NASA scientists working with the Curiosity rover say they've found hints of remnants of organic compounds on Mars, dating back to when the planet was probably a whole lot more hospitable.
Specifically, the Curiosity rover's Sample Analysis at Mars (SAM) instrument, a tool that vaporizes samples to determine what they're made of, picked up signs of organic salts, which can be thought of as the broken-down remains of ancient organic compounds, according to research published in the Journal of Geophysical Research: Planets back in March.
The presence of organic compounds doesn't necessarily mean that there was once life on the Red Planet, but it does help scientists determine whether Mars ever had the right conditions for sustaining life — and now NASA needs to get to work verifying these salts with more samples from Curiosity, Perseverance, and Europe's upcoming ExoMars rover mission to piece it all together.
Futurism caught up with the lead author of that study, James Lewis, a researcher at both Howard University and the NASA Goddard Flight Center, to learn more about what the study means, what it will take to verify these ancient organics, and what it all means for the possibility of life on Mars. Here's our conversation, edited for length and clarity.
Futurism: Before we get into anything specific, tell me a little bit more about your background. What kinds of questions do you like to answer?
James Lewis: I'm a geologist by training but I'm dabbling a little bit of biology now and then. My broad interest is in the intersection between mineralogy and organic chemistry. If life had left behind organic molecules, how would they be preserved in different minerals? And then when we're trying to analyze those organic molecules, how will the different minerals that they're preserved in impact our ability to analyze them?
So the two sides I'm interested in, one is looking at environments that might preserve evidence of life, and the other is how do we actually go about analyzing them? When we're doing these investigations on another planet, we're severely limited in the kind of techniques we can fly out there, so we can also do analog studies on Earth.
The big news is that Curiosity found signs of what might be organic salts on the surface of Mars. You were just talking about what it's like trying to piece together whether organic compounds existed a very long time ago — is the idea that these salts are kind of like their fossils?
We were thinking about what would be the best analogy to use. There's a danger because a fossil was explicitly life and with organic salts, in a way, life would be the exceptional explanation for why they're there.
The more likely explanation is that they're purely geological in origin. So we try to steer clear of "fossil" because I think as soon as a non-scientist hears fossil, they think "Oh, we found life on Mars." NASA was constantly saying to me "Make sure you don't say that life has been found on Mars."
They're like the last residue of organics that might be left behind. I was looking at the things that might be left behind in areas where organics were being broken apart and destroyed. The analogy we came up with was pieces of broken pottery that an archeologist might be looking at. It's like we're seeing a fragment of something and piecing together the story of where they came from. It could be super interesting but also very difficult.
Can you talk a little bit about what Curiosity actually seems to have found? What are we talking about when we say "salt?"
The salt most people think of is sodium chloride. And with an organic salt, instead of the chlorine atom, you have some sort of organic iron there. So the things I'm interested in are things like acetates. These can originate from the molecules that make up a lot of living things. When you expose them to radiation or oxidation, they eventually break down into these organic salts. And I was interested in two kinds of organic salts, specifically: acetates and oxalates. To me, they're interesting because on Earth we see them in a lot of different geological environments. And the oxalates, particularly, are very resistant to weathering. The only thing that will move them around is a highly acidic fluid.
The instrument I work with is called SAM, and the way that SAM works is it will take the scooped or drilled piece of Mars, ingest it into the instrument, then heat it up. And the way we heat it up is we basically start it at low temperatures and then progressively heat it up to high temperatures and throughout the heating, we're monitoring and identifying the gases that come off with something called a mass spectrometer. So the different chemicals we're seeing and the temperatures they come off at can be pretty diagnostic for a bunch of things. It can help us work out what minerals might be there. But it is also a way that we look for organic matter as well. One thing we see all the time in SAM is carbon dioxide and carbon monoxide. There are a number of different things that could be producing these, but I was interested in looking at whether organic salts of things like oxalates and acetates could be contributing to these. So I wanted to go into the lab, do the closest equivalent to a SAM experiment that we could do, and then look at how CO2 and CO and a few other gases are evolving and see how well it fits with SAM data. What we found is that a lot of the different mixtures we made matched extremely well with what we saw in SAM.
What we're saying in our paper is we think that these organic salts are a really good explanation for a lot of the SAM data we see because they only produce very simple gases that could be coming from other places. We can't 100 percent say that we're seeing these things, but there's a lot of compelling evidence to say that they could be there.
So the idea is that you're back on Earth, recreating the experiment so that you know what compounds are actually going into the instrument. And comparing those readings to the ones from Curiosity where you don't technically know what went into SAM, seeing those similarities in the readout, helps you piece together what's on Mars and whether it's organic, right?
That's correct. One of those inorganic phases could be things like carbonates, like limestone that's made of calcium carbonate. But they tend to break down at slightly higher temperatures than where we see a lot of our CO2 and CO. So organic salts are really good for explaining any CO2 or CO we see at low temperatures, between like 100 to 500 Celsius or so. So there's always a case of "It could be this or it could be that," but we're saying "These seem to be a very promising fit with a lot of our data."
There is another instrument on Curiosity that supposedly would be able to conduct a more direct analysis and verify the SAM data. But it hasn't yet. Right?
That's correct. Curiosity is a robot geologist that has managed to bring the entire laboratory along with it. So whenever it sees something of interest, it can analyze it in a bunch of different ways. So SAM is looking at the inorganic and organic chemistry of the sample and looking at very small amounts of gases coming of. The other instrument, which is called CheMin or Chemistry and Mineralogy, basically will fire X-rays at the sample. And the way those X-rays diffract through your sample tells you what minerals are inside. For a geologist, that's very useful because the minerals we see tell us what environment deposited those rocks. So we're particularly interested in any environment that may have had liquid water. And we found early on in the mission that there was ancient liquid water that would have been habitable for life as we know it. But CheMin is good at spotting things that make up one or two percent of the rock or greater. With anything below those concentrations, it's really going to struggle to identify them because when the X-rays go through the sample, they produce an X-ray pattern. If you say "Oh, I know what the pattern for this particular mineral is and this mineral, and then if I put them together I get the same pattern as what you see on Mars," well, obviously the samples are very complex. And once you start going down to only one or two percent of the sample or lower, it becomes harder and harder to pull out individual minerals from that data.
So one of the things we did in our paper was say "This is how much carbon dioxide we see being produced by our organic salt samples [on Earth] and this is how much CO2 we see in SAM experiments." And that allows us to kind of set an upper limit for how much organic salt might be in the sample.
What we're saying is SAM can highlight samples where we should really look at the CheMin data in great detail. But we also need to be really lucky to see an organic salt because we probably need one type of organic salt to be contributing the majority of that CO2. So that would mean one species of organic salt making up the bulk of the CO2 contribution.
So it's not that the other instrument implied that the SAM data was wrong by any means. It just hasn't found it yet.
The thing to remember is that they have very different missions and detection limits. SAM is trying to understand inorganic and organic chemistry and it is interested in things that make up thousandths of a percent [of the sample] or something, whereas CheMin is interested in the things that are present about one percent or greater. So SAM can see hints of things that could be there at very low concentrations that would be below the CheMin limit.
If and when the ExoMars mission lands, it will have another instrument on it that's capable of digging into and analyzing soil. Is that what it would take to get a more definitive answer? And if it's not that, is there something else that would lead to a more confident confirmation of these salts?
You would hope that when we drill deep that we wouldn't see organic salts because organic salts tell you "Oh, there may have been more complex organic matter, but it's subsequently been destroyed."
If we drill down one or two meters and we're still seeing them and nothing else, we'll be like, "Oh dear, that seems to be like an environment that is very destructive to organics." Part of the reason we wanted to put this paper out is this is a super exciting time for Mars exploration because you have the European and Russian mission ExoMars that also has a mass spectrometer that Goddard built onboard. And there's a bunch of different instruments that could potentially look for organics. The 2020 rover, Perseverance, can also do spectroscopy.
But the other thing Perseverance is doing is collecting samples and caching them. And then we're working to then do sample return. We're going to launch those samples off the surface and bring them back to Earth and analyze them in our labs. Because I mentioned whenever we try and do these analyses on another planet, we're very constrained in terms of power and space. So we can bring the samples back to us as things like OSIRIS-REx and some of the Japanese missions have done from asteroids, we can do our full suite of laboratory analyses with significantly fewer constraints. So if we don't manage to conclusively detect organic salts with a surface mission, I feel when we return samples to Earth, that's going to be probably our best chance of detecting them.
The big question is whether Mars once hosted microbial life in ancient history or, in a less-likely scenario, still has life on it today. Would confirming your latest findings have any implications for ancient extraterrestrial life?
If we see organic salts it's like flashing a signal that says, "There could have been a process in the past that concentrated organic matter in this particular location." So that's kind of the limit of what we really want to say, because there are both biological and non-biological processes that can concentrate organic matter. The issue we have to deal with on Mars is we think it had a thick atmosphere in its ancient past and that there was liquid water on the surface, but subsequently, Mars lost much of its atmosphere and it no longer has a strong magnetic field.
Looking for organics from that period could be interesting because if Mars is like Earth, there's been time to evolve life. Could there be life at this transition period? It's something we're going to be delving into. If we saw an organic salt, we'd be wondering what processes during this transitional period were potentially concentrating organic matter.
Is there a step today that you, or NASA, or other researchers could take to determine whether it was a biological or not biological process that led to these compounds? Or is that an entirely different endeavor once you figure out that they existed?
I think because that would be such an extraordinary claim, we'd need multiple lines of evidence to support saying it was biological. An organic salt on its own is only evidence of some sort of concentrating mechanism. What we would be looking for with SAM is whether there are other organic molecules.
So SAM, the basic experiment it does, is to heat up samples and send the gases to a mass spectrometer. But it's actually a pretty comprehensive laboratory and if it thinks there's a chance of interesting organics. It has two wet chemistry reagents that will make molecules that might be interesting in terms of life more amenable to our analyses. One of the issues is the molecules that life uses tend to be big and quite complex, so it's hard for our instruments to work with them. If we react to them with these wet chemicals, it basically makes them easier to analyze. So if we encounter an environment that we think could be preserving organic matter, that's where we break out these wet chemicals to try and get a more comprehensive organic picture.
The other interesting thing about organic salts is that life on Earth can use them for energy and carbon. So if we see an environment billions of years ago that had concentrations of oxalates or acetates, that would be interesting in terms of habitability. We know from our earlier analyses that Mars had water that was habitable for life. We know that organic matter does seem to be preserved on the surface and that could be a resource for life. If we're also seeing organic salts, that's another carbon and energy resource that life could be using. So we'd be increasingly building towards a picture that says ancient Mars had most of the resources that life would need to survive. Our real challenge now is trying to find if there is any evidence that life ever existed.
Well, what do you think about the prospects of life on Mars? Did this study alter your opinion in any direction?
I'm very cautious about linking research like this to life, and I know NASA and [the Jet Propulsion Laboratory] feel the same way. My paper is an organic matter story and not a life story. It's a step towards a better understanding of the Martian organic record but we are very far away from understanding what that record tells us about life.
This was a very sciencey, sitting on the fence answer. Sorry!
This paper came out in March. Has anything progressed at all since then? Are there updates you can disclose or is that paper the state of things as they are right now?
What's been challenging is that the pandemic has seriously impacted our ability to do lab work. Now that everyone is getting vaccines, we're slowly going back to working on-site. What hasn't stopped in this process is the rover. I actually help out a little bit with rover operations and it's been amazing to watch because this is a team of hundreds of people working remotely, still managing to operate a rover on another planet. So we're still acquiring samples, investigating different regions. It's been really interesting to see that. And the point of my paper is saying if SAM detects a really strong CO2 signal, that's sending a flag to CheMin to take a very close look for organic salts in their data. Every time we drill a sample, that's something we're looking out for.
What I'm currently doing is working with an intern to look at how the detectability of organic salts varies in different mineral mixtures. I mentioned the way that CheMin works is that it fires X-rays through samples and looks at the X-ray pattern that comes out from that. And the different minerals you have in there can have an impact on the interpretations. So when we look for organic salt signals, there could be a mineral that also produces a peak in that region that would complicate things. We're figuring out whether there are mixtures that would seriously complicate CheMin's efforts to look for organic salts or mixtures that would really help it. It seems like the X-ray techniques that CheMin uses are probably our best opportunity to detect organic salts in the near future. Refining our understanding of how organic salts might behave in CheMin could be really valuable.
Is that because other kinds of spectroscopy research would still produce the same indirect findings rather than a more direct confirmation?
Spectroscopy has similar issues to the X-ray methods where you would need a decent amount of organic salts to be sure. And because the inorganic chemistry of Mars is so complex, I mean, you have a lot of dust around as well. I imagine, trying to pick out organic salt signals from such a complex mess can be particularly difficult.
Before I let you go, is there anything else that you wanted to add?
One thing from the paper we didn't really touch on: I mentioned that with spectroscopy there might be inorganic phases on Mars that interfere with our ability to detect organics. We actually have similar issues in SAM because when we're heating up the samples, any phase that releases gases like oxygen or chlorine can be very problematic for organic detection because those gases are going to react with organics of interest. Because Mars likes to challenge us, our recent rover missions are suggesting to us that there's a salt called perchlorate that is pretty widespread on the surface. And perchlorate releases both oxygen and chlorine at relatively low temperatures. So one of the things we're trying to resolve in SAM data is determining how organics of interest would behave when you have them mixed with these perchlorate salts.
That's one of the things we did in this paper is to say "All right, if you have an organic salt and you mix it with perchlorate, how does that change how it behaves during a SAM analysis?" Now, what's interesting is we found actually when we mixed organic salts with the perchlorates, we actually got better fits for SAM data than when the perchlorates were absent. So in a way, it's pretty cool because we know that perchlorate is problematic for organic detection, but they're not for the organic salt story specifically. [The data is] not saying organic salts aren't there, it's saying these mixtures of organic salts and perchlorates are actually pretty compelling explanations for a lot of the data we're seeing with SAM.
Editor's Note 5/28/2021: An earlier version of this story mistranscribed the word "oxalates" as "oxides." It has been updated.
More on the Martian salts: NASA Rover Finds Likely Remnants of Organic Compounds on Mars