The prospects for life on Venus vanish, but they are not dead yet

The signs of phosphine gas in Venus’ atmosphere have faded somewhat, but are still there, according to a new data analysis.

In September, an international team of astronomers made headlines when they reported finding phosphine – a potential indicator of life – in the planet’s atmosphere.¹. Several studies quickly followed that questioned their observations and conclusions. Now the same team has re-analyzed some of its data, citing a processing error in the data originally used. Researchers have confirmed the phosphine signal, but say it is weaker than before.

The work is a major step forward in resolving the most exciting Venus debate in decades. “I’ve been waiting for this all my life,” says Sanjay Limaye, a planetary scientist at the University of Wisconsin in Madison, for whom the debate has reinvigorated the field.

The new analysis, based on radio telescope observations at the Atacama Large Millimeter / submillimeter Array (ALMA) in Chile, concludes that the average phosphine levels on Venus are about 1 part per billion, about one seventh of the previous estimate. Unlike their original report, scientists now describe their discovery of phosphine on Venus as “tentative”².

It is the team’s first public response to the criticisms that have been leveled against them in the past two months. “The scientific process is working,” says Bob Grimm, a planetary scientist at the Southwest Research Institute in Boulder, Colorado who is not involved in any of the phosphine studies. Researchers tend to follow up on large claims with great effort to gather evidence and prove or disprove it.

Taking another look

In its September report, the team used data from ALMA and the James Clerk Maxwell Telescope (JCMT) in Hawaii to make their discovery. Jane Greaves, team leader and astronomer at Cardiff University in the UK, said she and her colleagues redid the work because they learned that the original ALMA data contained a spurious signal that could have influenced the results. ALMA released the corrected data on November 16, and Greaves’ team ran a new analysis and posted it before peer review that evening on the arxiv.org preprint server. “We worked like crazy,” he told a meeting of the Venus Exploration Analysis Group, a community forum for NASA, on November 17.

According to Greaves and his colleagues, data from ALMA show the spectral signature of phosphine, a molecule made up of a phosphorus and three hydrogen atoms. They say no other compound can explain the data. Finding phosphine on Venus would be tempting because microbes produce the gas on Earth. If the signal is real and actually due to phosphine, it is possible that the microbes living and moving in the clouds of the planet are producing the gas.^4,3 – but it is also possible that there may be a non-living source of phosphine that scientists have yet to identify. To determine if either of these scenarios is true, researchers must first confirm the presence of phosphine.

In a critique of the original study, the researchers suggested that the signal reported as phosphine may indeed have come from sulfur dioxide, a gas that is common in Venus’ clouds but is not produced by life there.⁵. Greaves and his team responded in their latest report that that cannot be the case, based on how the phosphine fingerprint appears in data collected by the second telescope they used, the JCMT. Other criticisms have focused on the difficulty of extracting a phosphine signal from complicated data.

The new analysis found that phosphine concentrations in Venus’ atmosphere occasionally peak at 5 parts per billion. This means that gas levels can rise and fall over time at different points on the planet, Greaves said – a situation similar to the methane spikes appearing on Mars.

Another new stream of evidence supports phosphine on Venus. Inspired by Greaves’ report, a team led by Rakesh Mogul, a biochemist at California State Polytechnic University in Pomona, California, recently digged through decades-old data from NASA’s 1978 Pioneer-Venus mission. This spacecraft launched a probe through the planet’s atmosphere that measured the chemistry of the clouds as it fell. It detected a phosphorus compound which could be phosphine or another phosphorus-based molecule⁶. But “we believe the simplest gas that fits the data is phosphine,” Mogul said at the November 17 meeting.

Keep working

Where phosphine comes from remains a mystery. Even at the 1 part per billion level, there is too much of it to be explained by volcanic eruptions on the planet’s surface or lightning strikes in the atmosphere, several scientists said at the meeting. But phosphorus-based compounds could be produced by geological processes and then turn into other chemicals, such as phosphine, as they rise into the clouds, Mogul said.

The only spacecraft currently orbiting Venus, the Japanese Akatsuki, does not carry tools that could help resolve the debate. The Indian Space Research Organization is planning a Venus mission that would launch in 2025 and could potentially carry instruments capable of searching for phosphine. Meanwhile, Greaves and other researchers are taking longer on ground-based telescopes, including ALMA.

Researchers are studying many other aspects of Venus besides just phosphine, says David Grinspoon, an astrobiologist at the Planetary Science Institute based in Washington, DC. “There are 1,001 reasons to return to Venus, and if the phosphine” disappears “through further observation and analysis, there will still be 1,000 reasons to go.”