Lead author Candice Bedford, a research scientist at Purdue University, explained:

"Before we arrived at the Margin Unit, the main hypothesis — derived from orbital observations — was that the carbonate seen from orbit formed from interaction with the lake that existed in Jezero crater. But now we know that this location became a sort of crossroads for aqueous systems. The Margin Unit findings are important because Jezero crater sits inside one of the largest exposures of carbonate on Mars, so what we learn here reaches well beyond this crater."

Three episodes of water

The rover's analysis shows the rocks interacted with water on three primary occasions.

Episode one: groundwater. Carbon-dioxide-rich groundwater reacted with olivine, a greenish mineral common in volcanic rock. The reaction formed ridges of carbonate that run through fractures in the bedrock at low elevations. Those ridges are still visible today.

Episode two: the lake. Lake water then altered some of the rocks. Co-author Eleni Ravanis, a planetary scientist at the University of Hawaii at Manoa, described the evidence:

"Some of the Margin Unit rocks also contain silica. Turning olivine into carbonate can leave silica behind, and we see more of that silica in rocks that sat below the water line."

Silica is the mineral that makes up quartz and much of sand on Earth.

Episode three: hot water. The final episode left mineral veins at one location in the eastern part of the Margin Unit. The veins are about 10 inches (25 centimeters) thick and contain minerals such as calcium sulfate and fluorite. Their presence indicates that hydrothermal fluids — essentially hot water — once flowed through the rocks.

Olivine from an underground magma body

The team made another notable find higher up in the Margin Unit. There, the rocks are coarse-grained and crystalline — evidence of olivine that never touched water at all. This olivine formed in a body of hot magma underground, cooled slowly, and reached the surface only when the ground above it eroded away. Lower down, the olivine grains have silica packed between them.

The carbonate and silica carry an astrobiological twist. On Earth, when water interacts with olivine, the reaction can release hydrogen, which some microbes use as a food source. The same reaction leaves behind carbonate and silica — two minerals capable of preserving traces of those ancient microbes. The minerals are therefore promising places to look for biosignatures on Mars.

Last year, researchers in France independently reported evidence for ancient hydrothermal systems in Jezero crater, adding support to the picture of a geothermally active early Mars.

Surprises are the norm

Bedford cautioned that orbital data rarely tells the full story of what rovers find on the ground:

"If there is one thing I have learned after 10 years working with Mars rovers, it is that Mars constantly throws surprises at you. It is very rare that things are as we expect them to be from orbital data. I hope this work helps reshape how scientists view the history of water in Jezero crater and across Mars. Ultimately, I hope it helps planetary scientists reconstruct the changing climate and habitability of early Mars."

The findings come from a single, though geologically rich, location, and the interpretations rest on rover-based remote sensing rather than laboratory analysis of returned samples. Even so, because Jezero crater sits within one of the largest carbonate exposures on Mars, the lessons learned at the Margin Unit may reshape how scientists understand the Red Planet's watery — and hot — past.