It’s not a matter of if, but when.
We will find life on another planet, another moon, something. But will you and I live to see it? I’d say we’ve got a good shot at it.
I’m not necessarily talking about “intelligent” life, the kind that beams episodes of “The Zorklemooners” to us, although that would obviously be quite amazing. I’m simply talking about being able to point to another world, especially an explorable one, and say, “There are LIVING THINGS there.” That’s a drop-everything moment.
One day in the not-too-distant future we will see another Victorian botany era, where the race is on to find and paint and draw new species, and we’ll all sit in our parlors drinking tea and eating scones and marveling at the new sketches and descriptions of living things from faraway places we have never seen before. Just pure discovery.
Of course, we are talking about alien life here, so maybe some Edward Lear nonsense botany is more appropriate:
What I want to give you here is a summary of the chances we have to find life, how we’re going about it, and when we could possibly expect success. We’re not really dealing with things like SETI and the search for radio communications from intelligent beings here, although that is surely a worthy topic as well.
I’m going to focus mostly on our own Solar System, because as we’ve learned more about it, we’ve come to realize that it holds many very compelling possibilities for hosting life, substantially more than we thought even a few years ago.
But first I’ll start with a very recent and intriguing development that is an exception to that. It’s a possible way to sleuth out life on very distant planets, but it’s the newness and innovative nature of this one that inspired me to finally dig into this whole subject more completely.
A smoking gun in the spectrum
If we’re trying to find life on distant planets that can’t be explored, we’re pretty limited in what we can look for. True, we might see evidence of water or oxygen in the spectral signature, and while those are suggestive of a life-supporting environment, they don’t really prove anything.
But are there any signals that would be more of a smoking gun? Dr. Clara Sousa-Silva of MIT thinks so. She’d long been studying spectra of the atmospheres of gas giants like Jupiter and Saturn, so she already had a very good sense of what kinds of molecules you can detect and of the chemistry of how these compounds tend to arise. She wanted to maximize the information she could get from this stuff, so she started cataloguing thousands of compounds with these things in mind.
And she found that there’s at least one gaseous compound called phosphine that has a lot of happy coincidences going for it. I was ready to show a simple phosphine molecule, but then I found a photo of Sousa-Silva holding a model of one!
The paper she wrote with her colleagues is out now online and will be officially published early next year.
Phosphine can arise in spontaneous chemical reactions, but only at temperatures above about 1,000˚F. So you will find some when you look at the atmosphere of a gas giant like Jupiter. It forms down in the hot core, and because heat rises, it will circulate out to the surface.
But below 1,000˚F, phosphine cannot and does not form spontaneously. The energetics just don’t allow it. But it can and does form at cooler temperatures right here on Earth with the help of living organisms. We don’t know exactly how they do it, but bacteria that thrive in places where there’s no oxygen are quite good at forming phosphine. It turns up in swamps, in lake sediments, floating above piles of penguin poop, and yes, even in farts. Sousa-Silva explains why it’s a smoking gun for the presence of life:
“At some point we were looking at increasingly less-plausible mechanisms, like if tectonic plates were rubbing against each other, could you get a plasma spark that generated phosphine? Or if lightning hit somewhere that had phosphorus, or a meteor had a phosphorus content, could it generate an impact to make phosphine? And we went through several years of this process to figure out that nothing else but life makes detectable amounts of phosphine.”
Phosphine can be detected by looking at the infrared part of the spectrum (not visible to our eyes but easily visible to optical instruments). The wavelengths that phosphine absorbs are shown below in orange against the absorbances of all the other gaseous molecules you might find in a planet’s atmosphere:
That stands out like a sore thumb. It’s practically got the 4- to 5-μm (micrometer) wavelength range all to itself. That means that if phosphine is in a planet’s atmosphere, you’re going to detect it.
But the REALLY happy coincidence is that the James Webb Space Telescope — which just so happens to be designed to probe the infrared part of the spectrum and will be the most powerful space telescope ever — is finally going to launch in March 2021. (There’s a very good article about overcoming the technical challenges of that project here.) And it will indeed be looking at distant exoplanets, and those will surely include rocky planets that stay well below 1,000˚F.
Wouldn’t it be cool if phosphine showed up on one?
Enceladus, Saturn’s 6th-largest moon
Oh, Enceladus! Honest to goodness, if you took a cocktail of all of Earth’s microorganisms and dumped them into the water that lies under the surface of Enceladus, something would survive and start growing. The only question is, did life ever actually pop up there? If it did, and we find it, it’s game over. We’d be certain that the Universe is teeming with life.
We had NO IDEA about any of this until the Cassini-Huygens mission found in 2005 that Enceladus shoots plumes of water vapor into space because it has a gigantic ocean underneath its surface.
Enceladus spews so much stuff, in fact, that Saturn’s “E” ring is made up entirely of that stuff. The Cassini spacecraft took a picture from behind Saturn, when it eclipsed the Sun, and that allowed us to view the “E” ring clearly. It’s the bluish outermost ring you don’t normally see in pictures of Saturn:
Cassini really is the gift that keeps on giving, because in 2018, after the demise of the spacecraft, data retrieved from its Cosmic Dust Analyzer showed that the plumes of Enceladus contain some pretty large organic molecules, the kind associated with living things. These molecules have ring structures and molecular weights of up to 200, which means that the diagram below is not a bad set of examples to illustrate the kind of stuff that’s in there:
We also know that the ocean beneath the surface of Enceladus contains methane and hydrogen, especially after a 2019 study estimated their concentrations down there to be pretty substantial, and those are fuels that organisms on Earth can and do eat to survive and grow, even without any sunlight.
So ... don’t you want to go back to Enceladus?! Well, fortunately, NASA does, too. They are in the process of considering several mission ideas. It could be as simple as sampling the plumes again but with much better instruments, like even a microscope. We might also land on the surface to sample plume snow, which might be logistically easier to do. We could arrange to return samples to Earth, although it’s a 14-year round trip. It could even be as complicated as sending a vehicle down into the ocean.
We don’t know which option NASA will ultimately choose, but it’s clear that they want to do it, and I hope they make a decision soon. We need to get back there!
Europa, Jupiter’s 4th-largest moon
We’ve had the idea that Europa might have a subsurface ocean since the late 1990’s, based on imaging and also on magnetic and gravity measurements from the Galileo mission. The Hubble Space Telescope, back in 2012, had spotted plume-like activity similar to that of Enceladus that appeared to include hydrogen and oxygen. But just a month ago, water vapor was detected directly in Europa’s atmosphere thanks to some dedicated people at the Keck Observatory. They found that it appeared in a burst, suggesting a plume, and therefore, most likely a subsurface ocean.
Happily, NASA has committed to return to Europa for a much closer look. The Europa Clipper mission has been confirmed and will launch between 2023 and 2025. It will do 45 flybys of Europa, coming as close as a few miles. Its job will be to scope out the dimensions of the ocean but also to confirm water plume activity. If it does, it will fly through the plumes and make careful measurements of their composition. None of this would prove life existed, but it would set the stage for a subsequent mission, the Europa Lander, which NASA has proposed but not quite confirmed yet. That lander would have its best shot at confirming life in exactly the way you’d think: taking a microscope and just looking.
Our best idea of what the ocean under Europa’s surface looks like is this:
Titan, Saturn’s largest moon
Titan is a real wild card. Astrobiologists range from those that think it’s the most likely place to discover more life in the Solar System to those that think it’s very unlikely. Even though Titan’s got a thick atmosphere full of organic molecules, its surface is very cold. So cold, in fact, that it has lakes of methane and ethane — basically liquid natural gas.
There’s no particularly good reason to believe that much water exists on Titan, so if there is life there, it would have to be quite different from that on Earth, using really cold hydrocarbons rather than water to live and grow in. But whatever you think of the chances, we’re going there!
On June 27 of this year, NASA approved the Dragonfly mission, which will send a helicopterish, dronish vehicle called a rotorcraft to fly a planned total distance of 108 miles across Titan’s surface. The images by themselves will make the trip worth it, but we’ll also learn a lot more about liquid above and below the surface and any chemical signs that are indicative of life, or at least the ability to host it. Dragonfly launches in 2026 and arrives in 2034. Its adventure will begin like this:
Hard to believe that’s real. Can it be? Ohhh, it be.
And there are many other possibilities...
...even within our own Solar System! There is evidence of varying degrees of liquid water within Triton (Neptune’s largest moon), Pluto (!), Ceres (the largest “asteroid”, actually a dwarf planet), and yes, even good old Mars. I only gave you the most-developed cases above, but life could absolutely be found in any of these other places as well, and efforts are underway to visit or revisit all of them, and I wish I could do all of them justice here.
One way or another, quite possibly within this generation, we’re going to find life somewhere other than Earth.
Don’t doubt it!
If you think all this subsurface life sounds implausible, consider that about 70% of microbial life on Earth lives below the surface, not merely all the way down to the ocean floor, but at least three miles beneath it, and possibly as deep as 12 miles underground. Some findings from the Deep Carbon Observatory, made up of over 1,000 scientists worldwide (the real “Deep State”!), in their research summary entitled “Life in Deep Earth Totals 15 to 23 Billion Tons of Carbon—Hundreds of Times More than Humans”:
- The deep biosphere constitutes a world that can be viewed as a sort of “subterranean Galapagos” and includes members of all three domains of life: bacteria and archaea (microbes with no membrane-bound nucleus), and eukarya (microbes or multicellular organisms with cells that contain a nucleus as well as membrane-bound organelles)
- Deep microbes are often very different from their surface cousins, with life cycles on near-geologic timescales, dining in some cases on nothing more than energy from rocks
- The genetic diversity of life below the surface is comparable to or exceeds that above the surface
- The absolute limits of life on Earth in terms of temperature, pressure, and energy availability have yet to be found. The records continually get broken. A frontrunner for Earth’s hottest organism in the natural world is Geogemma barossii, a single-celled organism thriving in hydrothermal vents on the seafloor. Its cells, tiny microscopic spheres, grow and replicate at 250˚F
- The record depth at which life has been found in the continental subsurface is approximately 5 km; the record in marine waters is 10.5 km from the ocean surface, a depth of extreme pressure; at 4000 meters depth, for example, the pressure is approximately 400 times greater than at sea level
Life is unbelievably resilient, and there are several places right here in our own Solar System where it has everything it needs.
I leave you by introducing some of our friends found deepest within planet Earth: