Does This Mars Rock Show a "Potential Biosignature"? Or Just "Wet Chemistry"?

A fresh round of excitement is sweeping across NASA's Perseverance team as the Mars rover nears the completion of its survey of an ancient riverbed. On July 25th, the team announced the discovery of multiple types of “potential biostructures” in the area. The claim comes from the rover’s recognition of organic material in certain small structures — but the only way to tell if life formed those structures is to send samples to better-equipped terrestrial labs, says project scientist Ken Farley (Caltech).

If this sounds familiar, it's because the Perseverance team reported other “potential biostructures” about a year ago at a different site in the 45-kilometer-wide Jezero Crater. However, the rover’s equipment couldn’t distinguish if those materials were formed by life or by inanimate processes.

Perseverance performed the new tests while parked beside a rock dubbed “Cheyava Falls,” after a waterfall in the Grand Canyon (it’s Hopi for “intermittent waters”). The 1- by 0.6-meter (3.2- by 2-foot) rock is composed of sediment deposited 3.5 billion years ago by a river flowing into a body of water in Jezero that was the size of Lake Tahoe.

A Rock with "Leopard Spots"

Like other river deltas, Cheyava Falls collected diverse rocks and other material that settled to the bottom where the river water slowed. This photo taken on July 18th shows the intriguing complexity of the rock:

“One thing that jumps out when you look at this beautiful rock is a band of reddish rock in which are tannish blotches about a millimeter across which the team calls leopard spots,” says Farley.

Look closely and you can see a thin black ring separates the tan leopard spots from the brownish area. This color pattern carries an important message.

“The red, tan and black colors indicate that a chemical disequilibrium occurred,” Farley explains. The splotches are clear evidence that a chemical reaction followed to bring the material to a more stable condition when Mars still was wet.

Such spots form on terrestrial rocks when chemical reactions with water turn hematite, an iron-based mineral, from red to white; on Mars, similar reactions would change the red of the background rock to the tan of the leopard spot. The reactions also release iron and phosphate, which is what makes up the black rings, according to measurements using another Perseverance instrument named PIXL.

These reactions are not necessarily biological in origin. However, Farley points out that on Earth, such reactions sometimes supply energy for microbes.

"The potential of this finding is certainly exciting!" says Paul Byrne (Washington University in St. Louis), who was not involved in the study. "Biosignatures can come in a huge range of forms, and certainly these chemical reactions that are suspected to have taken place in this rock are consistent with a biosignature."

"But as the team carefully points out," Byrne adds, "this finding isn't definitive of anything. They're right to be careful and cautious, and I laude them for being very clear about this uncertainty."

The new study was the first to employ SHERLOC, short for Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals. The team designed this instrument to search for organic compounds and other potential signs of life using specialized spectroscopic techniques. SHERLOC found organic material in all the rocks exposed on the Cheyava Falls surface. While it didn’t identify specific organic compounds, Farley says the data hint at the presence of kerogen, a solid, insoluble gunk of organic compounds. This gunk is sometimes found in fossils; it also accumulates in sedimentary rocks and asteroids.

Another puzzle was the presence of small, dark-green crystals of olivine, a mineral that crystallizes out of magma, found together with white veins of calcium sulfate on the surface of Cheyava Falls; the latter were likely deposited by the flowing river. However, what their combined presence means isn’t clear, Farley says: “Nobody has ever seen a set of minerals with olivine and sulfate together.”

On July 21st, Perseverance drilled a 6-cm (2-inch) core containing a leopard spot as its 22nd rock sample. Examining the core’s bottom showed that the leopard spot appears on both ends of the core, which means it’s three-dimensional rather than just a stain on the surface. 

It all leaves Farley filled with a sense of wonder. Lakes and rivers on Mars and Earth “might not have been that different 3.5 billion years ago,” he says. “Then something happened, and life was extinguished. That's the way planets can go.”

The Quest for Sample Return

Despite the amazing things Perseverance has discovered so far, the car-size rover can only carry a limited suite of scientific instruments to tease out Martian secrets. The only way to tell whether these “potential biostructures” are artifacts created by living things or just mineral reactions is to collect samples and send them to terrestrial laboratories equipped with the latest and greatest instrumentation.

NASA and the European Space Agency have been planning to bring samples home from Mars for years. Originally, the plan called for NASA to launch a Sample Return Lander in 2028. Inevitably, the schedule slipped and the budget expanded. An independent review earlier this year found the budget would either have to soar to $8-$13 billion, or the schedule would have to slip to 2040. On April 15th, NASA Administrator Bill Nelson said the plan should go back to the drawing board for something quicker and cheaper. Spending bills in the House and Senate are now calling on NASA to move forward with the mission.

The good news is that the new Perseverance results show we have a good reason to send samples from Mars to Earth for analysis. "It was already important to bring these samples home," Byrne says, "but now we have an even greater imperative to do so."

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Editorial note (August 3, 2024): Comments from Paul Byrne, an expert in the field who was not on the research team, were added to the story.