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So, astronomers *may* have found evidence of life on Venus
A team of astronomers has announced that they have found what may be evidence of life on Venus.
Or to be more accurate, above it: In the thick cloud layer that perpetually covers the planet.
To be clear, they didn't see aliens flying through the air there or anything like that. What they did find was a chemical called phosphine, which, by all accounts, shouldn't exist in the atmosphere of Venus at the levels they detect. On Earth, phosphine is either made industrially, or by microbes.
Venus, as far as we know, doesn't have a burgeoning industrial economy, so...
… well, let's not jump to conclusions. The scientists involved certainly haven't. They're careful to say that what they've found is consistent with the presence of life in the Venusian atmosphere, but they don't come right out and state that it is the product of bacterial belches. Which is prudent; it may yet be from some as-yet-unknown non-biological chemistry going on there.
I would categorize this discovery as cautiously interesting.
One way to look for life on other worlds is to seek out biomarkers, chemicals that are produced by life. Free oxygen is a good one, because it's indeed produced by life and is so reactive that you wouldn't expect to see it in large abundances in an alien atmosphere without life constantly replenishing it.
Recently, phosphine has been examined the same way* and found to be a good biomarker for the same reasons. On Earth, phosphine (PH3) is only made by humans artificially, or by anaerobic bacteria, generally in rotting corpses. Finding it in an alien atmosphere at relatively high levels would be a decent indicator (though not proof) of biological processes.
Venus is not the first place you'd expect to find biology, though. The atmosphere is crushingly thick, with a surface pressure 90 times Earth's — the same pressure you'd feel 900 meters under water on Earth! It's also mainly carbon dioxide, a fierce greenhouse gas, making the surface of Venus extraordinarily hot, about 470°C (900°F), hot enough to melt tin and lead. Also, its clouds are made of sulfuric acid, because why not throw that in there too. Clement it is not.
But at higher altitudes, at 50 km or so above the surface, the temperature drops to 20-30°C, comfortable by Earthly standards. The pressure is much closer to Earth sea level as well. If life like anything we understand exists on Venus, that would be a good place to look for it.
With that in mind, the astronomers observed Venus with the James Clerk Maxwell Telescope (JCMT). Phosphine is known to absorb light with a wavelength of about a millimeter (well outside what the human eye can see, past even infrared), where JCMT is sensitive.
And that's just what they saw, a dip in light at that wavelength. However, while the observations were good, they weren't convincing, so the team got time on ALMA, another millimeter telescope in Chile that is larger and more sensitive than JCMT. The data from that were far clearer, again showing absorption at the right wavelength for phosphine.
To their knowledge, no other molecule or atom could absorb at that specific wavelength. Still, to be clear, finding a single absorption feature in the spectrum is a little bit iffy. I'm reasonably convinced they found phosphine, but I'd be happier if they found it at different wavelengths as well. They do plan on observing more to look for it.
Their observations indicate a level of phosphine at 20 parts per billion (so, 20 molecules of phosphine out of a billion of everything else). That may sound low, but it turns out that from what we currently know, on Venus it's hard to make anywhere near that much. The phosphine was seen at altitudes of at least 50 km. The environment there is pretty hostile to phosphine, which would decompose fairly rapidly. They estimate that that it should all be gone in less than a thousand years at that level, and probably much faster.
So something must be actively making it to keep the levels up. But what?
The scientists looked at a variety of known pathways to make phosphine, but none really fits the bill. The list included sunlight (not enough to make the right amount of phosphine), geological processes on the surface and subsurface (scratched off because those can't make enough either), lightning (too rare on Venus by a factor of ten million to produce the levels seen), meteoric activity (again, too low), and volcanoes.
That last one would need volcanic activity at least 200 times higher than Earth's to account for the phosphine levels. I wouldn't 100% cross that off just yet; there's evidence of volcanic activity on Venus, but we don't know the rate. Still, 200X is the lower limit, and it might take far more (hundreds of millions times more!) to make the phosphine, so it seems pretty unlikely.
So no non-biological process known can make phosphine at the levels needed to explain the observations. That doesn't rule out unknown processes, of course. You can't cross off the list something you don't know about. Clearly, more research needs to be done about that. We really don't know all that much about the chemistry of the clouds of Venus, so it's entirely possible there's just something weird but non-biological going on here.
So, could it be life?
Well, yeah, maybe. But that's a big leap, and again one they don't affirmatively make in their paper. They do discuss it, though, as they properly should, since they can't rule it out.
Interestingly, when observed in ultraviolet there are weird dark patches in Venusian clouds that are very difficult to explain. A semi-whacky idea proposed a while back is that there are huge clouds of bacteria floating there that absorb the UV light. It's a bit out there, but not too far out there. And here we have phosphine at about the same altitude over the surface of Venus. Hmmm.
Where does this leave us? Well, it's intriguing, certainly! But it's not conclusive. What's needed are more observations (as usual) to pin some things down. Is this really phosphine (likely, but more evidence is always nice)? At what altitude exactly is it above the Venusian surface? What's the temperature and pressure there? What else is floating around that might interact with the phosphine (sulfur clouds, acidic raindrops, minor constituents, etc.)? Also, what other non-biological processes might be at work here?
What we really need is a closer look. NASA and ESA both have Venus missions in the early stages. The private company Rocket Lab wants to send a mission there in the near future as well, specifically to look at the atmospheric layer where life might be.
We've been looking to Mars for scientific evidence of life for a long time, and it's certainly a good place for it. But I find it delightful that Venus — perhaps the last place in the solar system you'd expect to find life — could turn out to be where we find it first. But we're a ways from that, certainly.
But it does make things more interesting, doesn't it?
My thanks to Nicole Mortillaro, and to the study's lead author Professor Jane Greaves for her helpful conversation.
*Note that four of the authors on the linked paper about phosphine as a biomarker are also authors of the paper about phosphine at Venus.