Does this Martian rock show a 'potential biosignature'? Or just 'wet chemistry'? – Sky & Telescope

A new round of excitement is sweeping through NASA's Perseverance team as the Martian explorer nears completion of its study of an ancient riverbed. On July 25, The team announced The discovery of multiple types of “potential biostructures” in the area. The claim is due to the rover’s recognition of organic material in certain small structures, but the only way to know if life formed those structures is to send samples to better-equipped terrestrial laboratories, says project scientist Ken Farley (Caltech).

If this sounds familiar, it’s because the Perseverance team reported on other “potential biostructures” about a year ago. in a different place in the 45-kilometer-wide Jezero Crater. However, the rover team was unable to distinguish whether these materials were formed by life or by nonliving processes.

Perseverance conducted the new tests while parked next to a rock named “Cheyava Falls,” named after a waterfall in the Grand Canyon (which means “intermittent waters” in Hopi). The 3.2-by-2-foot (1-by-0.6-meter) rock is made of sediment deposited 3.5 billion years ago by a river that emptied into a body of water the size of Lake Tahoe at Jezero.

A rock with “leopard spots”

Like other river deltas, Cheyava Falls accumulated various rocks and other materials that settled to the bottom where the river water slowed down. This photo taken on July 18 shows the intriguing complexity of the rock:

“One thing that stands out when you look at this beautiful rock is a band of reddish rock that has tan patches about a millimeter wide that the team calls leopard spots,” Farley says.

If you look closely, you can see that a thin black ring separates the tan leopard spots from the brown area. This color pattern conveys an important message.

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

These spots form on Earth's rocks when chemical reactions with water cause hematite, an iron-based mineral, to turn from red to white; on Mars, similar reactions would change the red of the background rock to the brown of the leopard spot. The reactions also release iron and phosphate, which may have formed the black rings.

These reactions do not necessarily have a biological origin, but Farley points out that on Earth, these reactions sometimes provide energy to microbes.

“He potential “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 wide 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 is not definitive. They are right to be careful and cautious, and I commend 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 possible signs of life using specialized spectroscopic techniques. SHERLOC found organic material in all of the rocks exposed on the surface of Cheyava Falls. While it did not identify specific organic compounds, Farley says the data points to the presence of kerogena solid, insoluble substance made of organic compounds. This substance is sometimes found in fossils; it also accumulates in sedimentary rocks and asteroids.

Another mystery was the presence of small dark green crystals. olivinea mineral that crystallizes from magma, found together with white veins of calcium sulphate on the surface of Cheyava Falls; the latter were likely deposited by the flowing river. However, it is not clear what their combined presence means, says Farley: “No one has ever seen a mineral assemblage with olivine and sulphate together.”

On July 21, Perseverance drilled a 2-inch (6 cm) core containing a leopard spot as its 22nd rock sample. Examining the underside of the core revealed that the leopard spot appears on both ends of the core, meaning it is three-dimensional and not just a spot on the surface.

All of this leaves Farley with a sense of wonder. The lakes and rivers on Mars and Earth “might not have been so different 3.5 billion years ago,” he says. “Then something happened and life became extinct. That’s how planets can evolve.”

The quest for sample return

Despite the amazing things Perseverance has discovered so far, the car-sized rover can only carry a limited set of scientific instruments to unravel Martian secrets. The only way to know if these “potential biostructures” are artifacts created by living beings or simply mineral reactions is to collect samples and send them to Earth-based labs equipped with the most modern, highest-quality instrumentation.

NASA and the European Space Agency have been planning to bring home samples from Mars for years. Originally, the plan called for NASA to launch a sample-return lander in 2028. Inevitably, the schedule was pushed back and the budget was expanded. An independent review earlier this year concluded that the budget would have to increase to $8-13 billion or the schedule would have to be pushed back to 2040. On April 15, NASA Administrator Bill Nelson said the plan should be back to the drawing board for something faster and cheaper. Spending bills in the House and Senate are now Calling NASA to continue the mission.

The good news is that Perseverance's new results show we have a good reason to send Mars samples back 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 part of the research team, have been added to the story.