What mud cracks on Mars tell us about whether life could have formed on the planet

Multi-billion-year-old mud cracks on the surface of Mars are helping researchers piece together the ancient history of our planetary neighbor’s climate.

Observed by NASA’s Curiosity rover in 2021, the hexagonal cracks are the subject of new research published in the journal Nature that lays out how they came to be and how they connect to Mars’ — and possibly Earth’s — early history.

These cracks emerged on a stretch of the Martian surface that researchers believe experienced a regular — potentially even seasonal — cycle of saturation and dryness. That’s important because scientists suspect that sustained wet-dry cycles are key to creating conditions that could cultivate the building blocks of life.

“We’re studying Mars in order to sort of understand what Earth was like when life was likely to form [here],” said Patrick Gasda, a research scientist at Los Alamos National Laboratory who coauthored the paper.

Animation by Megan McGrew/PBS NewsHour

But still, what’s the big deal about patterns in the mud? We’d probably skip right over them if we saw them on our planet (unless you’re Mona Lisa Vito, about to crack a murder case wide open).

The Martian surface serves as a kind of planetary fossil. Both Mars and Earth are over 4 billion years old and though each faced a number of dynamic, destructive forces, it’s easier to observe Mars’ distant past than get an accurate sense of what our planet looked like in its infancy. A combination of factors like tectonic plate activity and the imprint of living things have altered or destroyed much of what we might have found in the ancient terrestrial geologic record, Gasda said.

There’s a lot we still don’t know for certain about Mars, but researchers believe the planet was able to support a liquid water cycle for a fairly short fraction of its history, Gasda noted — potentially something on the order of a 10- to 100-million-year time frame out of the billions of years it has been around.

At some point, Mars’ atmosphere thinned, the temperature dropped considerably, and all of the activity attributable to flowing surface water stopped, he said. The geologic record from this brief era has been more or less frozen in time ever since, aside from changes brought by wind erosion and the occasional volcanic eruption or impact from another cosmic body.

“On Earth, we argue about individual little mineral grains that are left over from 4 billion years ago and try to use those to interpret everything about the ancient Earth,” said Briony Horgan, a member of the Mars 2020 mission science team and professor of planetary science at Purdue University who wasn’t involved with the paper. “But on Mars, we actually have the real rocks sitting here from 4 billion years ago right at the surface.”

Where Curiosity spotted the mud cracks

These specific mud cracks are located on Mount Sharp inside Gale Crater, which researchers believe was once home to a saline lake.

Since Mount Sharp is composed of sedimentary layers, Curiosity’s observations move forward through time as it ascends the slope of the mountain, said William Rapin, a CNRS research scientist at Institut de Recherche en Astrophysique et Planétologie in Toulouse, France, and lead author of the paper.

Both the mountain and the mud cracks have been subject to erosion but enough has been preserved and exhumed over time to allow researchers to observe them via the rover.

This view of Mount Sharp is derived from a combination of elevation and imaging data from three Mars orbiters. The view is looking toward the southeast. It shows where the Curiosity rover landed in Aug. 2022. Image courtesy NASA/JPL-Caltech/ESA/DLR/FU Berlin/MSSS

“It’s 3.8 billion years old — this is extremely old. We cannot dream of rock preserved at that age on Earth in that preservation state,” Rapin said. “It’s pristine.”

When Gale Crater was home to a lake system billions of years ago, water levels rose and fell over time, just like they do in lakes on Earth. The cracks formed in an area where the ground that was once rich with clay began to contain more sulfate, Gasda explained. Researchers believe that moment represents the start of a drying out process.

Rovers have observed T-shaped mud cracks elsewhere on Mars. But these cracks are unique because they’re hexagonal, a shape that indicates they were hydrated and dehydrated regularly by a sustained water cycle that, Rapin said, may have fueled seasonal changes for thousands or even millions of years.

How the mud cracks may have formed

The water that led to the formation of these mud cracks likely came from a combination of the flooding of water on Mars’ surface and rising groundwater, which periodically evaporated before the cycle repeated itself again, according to the paper.

When the water in the crater dried, it left behind evaporites (or rocks/crystals) in the form of sulfate salts, which he noted are concentrated around the perimeter of each hexagonal crack.

Researchers identified the sulfates using instruments aboard Curiosity like ChemCam, which documents the mineral and chemical composition of materials it analyzes. It observed more and more sulfur as the rover traveled uphill, which Gasda said researchers attribute to the sulfates.

“The sulfate that’s filling the cracks, that’s a little bit harder than the rock — basically, the lithified mud that’s around it — and so the mud gets eroded away and leaves behind these really nice polygons of the salt around it,” Horgan said. “That’s why they pop out in the images so nicely.”

A similar phenomenon plays out in the southwestern United States, Gasda noted. In places like Death Valley, intermittent flooding leaves behind deposits of evaporites as water evaporates.

This mosaic, taken by the Mars Curiosity rover, looks uphill at Mount Sharp in the middle of Mars’ Gale Crater, where the rover discovered the mud cracks. The mosaic was assembled from dozens of images taken by Curiosity’s Mast Camera (Mastcam) on Sol 1931 in Jan. 2018. The scene has been white-balanced so the colors of the rock materials resemble how they would appear under daytime lighting conditions on Earth. Image courtesy NASA/JPL-Caltech/MSSS

Exploring Mars through rovers has been instrumental in helping us understand just how much liquid water existed on the planet’s surface. Before Curiosity, the general consensus was that Mars was mostly cold and dry with only a brief period of warmth, Gasda said. But he noted that Curiosity’s mission showed that Gale Crater was much more wet compared to other parts of the planet, which suggests that at least part of the planet had a long, active water cycle.

“Curiosity was the first one to explore the remnants of a lake, [which] formed a habitable condition,” Rapin said. “From an Earth standard, that means if you bring life as we know on Earth, it would have thrived on the ancient Mars.”

NASA plans to attempt a complicated sample return mission over the next 10 years involving the Perseverance rover. The samples collected so far include sulfate salts similar to the ones in Gale Crater that may contain trapped droplets of water and bits of Martian atmosphere, Horgan said, all of which would allow researchers to more accurately reconstruct the planet’s ancient climate.

What Mars can tell us about ancient life

Liquid water is key to sustaining life on Earth, but for life to evolve, wet-dry cycles may be a crucial component.

The genetic material that makes up organisms on Earth — RNA and DNA — first emerged after a lot of trial and error involving rudimentary molecules like amino acids and nucleobases, Gasda said. Nucleobases are a bit harder to forge, but amino acids crop up fairly easily, including on extraterrestrial bodies like comets.

“These building blocks of life are kind of sitting around everywhere,” Gasda said.

The next step toward life is polymerization, or amino acids connecting to one another in chains to form proteins. But if two amino acids beside each other are submerged in water, Gasda said, they won’t polymerize. That’s because water is a byproduct of the polymerization process, he explained, so that chemical reaction doesn’t happen under wet conditions.

But if that water dries up — like the conditions under which the evaporites formed in the mud cracks — those amino acids become concentrated and the likelihood that they polymerize rises.

“You could have this sort of seasonal wet-drying cycle happening on Mars where these types of reactions would be favored to happen,” Gasda said. “And then you could have the polymers and then, who knows, maybe later, enough polymers could get together and that could form a chemical system that would look like life, almost. But we have no idea if that happened.”

He added that this process happened repeatedly across Earth in its early days before one set of polymers won out. All life on Earth, from bacteria to humans, share the same basic set of amino acids, Gasda said.

There’s no evidence life ever existed on Mars, but the red planet did have many of the right ingredients and — at least temporarily — climatic conditions that could have allowed it to form.

Rapin noted that we’re currently at a “dead end” on Earth when it comes to understanding how life arose because we have no preserved geologic evidence from that time. The oldest known fossil is an imprint of bacteria, which he said captures a time when life had already become complex.

On Mars, we may have the chance to see what happens earlier in the arc of evolution, and whether there are paths beyond RNA and DNA for life to come into being, he said.

“Maybe life never arose on Mars,” Rapin said. “But at least, because it never arose and never colonized completely like it did on Earth, [the planet] may have preserved the memory of what happened earlier.”