If the phosphine detection is confirmed, could there be some non-biological process that’s missing from the team’s models that could explain it?
The researchers tried modeling the complex atmospheric chemistry of Venus to see if they could explain the levels of phosphine they detected. But they could only reproduce a signal about a thousandth as strong as what they observed. More exotic ideas fell short, too, including lightning and meteorites. (The details of their full modeling analysis, led by MIT's Williams Bains and Janusz Pekowski, are being published in a separate paper currently going through peer review.)
The team also argues that observed volcanic activity on Venus can’t account for all the phosphine. However, Filiberto thinks that conclusion may be premature.
He has co-authored two papers in the last year reporting evidence of fresh lava flows on Venus’ surface. That would mean the planet "is a lot more volcanically active than we thought,” he says. “And we don't know what gases are coming out of those volcanoes." (An independent team at ETH Zurich and the University of Maryland reported further evidence of venusian volcanism in July.)
These volcanoes could be pumping phosphine directly into the atmosphere, Filiberto says. They could also be belching hydrogen, which might allow phosphorus acid from the atmosphere to react and form phosphine, thanks to the high temperatures near the surface. "I don't think we can discredit this at this point and say it can't be volcanoes, or at least that there can't be a volcanic contribution," he says.
There's also the intriguing possibility that the chemistry of Venus is simply stranger than expected.
"The team, I think, did a nice job in kind of presenting a set of first-order models," says Ehlmann. "But now we can dig a little deeper and consider weird chemistry." For instance, she says, perhaps the modeled chemical reactions behave differently in Venus' extremely acidic environments, or maybe air moves between atmospheric layers in unexpected ways.
Then again, maybe the chemistry isn’t even really that strange, given how little we know about surface conditions on Venus. “Phosphine is easy to make,” tweeted Lee Cronin, an inorganic chemist at the University of Glasgow. “Rocks [could] get thrown into the air by some process and react in the atmosphere,” he added. “There are just so many…possible options.”
There’s also the possibility the phosphine is coming from a totally unknown source. Sarah Hörst, a planetary scientist at Johns Hopkins University, likewise took to Twitter to point out that in the early 1980s, astronomers detected carbon monoxide on Saturn's moon Titan. Models failed to explain that find for decades. Then in 2008, the Cassini mission discovered that another Saturnian moon, Enceladus, had cracks on its surface that were spraying water into space, effectively injecting it into Titan's atmosphere. Researchers hadn’t included that possibility in their models.
"The less you know about an atmosphere,” Hörst tweeted, “the harder it is to use a model to draw conclusions about it, and the more careful you have to be about how you use it.”
For now, the team behind the phosphine detection is letting the rest of the community digest their work, as well as waiting to see if someone else can explain it.
"When I first heard about it, honestly, I was very skeptical too," says Seager. When the team's models failed to find a non-biological explanation for the phosphine, she admits to having mixed feelings. "Dare I even say, we wanted it to go away,” she says. “Like, no one wants to be out there claiming there's life."
“And when we got better data,” Seager adds, “eventually I had to [say], 'Wow, this is real.'"
Now that the work is out there, the team is prepared — even eager — for other researchers to challenge their assumptions. But so far, Seager thinks many of the critiques being raised are already addressed by the team in their analysis.
"The team has had years … to digest this, and to criticize, and to work through our self-criticism," she says. "We've had reviewers take months to give us more criticisms. So, we've had a long time to sort of cycle through all these. It's been interesting, watching everybody trying to digest this in a day or two, right? All their questions are legit and natural, but they do need to read the paper."
Mission to Venus?