In September, it was announced that in the journal Nature that phosphine had been discovered in the atmosphere of Venus, sparking a debate about the possibility of life on the planet. Dr Colin Wilson of the Oxford department of Physics cautions that the question of life on Venus is not necessarily a simple one.
“Its interpretation needs to be somewhat nuanced. And it’s never as simple as the presence of phosphine means life.”
Phosphine is a biomarker, which means that it is considered a sign of possible life. This is because, on Earth, much of the phosphine in the atmosphere is made via biological processes. “Most of the phosphine, almost all the phosphine found in the atmosphere comes from biological sources. So because it’s from anoxic environments, you get it formed in the gut. You get it formed in swamps, away from the air.”
Still, “there are lots of other ways you can get phosphine. So other sources of phosphine in the Earth’s atmosphere include the breakdown of phosphorus containing dust and minerals, possibly some kinds of volcanism. You can also have lightning breaking down atmospheric phosphine containing molecules and having phosphine as a by-product.”
“It’s only by eliminating all other abiotic forms of production of phosphine that we would be able to conclude that phosphine [on Venus] is produced by life.”
“Even eliminating all the abiotic forms of producing phosphine wouldn’t be enough because life as we know it cannot exist on Venus either. So before saying it must be life, you’d have to find the kind of life which could be producing it.”
Even the presence of phosphine on Venus is contested. “One of the first things we have to do as a science community is follow up on that detection to try to see whether it’s repeatable as a detection. Lots and lots of corroborating observations are going on as we speak from various telescopes on Earth.” So far, the researchers have already found a processing error in their original data. They say that while the phosphine signal is still present, it’s fainter than before.
Dr Wilson’s work focuses on the three primary candidates for life in our solar system: Venus, Mars, and Titan, one of Jupiter’s moons. Atmospheric physics, his field of science, is one filled with both potential risks and challenges.
“I did my PhD here in Oxford working on the wind sensor for Beagle to Mars Lander, UK Mars Lander, which went to Mars in 2003. Unfortunately, that Mars Lander crashed and I managed to get that same wind sensor on the next European Mars lander, which went in 2016. And unfortunately, that crashed as well. So this illustrates certainly some of the high risk nature of planetary exploration.”
“There’s a limit. Fundamentally, it’s a spatial resolution limit. It’s what you can tell. So we can tell sort of large scale weather patterns. But a lot of the details of what’s going on on the planet can’t be sensed from ground.”
The next step for many planetary scientists is to send an orbiter to the atmosphere of the planet, but often chemistry and composition still can’t be fully understood from that vantage point. The ultimate aim for scientists, then, is to get landers, rovers, and balloons onto the planets that they are studying.
The Titan Dragonfly mission, which Dr. Wilson is developing a wind sensor for, is aiming to do just that. The Dragonfly is a nuclear powered rotorcraft which is capable of flying to a new site on the moon every few weeks, and is currently set to launch in 2027.
“Titan is a world like no other. It has. It’s the only moon we know of in the solar system with a substantial atmosphere and the atmosphere is mostly nitrogen. So the only nitrogen rich atmosphere, which we know is the earth, really. And so Titan is another body like that.”
“It’s also [got] lots of organic chemistry. It’s got huge amounts of methane and ethane. You may raise your eyebrows thinking, you know, where do hydrocarbons come from on earth? It’s sort of fossil fuels, decaying ancient forests. So we don’t think that’s where Titan’s hydrocarbons have come from.”
An area for investigation, then, is the origin of these hydrocarbons. “In some ways, this is an ideal laboratory for understanding sort of prebiotic chemistry, the organic chemistry stages you go through before you get to life.”
Other bodies in our solar system are often gateways to furthering our knowledge of things here on Earth. Dr. Wilson has also been involved in a proposal known as EnVision Venus, which sets out to explore the geological character of the planet. In the past, he worked on the Venus Express, an orbiter focused on the atmosphere of the planet. “But actually, some of the most intriguing and tantalising results it sent back were about current geological activity on the surface”
“It’s covered in volcanoes. There may be a million volcanoes or more on Venus and lots of rifts and signs of tectonic activity. We have a global map of the surface showing all this, but it was effectively a static picture obtained in the early 90s by one of the first planetary radar missions. And we don’t know whether any of these volcanoes are active today.”
“Why do we care about this? Well, it’s like a parallel earth. It’s the same size as Earth, made of the same sort of materials formed around […] the same amount of time ago. But it’s evolved really differently with this huge greenhouse effect.”
“The science is split […] between whether Venus used to be Earth-like and inhabitable with liquid water oceans in the past, or whether its evolution bypassed that stage of being nice and clement and inhabitable. And so our investigations of its geological state should tell us that difference, whether it was once an earth-like planet.”
Coming back to the question of life on Venus, it is some of these very conditions that make life on the planet a complex issue. “The surface of Venus is up at four hundred fifty degrees centigrade or so. So nothing, nothing is going to survive there.”
“Even life adapted for other environments is going to have difficulties because really most life as we know it, or as we can conceive of it, needs liquid to mediate the interactions between different parts of the cell and so on.”
Life on Venus, then, would not necessarily be life as we’re used to seeing it. “People have been talking about life surviving in the clouds, because as you go up in altitude, as on earth, the temperatures drop. And by the time you’re 50 to 60 kilometres above the earth, above the planet’s surface, you have environments which are rather pleasant, rather comfortable even.”
“I don’t think anyone [is] suggesting that you’d have birds or condors soaring around because it takes great big evolutionary processes to get to that stage. So we’re probably looking at bacteria.”
“Some of the very, very earliest rocks we can find on Earth already bear the signs of life having evolved very soon after the Earth became habitable as an environment for billions of years. […] So that tends to suggest that it’s maybe it’s quite easy to create single celled, single cellular life, relatively easy.”
Still, Dr. Wilson seems to think that the most likely version of Venus with life is one that no longer exists. “I think that the places most likely to form some sort of life would be possibly ancient Mars and ancient Venus, because there are strong suggestions that both of them were once more habitable in their climates.” With no orbiters currently able to confirm phosphorus directly from the planet, it might be a while before we hear more on the subject.