A biosignature on Mars? Unpacking Perseverance's Cheyava Falls find

On July 21, 2024, NASA’s Perseverance rover examined a rock partially buried in a dry riverbed on Mars. The rock, named “Cheyava Falls,” was within a formation named “Bright Angel.” This particular rock showed signs that water had once flowed over it, and had signs of organic molecules, the building blocks of life. It also had a smattering of dark and bright spots along its surface — patterns that, on Earth, are mainly known to form through the living processes of microbes or through chemical reactions that can provide fuel for life. Perseverance collected a sample, which has been named “Sapphire Canyon.”

On Sept. 10, 2025, Perseverance scientists published a peer-reviewed paper in the scientific journal Nature, stating that they had found no particularly strong evidence for any of the abiotic (non-biological) ways these patterns could have formed on the Martian rocks. 

While this doesn’t mean that the only explanation is alien microbes, this announcement marks a major step forward in the search for life beyond Earth. 

Joel Hurowitz of Stony Brook University, New York, is one of the scientists working on the Perseverance mission, and is the lead author of the paper about this discovery. He joined Planetary Radio host Sarah Al-Ahmed for a discussion of the findings, which aired on Oct. 1, 2025. 

The transcript below has been edited for length and clarity. Listen to the full episode here.

Sarah Al-Ahmed: What has happened in the last year, and more recently, that's brought this story back into the press?

Joel Hurowitz: We were getting our first observations of the Bright Angel formation that the Cheyava Falls rock is a part of in the month leading up to that [first announcement]. We kind of knew right away that there was something really interesting and exciting going on in these rocks. 

And then in the interim time since then, we spent, I want to say, another month or two roving around inside of the Neretva Vallis channel, investigating more of the rock outcrops that the Cheyava Falls rock was a part of, really trying to firm up our understanding of the environment that those rocks were deposited in. And around maybe September of 2024 was when we really started interpreting the data that we were collecting. And that went on for probably two months or so. It was probably the fastest I've ever turned a paper out into something that was ready for submission.

We submitted the paper in November after a bunch of team internal discussions and reviews, making sure that everybody was on board with the interpretations. Then, between November and now, was the review process. We went through two rounds with our peer reviewers, and then the rest of that time was just getting everything lined up and scheduled at the journal for publication. So basically, we were just working and responding to reviews during that entire time period.

Sarah Al-Ahmed: It's a huge claim to say there are potential biosignatures in a rock on Mars. But now that it's been reviewed so thoroughly, we can confidently say not that this is actually evidence of life on Mars, but that we found something really interesting here that really makes a case for bringing these samples back.

Joel Hurowitz: Yeah, that's absolutely right. The observations that we made with the full instrument suite on board the rover are everything that this payload was designed to be able to do. 

The observations that we've collected extend from the outcrop scale images to the subsurface ground penetrating radar data, and that really lets us build a picture of the environment at the time that the rocks were being formed — all the way down to the micro scale of these little nodules and reaction fronts and their interesting mineral enrichments in organic matter that could plausibly be explained by the activity of microorganisms that do similar things on Earth, but that might also have other non-biological explanations. 

But I think that's about as far as anybody can be expected to go with a rover 200 million miles away from the Earth. The key next step is to bring this sample that we collected back to Earth. 

This paper and the analyses we did raise a large number of open questions. There's some amount of work that people are going to be able to do here on Earth without the sample in laboratories. They can go out into the field and look for analogous settings on Earth where suites of minerals and chemical reactions are taking place, similar to what we observed in the Bright Angel formation.

But my suspicion is that we'll get to a place where there's a community of people who have said, "I've figured out a way to do this without biology," and a community of people who have said, "Well, yeah, but I've also found ways to do it with biology." Hopefully, they will develop the tests that we would want to run on this sample to determine which of those two is the right option. But that means the sample's got to come back.

Sarah Al-Ahmed: Perseverance landed back in Jezero Crater in 2021, and since then, it's been exploring around that river delta and then making its way up to the rim of the crater. Where is this Bright Angel formation in the context of those travels?

Joel Hurowitz: The Bright Angel formation is right on the off-ramp out of the crater. So we landed on the crater floor and investigated the rocks there, and they turned out to be a suite of igneous rocks. There are a bunch of lava flows, and then potentially some rocks that represent magmatic intrusions into the crater floor itself. And then we drove off that crater floor unit up into the delta. So we investigated the front of the delta, and then the top of the delta, and confirmed that indeed it was a delta, formed in the way that we expect it to have formed via similar processes that we observe on Earth. It's a river flowing into a lake and depositing sediments into that lake.

And then we went into this unit called the Margin Unit, which is kind of this strip of rock that sits between the delta and the rim of the crater. The Margin Unit has been kind of a puzzle for us, and still is, despite all our capabilities. We have a variety of opinions on the team about exactly what that unit is and how it was formed. But it's geochemically and mineralogically really exciting. It looks like it's a bunch of igneous minerals that have been converted into carbonate minerals and silica. So there's a whole history of water-rock interaction recorded in those materials, and whatever they turn out to be, we'll figure it out when we get those samples back to Earth.

But just before we drove off the Margin Unit and out onto the crater rim, we had been planning to drive down into this river channel that cuts through the Margin Unit and was basically the feeder system for the sediments that formed the delta. So we drove down into that river channel, and in the walls of that river channel, that's where the Bright Angel formation is located, and that's where all of these findings come from. It wasn't happenstance that we ended up in that river channel, but boy, did we ever have some good fortune because they turned out to be super exciting rocks.

Sarah Al-Ahmed: What makes the mudstone, the conglomerates, and this Bright Angel formation stand out compared to the other layers in Jezero Crater?

Joel Hurowitz: There are a bunch of things. Looking down at the planet from orbit, they kind of stand out as being light-toned and layered-looking rocks that you can see exposed in the walls of the river channel. But when you see them up close, they are the finest-grained sediment that we've really seen on the mission. There are some similar-ish rocks down in the delta front, but they have a very different chemical character to them, and they may or may not be quite as fine-grained as these rocks.

For me personally (and I'm not a sedimentologist, so maybe a sedimentologist wouldn't be surprised by this), I wasn't expecting to find the finest-grained mudstones that we'd see in a river channel. Usually, when you think of a river channel, you think water's moving by really quickly, you're going to have lots of coarse sediment in there, and the mud will bypass that part and get deposited way out in the lake. But somehow the mud's settled inside of this river channel by a process that may have included the river channel actually getting dammed up at one point and kind of backing up behind that dam, perhaps by a landslide or something like that. There's a paper that's actually submitted to a journal that suggests that's the right way to interpret why this river channel got filled up with mud.

And then not only are they super fine-grained, but chemically they're incredibly distinctive from any other sediments that we saw lower down in the delta. They're really oxidized, meaning they're really rusty. And they've been chemically leached of a number of elements, like their magnesium and calcium have all been removed from the rock. That's the kind of thing that happens when you have rock exposed somewhere outside of the crater that's being chemically weathered. It may have been leached by rainfall pumping through it, and then that material gets flushed down into the river system and deposited as these muds.

We didn't see anything like that earlier down in the delta that was either that leached or quite that oxidized. I think what's cool about that is that it tells us that, at some point in the history of this river lake system, the environment was not really doing much chemical leaching. It wasn't very oxidizing. And then the climate and atmosphere changed in a way that provided a new type of sediment where the material was being oxidized, it was being chemically leached. I think it's giving us a sense that the sort of climate of the environment around Jezero Crater was dynamic and changing rapidly in time.

Sarah Al-Ahmed: Which gives us a good way of learning more about the history of that area. And Perseverance's SHERLOC instrument picked up evidence of complex organic carbon in this mudstone as well. It did it by specifically looking at this kind of G-band signal, a spectroscopic peak that's kind of a fingerprint of more aromatic carbons or graphitic stuff. Can you explain what that signal revealed to us about the types of organics that are present in these rocks?

Joel Hurowitz: What this means is that we can identify that these rocks have a complex macromolecular organic carbon in them. That type of carbon can have a variety of sources. The Nature paper is probably the first wave of information that's going to come out about these rocks. There is another paper that is going to follow the Nature paper that describes in much more detail exactly what we can say about that organic matter.

But it's the kind of high molecular weight carbon compound that can form by a variety of processes. You can find it in meteorites, and you can find it being synthesized in hydrothermal systems as a result of high-temperature water-rock reactions. You can also find it as the degradation product of biologically sourced carbon. So there are a bunch of ways that you can get carbon like that into these rocks. And again, this is one of those questions where to really determine what the origin of the carbon in these rocks is, we're going to need laboratory analyses back here on Earth.

Sarah Al-Ahmed: We've found these organics in some places [in Jezero Crater], like Cheyava Falls or Apollo Temple, but not in other places, like Masonic Temple. Why is that the case? Because it's not surprising necessarily that there are complex organics, but it's surprising to me that there's such a different kind of population of them depending on where you are within Jezero Crater.

Joel Hurowitz: A couple of things kind of stand out to me in reflecting on that question. One, why is it that the most oxidized sedimentary rocks that we've come across are the ones that have organic matter in them? That's kind of interesting, and maybe not what you would've expected, just kind of knowing that oxygen and oxidants and organic matter don't really like each other very much. But there was something about the environment at that time that favored the accumulation of organic matter in that part of the lake when Bright Angel was forming. Maybe it's just because these are muds, and muds are really good at preserving whatever organic matter is raining out of the water column along with the mud and protecting it. That would make some sense.

But the other thing to your question is, why is it present in some types of rocks in the Bright Angel formation and not in others? My gut sense on this one is that when the mudstones were being accumulated in the sort of north side of the Neretva Vallis channel, those muds were slowly accumulating as mud settled out of the water column to the lake bed. And under those conditions, whatever organic matter was in the water column was also settling down onto the lake bed with the mud.

In the other places where we didn't see the organic matter, like over in Masonic Temple, those are conglomerates. And so they're deposited really quickly, probably because of debris flows or things like that, maybe coming in off the crater walls. And so maybe there just wasn't time for the organic matter to accumulate in the same way because those sediments came in as a pulse rather than through gradual accumulation. So that's my guess as to why those differences are there.

Sarah Al-Ahmed: As we're looking at this rock, there are two very distinctive features: poppy seeds and leopard spots, as they're called. So let's start with the poppy seeds. What are these things in the rock?

Joel Hurowitz: They are 100 to 200-micron-diameter mineral accumulations containing both iron and phosphorus. And based on their chemical properties, their color properties, and some of the elements that they don't contain, like aluminum, we think that they represent little nodules of a mineral called vivianite and some water molecules. And there's a good chance that they're not pure vivianite anymore, because on Earth, if you expose vivianite to air or any oxidants, it starts to change its mineralogy to something a little more oxidized. Our guess is that it started out life as vivianite, and then as it's been exposed to the environment on Mars, it's probably changed its character to something a little bit more oxidized than the original vivianite that was there.

Sarah Al-Ahmed: So the host rock is mostly made out of this oxidized rusty iron. And then you have these poppy seed nodules that are made out of this iron mineral called vivianite. Why is finding this kind of reduced iron phosphate inside an oxidized rock important?

Joel Hurowitz: It is providing evidence that a redox reaction took place. There's an electron transfer process that took the iron in the mud, this ferric iron, iron 3+, and turned it into iron 2+ via reduction. And the partner, the thing that actually donated those electrons, well, we think it's the organic matter in that rock. So there's a ferric iron in the mud and organic matter in the mud. And as those two things settled out on the lake bed, the organic matter and the ferric iron reacted with one another to produce iron 2+. It reduced the iron in the mud to this other form of iron that could then combine with phosphorus to precipitate vivianite.

And that reaction between ferric iron in the mud and organic matter in the mud, this is one of these things that has a potentially biological origin. Because when we see those two ingredients being deposited in muds around the world today in marine settings, lake settings, and estuaries, there's a population of microbes that are basically eating that organic matter and facilitating that redox reaction that ends up making vivianite as a byproduct.

Sarah Al-Ahmed: So those are the poppy seeds. And then we have these leopard spots. What makes them so visually and chemically distinct from, say, the poppy seeds?

Joel Hurowitz: They're bigger, more like a millimeter or two in diameter, and they have a dark rim and a bleached, white-toned interior, so they kind of stand out against the reddish-colored mud. The rim on the leopard spots is more of that vivianite material, so it's the iron phosphate that makes up the rim.

And then inside of the leopard spot, the reason we think that it has this sort of bleached color to it is because the rusty red iron has been removed from the core of the leopard spot and exported into the new mineral phases, the vivianite. And then in the middle of the leopard spots, there's another new mineral, this greigite that is an iron sulfide mineral. So there's almost like a stratigraphy from the inside to the outside of the leopard spot of different types of minerals.

Sarah Al-Ahmed: How do we know that these spots represent reaction fronts that formed in place rather than something that was deposited around it?

Joel Hurowitz: Yeah, it's a good question. We thought this through some. And what we concluded was that if you had 100 or 200-micron-diameter vivianite grains rimmed in iron phosphate and at their core had iron sulfide in them, they would have a different density than the surrounding sediment that they're deposited in. But it doesn't look like they were deposited and separated by density differences. And honestly, the reaction fronts, the leopard spots, there's no way those things are grains. They just look like they formed right there. I would have a hard time making those into grains just based on their visual appearance.

Sarah Al-Ahmed: So how is your team interpreting the sequence of redox reactions that transformed all this ferric iron into these reduced minerals?

Joel Hurowitz: The way we imagine this happening is that you had little bits of organic matter in the mud, and by whatever process, they were reacting with one another and with the mud in a way that would consume the organic matter and make new minerals. And in the places where the poppy seeds form, maybe it just ran out of organic matter in that little local environment. There just wasn't enough for it to continue reacting.

Whereas for the leopard spots, perhaps there was, I don't know, I almost want to call it a bigger chunk of organic matter. So, as you were reacting that organic matter with the surrounding mud, the vivianite-producing front continued to migrate outward away from that organic matter. And at some point, you run out of ferric iron to react with. And so the next thing that might be available to react with would be sulfate that is maybe dissolved in the mud-water slurry at the bottom of the lake. And so the organic matter then might start reacting with the sulfate to make reduced sulfur that can then combine with ferrous iron to make greigitel. So it's kind of almost like a ladder of redox reactions taking place.

Sarah Al-Ahmed: The paper describes this kind of inverse relationship between the abundance of reduced minerals and how red the surrounding rock is. Why is that so important?

Joel Hurowitz: I think what it tells us is that these leaching reactions that were taking this initially rusty red mud and turning it into new mineral phases, they went as far as they could based on how much organic matter was there to begin with. So in cases like Apollo Temple, which is the abrasion patch right next to Cheyava Falls, there was enough organic matter there that these redox reactions were actually able to almost completely bleach the rock of its ferric iron. And in other places, like at one of the other targets, Walhalla Glades, the bleaching wasn't quite as extensive. So the rock still has a kind of tan color to it. And maybe what that means is there just wasn't as much organic matter available there to complete that bleaching process. And that bleaching process is a direct result of these redox reactions that make these new mineral phases.

Sarah Al-Ahmed: So we've been talking about a lot of rock reactions, what this has to do with organic chemistry, but the real headline here is that all these might be potential biomarkers. And from a geochemical standpoint, how plausible is it that this vivianite and greigite could have formed at these low temperatures without any kind of biological process?

Joel Hurowitz: I break it out in terms of the two mineral phases. I think it's easier to make the vivianite in the absence of biology. I have not seen the laboratory experiment where somebody went in and took macromolecular organic carbon and ferric iron-rich mud and incubated them together and turned it into vivianite. We went through the literature to try to find examples that might say that this can be done completely in the absence of biology. We didn't find that experiment. It doesn't mean that someone won't, in a week, go off and do that experiment and publish that finding. And it's well known that ferric iron and organic matter are quite reactive toward one another. And so, the redox reactions that make vivianite, it feels plausible to me that someone will figure out that that can be done completely abiotically.

The production of the iron sulfide mineral, the greigite, is a tougher ask. And the reason is that the reaction between sulfate and organic matter at room temperature conditions is incredibly slow. So slow that we don't actually observe it happening in natural environments on Earth. Where we do see it happening is where biology is involved. Sulfate reduction is a metabolism that microbes will make use of to generate energy. The other place that this can happen is if you take sulfate and organic matter, and you cook them together at temperatures above about 120 to 140 degrees centigrade. Then you can overcome that kinetic inhibition that exists at lower temperatures, and you can drive the production of reduced sulfur-bearing minerals.

Now, two things on that. One is, again, I don't know if the experiment is out there where somebody tried to replicate the initial state of the Bright Angel system and then heated it up to make these minerals, but someone needs to go out and do that experiment. I think it's really important. The other aspect of this, of course, is that we spent a lot of time in Neretva Vallis and in our paper trying to figure out whether we could find any evidence that the rocks had been heated. And within the limits of our payload's capability, it's just not obvious to us that it has been. So to us, the sulfate reduction reaction seemed more plausible at a low temperature. So I think that's probably the one that has a less obvious non-biological origin. Again, unless it turns out that we just can't tell when rocks have been heated to 120 degrees with a rover payload, in which case that will have been the answer. We weren't able to detect that subtle temperature difference.

Sarah Al-Ahmed: Your co-authors also do stress that even though there could be biological reasons for this, there could be abiological reasons for this. And there are alternatives to this being life on another world. But just on the face of it, it's hard to explain.

Joel Hurowitz: Yep.

Sarah Al-Ahmed: So I get why people are so excited. But we do have to be very mindful when we're making claims like we found life on another world. I know people want to jump to that. And we all, as scientists, want to get to that answer, but we have to take this step by step. And NASA and the astrobiology community often refer to this confidence of life detection scale, and that's part of how we assess these discoveries. Can you talk a little bit about that scale, and where you would place this Sapphire Canyon sample on that scale today?

Joel Hurowitz: Another one of the educational things for me is learning about the CoLD scale, the confidence of life detection scale, as I've sort of grown into this role. I'm not an astrobiologist, but I play one on TV.

So I think that it sits at step three on the CoLD scale, which is interesting. Step one is an interesting, potentially biological signal. Step two is that you have ruled out contamination as a source of that signal — that the organic matter in those rocks is not just a surface phenomenon that shed off the rover. [In the Sapphire Canyon sample,] it's present in the abraded patches and its distribution in the rocks makes sense from a paleoenvironmental perspective. I don't know how the poppy seeds and leopard spots would be contamination, and I don't think the organic matter is either. So that takes us past step two.

And then step three, at least in my interpretation, is that the signal is coming from an environment that we know is a plausible host for biological processes. So it's sort of building that geological context, and I think we've established that this is a plausibly habitable environment that could have had biology in it and preserved signatures of that biology.

Step four is a really big step. It's that you've ruled out all known non-biological processes that could make that signal. I feel like we're on step three and we started lifting our foot off of step three and are trying to put it down on step four, but we're not there yet. And I think getting up to step four is going to be the work that follows this paper, the work that people do in labs, the work that people do on Earth here, and then ultimately the work people will do on the sample if we ever get it back.

Sarah Al-Ahmed: We'll call it step 3.5. But really, there's so much work that needs to be done in order to figure this out. We need to figure out if there are abiological processes for creating these rocks. But more profoundly, I think we just need to get those rocks down here to Earth so we can do some testing on them. What kind of analysis on these rocks would you personally be most interested in seeing happen?

Joel Hurowitz: There'd be a couple of things that I would be most excited about. One class of analyses would be isotopic analyses. I would love to see paired analysis of the iron isotopic composition of the mud and the iron in the greigite and the vivianite because those things are related to one another by a redox reaction. And the magnitude of the difference between them and the sign of the difference in their isotopic compositions could be very telling in terms of whether or not biology was involved.

This is a tool that we use in trying to understand whether the oldest rocks on Earth have been formed as a result of biological processes. We always go to isotopes. And the fact that we have the mineral pairs all in the same rock, that's exactly what you want. We can do the same on the sulfur between the greigite and the surrounding sulfate-bearing mudstone. And then there's carbon in multiple redox states. There's the organic carbon, and then there's a little bit of carbonate in the rock. So we can throw three isotope systems at the problem to try to see whether they're telling an internally self-consistent story about differences in isotopic composition that are offset in ways that we know biology does on Earth.

And then of course, there's all the things that an organic geochemist would do. What is the organic matter? Are there little lipids preserved in the rock? I can imagine all kinds of microscopy and things that would be done on these rocks to try to figure out what other potential biosignatures might be contained in them. But that's kind of not my area of expertise. 

Sarah Al-Ahmed: I really hope that this Mars Sample Return mission happens and that we get these samples back here. Even if it's not just this sample, the entire history of everything we've picked up on Mars with this rover tells such a compelling story about how this world has evolved and what it might have been like in the past. It just kills me that those samples are just sitting there on Mars waiting for us.

Joel Hurowitz: And we're still collecting. We're not done yet. We've still got six tubes left to fill, and there are more exciting things left to collect before we finish that job.

Sarah Al-Ahmed: But in the meantime, this is one of the most compelling bits of evidence of potential biosignatures on another world that we've detected so far. So how do you balance the public's understandable excitement about a story like this with the true story of this, which is that it's really complicated and we're still trying to figure it out.

Joel Hurowitz: I think all we can do as scientists is convey how excited we are about this and the potential of this discovery, while also conveying that there is uncertainty here, and that there are steps that need to be taken to reduce that uncertainty. So I think we have to be really careful not to say anything like this is a slam dunk, that we've discovered life on Mars. That's not what we're saying.

But we're saying we've discovered something really exciting that, with additional work, might tell us whether or not Mars was ever inhabited. And maybe that kind of speaks to a bigger picture question: Wouldn't you love to know the answer to whether or not there was ever life on a planet other than Earth? In that little tube, the answer might be there. So it's a tricky balancing act. But hopefully, if nothing else, folks get the sense of our excitement, and maybe that rubs off in a way where they think, “Wow, there's something really cool going on here that we want to know more about.”

Sarah Al-Ahmed: Anytime we're doing our space advocacy work in Washington, D.C., and walking around with Bill Nye, he has a 3D-printed sample container from Perseverance that he keeps in his pocket at any given moment. Because it's things like that. It's physically holding it in your hands, and imagining a world where that's in our science labs and we're testing those rock cores. I feel like that's one of the most compelling things I've ever seen when we're talking about why we love this kind of science so much and why it deserves so much attention. So I just want to send a thank you to you and everybody else who has done such thoughtful work on this because you're not jumping to conclusions, but you're also giving us hope that we'll be able to answer one of the greatest questions humanity has ever posed, maybe in our lifetimes. And that is super exciting.