Perseverance Found Igneous Rock at Jezero Crater. It Holds a Three-Chapter Water Story.

NASA's Perseverance rover found igneous rocks at Jezero Crater's Margin Unit in September 2023 showing evidence of at least three separate water interactions, revealing early Mars had more complex aqueous systems than orbital data suggested, according to findings published Monday in the journal Communications Earth and Environment.

The discovery surprised the mission team. Scientists expected sedimentary rock at the Margin Unit, a geologic area that stretches along the shoreline of an ancient Martian lake. Orbital observations had detected strong signals of carbonate minerals there, and on Earth, carbonates frequently form in shallow lake and ocean environments. Sedimentary rocks, built up from layers of sand and silt, are also good at preserving traces of past microbial life. What the rover found instead was igneous rock, the kind that forms from magma deep underground or from volcanic activity at the surface.

That was not a disappointment. It turned out to be a more complicated story.

"Before we arrived at the Margin Unit, the main hypothesis was that the carbonate seen from orbit formed from interaction with the lake that existed in Jezero Crater," said Candice Bedford, a research scientist at Purdue University and the study's lead author, in the NASA account of the findings. "But now we know that this location became a sort of crossroads for aqueous systems."

The igneous rock preserved that crossroads in detail. Mineral crystals inside igneous rock lock in a record of the conditions at the moment they formed, and the Margin Unit rocks captured at least three distinct water events, each of which altered the rock's chemistry and appearance, according to NASA.

The instrument that read those records is SuperCam, which sits on the rover's mast and identifies the mineral makeup of rock by analyzing the light it reflects. When the science team spots a target, they can command SuperCam to fire a laser up to 21 feet, or 6.5 meters, away. The resulting burst of plasma produces a spectrum that reveals the target's chemistry. Perseverance analyzed more than 185 bedrock targets across the Margin Unit this way, NASA said.

The rover explored the Margin Unit across roughly 870 feet, or 265 meters, of elevation, according to NASA. At higher elevations, the rock was coarse-grained and crystalline, rich in olivine, a mineral made of magnesium and iron, with almost no sign of water contact. That olivine formed in a body of magma deep underground, cooling slowly enough for its grains to grow large, and reached the surface only after the ground above it eroded away, NASA said.

Lower in the unit, closer to the ancient lakebed, the picture changed. The olivine grains were fractured, with silica filling the gaps between them.

The team can read the sequence of water events, though not their age, NASA said. The first event brought carbon-dioxide-rich groundwater into contact with the olivine, producing ridges of carbonate that now run through fractures in the bedrock at low elevations. Those carbonate-filled fractures stand out today because the softer rock around them has worn away.

The second event may have been the lake itself. "Some of the Margin Unit rocks also contain silica," said Eleni Ravanis, a planetary scientist at the University of Hawaii at Manoa and a coauthor of the study, in the NASA account. "Turning olivine into carbonate can leave silica behind, and we see more of that silica in rocks that sat below the water line."

The third event left mineral veins about 10 inches, or 25 centimeters, thick at one location in the eastern part of the Margin Unit, NASA said. Those veins contain calcium sulfate and fluorite. Fluorite typically forms when hot water circulates through volcanic rock, which tells the team that a heated underground-water event came through after the others.

Each of those minerals carries weight in the search for ancient life. When water interacts with olivine on Earth, the reaction can release hydrogen, which some microbes use as a food source, and it leaves behind carbonate and silica, two minerals that can preserve traces of microbial activity, according to NASA.

Bedford put the broader significance plainly in the NASA account. "Jezero Crater sits inside one of the largest exposures of carbonate on Mars," she said, "so what we learn here reaches well beyond this crater."

The team cannot yet say when any of the three water events occurred, NASA said. That is the open question the record leaves.

Perseverance's mission includes astrobiology, the search for signs of ancient microbial life, as well as characterizing Mars geology and past climate and collecting and storing rock and regolith, according to NASA. Jet Propulsion Laboratory, managed for NASA by Caltech, built and manages rover operations on behalf of NASA's Science Mission Directorate. SuperCam is co-led by Purdue University, Los Alamos National Laboratory, and two French institutions: IRAP, the Research Institute in Astrophysics and Planetology, and CNES, the Centre National d'Etudes Spatiales, both in Toulouse, NASA said.

What comes next for Perseverance at Jezero is not detailed in the published findings. The study itself, and the full mission record, are available at science.nasa.gov/mission/mars-2020-perseverance.

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