When NASA’s Perseverance rover launches this summer, it will face one of the most ambitious missions in any space exploration project to date: To search for evidence of life on Mars. If there ever was life on Mars, there almost certainly isn’t now — so how do you go about hunting for evidence of something billions of years old on another planet?
The answer involves the heaviest rover ever sent to another planet, a dried-up lake bed that is millions of years old, and a superpowered laser that vaporizes samples from 20 feet away. We spoke to two NASA Mars experts to find out more.
A brief history of Mars
Mars today is a cold, barren planet with a very thin atmosphere that is inhospitable to life. But billions of years ago, it was a very different place, covered in surface water and possibly even hosting an enormous ocean spreading across its northern hemisphere. These factors mean it could once have hosted life.
“What we do know is that there was abundant water at the surface of Mars in its distant past,” Katie Stack Morgan, a researcher in Martian geology at NASA’s Jet Propulsion Lab, said. “We have abundant evidence for that in … the minerals that we observe at the surface, the land forms that we see, the valley networks carved into the surface of Mars, the presence of these deltas in ancient crater lake basins. We know that water was there at the surface.”
That knowledge leads to other inferences such as that the surface temperature must have been warmer, as today it’s too cold for water to exist continuously as liquid on the surface. It also suggests that Mars’s atmosphere was likely thicker and richer than it is today.
There is some debate over exactly how long water was on the surface for, but scientists agree that it was there for what Stack Morgan described as “geologically significant periods of time.”
And where there is liquid water, there is the potential for life to have existed.
What might life on Mars have looked like?
Researchers are careful to emphasize that they are searching for life as we know it — because it would be impossible to search for something entirely unfamiliar. But there are good reasons to assume that if there were life on Mars, it would be at least comparably similar to life here on Earth.
“There is variability of microbial life here on Earth,” Stack Morgan said, depending on environmental factors like humidity, temperatures, altitude, and many others. “But one of the reasons we expect life, if it existed on Mars, to be at least recognizable, is that as far as we can see, the types of settings on Mars were once very similar to the kinds of settings we have on Earth.”
We know that there were lakes on Mars, just like those on Earth, as well as features like deltas and mountains. We know that there are organic molecules on Mars, which could be created by life but could also have arisen from other natural processes. At some point in the planet’s history, it might have been not so different from Earth today.
“We have every reason to believe that microbes, if they existed on Mars, would adapt in the same ways that microbes on Earth have adapted,” Stack Morgan said. “As far as we know, we had the same ingredients for life on Mars as we had here on Earth. So that creates confidence that if life on Mars did once exist, we would recognize it.”
What evidence of life looks like
So how do we spot something that may once have been alive?
Unfortunately, “there’s no tricorder,” Luther Beegle, principle investigator of the SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals) instrument on the Perseverance rover, said. “There’s nothing you can point at something and say, ‘Oh, there’s life.’ It’s a lot of information you have to wade through, to look at everything together and come up with a scientific conclusion.”
“We are looking for what we call potential bio signatures,” Beegle explained. “On any given body in the solar system, unless something is waving at you, I’m not sure whether you could call it life or not. We have serious scientific debate in this community about what life is and how you detect it.”
It would be easy to detect currently living microorganism communities such as bacterial mats. But it’s very unlikely that we’d find currently living organisms on Mars, so scientists instead look for evidence that these communities might have existed in the past.
“But it’s hard to say what these communities would be like after two [billion] to three billion years of sitting on the surface,” Beegle said. “So it’s hard for us to know what one measurement we could take that would enable us to say: ‘This was definitely alive.’
“What we can do is say, ‘This is a really interesting sample. There’s a good chance this was alive a long time ago. We should bring this sample back and look at it in a terrestrial lab.’ And then you can come to a scientific consensus.”
How to hunt an alien fossil
When it comes to actually locating evidence in samples, the first and most obvious method is simply to look for it.
“The first way that you look for signs of ancient life is with your cameras,” Stack Morgan explained. “You image the terrain around you and you look for what we call morphological features — shapes and textures in the rocks — that seem unusual or that they might not have been formed by physical processes. So, the easiest example you could think of here on Earth is a dinosaur bone, in terms of examples of macroscopic evidence of life and charismatic megafauna.
“But we expect the search on Mars to require more subtlety. Because previous rover missions have not observed megafauna in any way, so if we’re looking for signs of life, it’s likely at the microbial scale.”
So to understand what evidence of microbial life on Mars might look like, we can look to the rocks here on Earth and how they preserve signs of ancient life. “We look for very fine scale shapes and textures in the rocks,” Stack Morgan said. “But also things like rock layers, which perhaps crinkle in an unusual way. Or perhaps patterns that we wouldn’t expect.”
The other way to look for signs of life is to focus on the composition of rocks, especially the presence of potential organics. The presence of organics and the unusual rock textures in combination can suggest that life once lived there.
This combination of composition and texture is exactly what Beegle’s instrument SHERLOC was designed to investigate. And unlike previous rovers, it can investigate samples without destroying the texture of rocks. “That’s exactly how we go about looking for evidence of ancient life in our own rock record here on Earth,” Stack Morgan said. “And we can now do that on Mars.”
Using light to analyze rocks
SHERLOC’s most important tool is its spectrometer, which uses light to see what a sample is made of. “You shine a light on something and you look at the wavelength of light it emits, which tells you what color it is,” Beegle explained. “And by looking at that color, you can tell something about the sample.”
There are many different types of spectroscopy, such as the laser-induced breakdown spectroscopy performed by Perseverance’s SuperCam instrument, in which a high-powered laser vaporizes a sample and analyzes the compounds given off. But to search for evidence of life, you need to look on a smaller scale and preferably use a nondestructive method, so you don’t have to destroy a sample in order to analyze it.
SHERLOC uses a nondestructive method called raman spectroscopy. “In raman spectroscopy, you can tell whether something is an amino acid, or whether it’s a carbonate, or whether it’s a coal, or something else,” Beegle explained. SHERLOC can also perform fluorescent spectroscopy, which can detect the presence of organic molecules.
Used together, these methods can give information about a sample such as whether it is organic, whether it formed in a liquid environment, whether is was at a high temperature, and so on. The SHERLOC data can also be combined with data from other Perseverance instruments like PIXL (Planetary Instrument for X-ray Lithochemistry) or the cameras on Mastcam-Z to give a more complete picture of what any given sample is composed of.
Particularly valuable for study are sedimentary rocks which are formed in layers over time. If Perseverance can find and analyze such a sample, it could potentially see how the environment on Mars developed over thousands of years — and it might even get a glimpse of something like a carbonate layer within a bunch of basaltic layers, which would suggest that something rare and important happened at one particular point in time in the region’s history.
Picking a landing spot
To hunt for signs of life, not just any spot on Mars will do. NASA has specifically chosen the Jezero Crater for the search, as it has particular features that make it the most likely location we’ve found so far to have preserved evidence of life.
“Jezero is a very special place on Mars,” Stack Morgan said, due to the presence of a delta there. “There are hundreds of ancient crater basins that people think had lakes, including the Gale Crater [where the Curiosity rover is currently exploring]. But not every crater has a delta preserved in it. A delta is the land form produced when a river opens up into a large basin and deposits its sediment.”
A delta provides further evidence that water was once at the site, and means that there will be interesting rocks to explore.
“What also makes Jezero very special is that it has an inlet valley where the water flows in, but what makes it almost unique is the presence of an outlet valley,” Stack Morgan said. “It’s a simple, subtle thing, but it’s remarkable how important that is, because if you have an inlet valley, you know that the water had to flow in. But if you have an outlet valley, you know that the water had to fill up to the level of the outlet valley.”
If a lake were shallow, it might have dried up intermittently and would not have been hospitable to life. But if a lake was deep enough to be a standing body of water for a long time, that would be a much more likely location for life to develop and take hold.
“Jezero has not only the land form that shows us there was water there, but we also have evidence that the entire crater filled up,” Stack Morgan said. “That’s what helps increase our confidence that Jezero is a good place to look for life, in a way that other places including Gale are a bit more of a gamble.”
Another thing that makes Jezero unique is the minerals that we can observe there. “Jezero Crater is the only one of these ancient crater lake basins that has carbonate minerals,” Stack Morgan said. Carbonates on Earth form the structural basis of fossils and are found in coral reefs, like the Great Barrier Reef in Australia. Finding them in a lake basin on Mars could indicate the same thing.
Not only are carbonates present — they’re also located around the inner rim of the crater, where the lake would have been shallow, which is where we’d expect to find them. Carbonates are “really good at preserving evidence for life,” Stack Morgan said. “So if you had to pick a place on Mars to go to search for life, you would go to the carbonate inner ring of a shallow lake environment” — which is exactly what the Jezero Crater offers.
Getting samples back to Earth
Although the public often has the idea of a magic machine that can instantly analyze samples and see what they’re made of, à la CSI, the reality is that the process of sample analysis takes a long time and consists of many steps that have to be painstakingly followed. It’s not possible to shrink an entire suite of analysis tools into the tiny amount of space available on a rover — some of the instruments are the size of a house, and the available space on the rover is the size of a shoebox — so to really understand what a Martian sample consists of, we need to get it back to Earth.
That’s why the next step in the search for life on Mars after Perseverance is a sample return mission, in which one or more spacecraft are sent to Mars to collect the samples of rock and soil that Perseverance has collected and return them to Earth.
“If you’re going to look for life, a sample return mission is an essential next step,” Beegle said. “Because it allows you to bring back a sample, you can put it in a lab, you know a little bit about it, and then you can plan everything out from there.
“What every space mission does is assume what you’re going to find there — and that’s how you design your instruments. But with sample return, you can bring it back, you identify a little bit more about the sample, you use a lot of nondestructive technologies like CT scans and X-ray tomography, and you understand more about the sample so you can tailor your experiments to what the sample is.
“So sample return is really valuable, and really important … It’s vital to the question of whether or not life existed on Mars. I don’t know how you’d do it without it,” Beegle added.
The hunt begins this summer
The Perseverance rover is set to launch this summer, some time in a two-and-a-half week period beginning on July 17. It should land on Mars on February 18, and from there is can begin exploring its surroundings and taking samples, and perhaps even find evidence that Earth isn’t the only planet to have hosted life.
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