Life on Venus? That outrageous-sounding possibility understandably made headlines a couple weeks ago. In part, the news grabbed people's attention because Venus seems like such an unlikely place to find anything alive. The surface temperature there averages 460 degrees Celsius, and the pressure at sea level is a crushing 93 times the atmospheric pressure on Earth — except, of course, there is no actual sea on Venus.
There was another startling aspect of the life-on-Venus story, however: the nature of the evidence itself. There are no rovers rolling across Venus. We do not have any samples of Venusian rocks to put under the microscope. We have no physical samples at all of the planet. The search for life was conducted from afar, entirely indirectly, using radio telescopes. Those provocative hints of life came in the form of an extremely slight radio shadow indicating the presence of a molecule known as phosphine, a phosphorus atom bonded to three hydrogen atoms.
Jane Greaves, an astronomer at Cardiff University in Wales, and her colleagues used the James Clerk Maxwell Telescope in Hawaii and the ALMA radio observatory in Chile to scan for phosphine in Venus's atmosphere. To their surprise and delight, they found traces of the molecule, at a level of about 20 parts per billion, mixed in with the planet's acidic clouds.
On Earth, phosphine is primarily generated by anaerobic bacteria. Greaves and her team could come up with no plausible geological process that would produce the observed concentration of phosphine on Venus. Then again, the fact that they couldn't think of a non-biological process doesn't mean that one does not exist, as the researchers themselves openly noted in their paper.
The discovery of phosphine on Venus was an unexpected, confoundingly obscure signal of potential alien life. It was also a telling preview of things to come.
As scientists expand their search for life across the solar system, and on planets orbiting other stars, there will be more and more reports of possible life detections. As with phosphine on Venus, they will come from indirect data. They will be ambiguous. They will be fiercely debated. And those signals of alien life will all, with near-certainty, take the form of a molecule.
Astrobiologists call any detectable evidence that life is present now, or that it was present in the past, a "biosignature." In principle, a biosignature could come in the form of a flying saucer with aliens parading down a set of impeccable white stairs, or of a radio signal containing instructions on how to build an interstellar gateway. I can't say that these things are impossible. Nobody can. But the overwhelming likelihood is that the first indications of alien life will take the form of molecules like phosphine — molecules that are commonly associated with biological processes, and that seem difficult to explain without them.
If you are a skeptical reader (and I hope you are!) you might well ask at this point, "How can you know?" Since we have no idea what kind of life, if any, exists in the universe beyond Earth, how can we possibly claim to know what kinds of detections are more likely than others? Fortunately, we're not flying completely blind here. We can take some important lessons from our technological limitations, from the nature of life on Earth, and from some of the fundamental physical principles that limit what life can do.
Right now, scientists have in hand physical samples of exactly one other planetary body, Mars. There are 277 meteorites that originated on Mars, blasted off its surface by an asteroid impact before eventually landing on Earth. There are no known terrestrial meteorites that originated from any other planet, and there are currently no plans to bring back samples from any planet other than Mars. (We have quite a few meteorites that came from the Moon, along with a great deal of lunar material returned by the Apollo missions, but nobody is seriously thinking about the Moon as a place to look for life.)
For any other location in the universe, then, any life detection will have to rely on remote sensing. Now consider the kinds of remote biosignatures we could look for. Signature of a technological civilization would be exciting but, statistically speaking, have a very low probability.
Suppose scientists found another planet identical to Earth. We might be seeing it at any point in its history. Through most of the Earth's history, there was no intelligent life on this planet. Through most of the history of intelligent life on this planet, humans have had no technology that could readily be detected from afar. Only since the Industrial Revolution have humans made changes that might plausibly qualify as a remote biosignature — most notably, emitting artificial light from cities and broadcasting radio signals.
The appearance of photosynthetic life began to radically alter Earth's chemistry about two billion years ago, flooding the atmosphere with oxygen. An oxygen-rich atmosphere is not geologically stable unless it is constantly replenished, and life is the only process we know of that can steadily generate oxygen on the necessary scale. Earth therefore has had an obvious molecular biosignature for at least two billion years. Earth has had an obvious technological (non-molecular) biosignature for less than two centuries.
By sheer dumb chance, your odds of finding a parallel Earth during its technological phase are 1 in 10,000,000. (How long will this phase last into Earth's future? Nobody knows, and I'm not going to deal in hypotheticals.) There are obvious limitations in extrapolating from a sample of one, but for now Earth is the only case study we can work with. Intelligent life seems to take billions of years to evolve, so even if living planets are common, we will almost surely have to rely on molecular signals for finding life on them.
Now take a look at the most promising places to look for extraterrestrial within our solar system. For decades, most astrobiologists focused on Mars. More recently, the moons Europa and Enceladus have joined the short list, because they have extensive, warm oceans beneath their icy crusts. A few researchers have made the case for possible life on Saturn's haze-cloaked moon, Titan. With the reported discovery of phosphine on Venus, we can add that planet to the list.
What all of these locations have in common is that they are severely constrained in resources and in available energy compared to Earth. It seems unlikely that any of them could sustain complex, multicellular life; Europa seems the best bet in that regard, since it has a well-protected ocean that has probably remained stable for billions of years. And even if complex life does exist, or did once exist, on one of these worlds, we would be hard-pressed to find it.
It's conceivable that scientists could find fossil traces of ancient life on Mars locked away in rock samples brought back to Earth sometime in the coming decade. A quarter-century ago, a group claimed to have found micro-fossils in the Martian meteorite known as ALH84001. Their findings were discredited, but the basic approach is reasonable. Who knows? With enough digging, maybe we could find the fossil remains of an ancient Martian clam. But almost all life-related research on Mars today focuses on molecular evidence, just as with the very first Martian-life experiments aboard NASA's Viking landers in 1976.
Studies of all the other potentially habitable worlds in our solar system likewise is all about the molecules. The Cassini spacecraft detected traces of methane, carbon dioxide and even complex organics in plumes of water shooting from fissures in Enceladus's crust. That makes a tantalizing case for life in the underlying ocean — but actually exploring that ocean, or the even bigger one on Europa, would require landing, drilling through potentially kilometers of ice, dropping a probe into a pitch-black ocean and doing extensive exploring.
The technology to mount such a mission does not yet exist, and at any plausible level of funding it won't exist for decades to come. For now, molecules it is. NASA is sending a flying robot called Dragonfly to Titan in the 2030s. The only biosignature it could look for is molecules. Next year, NASA may approve a new mission to Venus. The concepts under consideration would be able to search for molecules only. The same goes for a potential private mission to Venus that could fly in the mid-2020s.
As for finding life on planets around other stars, there are really only two kinds of data to look for: messages and molecules. I'm a big fan of SETI (the Search for Extraterrestrial Intelligence). It doesn't cost much, it produces many side benefits in exploring the universe, and the potential payoff is huge. Huge! Finding proof of intelligent life elsewhere in the universe would perhaps be the greatest scientific discovery of all time.
That said, we have no way of knowing if there is anyone out there to talk to. The famous Drake Equation provides only a framework for how to ask the question. For instance, we have no idea if alien life (if it exists) inevitably evolves toward intelligence, if the rate at which that happened on Earth is at all typical, or if alien civilizations would have any interest in communicating with us. I'll be ecstatic if someone detects a message, but I sure don't expect it to happen.
At the other extreme, we know that pre-biotic chemistry is commonplace throughout galaxy and beyond, and that simple life appeared on Earth pretty much as soon as the surface was potentially habitable. Those two details, along with some other circumstantial evidence, strongly suggest that simple microbial life could be abundant across the cosmos. The only message such life will broadcast is a chemical, molecular one.
That is why scientists are working so hard to find potentially habitable, Earth-size planets around other stars. If those planets happen to pass directly in front of their stars, as is the case for the worlds of the TRAPPIST-1 system, it would be possible to study starlight streaming through their atmospheres and to study their composition. With a sufficiently powerful telescope, it would also be possible to directly observe an Earth-like planet around another star and study its composition spectroscopically.
The upcoming James Webb Space Telescope (whenever it finally launches) will be the first telescope that has at least a chance of finding such molecular biosignatures around planets orbiting other stars. The generation of telescopes after Webb stand a better chance. This is extremely delicate, challenging work. We are looking for traces of gas in a sliver of atmosphere around a planet passing in front of a star that is tens or hundreds of trillions of kilometers away. I mean.
And yet that isn't even the final challenge. The detection of phosphine on Venus previews the debates and disputes that are likely to erupt — disputes that may well go on for years or decades. As soon as Greaves and her collaborators announced their discovery of phosphine on Venus, the questions began. The easy question is, did they really detect the phosphine correctly? (The results look robust, but should of course be independently verified.) The hard question is, what does the detection of phosphine actually mean?
On Earth, phosphine comes from microbes. On Jupiter and Saturn, though, extreme physical conditions allow phosphine to form without the presence of life. It's possible that phosphine forms via some abiotic mechanism on Venus as well; one recent paper proposes that it may be the result of intense Venusian volcanism. Even if nobody can find a plausible alternative explanation, phosphine can never be taken as proof of life on Venus. All it can do is stimulate searches for other lines of evidence that would help make the case.
Eventually, a space probe could drop a balloon into the clouds of Venus and look for microbial life directly. For planets around other stars, that's not an option. There, we will have to rely entirely on molecular evidence. Anything out of chemical equilibrium is automatically interesting. Finding an oxygen atmosphere would be a provocative sign of life, but far from proof. Finding oxygen plus methane would be more convincing, because they can't coexist unless constantly replenished.
It's hard to imagine that any mix of molecular signals could ever be fully convincing. As astrobiologist David Grinspoon recently commented on Twitter, "Someone can always come up with an abiotic geophysical Rube Goldberg mechanism. And for exoplanets we won’t have any 'ground truth.' "
The answer may ultimately come down to statistics. MIT astronomer Sara Seager told me that her hope is ultimately to find 50 or 100 planets with interesting molecular biosignatures. If that happens, it will seem highly improbable that every single one is a false alarm; it will seem highly probable that simple life, at least, really is commonplace in the universe.
But those molecular signals will not give the kind of fully satisfying answer we crave to the question of "are we alone?" For that, we have three options. We can hope for a SETI message or wait for a flying saucer. Or we can start doing some serious digging on Mars and drilling on Europa, hoping that we will find an answer closer to home.
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