What three water episodes did the rocks record?

The Margin Unit rocks interacted with water at least three distinct times, and each encounter changed their chemistry and appearance.

• First episode: Groundwater rich in carbon dioxide moved through fractures and reacted with olivine, depositing carbonate in lower elevations. Erosion later exposed these harder ridges.
• Second episode: Possibly connected to the ancient lake that once filled Jezero Crater, water left additional silica where rocks sat below the waterline.
• Third episode: Hot fluids circulated through the rock, depositing mineral veins about 10 inches (25 centimeters) thick, including calcium sulfate and fluorite.

Coauthor Eleni Ravanis of the University of Hawaii at Manoa noted that silica content rises in rocks that once sat below the lake's waterline — a sign of repeated reaction between olivine and water.

Why does fluorite matter?

Fluorite commonly forms when heated water moves through volcanic rock. Its presence indicates Jezero Crater hosted a later period of hydrothermal activity, after the earlier groundwater and lake interactions ended.

What does this mean for habitability?

Carbonate and silica are priority targets in the search for biosignatures. On Earth, water reacting with olivine can release hydrogen — an energy source certain microbes can exploit. The same reactions can also create minerals that trap and preserve biological traces.

The area's importance extends beyond Jezero Crater itself. Bedford stressed that the crater sits inside one of the largest carbonate exposures on Mars.

"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," Bedford said. "But now we know that this location became a sort of crossroads for aqueous systems."

What did higher elevations show?

At the upper parts of the Margin Unit, Perseverance found coarse crystalline rock rich in olivine with almost no water alteration. Olivine contains magnesium and iron.

The team concluded this material crystallized slowly deep beneath the Martian surface from a magma body. Later erosion stripped away the overburden and exposed the rock.

Lower down, near the ancient lakebed, the olivine appears heavily altered. Its grains fracture, and silica fills the spaces between them.

What comes next?

Researchers cannot yet date each water episode precisely, but the order of events is now clearer. The Margin Unit record offers a window into changing climate and habitability on early Mars.

"If there is one thing I have learned after 10 years working with Mars rovers, it is that Mars constantly throws surprises at you," Bedford said. "I hope this work helps reshape how scientists view the history of water in Jezero Crater and across Mars."

The journal paper was published on September 30, 2026. It runs under DOI 10.1038/s43247-026-03997-9.