Small molecules, bidetection of Venus
On the local morning of September 14 th, the Royal Astronomical Society tweeted an announcement for an afternoon press briefing about a significant discovery. Within hours, “phosphine” and “Venus” were being proposed by some on social media as the discussion topic. In combination with shared links to the January 2020 Astrobiology article “Phosphine as a Biosignature Gas in Exoplanet Atmospheres,” some were already anticipating a most remarkable event.
An international team from the UK, USA, and Japan, led by Prof. Jane Greaves of Cardiff University, released their open access article “Phosphine gas in the cloud decks of Venus” in Nature Astronomy as the press briefing was taking place. The cautious excitement of Greaves, Dr. Anita Richards (Jodrell Bank Centre for Astrophysics), and Dr. William Bains and Prof. Sara Seager (MIT) over bringing such a potentially groundbreaking and multi-disciplinary discovery to fruition was obvious to all in attendance. No topic in science generates more speculation than extraterrestrial life, and the discoverers will go down in history for that achievement − this would clearly be a milestone as the names “Gagarin” and “Armstrong/Aldrin/Collins” conjure. One assumes the most benevolent intentions of such a briefing, especially with the presenters stating that their work is of a molecular detection and an interpretation in the absence of any known non-biological explanations. That said, one can only imagine the instinct to be the first to make such a discovery, however preliminary, known to the world as soon as possible.
What has followed is a story of measurement, interpretation, reserved speculation, and controversy − not over the discovery of phosphine, but over its indication as a possible biological marker, which has driven discussion beyond just the astronomical community. What is most clear is how little we still know about our own solar system, as a simple molecule can lead to heated debate because of our incomplete understanding of the chemistry and geology of our nearest planetary neighbors. What we know and do not yet know about the production of phosphine, PH3, on Venus are central to the past few weeks of speculation. On Earth, phosphine can be produced by lightning and volcanism, both common on Venus. That said, the atmospheric and geochemical routes to phosphine could only account for a fraction of the amount detected based on our current understanding of both phenomena. The possible explanation put forth by the authors applies the predominant method of phosphine production on Earth – biochemical processes in low-oxygen conditions.
As is obvious from the many design features of the few successful Venera and Pioneer missions to land probes during the 1970’s and early 1980’s, the conditions on Venus can be brutal, with a mean surface temperature of 460 °C. As one moves higher in altitude, the temperature and pressure decrease to where some atmospheric zones might feel quite pleasant. That said, only the heartiest microbe or properly suited human would find the zone safe even for short periods. The Venusian atmosphere is reactive to many small molecules of biological interest including phosphine. The 96.5% carbon dioxide content drives the “runaway greenhouse effect” that causes the extreme surface temperatures. Sulfuric acid clouds obscure the Venusian surface, making radar the source of our mapping from orbit of the near-entirely of the planet.
Beyond the 3.5% additional nitrogen contribution, all other gases are measured in parts-per-million (ppm) or billion (ppb). The 20 ppb-level phosphine detection was accomplished with the James Clerk Maxwell Telescope, located high above water vapor at the Mauna Kea Observatory in Hawaii, and was confirmed using the Atacama Large Millimeter/submillimeter Array (ALMA), itself located high in the arid Atacama Desert in northern Chile.
During the press briefing, you could sense the scientists trying to reign in the rampant speculation of viewers to focus on how the detection of phosphine is of consequence because we do not have an explanation for how such concentrations could exist in the Venusian atmosphere given what we know on Earth about its production. Such an interpretation is another example of the principle of mediocrity, a topic of the July-August 2020 issue. If phosphine is produced on Earth by life and we’ve no other mechanism for its production in significant quantities, we can at our potential peril and misunderstanding − attribute its presence elsewhere to similar biological processes.
This detection adds a piece of hard data to a debate that, for much of our history, has been driven merely by our knowledge of other planets. The ability to apply hard science to the exobiology question was founded largely on scientific developments at the end of WWII and the political race to prove the superiority of political ideologies during the Cold War – radar technology, detectors across the electromagnetic spectrum,satellites, and interplanetary propulsion. This is why the appearance of more datadriven speculation in the scientific literature, as well as more focus about life far less technologically advanced than our own, only began starting in the 1960’s. A relevant paper was published in Science by CarlSagan in 1961, titled “The Planet Venus.” In it, he comments on the likelihood of detecting life on the surface: “…it appears quite certain that terrestrial organisms deposited on the surface of the planet would quickly be killed. Consequently, there seems little danger of biological contamination of the surface of Venus.”
Enough was then known about the Venusian atmosphere that its possible hosting of simple lifeforms could not so easily be ruled out. Sagan wrote: “conditions are much more favorable at higher altitudes, especially just beneath the cloud layer, and there is the distinct possibility of biological contamination of the upper Cytherean atmosphere.” The most recent data-driven analysis of Venusian life harkens back to the days of Percival Lowell and his Martian canals, with the Russian scientist Leonid Ksanfomaliti claiming to have found life in images sent back from the Soviet Venera 13 lander mission in 1982. The consensus now is that the disc-like and scorpion-like creatures were, in fact, lens caps and processing artifacts – a clear example of the inverse relationship between data and speculation.
Anyone basing Venusian life on the singlecelled organisms inhabiting ponds on Earth would have to take dramatic leaps in biology to make such organisms robust enough to survive the hyper-acidic, poisonous, and water-poor conditions of the upper Venusian atmosphere. That said, similar leaps already abound on Earth. A few examples include thermococcus gammatolerans, which can survive high doses of gamma radiation, pyrococcus furiosus, which thrives at the boiling point of water (we simple surface-dwellers rely on boiling water to kill harmful bacteria), or clostridium paradoxum, which is found in acidic mine drainage and volcanic springs. Extreme conditions call for extreme adaptation. We find organisms on Earth existing in places, including the upper atmosphere, where we would not even think to look for them if not for the constant reminder that life, within a very wide range of environments, always finds a way.
Even without an actual lifeform, the presence of certain small molecules in planetary atmospheres and their attribution to biological processes is also nothing new, even in our own solar system. Seasonal methane variations in the Martian atmosphere are one example. Methane, CH4, is broken down by UV radiation. The thin Martian atmosphere provides little protection from solar radiation, resulting in what should mean the eventual depletion of methane. That it is found at all – and changes seasonally – indicates that either some geochemical process is producing methane cyclically or that, possibly, some biological process is occurring that is seasonally restoring atmospheric methane.
While the detection of methane around a rocky exoplanet would be a significant discovery, there is another molecule that would make all of exobiology take notice. Oxygen presently makes up 21% of Earth’s atmosphere, but this percentage would itself eventually reduce to zero if not constantly replenished through photosynthesis. In fact, the evolution of life did not begin with aerobic, or oxygen-consuming, respiration. It was the evolution of singlecelled organisms capable of photosynthesizing CO2 and producing O2 as a waste product that ultimately changed the atmosphere, and evolutionary history, of Earth.
As we search today, the detection of oxygen in an exo-atmosphere does not then mean a wealth of multicellular organisms but may mean that some simple lifeform producing oxygen as a waste product might be responsible for what we detect.
Phosphine, from our current understanding, falls into the same category as meth-ane and oxygen. Without some source to replenish it, it simply should not be detectable in the atmosphere of a rocky planet based on what we currently know of its chemistry. If Venus samples reveal a lifeform producing phosphine (or any molecule!), Earth would go from being alone in the universe to the second planet in its own solar system. Technological sophistication aside, evolution on two close planets with different histories and surface chemistries dramatically expands what exobiologists know to look for. But the ramifications of one or more new lifeforms will not be fully known until scientists are able to determine beyond all doubt that the lifeform is a product of Venus alone. You can, after all, buy Mars rocks − proof that the rocky inner planets have exchanged materials regularly over the history of the solar system.
Consider just one aspect of Earth biology: if the DNA of a Venusian lifeform, if it has such an information repository, were identical to that of the bases and pairings (“A-T” and “G-C”) of life on Earth, it would be easier to accept that either the Venus life or the Earth life came first and one ended up establishing a foothold on the other planet − remembering that early Venus, like early Mars, may have been more hospitable to the chemical processes that began the tree of life we see on Earth today, and that Earth might have become the unanticipated beneficiary of eons of fundamentally different chemical processes elsewhere. If Venusian genetic material were similar, but not quite like DNA, did a small number of resilient extremophiles from Earth/Venus survive the long, cold trip to Venus/Earth and begin to incorporate similar molecular fragments, or have we possibly learned that a stable chemical information repository (like DNA) now is even more established as at the core of biological life? If Venusian genetics was fundamentally different from that on Earth and there was no possible way that stochastic chemical processes could have converted one form into another after some ancient panspermic event, then the case for an entirely separate evolutionary course becomes the accepted theory until future studies can challenge the separate-evolution theory. In the Nature Astronomy paper, the authors take known non-biological processes for phosphine production, including from the known chemistry of the atmosphere and surface of Venus, to further argue for a potential biological origin. The study of non-biological phosphine sources will undoubtedly come to be a new research focus. Despite our great hope of finding extraterrestrial life, the scientific community is always ready to apply Occam’s Razor to any such claim. Scientific skepticism at extraterrestrial claims might always be treated as if an alternative explanation is the more likely one. As Sagan said, “extraordinary claims require extraordinary evidence.”
Of all of the statements made about the phosphine discovery and analysis, perhaps the most complicated one was published by the Organising Committee of Commission F3 of the International Astronomical Union itself, which seems to admonish the authors for even pushing the biological origin hypothesis. From the statement: “the Commission is concerned with the way the potential detection of phosphine has been covered for the broad audience. It is an ethical duty for any scientist to communicate with the media and the public with great scientific rigor and to be careful not to overstate any interpretation which will be irretrievably picked up by the press and generate great public attention in the case of life beyond Earth.” The interpretation of the statement among several members of the astronomy community has been varied, but Oxford Professor and BBC The Sky at Night host Dr. Chris Lintott has summed up the thoughts of many, describing the statement as a “grumpy response of a bunch of senior people not involved in work that got public attention.” With the IAU executive presently calling for the F3 statement retraction, there has officially been a controversy over the possible detection of biological processes on Venus, a controversy over the F3 response to the controversy,and now any future IAU controversies over the F3 controversy over the response to the controversy. Social media astrophiles are free to sit back and watch how the professionals do it.
As will be true for exoplanets, it is possible that a strong marker of Venusian biology is already visible to us, but we do not yet recognize it as a marker either food, waste, oxidant, or whatever life does as part of its existence. The phosphine discovery bridges one possible gap and, if life always happens to find a similar way in the universe, its detection could be the beginning of our new chapter in the biology of the universe. Until then, there remain a great many studies.