A false-color image of Venus taken in ultraviolet light
JAXA/ISAS/Akatsuki Project Team (CC-BY)
Scientists have detected an unstable and unexpected gas in Venus’s atmosphere that, on Earth, is commonly produced by some microbes. The find could be a sign of life in the clouds of our nearest planetary neighbor—or, it could merely be evidence of some weird, as-yet-unknown chemical processes taking place there.
The gas “is present at levels much higher than can be explained via known methods of production,” says Lewis Dartnell, an astrobiologist at the University of Westminster who was not involved in the new study.
Researchers detected a distinctive signature of the gas—phosphine—in the venusian atmosphere in June 2017 using a ground-based telescope. The observation was later confirmed in March 2019 with another such telescope. The instruments showed a faint reduction in the light at a wavelength only known to be absorbed by the gas, Jane Greaves, an astronomer at Cardiff University, and her colleagues report today in Nature Astronomy. The absorption levels suggest phosphine is present at concentrations of up to 20 parts per billion at altitudes above 53 kilometers, Greaves says.
Although 20 parts per billion sounds like a trifling amount, there shouldn’t even be that much. Phosphine is relatively unstable and in the harsh, superacidic conditions found high in the venusian atmosphere, the average lifetime of a molecule is a mere 16 minutes or so. To counteract ongoing destruction of the gas, there must be a steady—and prodigious—source of it.
Greaves and her team tried to figure out where all that phosphine is coming from. Phosphorus-containing minerals, one possible raw ingredient for phosphine, aren’t likely to waft up to high altitude from the planet’s surface. Lightning and sunlight-driven chemical reactions also wouldn’t produce enough of the gas. Volcanoes on Earth spew very small amounts of phosphine, but there’d need to be about 200 times as much volcanic activity on Venus to account for the levels seen there.
On Earth, a variety of microbes that thrive in low-oxygen environments produce phosphine. And those organisms would only need to pump out 10% of the phosphine they do here to explain the levels seen on Venus, the team notes. At altitudes between 53 and 61 kilometers above the venusian surface, temperatures are a balmy 30°C. That’s certainly more microbe-friendly than the hellish, lead-melting temperatures of about 900°C down at ground level. However, life as we know it would have a tough time in the hyperacidic, dehydrating conditions of Venus’s atmosphere regardless of the temperature.
The presence of phosphine isn’t a sure sign of life, says study co-author Sukrit Ranjan, a planetary scientist at the Massachusetts Institute of Technology. The chemistry of phosphine isn’t well known, and it’s possible that the gas could persist more readily at the lower, temperate layers of the venusian atmosphere, which could be shielded from sunlight that drives phosphine-destroying photochemical reactions.
The team’s observations are “curious and unexplained,” Dartnell says. Even if tiny venusians aren’t responsible for the phosphine, he notes, “at the very least they’ve uncovered some interesting new atmospheric chemistry.”