Why we need to return Martian rock samples to Earth to test for signs of life

Is there life on Mars? It's a big question, one that David Bowie has been asking for a long time now. So, the majority of rovers and missions that we send to Mars have this focus of trying to figure out does life exist on Mars currently or more likely we think existed at some point in the history of Mars but no longer. So from the rovers of the past like Opportunity or future like the Roslin Franklin rover of ISA's ExoMars mission and of course to Perseverance and Curiosity rovers which are currently operating on Mars. But there's always been some doubt of whether the rovers are actually capable of detecting trace amounts of life if it's there. So, the Perseverance rover is currently collecting rock samples and leaving them on the surface of Mars for a future mission to collect and return to Earth. But why? Why do we need to return the samples to Earth to test for this when essentially the rovers are just scientific lab instruments on wheels? Well, a few years ago, this paper was published by Azu Bust and collaborators who used the techniques available to current and planned Mars rovers to test whether they could find signs of life in a sample of rock that was taken from Earth in the Atacarmama Desert. And spoiler alert, they couldn't. So, in this video, we're going to dive into this paper and chat first about why Azoua and collaborators chose the Atakama desert for their experiment. Then second chat about what kind of test the zoos and collaborators did in the lab here on Earth to detect signs of life in the Atakama desert sample. And then finally third, what would the Mars rover instruments detect in this sample from the Atakama desert instead? And if you love deep dives into the cutting edge of science just like this, then why don't you check out Nautilus, who are the sponsor of this week's video. Nilus is my favorite science magazine because it explores the big ideas in science. You know, the ones that will still be long debated into the future. Now, some of the world's greatest scientific minds both read and write for Nautilus, including literary giants like the late Cormick McCarthy, who published the only non-fiction piece of his career in North or senior scientist for astrobiology at NASA as research center, Caleb Sha, who regularly contributes reflections on our place in the universe, which is why I know all of you will especially enjoy their cosmos section, which explores the science and philosophy of studying the universe and does so with such a wonderful style with this like merger manager of art, culture, science, and discovery. So, join me in reading Nautilus by heading to join nautilus. com/dbecky, and you'll get 15% off your membership. You can join as a digital only member or in print to receive six beautifully illustrated, award-winning collectible editions per year that look wonderful on your coffee table. So, thanks again to Nautilus for sponsoring this video. And now let's dive into the details of testing for whether life exists on Mars and start with why Azoua and collaborators chose

the Atacarmama Desert for this experiment. Specifically, an area known as Redstone, which if you look at the geological history of the rocks, it tells us that it used to be a river delta. So, a delta is formed when a river flows towards the sea. And when it reaches the sea at the mouth, the water slows down and loses its ability to carry sediment. So, things like sand, silt, and clay. And the sediment is then deposited building up over time to form the delta. Now due to the shifting of earth's tectonic plates over billions of years and the change in climate on Earth, the place where this river used to flow in redstone is now underwater. It's under the Pacific Ocean, whereas the delta is on land in the Atakama desert. The rock there is made of lots of quartz, sandstone, and hematite, which is what gives it its red color. And everything I've just said, we also think applies to Mars. We think water once flowed on ancient Mars, which looks very different to what it does today. We think it started out with a very thick atmosphere, but over time, radiation from the sun stripped that atmosphere away because it didn't have a magnetic field like Earth does to protect it from solar radiation. As the atmosphere boiled off, so too did the water, leaving behind a rocky desert-like planet where water used to flow. And we see features on Mars that look very similar to features here on Earth that are created by flowing water, like big canyons carved by flowing water or like dried up river deltas. And it's these places that we land rovers and probes on Mars in. First of all, because rubber deltas tend to be very flat, which is great for landing, but also if you're looking for signs of life, at least life as we know it here on Earth that needs water to survive, then we should land in the places that used to be wet. So, Bostos and collaborators argue that redstone in the Atama desert is the most Marsike place you can get on Earth. So, if you want to be able to test whether the current instruments and techniques that are available to us on the rovers on Mars would be capable of finding trace amounts of either current or past life on Mars, then you've got to test whether those same techniques are actually capable of doing it with a

sample found here on Earth as well. Which brings me to part two. What kind of test did Azou and collaborators do in the lab on Earth to detect signs of life? So when you're trying to work out whether a sample you have whether that is rock or if it's a liquid gas if you're trying to work out what that's made of the first thing you have to do is split it into its individual component molecules or elements and then you can measure the properties of those different components that you split it in things like mass and charge to be able to work out okay what actually have you got here. So Azou collaborators used a combination of both gas chromatography and mass spectrometry to do this. So gas chromatography is essentially when you boil your sample really slowly and you wait for all of these different things to turn into their gases and that will happen at different boiling points and if you record the different boiling points then you know what compounds and what molecules you have. Mass spectrometry is when you pass the different components of a sample through a magnetic field. So the more charged a molecule is the more it will get deflected by the magnetic field as it moves through. But also the heavier a molecule is the more it will resist the deflection. So the amount of deflection you get for each of these different components tells you the ratio between the mass and charge of the molecule which allows you to identify which molecule it is. So by combining these two techniques together, you get a lot of detail about what your sample is made of and you're able to detect even trace amounts of specific molecules like for example biomarkers that identify the building blocks of life or something that only occurs after biochemistry life existing. So this table shows what Azou found when they did this and looked for these classic biomarkers organic chemistry molecules. You can see that the total organic carbon containing molecules here, the TOC had a total dry weight in the sample of just. 1%. So in a sample taken from right here on Earth, a planet that is quite literally teeming with life, only 0. 1% of the sample taken from redstone in the Atakama desert in Chile would point towards the existence of life here on Earth. But that's just indirect evidence. What about finding direct evidence of life? Like for example, detecting DNA molecules themselves. Well, Azouas and collaborators used a dye called SYBR green which when it binds to DNA molecules glows green so that we can see it at least under a microscope anyway. Similarly, there's a technique that can do the same for bacteria itself, microorganisms, which Azoutos and collaborators use to find low levels of bacteria in the sample. Just 600,000 bacteria cells per gram of sample. That might sound like a lot, but compare that to a typical gram of soil, which would have 10 billion bacteria cells per gram, or even your own colon, which would have 100 trillion bacteria cells per gram. In comparison, 600,000 bacteria cells per gram is right on the edge of what's possible for us to detect using the best lab techniques we have available to us right here on Earth. So if that's the case, then what would the

Mars rover instruments detect in this sample from the Atakama desert? Because it's the same techniques that are available on the rovers. They also have spectrographs. this technique where you split the sample by passing it through a magnetic field and all the different molecules deflect a different amount based on their ratio of mass to their charge. So Azoutos and collaborators used the same techniques that are available say on the Curiosity and Perseverance rovers to reanalyze the redstone sample and found that while they could give the same results in terms of like the main constituents of what the rock was made of, they really struggled to identify any organic molecules in the sample. the organic molecules being the biomarkers of life that we look for to indicate like indirect evidence that life is present. You can see how in this figure the gray shaded regions show where the signatures of organics found in the lab on Earth in this sample should show up, but there's nothing there. Okay, so what else can we do? Well, Curiosity Rover also has an instrument on board that can do what's known as pyrolysis, where you heat up a substance in the absence of oxygen. It's actually what we use to turn wood into charcoal here on Earth. But in terms of chemistry, it can once again reveal like what a sample is made of. And using this technique, Azoubusttos and collaborators did manage to reveal organic molecules, but right at the detection limit of the kit they were using that's just a standard commercial off-the-shelf product. The problem is that the instrument on board Curiosity Rover has a detection limit 10 times lower than that. So, it's unlikely that Curiosity would be able to spot anything if it was there. Similarly, issa's future Rosalyn Franklin rover will have an instrument called MOMA on board which does a form of pyrolysis. But again, using this exact technique, no organics were detected. It was well below the instruments detection limit. Which just goes to show how important it is to have these tests in place, these analog tests where you can say, okay, this is what we're planning to put on a rover. Is it capable of detecting these lower limits that we've detected in an analogous environment here on Earth? which is why Azua and collaborators also tested an instrument known as solid LD chip which is currently in development by the Spanish Astrobiology Center in collaboration with NASA. It's currently undergoing a load of testing so that NASA can consider using it in future missions because it is a little bit different. It's not really like a chemistry experiment. It's more of a biology experiment. It actually has specific antibodies that are in place to detect specific antigens, as in proteins and organics, but also microorganisms themselves. And Azuas and collaborators found that with this they could find evidence for the bacteria that was present in the redstone sample, which is great, but that's not on any of the current rovers that are on Mars. Right now we just have the mass spectrometers and the pyrolysis techniques which apparently are under the detection limits at least for the redstone sample. And if you're thinking well why do we send these things? Why don't we send like the cuttingedge you know amazing laboratory techniques that we have here on Earth? But you've got to remember right they involve a human and not a robot doing things. And also if you're sending a probe up to Mars you need it to be as light as possible. So either we need to send another rover with the instruments on board we know are capable of detecting at least these low limits found in an earth sample or we need to bring rock samples back to Earth. And the good news is that there are plans to do both of those things. But the bad news is that both of those things are currently at risk thanks to politics and funding. is Exom Mars mission with the Roslin Franklin rover which was supposed to launch in 2022 on a rocket that was developed by Roscosmos the Russian space agency and the mission was supposed to be like the actual rover itself delivered from orbit around Mars to the surface of Mars by a lander developed by Roscosmos but then after Russia's invasion of Ukraine issa halted all collaborations with Roscosmos instead Airbus will now build the new lander and the plan is to launch in 2028 with NASA who will source a commercial rocket. But whether that will even go ahead given the drastic cuts to NASA's budget that have been proposed by the current US government, we don't know. What we do know is that if those proposed cuts make it all the way through Congress and get approved, one thing that is definitely going to be cut is the Mars sample return mission. the robotic mission to return these rock samples drilled by Perseverance back to Earth by the mid 2030s. Instead, this newly proposed NASA budget says the priority is sending humans to Mars and therefore humans will bring back the rock samples. But do we even want to send humans to Mars from a scientific perspective? because you risk contaminating the surface of Mars and contaminating any rock samples you collect so that you can never actually answer the question of is there life on Mars. So, if we're finally going to answer David Bowie, then either we need to send rovers to Mars with instruments that are actually capable of detecting the presence of life below these limits that have been revealed by this new study, or we're going to have to bring the rock samples back to the lab here on Earth to test them. We're just going to have to think very, very carefully about how we do that without messing everything up.

That might sound like a lot, but compare that to a typical gram of soil, which would have 10 billion bacteria cells. There is a plane going over. I will take this opportunity to have a drink of water, not get angry at the plane.