We may have another hint that there really is phosphine – a gas that may be a sign of life – in the clouds of Venus, and it comes from old data collected by a spacecraft that visited the planet in 1978.
Last month, Jane Greaves at Cardiff University in the UK and her colleagues announced that they had found phosphine in the Venusian atmosphere after using two telescopes to spot it absorbing light that bounced through the clouds of Venus. They couldn’t link that phosphine to any known chemical processes of Venus, leading them to suggest the possibility that it came from living organisms.
“There was some controversy in terms of the veracity of the signal, and I was inspired from that to look for other evidence that could support that detection,” says Rakesh Mogul at California State Polytechnic University, Pomona. He and his colleagues re-examined data from NASA’s Pioneer Venus Multiprobe, which measured the masses of various compounds as it sank into the crushing Venusian atmosphere in 1978.
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Mogul and his colleagues found previously unreported signs of phosphine consistent with the levels that Greaves’s team spotted from Earth, along with other chemical compounds that are expected to form as phosphine breaks down.
Data like this, from a mass spectrometer, is notoriously difficult to interpret, so this finding isn’t completely definitive, says David Grinspoon at the Planetary Science Institute in Arizona. “I’m still not 100 per cent ready to declare there’s definitely phosphine in the clouds of Venus, but it’s more strongly indicated now than it was before there was this second hint.”
Mogul’s team also reported signs of several other molecules, such as methane and nitric oxide, that we didn’t expect to be present in Venus’s clouds. That isn’t necessarily surprising, says Grinspoon. “If phosphine is there, then there’s some chemistry that we didn’t know about going on there.” That wouldn’t just mean the addition of phosphine to our current models. “There would be other things too,” he says.
Methane in particular is interesting because it has long been considered a potential biosignature, although there are other ways to make it. “If there is life in Venus’s clouds – which is, of course, extremely speculative at the moment – we would expect a ton of biosignatures, not just phosphine,” says Clara Sousa-Silva at the Harvard-Smithsonian Center for Astrophysics in Massachusetts, who worked with Greaves on the phosphine discovery.
We still have a long way to go before we understand Venus enough to figure out where phosphine and these other unexpected compounds come from, she says. Combing through old data with new eyes is a start, but eventually we will need more space missions to really understand what is going on.
“It’s amazing that here we are, trying to glean every bit of meaning out of data that is from a spacecraft that was launched in 1978, when we know how to build instruments that would absolutely nail this answer if we sent them to Venus today,” says Grinspoon. “If anything, it’s a testament to how unexplored Venus is.”
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