Recent research has revealed that Venus might have looked like Earth for three billion years, with vast oceans that could have been friendly to life leading to recent speculation that it may have been the first life-bearing planet. A conjecture spurred by the announcement in September of 2020 by an international group of researchers reported in the journal Nature Astronomy, that there may be a whiff of life in the famously inhospitable planet’s atmosphere in the form of traces of phosphine, a gas that is associated with life where there is no oxygen.
Plenty of time for evolution to kick in
“That’s what sets my imagination on fire,” says Darby Dyar, a planetary scientist at Mount Holyoke College with NASA’s Solar System Exploration team, about the 2020 announcement, which led her to surmise: “If that’s the case, there was plenty of time for evolution to kick into action,” and conclude that Venus may have been the first habitable planet in the Solar System — “a place where life was just as likely to arise as it was on Earth.” On Earth, says Dyar, phosphine is found “in sewage facilities and in the guts of living animals.”
“The problem,” observed Dyar, “is that we haven’t thought too much about whether phosphine can be created abiotically on Venus, in part because we know so little about the planet and its chemistry.”
Spectral signatures of almost 1000 atmospheric molecules
Fast forward to 2021 research released by an international team, led by scientists at the University of New South Wales (UNSW) in Sydney, Australia , who have made a key contribution to this and any future searches for life on other planets by demonstrating how an initial detection of a potential biosignature must be followed by searches for related molecules, revealing the spectral signatures of almost 1000 atmospheric molecules that may be involved in the production or consumption of phosphine.
Spectral Data collected by telescopes – our one and only tool
“The only way we’re going to be able to look at exoplanets and see whether there’s life there is to use spectral data collected by telescopes – that is our one and only tool,” says quantum chemist and molecular physicist, Dr Laura McKemmish at the University of New South Wales, referring to the upcoming launch of the infrared James Webb Space Telescope (JWST). The JWST will be able to identify the spectral signatures of atmospheric molecules that may be involved in the production or consumption of phosphine that may indicate evidence of life if found in the atmospheres of small rocky planets like our own, where it is produced by the biological activity of bacteria.
“To identify life on a planet, we need spectral data,” says McKemmish about the new crossroads in the search for life beyond Earth with the ability to point a telescope at a planet and with the right spectral data determine what molecules are in the planet’s atmosphere such as phosphorus, an essential element for life. “Up until now,” she says, “astronomers could only look for one polyatomic phosphorus-containing molecule, phosphine.”
When an international team of scientists last year claimed to have detected phosphine— a chemical compound made of one phosphorous atom surrounded by three hydrogen atoms (PH3)– in the atmosphere of Venus, it raised the prospect of the first evidence of life on another planet – albeit the primitive, single-celled variety. Some scientists however questioned whether the phosphine in Venus’s atmosphere was really produced by biological activity, or whether phosphine was detected at all.
“Unusual Chemistry or Little Green Men?”
In a paper published in the journal Frontiers in Astronomy and Space Sciences, they described how the team used computer algorithms to produce a database of approximate infrared spectral barcodes for 958 molecular species containing phosphorous.
“Phosphine is a very promising biosignature because it is only produced in tiny concentrations by natural processes. However, if we can’t trace how it is produced or consumed, we can’t answer the question of whether it is unusual chemistry or little green men who are producing phosphine on a planet,” says McKemmish, who brought together a large interdisciplinary team to understand how phosphorus behaves chemically, biologically and geologically and ask how this can be investigated remotely through atmospheric molecules alone.
“What was great about this study is that it brought together scientists from disparate fields – chemistry, biology, geology – to address these fundamental questions around the search for life elsewhere that one field alone could not answer,” says astrobiologist and co-author on the study, Associate Professor Brendan Burns.
“At the start, we looked for which phosphorous-bearing molecules – what we called P-molecules – are most important in atmospheres but it turns out very little is known.,”says McKemmish. “So we decided to look at a large number of P-molecules that could be found in the gas-phase which would otherwise go undetected by telescopes sensitive to infrared light.”
Barcode data for new molecular species are normally produced for one molecule at a time, McKemmish says, a process that often takes years. But the team involved in this research used what she calls “high-throughput computational quantum chemistry” to predict the spectra of 958 molecules within only a couple of weeks.
“Though this new dataset doesn’t yet have the accuracy to enable new detections, it can help prevent misassignments by highlighting the potential for multiple molecular species having similar spectral barcodes – for example, at low resolution with some telescopes, water and alcohol could be indistinguishable.
“The data can also be used to rank how easy a molecule is to detect. For example, counter-intuitively, alien astronomers looking at Earth would find it much easier to detect 0.04% CO2 in our atmosphere than the 20% O2. This is because CO2 absorbs light much more strongly than O2 – this is actually what causes the greenhouse effect on Earth.”
Extend the technique to the radio wavelengths
“Our paper provides a novel scientific approach to following up the detection of potential biosignatures and has relevance to the study of astrochemistry within and outside the Solar System,” says McKemmish. “Further studies will rapidly improve the accuracy of the data and expand the range of molecules considered, paving the way for its use in future detections and identifications of molecules.”
Fellow co-author and Commonwealth Scientific and Industrial Research Organization (CSIRO) astronomer Dr Chenoa Tremblay says the team’s contribution will be beneficial as more powerful telescopes come online in the near future. She says although the team’s work was focused on the vibrational motions of molecules detected with telescopes sensitive to infrared light, they are currently working to extend the technique to the radio wavelengths as well, which will be important for current and new telescopes like the upcoming Square Kilometer Array to be built in Western Australia.
Image credit: Shutterstock License, thick clouds over Venus. 3D illustration by Jurik Peter