Venus Isn’t The First Place We’ve Found “Alien” Chemistry

A chemical in the upper cloud layers of Venus might be evidence of alien life floating in the noxious haze, a group of astronomers announced in a paper published earlier this week. Here on Earth, the chemical compound phosphine is produced when bacteria break down organic matter; phosphine is a waste product, the bacterial equivalent of poop and exhalation combined. On Venus, the astronomers couldn’t find any other explanation for the amount of phosphine in the atmosphere – and they tried (more on that below).

However, the detection of phosphine on Venus isn’t the first time scientists studying other worlds have discovered chemicals that look like they could be the work of alien microbes. It probably won’t be the last time, either. And someday, the mysterious chemical signature making headlines will actually turn out to be alien farts – but probably not on Venus.

How To Look For Aliens

The most obvious way to search for alien life is to actually go visit another world and meet the inhabitants – whether that means shaking hands with a talking mass of tentacles or just running a bunch of soil samples through a microscope in search of living microbes. But that’s not so easy when you’re trying to explore other worlds from a distance, through the lens of a telescope or the robotic cameras of an orbiter or rover.  In that case, your best bet is to look for chemicals that suggest the presence of life.

First, you might look for signs that it’s possible for anything to live on your hypothetical planet. Life as we know it can’t exist without water, so that’s a good start. That’s why all the evidence for water in various forms on Mars has been so exciting; it proves that the planet was once extremely habitable, and suggests that parts of it may still be extreme habitats for especially hardy microbes. We’ve also found evidence of liquid water on Europa and Enceladus, icy moons of Jupiter and Saturn, respectively.

(As an aside, if you take nothing else from this article, take a moment to marvel at the gloriously bizarre worlds in our solar system. One world used to be habitable but now swelters beneath clouds of sulfuric acid and a surface temperature hot enough to melt lead. Another orbits a gas giant and has a globe-spanning ocean hidden beneath a thick icy crust. Yet another has seas of liquid methane and ethane washing bits of tar ashore onto ice frozen as hard as rock. One spews vast plumes of water vapor and ice crystals out into space, slowly building one of the rings of the gas giant it orbits. And there’s even one with some sort of life form on its surface, launching robots to explore all the others. Weird but true.)

Water will be one of the first molecules astronomers look for when the James Webb Space Telescope finally launches and starts observing the actual atmospheres of exoplanets. But just because a place is livable doesn’t mean that anyone has actually moved in; the next question is whether the right chemicals are available to make cell membranes, DNA, and the enzymes that keep life’s chemistry going.

The organic molecules (long chains of carbon with various combinations of elements like hydrogen, oxygen, and nitrogen) that form the building blocks of life have turned up in the rocky material of asteroids, in the plumes of water vapor and ice erupting from the cracks in Enceladus’ crust, and on Saturn’s hazy orange moon Titan. The hydrocarbon world Titan is not a place you’d want to live, but it has spawned all sorts of interesting discussions about what a methane-based cell might look like.

Those sorts of chemicals are clues that life is possible, but what if you want to find evidence that alien organisms are actually living, breathing, eating, and excreting on the surface (or in the clouds or the hidden seas) of an alien world? Here on Earth, plants and cyanobacteria exhale oxygen as a waste product of photosynthesis, so oxygen and ozone in a planet’s atmosphere are good clues that something might be alive down there. Some bacteria excrete methane, and others make nitrous oxide.

It Hasn’t Been Aliens Yet

Others (and we’re not even sure which ones, exactly) produce phosphine. That’s why everyone got so excited when astronomers announced that they’d found the spectral signature (radio waves emitted from Venus were blocked at a certain wavelength because phosphine absorbs that wavelength) of phosphine in the Venusian clouds.

But it’s not the first time we’ve found something that looked like a biomarker. Mars, for instance, has been taunting us for decades with trace evidence that might be signs of life or might just be the work of geology and soil chemistry (which are super interesting in their own right, but if you’re looking for aliens, even really interesting geological or chemical explanations are a bit like pulling the mask off a Scooby Doo villain).

In 1975, NASA sent two robotic landers, Viking 1 and Viking 2, to Mars. Part of their job was to search for evidence of alien microbes in the Martian soil, busy consuming nutrients and excreting waste chemicals, like microbes do. The experiment scooped up a sample of Martian soil, added a drop or two of a nutrient mix, and measured the gases that wafted up from the soil afterward. The carbon in the nutrient mix was the radioactive isotope carbon-14, which acted as a glowing tag to help identify any waste products it turned into.

The first two attempts produced mildly radioactive carbon dioxide – which looked like clear evidence of microbes chowing down in the Martian soil. And to this day, at least one of the scientists involved in the experiment is still convinced.

A week later, the Viking team repeated the experiment again, with the same soil samples. If their nutrient mix had fed a population of Martian microbes, then they should have grown and multiplied, and they should exhale even more tagged carbon dioxide the second time around. But nothing happened. Scientists generally agree that the tests were inconclusive – they didn’t prove there were aliens on Mars, but they also didn’t prove there weren’t aliens on Mars.

The best explanation for the Viking lander tests was soil chemistry. If chemicals in the soil on Mars reacted with the chemicals in the nutrient mix, they could produce carbon dioxide – but that reaction would use up all the reacting chemicals in the soil sample. That’s why nothing happened during the second attempt. In 2008, another intrepid Martian robot discovered oxidizing chemicals called perchlorates in Martian soil – mystery solved.

Mysterious Martian Methane

Remember that some bacteria here on Earth produce methane when they metabolize tasty, tasty organic nutrients? In 2004, the ESA’s Mars Express Orbiter spotted methane in the Martian atmosphere. A while later, the methane reappeared; that’s important, because methane shouldn’t last long in Mars’ atmosphere. UV radiation from the Sun, combined with other chemicals in the planet’s atmosphere, should break down methane molecules pretty quickly. So the fact that methane kept turning up meant that something on Mars was producing more methane.

In 2018, the Curiosity rover noticed a large spike in Mars’ methane production (don’t laugh), and eventually figured out that there’s a seasonal cycle of methane highs and lows on the planet.

That could be a sign of life. It could also be a sign of geological activity. We’re still not sure what the answer is. The ratio of hydrogen to methane in the Martian atmosphere could help tell the difference. Here on Earth, scientists are running experiments and computer simulations to test how well methanogen bacteria could survive in an environment like Mars. And eventually a Mars mission may bring back evidence of life that doesn’t involve methane at all.

What About Life On Venus?

Clearly, just because you’ve found a potential biomarker, you haven’t necessarily found aliens. Decades of searching for life on Mars should teach us that it’s important to follow up on discoveries like this one, but a biomarker on its own is usually not conclusive. There are nearly always more questions to answer before we can say for sure that we’ve found aliens, volcanoes, weird soil chemistry, or something else entirely.

And on Venus, that last option actually looks most likely. As Ethan Siegel reported, the authors of the recent studies tried several alternatives and none of them produced the amount of phosphine they observed in the planet’s atmosphere. The only thing that they could find that explained it was life – and even then, the habitable band of the Venusian atmosphere would have to be teeming with it, and they’d have to be pumping out a lot of phosphine. So for that reason, as well as the fact that Venus is pretty much what you’d get if 2020 was a planet, alien cloud microbes are a bit of a stretch.

Instead of aliens, the astronomers have probably discovered a new kind of chemical reaction that we didn’t previously know about. That’s similar to how Martian methane could teach us something about either life on Mars or geological activity on the planet. Either of those things would be really interesting.

And in the case of Venus, it’s also a question we’d better answer if we want to look for life on exoplanets when JWST eventually launches, because phosphine is one of the easiest biomarkers to spot, and because a recent paper suggested that it was one of the clearest biomarkers available. We need to be very sure of that, and we need to know whether we can rule out other options – which means we need to know what those options are.

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