Scientists Devise New Plan to Study the Most Exciting Rock on Mars

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The most exciting rock known to science is a school-desk-sized chunk of mudstone currently stuck on Mars.

Formed from fine, water-washed sediments on the floor of a long-lost lake—some 3.5 billion years ago, when Mars was a warmer, wetter world—the rock was found in 2024 by scientists using NASA’s Perseverance rover to explore what’s now known as Jezero Crater. Dubbed Cheyava Falls, the mudstone stood out to the researchers because its surface was spangled with strange speckles and ring-shaped blobs, which they referred to as poppy seeds and leopard spots. They also discovered that it was packed with organic matter—chemical compounds of carbon, the elemental cornerstone of biology as we know it.

Organic-rich rocks right here on Earth sometimes contain similar features, which tend to be created by microbial life. And after painstaking follow-up studies with the rover, the Perseverance team announced earlier this year that ancient alien microbes might be the best explanation for the Martian rock’s spots and seeds as well.

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To know for sure whether Cheyava Falls is proof of past life on Mars—or instead just a weird quirk of lifeless organic chemistry—astrobiologists want to bring some of the rock back to Earth for closer study. But the NASA-led international program to do just that, known as Mars Sample Return (MSR), is in political limbo, beset by ballooning costs and flagging federal support. Even if MSR does go ahead as planned, Perseverance’s hard-won samples of Cheyava Falls and other Martian materials wouldn’t arrive before 2040.

Not content to sit idle, a cadre of scientists organized by NASA’s Jet Propulsion Laboratory (JPL) is pursuing an audacious plan B. Rather than wait for pieces of Cheyava Falls to reach Earth, the researchers will try growing the rock’s most mysterious features for themselves in carefully curated or manufactured mudstones. By subjecting these simulacra—some of which will bear terrestrial microbes, while others will be slow baked and sterilized—to lab-based conditions mimicking what’s known of early Mars, the team hopes to learn how Cheyava Falls really got its spots.

“Take your best guess as to what was in the mud. Take your best guess as to what the nature of the organic matter is. Stir them up together, let it all settle to the bottom, and watch what happens,” says Joel Hurowitz, a geoscientist at Stony Brook University and a member of the Perseverance science team, who is familiar with the work.

This approach won’t be able to definitively prove or disprove the existence of past life on Mars. But by mapping out every conceivable way to make the seeds and spots in a lab, scientists can determine whether it’s likelier that the features evident in Cheyava Falls were made with microbes or without them.

Cooking Up Poppy Seeds and Leopard Spots

The universe owes a lot to the behavior of electrons. Whether we are talking about the explosive deaths of stars, the formation of planets, the weather or the critters that live under it, electrons often drive the chemistry that makes things happen.

One particularly important type of chemical drama is known as an oxidation-reduction, or redox, reaction. Oxidation involves the loss of electrons, while reduction is the gain of electrons. Redox reactions happen everywhere, all the time, in all sorts of environments—and they are essential to the normal functioning of living things, allowing organisms to obtain energy, to maintain basic cellular operations and even to shield themselves from external dangers.

Nobody expected to see fossilized creatures, or even necessarily preserved microbial corpses, on the surface of Mars. But finding trace evidence of biologically driven redox reactions was far more plausible, and the Cheyava Falls outcrop—and its wider environment—is a near-perfect place to look.

“They record an ancient, habitable environment,” says Sanjeev Gupta, an Earth scientist at Imperial College London and a member of the Perseverance rover team. Within it, Perseverance picked up a bounty of organic material. It also saw tiny nodules and larger, halolike features: the poppy seeds and leopard spots, respectively. Both the poppy seeds and leopard spots are the graffiti left behind by the redoxlike shuttling about of electrons.

The poppy seeds contain a reduced form of iron, Fe(II), found in a mineral named vivianite (seen as black specks). Fe(II) is produced when preexisting Fe(III) gains an electron. Fe(III), the oxidized version of this iron, was found within the original Cheyava Falls muds.

The leopard spots also have Fe(II) in two different mineral forms: vivianite (which appears as dark rims) and greigite (which is within the spots’ interior). The spots also contain sulfides, a reduced form of preexisting sulfates that is also found in the Cheyava Falls muds; the sulfides are also part of the mineral greigite.

The seeds and spots are essentially “a fossilized chemical reaction,” Gupta says. And any experiments on Earth hoping to re-create them will take one of two possible pathways: one that deploys microbes and one that doesn’t.

First, let’s look at the nonbiological options. One way to turn Fe(III) and sulfates into Fe(II) and sulfides is to heat up the ingredients found in those muds and wait. “That’s a reaction that can happen without life. But it’s incredibly slow,” says Michael Tice, a geobiologist at Texas A&M College of Arts and Sciences and a member of the Perseverance science team. And by slow, he means potentially millions of years.

A good analogy is sugar and oxygen. The two can react to unleash bountiful chemical energy—but sustained, strong heat is what really makes that happen. Sugar doesn’t much react with oxygen just sitting on your kitchen table. Similarly, you wouldn’t get the Cheyava Falls features unless you baked the original muds at high temperatures—150 degrees Celsius or more. Yet NASA’s Perseverance rover has uncovered no evidence of such cooking for Cheyava Falls, and it seems the seeds and spots were created shortly after the mud was deposited.

Now let’s look at the microbial route. If that mudstone had instead formed from a lake bed on Earth, one would expect prevalent microbes to “consume” the organic matter and gain energy effectively from the reduction of Fe(III) and the sulfates. This would happen relatively fast because Earthly microbes deploy potent enzymes that ease the reaction’s energetic thresholds; no high-temperature cookery is required. And “it’s exactly where you’d expect microbes to be living,” Gupta says.

Whisper it: based on current evidence, it seems likelier that microbes made these seeds and spots than geological activity. But the problem is that the two chemical pathways “start with the same reactants and end up with the same products,” says Morgan Cable, a research scientist at the Laboratory Studies group at JPL and a member of the Perseverance science team. “The reaction is essentially the same. That’s where it gets tricky.”

Laboratory Alchemy

Thanks to orbital reconnaissance from spacecraft and ground truthing from rovers, we already have a pretty good idea of what Jezero Crater was like in its halcyon days eons ago. By Martian standards, it was an aquatic wonderland, with water flowing through channels to form and feed a crater lake, piling up sprawling deltas of swept-in sediments, all under a warmer, thicker carbon-dioxide-rich sky. Remarkably, scientists can re-create parts of this past realm in their state-of-the-art laboratories.

Test chambers can be kept at the right temperatures to simulate Martian conditions and can be filled with myriad mixtures of gases to reproduce atmospheric pressures and compositions that prevailed on the planet in its deep past. Synthetic mudstones custom-concocted in labs or purchased premade can incorporate various recipes informed by Perseverance’s measurements, with fluctuating amounts of oxygen, organic matter, acidity, salinity, and so on.

Over time, as these simulacra unfold under different environmental conditions, watchful scientists can see what happens—and adjust accordingly to explore the truly vast landscape of possibilities. For better or worse, “the range of experiments to engage with is endless,” Hurowitz says.

On Earth, life famously gets everywhere. Heat can ensure certain mudstones are sterilized, Cable says, similar to how water can be boiled to deactivate any microscopic bugs. But you can’t just flambé the mudstones, as that would also alter their Mars-like starting chemistry.

A gentler, tried-and-tested method of microorganismal murder is known as dry heat microbial reduction, or DHMR. “It’s how we sterilize spacecraft,” Cable says. Things wouldn’t get scorching hot with this method; instead mudstones would be gradually warmed in dry conditions for hundreds of hours. “That usually kills or deactivates most forms of life, including bacterial spores,” she says. To be safe, experimenters using this technique can continuously assay the supposedly sterile soil to make sure there aren’t any microbes left in them.

For the deliberately biological experiments, the JPL team is spoiled for choice. Microbe-mediated reaction patterns resembling the poppy seeds and leopard spots can be found all over Earth, where they are often still associated with the diverse microbial ecosystems that made them. “You’d find them in mud, underwater,” Hurowitz says—in both the present and the distant past, from freshly deposited marine sediments off the shores of Taiwan to extremely old rocks in Scotland. Whatever the terrestrial source, the team simply needs to inoculate some of its Mars-like mudstones with microbes able to rapaciously gorge themselves on Fe(III) and sulfates and spark a population boom.

“We’ll start there and see where these reactions take us,” Cable says. “We’re going to go down so many different rabbit holes.” Besides trying to summon the poppy seeds and leopard spots into existence in fresh rocks, the team also want to know how to prevent them from growing in the first place.

Reducing Fe(III) produces more energy than reducing sulfates. But if microbes were involved, they switched from Fe(III) reduction (making the mineral vivianite) to sulfate reduction (making the mineral greigite). It’s not clear why, but it’s certainly strange—scientists would expect hungry bugs to prefer more energy-rich fare, so why would they leave Fe(III) “candy” untouched to munch on sulfate “broccoli” instead? These experiments could offer answers and put constraints on the types of microbes—and ancient chemical concoctions—that may have been present on Mars 3.5 billion years ago.

The Perseverance scientists expect that their work will eventually produce poppy seeds and leopard spots both with and without life’s help. But the environmental conditions leading to both will likely be radically different. Then the researchers can return their focus to Perseverance—still scooting around Jezero—to try sniffing out other rocks nearby that are closer geochemical matches to any mudstones they’ve coaxed into sprouting the telltale speckles, whether with biology—or without.

Ideally, the rover will encounter another exciting site—and uncover additional tantalizing hints of ancient Martian life. “You don’t just want one line of evidence. You want something completely independent of it pointing in the same direction,” Tice says.

But Cheyava Falls on its own is already a thrill to the Perseverance science team. Finding it was the easy part. “Now the hard work begins,” Hurowitz says.