA. There were several problems with this idea. Firstly, as a cryogenic space telescope that has to be kept with a few degrees of absolute zero to operate, SPICA could never be pointed close enough to the Sun to be able to observe Venus. And, secondly, as a candidate mission, there was no guarantee it would fly [it has in fact since been unceremoniously cancelled by ESA – but that is another story], and even if it did, it would not be until the mid 2030s.
Prof Greaves, and the rest of us who were interested in this idea, thus had to look elsewhere.
Fortunately the far-IR isn’t the only place where phosphine transitions are found. There is another at a wavelength just over 1mm that can be observed from the ground. We thus proposed to look for phosphine in the atmosphere of Venus with the James Clerk Maxwell Telescope (JCMT). We weren’t expecting to detect anything, but we could demonstrate the feasibility and then ask for more time to set a useful upper limit.
We were awarded about 8 hours of telescope time spread over several different mornings, when Venus was going to be visible. The data were taken and, over several months, were analysed by Prof Greaves, Dr Emily Drabek-Maunder and others. The data were not very nice since Venus is a very bright source, far brighter than the faint galactic and extragalactic sources that the JCMT is usually used to observe. This brightness produces ripples in the data that vary with time and the position of Venus on the sky. These have to be measured and removed before we can look for any sign of the weak absorption feature at a specific wavelength that would indicate the presence of phosphine.
We were thus very surprised when Prof Greaves told us she thought we had a detection, indicating about 20 parts per billion of phosphine at an altitude of about 55km in the atmosphere of Venus.
This left us with two problems.
Firstly, if we were going to claim something as surprising as the detection of a possible biomarker in the atmosphere of Venus we had to be very sure that it was real. The best way to do this is to get an independent observation with a different instrument. The obvious instrument for this was ALMA, the Atacama Large Millimetre/Submillimetre Array, so we applied for time there and got rejected.
Secondly, while it is easy to discuss the implications of a non-detection, the interpretation of a detection is a lot harder. You need to analyse how phosphine might be produced by normal chemical processes taking place on Venus. This is not a simple proposition – Venus is a complicated place, with volcanoes, lightening, multiple layers of atmosphere and more. We had a couple of Venus experts on the team, but nobody who was an expert on the chemistry that might produce phosphine in this environment. The fact that we couldn’t say anything definitive about the chemical processes that might be behind the presence of phosphine on Venus was one of the reasons we didn’t get time on ALMA.
So we were all left scratching our heads, until I attended an astrophysics group seminar at Imperial one Wednesday afternoon. This was a talk on looking for biosignatures on exoplanets and possible scenarios for the origin of life elsewhere. The speaker was Dr William Bains, and I have to say it was an excellent talk. After he had finished, I somewhat sheepishly approached him and said, “Do you think phosphine could be a biomarker, because we think we’ve found some on Venus?” This was something of a lightbulb moment, and I very soon realised I was talking to almost exactly the right person. It turned out that William had been working with a group at MIT under Prof Sara Seager looking at potential biomarkers on exoplanets. One of the molecules they had identified as such was phosphine, and they had Dr Clara Sousa-Silva (who uses the twitter handle @drphosphine), possibly the world’s greatest expert on phosphine, as part of their team.
By the end of the week I had put Prof Greaves and the MIT group in contact and things started moving much faster.
The chemistry details all came together, and we submitted a much improved proposal to ALMA for immediate observation, since Venus would be unobservable for much of the rest of the year. We got the time in March 2019 and the data arrived very quickly. A friend, ALMA expert and SF fan, Dr Anita Richards worked to reduce the ALMA data and, by July 2019, it looked as if we had strong confirmation of the presence of phosphine in the atmosphere of Venus.
It took some time for us to get these results published – we bounced off the journal Science but then moved to Nature Astronomy who accepted the paper with some pleasantly enthusiastic referees comments. The paper was published to media fanfare in September 2020. They even delayed Sky at Night by a day so they could cover our results.
It was at that point I really began to feel like a character in an SF novel.
Needless to say there has been some controversy. Various authors have taken pot shots at our data analysis and, to be fair, some problems in the ALMA observatory pipeline were found. This means that the ALMA results, while still confirming the presence of phosphine, were not as strong as we thought, and there are indications that the amount of phosphine in the atmosphere of Venus is probably varying with time and position.
There was also a possible confirmation of our result from data from Pioneer Venus Probe, a mission that sent a descent stage into the atmosphere of Venus in 1978. The mass spectrometer on board has a weak signal at the right molecular mass to be phosphine, though this would require as much as 100 parts per billion if it is correct.
We’re now at the point where the scientific world has had several months to look at our data. Despite some sniping the result seems to be standing up and, despite the expectations of our chemists, no cunning non-biological route to producing phosphine has been found. We ourselves (Dr William Bains in fact) had produced a huge paper looking at all the possible routes to phosphine production through non-biological processes (including volcanoes, lightening, infalling meteors, solar UV driven photochemistry and more) and found them all lacking, but you always wonder what you might have missed.
We are now getting more data from the ground, and looking at other ways to find out more about phosphine on Venus. This includes flyby observations by spacecraft en route to Mercury and Jupiter, and future missions to Venus itself.
All of this is going much slower than the plot of an SF novel. It’s taken 3 years to get to where we are from our first JCMT observations, and it will take many more before there can be a dedicated mission to seek confirmation. We’ve had some interesting controversies, which have resulted in us getting apologies from the IAU Astrobiology Commission, some of whose leadership didn’t like our press briefings (or rather what some of the press did with them), and from the lead author of one of the critical papers, who got a bit over enthusiastic in calling for our results to be retracted.
I’m pretty sure we’ve put a lot of noses out of joint in the Solar System and astrobiology communities, at least partly because we’re largely outside these communities and have come up with results that they would like to have come up with themselves.
Have we found life on Venus? I really don’t know. I would like it to be true – it would be the closest I’m likely to get to a Nobel Prize – but at this stage I really can’t tell. William thinks there’s a 90% chance it’s some kind of photochemistry that he hasn’t thought of, but I think he’s selling himself a bit short.
At this point only time and more data will tell us the truth. Until then, I get to be a character in an SF novel where we have discovered signs of life on Venus.
David l Clements is a Reader in astrophysics at Imperial College London where, among other things, he runs the annual Science for Fiction writers’ briefing on the latest science. He is also a science fiction writer, with publications in Analog, Nature, Clarkesword, Shoreline of Infinity and numerous anthologies. His first short story collection Disturbed Universes was published by Newcon Press in 2016. His non-fiction book Infrared Astronomy: Seeing the Heat was published in 2014.