Was Venus, described by Stephen Hawking as Earth’s “kissing cousin,” the solar system’s first habitable planet? Scientists have recently conjectured that Venus may have been radically different from the hellish Venus we see today, resembling Earth for its first three billion years, with vast oceans that could have been friendly to life. Fast forward to the 21st century –strange dark patches, enigmas, “unknown absorbers” have been detected floating inside the clouds of Venus capturing large amounts of solar radiation that may prove to be extraterrestrial microorganisms.
Venus atmospheric gases circulate amid cloud layers according to patterns that scientists don’t fully understand, with some scientists speculating that the dark patches –first observed by ground-based telescopes more than a century ago–might be forms of sulfur, ferric chloride or, as Carl Sagan once speculated, even microscopic life.
“These are questions that haven’t been fully explored yet and I’m shouting as loud as I can saying that we need to explore them,” said Sanjay Limaye, at the University of Wisconsin, Madison and former chair of NASA’s Venus Exploration Analysis Group in August of 2019 about the possibility that the dark patches may be evidence of microbial life. “It’s a possibility we can’t overlook,” says Limaye.
“The difference between Earth and Venus,” observed Limaye, “is that on Earth most of the energy from the sun is absorbed at ground level while on Venus most of the heat is deposited in the clouds.” Yet even on Earth, microorganisms — mostly bacteria — have been found alive at altitudes as high as 41 kilometers (25 miles) by scientists using specially equipped balloons.
“A Shock” –Extraterrestrial Aerial Life?
And this week, an international team of astronomers announced the discovery of a rare molecule — phosphine — in the clouds of Venus. On Earth, this gas is only made industrially or by microbes that thrive in oxygen-free environments. Astronomers have speculated for decades that high clouds on Venus could offer a home for microbes — floating free of the scorching surface but needing to tolerate very high acidity. The detection of phosphine could point to such extraterrestrial “aerial” life.
“When we got the first hints of phosphine in Venus’s spectrum, it was a shock!”, says team leader Jane Greaves of Cardiff University in the UK, who first spotted signs of phosphine in observations from the James Clerk Maxwell Telescope (JCMT), operated by the East Asian Observatory, in Hawaiʻi. Confirming their discovery required using 45 antennas of the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, a more sensitive telescope in which the European Southern Observatory (ESO) is a partner. Both facilities observed Venus at a wavelength of about 1 millimetre, much longer than the human eye can see — only telescopes at high altitude can detect it effectively.
Natural Non-Biological Sources?
The international team, which includes researchers from the UK, US and Japan, estimates that phosphine exists in Venus’s clouds at a small concentration, only about twenty molecules in every billion. Following their observations, they ran calculations to see whether these amounts could come from natural non-biological processes on the planet. Some ideas included sunlight, minerals blown upwards from the surface, volcanoes, or lightning, but none of these could make anywhere near enough of it. These non-biological sources were found to make at most one ten thousandth of the amount of phosphine that the telescopes saw.
To create the observed quantity of phosphine (which consists of hydrogen and phosphorus) on Venus, terrestrial organisms would only need to work at about 10% of their maximum productivity, according to the team. Earth bacteria are known to make phosphine: they take up phosphate from minerals or biological material, add hydrogen, and ultimately expel phosphine. Any organisms on Venus will probably be very different to their Earth cousins, but they too could be the source of phosphine in the atmosphere.
“To our great relief, the conditions were good at ALMA for follow-up observations while Venus was at a suitable angle to Earth. Processing the data was tricky, though, as ALMA isn’t usually looking for very subtle effects in very bright objects like Venus,” says team member Anita Richards of the UK ALMA Regional Centre and the University of Manchester. “In the end, we found that both observatories had seen the same thing — faint absorption at the right wavelength to be phosphine gas, where the molecules are backlit by the warmer clouds below,” adds Greaves, who led the study published today in Nature Astronomy.
Many Questions Raised
“Finding phosphine on Venus was an unexpected bonus! The discovery raises many questions, such as how any organisms could survive. On Earth, some microbes can cope with up to about 5% of acid in their environment — but the clouds of Venus are almost entirely made of acid,” said team member, Clara Sousa Silva of the Massachusetts Institute of Technology who has investigated phosphine as a “biosignature” gas of non-oxygen-using life on planets around other stars, because normal chemistry makes so little of it.
The team believes their discovery is significant because they can rule out many alternative ways to make phosphine, but they acknowledge that confirming the presence of “life” needs a lot more work. Although the high clouds of Venus have temperatures up to a pleasant 30 degrees Celsius, they are incredibly acidic — around 90% sulphuric acid — posing major issues for any microbes trying to survive there.
“The non-biological production of phosphine on Venus is excluded by our current understanding of phosphine chemistry in rocky planets’ atmospheres,” said ESO astronomer and ALMA European Operations Manager Leonardo Testi, who did not participate in the new study. “Confirming the existence of life on Venus’s atmosphere would be a major breakthrough for astrobiology; thus, it is essential to follow-up on this exciting result with theoretical and observational studies to exclude the possibility that phosphine on rocky planets may also have a chemical origin different than on Earth.”
More observations of Venus and of rocky planets outside our Solar System, including with ESO’s forthcoming Extremely Large Telescope, may help gather clues on how phosphine can originate on them and contribute to the search for signs of life beyond Earth.
Source: “Phosphine Gas in the Cloud Decks of Venus”in Nature Astronomy.
An accompanying paper by some of team members, titled “The Venusian Lower Atmosphere Haze as a Depot for Desiccated Microbial Life: A Proposed Life Cycle for Persistence of the Venusian Aerial Biosphere”, was published in Astrobiology in August 2020. Another related study by some of the same authors, “Phosphine as a Biosignature Gas in Exoplanet Atmospheres”, was published in Astrobiology in January 2020.
Image credit: ESO/M. Kornmesser/L. Calçada & NASA/JPL/Caltech
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