Perseverance finds signs of three water-alteration episodes in Jezero’s Margin unit

NASA’s Perseverance rover has found mineral evidence of three successive water-alteration episodes in the Margin unit along Mars’ Jezero Crater rim, according to a study published on September 21. The result matters because carbonate seen from orbit had helped make an ancient lake shore a leading explanation for the rocks. The new analysis instead points to several kinds of water-rock interaction, whose ages remain unknown.

The researchers analyzed SuperCam images, chemical measurements and reflectance spectra from more than 185 bedrock targets. Those observations reveal differences across the unit: high exposures retain crystalline, olivine-rich rock with little sign of water alteration, while lower exposures show a more complicated record. The team infers a sequence from the minerals and the relationships between rock features; it has not assigned dates or durations to the individual stages.

Three changes recorded in the rocks

In the proposed first stage, neutral-to-alkaline fluids rich in carbon dioxide circulated through the bedrock and formed carbonate in fractures. Erosion subsequently left some of those filled fractures standing as ridges. This interpretation makes groundwater an important part of the Margin unit’s story, rather than treating the carbonate signal seen from orbit as proof that the crater lake formed every deposit.

A later event moved some carbonate and deposited silica in pore spaces. The researchers identify two possible explanations: contact with Jezero’s paleolake water or a change in groundwater chemistry. The mineral association therefore supports another episode of alteration, but does not settle which water source supplied it. NASA reports that silica is more abundant below the proposed water line; study coauthor Eleni Ravanis also noted that alteration of olivine to carbonate can leave silica behind.

The contrast between high and low exposures is central to the interpretation. At higher elevations, the olivine-rich, slow-cooled crystalline rock shows little evidence of significant water exposure. Below the proposed second terrace level of the ancient lake, the study finds more extensive alteration, as well as signs that material was physically reworked near Neretva Vallis and the western fan. A single description of the entire Margin unit as lake sediment would miss that variation.

A shoreline with a more complicated history

Perseverance reached the Margin unit in September 2023, after orbital observations had identified carbonate there. Lead author Candice Bedford told NASA that the earlier idea linked that carbonate to the crater lake; she described the rover findings as pointing to a ‘crossroads for aqueous systems.’ That phrase reflects the team’s interpretation of several possible water pathways, not a direct measurement of where each fluid originated.

An independent stratigraphic study published in January 2026 adds a different kind of evidence. Using rover images and models of exposed rock, its authors distinguished two Margin unit subunits. They found that the origin of a western unit remained uncertain, though it could fit a carbonated olivine-rich igneous rock. Structures in eastern sandstones, by contrast, favored reworking by waves along a shoreline. Those results support a mixed history involving original rock formation, subsequent alteration and localized sedimentary deposits.

A separate 2024 orbital study found that the strongest marginal carbonate signatures generally do not overlap with the strongest olivine signatures in thermal-infrared data. That finding is another reason to be cautious about inferring one formation process for every carbonate-bearing outcrop from an orbital map alone. The new SuperCam work supplies a closer view of particular bedrock targets, while the earlier imaging and orbital studies address different parts of the geological picture.

What remains to be tested

The order of the inferred episodes is clearer than their calendar ages. NASA says the team cannot yet determine when the Margin unit encountered water. The January stratigraphic study argues that laboratory dating of original olivine crystals, carbonate products, and sedimentary grains or cements could help separate rock formation from later alteration. That would test whether the features now grouped into a sequence formed close together or across a longer span; the cited work does not yet answer that question.

Perseverance has collected Margin unit samples named Pelican Point and Lefroy Bay in the east and Comet Geyser in the west for potential return to Earth, according to the new study. It does not report Earth-based analysis of those samples, and the cited research establishes no return date. Until such tests are possible, the rover measurements support a relative sequence and competing interpretations for at least one water source, rather than a dated history of the crater’s water.

The authors describe the repeatedly altered rocks as being of astrobiological interest. Multiple water-rock reactions could help researchers examine how Jezero’s environments changed, but the reported minerals are evidence of those reactions, not evidence that life existed there. The unresolved ages, the uncertain source of the silica-forming episode and the differences between Margin unit outcrops remain important limits on what this result can establish.