NASA’s Perseverance rover has found evidence that one area of Jezero Crater on Mars experienced a much more complex water history than scientists had assumed. Rocks in the Margin Unit interacted with water at least three times: first with groundwater, then likely with the water of an ancient lake, and later with hot flows circulating beneath the surface.

The study was published in the journal Communications Earth & Environment. It helps reconstruct the history of climate change and habitable conditions on ancient Mars.

Instead of shoreline deposits, the rover found volcanic rocks

Perseverance arrived at the inner rim of Jezero Crater in September 2023. Scientists expected to see sedimentary rocks here that had formed along the shore of an ancient Martian lake.

Such rocks are especially interesting in the search for traces of ancient life: on Earth, clays and silts can preserve chemical and structural signs of biological activity. In addition, orbiters detected substantial deposits of carbonate minerals in this area, which on Earth often form in shallow lakes and oceans.

But the Margin Unit brought a surprise. Instead of the expected sedimentary deposits, Perseverance found igneous rocks — the kind that form when molten rock cools beneath the surface or when volcanic material hardens.

These rocks turned out to be a kind of geological chronicle. Their minerals preserved signs of several episodes of interaction with water, each leaving its own chemical signature.

Perseverance’s laser read the rocks’ chemical composition

Scientists obtained most of the data using the SuperCam instrument mounted on the rover’s mast.

When specialists select a rock target of interest, SuperCam can fire a laser at it from a distance of up to 6.5 meters. The laser vaporizes a tiny part of the surface and creates plasma whose spectrum makes it possible to determine the material’s chemical composition.

Using this method, Perseverance studied more than 185 bedrock targets within the Margin Unit.

“Before arriving at the Margin Unit, the main hypothesis based on orbital data was that the carbonates found there had formed as a result of interaction with the lake that once existed in Jezero Crater,” says the study’s lead author, Candice Bedford.

Now scientists believe the story was much more complex: this area became a kind of crossroads for several water systems.

Water first circulated below the surface

Perseverance investigated the Margin Unit across about 265 meters of elevation. In the upper part of the area, the rover found coarse-grained crystalline rocks rich in olivine.

They show very little sign of having been altered by water. Researchers believe these rocks originally formed deep beneath the Martian surface from magma. The melt cooled slowly enough for large mineral crystals to form inside it.

Later, erosion stripped away the overlying layers and exposed the ancient rocks.

Lower down, closer to the former lakebed, the picture changed. The olivine was much more heavily altered: its grains were fractured, and the spaces between them were filled with silica.

This is especially interesting from the standpoint of searching for life-friendly conditions. On Earth, the interaction of water with olivine can lead to the formation of hydrogen, which some microorganisms use as an energy source. Such reactions can also produce carbonates and silica — minerals capable of preserving signs of ancient biological activity.

The second stage may have been linked to an ancient lake

Scientists were able to reconstruct the sequence of events, although it is still impossible to determine the exact age of each one.

The first episode is linked to groundwater rich in carbon dioxide. When such water passed through the rock, it reacted with olivine. As a result, carbonate minerals formed in the fractures.

Later, erosion destroyed part of the surrounding, softer rock. The fractures filled with hard carbonate were preserved and now protrude above the surface like distinctive ridges.

Then, likely, came the period when the ancient Lake Jezero existed. This is indicated by the distribution of silica: there is more of it in the rocks that apparently lay below the water level.

“The transformation of olivine into carbonate can leave silica behind, and we see more of this silica in rocks that were below the waterline,” explains planetary scientist Eleni Ravanis.

Hot underground waters came last

The third episode turned out to be even more unexpected. It occurred later and is linked to hot water circulating beneath the surface.

In the eastern part of the Margin Unit, Perseverance found mineral veins about 25 centimeters thick. Among other things, they contain calcium sulfate and fluorite.

Fluorite turned out to be an important clue. On Earth, this mineral often forms when hot water circulates through volcanic rocks. Its presence therefore indicates that after the rocks interacted with groundwater and the ancient lake, the Jezero area experienced yet another phase — this time involving heated underground flows.

Thus, the same area of Mars was successively exposed to different water environments: first groundwater, then likely a surface lake, and much later hot water beneath the surface.

Martian water turned out to be more complex than expected

The new data changes the view of the Margin Unit as a place formed exclusively by an ancient lake. Scientists now see a far more complex history in which the same rock section was repeatedly exposed to water under completely different conditions.

This is especially important for studying the habitability of ancient Mars. The interaction of water with olivine could have created chemical energy sources for microorganisms, while carbonates and silica are capable of preserving evidence of conditions that once existed.

At the same time, researchers are not yet claiming that Perseverance found signs of life itself. The речь concerns geological evidence of the existence of various water environments that could potentially have been favorable for life.

“If there’s one thing I’ve learned in ten years of working with rovers on Mars, it’s that Mars constantly delivers surprises,” Bedford says. According to her, real data rarely fully matches what scientists infer from orbital observations.

Now Perseverance will continue studying these rocks, and the data it collects will help scientists reconstruct a more detailed history of water, climate, and Mars’s potential habitability in the distant past.