Mars had a secret water history: NASA rover finds ancient groundwater and hot fluids that reshaped the planet after its ancient lake vanished

Although it may be that Mars lost its rivers and lakes billions of years ago, the rocks found within Jezero Crater indicate that water did not vanish from the area altogether. The Perseverance rover that NASA is using to study Mars has found that the same stretch of the Martian surface has been changed by different water systems over time. The findings point to ancient groundwater, water associated with Jezero’s lake and a much later episode involving hot fluids moving beneath the surface.

The discovery comes from the crater’s puzzling Margin Unit, a geological area along the inner edge of Jezero Crater. Scientists initially expected Perseverance to encounter sedimentary rocks formed from material deposited by the ancient lake. Instead, the rover found igneous rocks that had formed deep underground and were later exposed and repeatedly modified by water.

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The study, published in Communications Earth & Environment, offers a more complicated picture of Mars’ ancient environment. Rather than one lake creating all the water-related features in the area, the rocks appear to preserve evidence of several distinct episodes.

Perseverance found the wrong kind of rock — and that mattered

When Perseverance reached the Margin Unit in 2023, researchers had good reason to expect sedimentary deposits.

Signatures of carbonates had been detected by orbital observation in the area, and the site is situated at the shoreline of the lake that used to exist on Mars in Jezero Crater.

Here on Earth, rocks composed of silt and clay may retain evidence of past environments, as well as provide information about chemical or structural features that can be utilised to examine the possibility of life-supporting conditions.

Rather than discovering sedimentary rocks, Perseverance discovered igneous rocks containing lots of olivine. Such rocks had originated from magma underneath the Martian surface, which allowed for slow cooling and formation of large crystals.

Only after that did the rocks become exposed to the environment due to erosion.

That gave scientists something unexpected to study: an ancient piece of Mars' interior that was subsequently exposed to several different water environments.

NASA's Perseverance team used the rover's SuperCam instrument to examine more than 185 bedrock targets across the Margin Unit. SuperCam can analyse rocks from a distance using a laser that produces a small plasma plume, allowing scientists to determine elements present in the target.

The result was not a simple record of an ancient lake. It was evidence of a much longer geological story.

The first water may have come from underground

The clues become particularly interesting at lower elevations within the Margin Unit.

There, the olivine-rich rocks show signs of significant alteration. The mineral grains are fractured, while silica and carbonate occur within and around the damaged rock.

Researchers think one of the earliest identified water-related events involved carbon dioxide-rich groundwater moving through the rocks.

With the interaction between the groundwater and olivine, chemical processes occurred, forming carbonates that were deposited in the fractures. Through erosion, the less resistant material was eroded away, making these fractures appear as ridges.

That sequence matters because it means the carbonate detected from orbit does not necessarily represent material deposited directly by Jezero's ancient lake.

Instead, at least some of the carbonate may have formed when groundwater interacted with the underlying rock.

The distinction changes how scientists can interpret similar mineral signatures elsewhere on Mars. A carbonate deposit seen from orbit might not automatically point to a former lake or shallow sea; it could also record groundwater moving through rock.

Candice Bedford, the study's lead author, described the Margin Unit as a crossroads for different aqueous systems. The area is particularly useful because Jezero sits within one of Mars' largest carbonate exposures.

Then the ancient lake left its own chemical fingerprints

Groundwater was not the only source of water to affect the Margin Unit.

A later phase appears to have been associated with the lake that once occupied Jezero Crater. The evidence comes partly from the distribution of altered minerals and silica within the olivine-rich rocks.

Scientists know Jezero once contained a substantial lake, with an inlet and an outlet providing evidence that water entered and eventually drained from the basin. Perseverance has previously documented a complicated history involving lake sediments and later flooding.

The new research adds another layer to that history.

Some rocks that sat below the ancient waterline contain silica. According to researchers, reactions that transform olivine into carbonate can leave silica behind. The location of these altered rocks therefore provides clues about how water interacted with the landscape.

That is important for the search for ancient habitability.

On Earth, water reacting with olivine can generate hydrogen, which some microorganisms can use as an energy source. The same processes can produce carbonate and silica, minerals capable of preserving evidence of ancient biological activity.

The findings do not establish that life existed at Jezero. Instead, they identify chemical environments that could be useful when scientists assess where ancient Martian life might have survived.

Mars later turned up the heat

The most unusual part of the story came from evidence of a later episode involving hot underground water.

In the eastern part of the Margin Unit, Perseverance identified mineral veins around 10 inches (25 centimetres) thick. The veins contain minerals including calcium sulfate and fluorite.

Fluorite is especially useful as an indicator mineral since it tends to form when hot water flows through volcanic rocks. Fluorite implies that there has been some hydrothermal activity in the area after the earlier ground and lake water alterations.

That gives scientists a sequence rather than a single snapshot.

First, groundwater moved through ancient rock. Later, the area was affected by the water associated with Jezero's lake. At a subsequent stage, hot fluids circulated underground.

Researchers can determine the relative order of these events, but they cannot yet assign precise ages to each episode. That leaves an important part of Mars' water history unresolved.

Still, the sequence demonstrates that Jezero's geological environment changed repeatedly.

The crater was not simply a lake that eventually dried up and froze into a geological time capsule. Its rocks continued to interact with water after the lake environment changed.

Why this matters for the search for life on Mars

The discovery is significant because water is only part of the habitability equation. Scientists also need to understand the chemistry, energy sources and physical conditions that existed when water was present.

The Margin Unit provides several pieces of that puzzle.

Olivine supplied reactive minerals. Groundwater provided a means for chemical reactions to occur. Carbon dioxide-rich fluids produced carbonate. The ancient lake created another aqueous environment. Later, heated fluids introduced a different set of chemical conditions.

Some of these environments could potentially have provided energy sources for microorganisms, although there is currently no evidence from this study that microbes actually lived there.

The rocks may nevertheless become valuable targets for future laboratory research if samples collected by Perseverance are eventually returned to Earth.

NASA has previously identified the olivine and carbonate-rich rocks around Jezero as important because they can preserve information about interactions between Mars' interior, atmosphere and water.

For now, Perseverance is continuing to build the geological record from the Martian surface. What looked from orbit like a carbonate-rich shoreline has turned out to contain a much older and more complicated story.

The bigger lesson is that Mars' water history cannot always be reconstructed from the presence of a lake alone. Groundwater can leave its own signature. Later hydrothermal activity can overwrite earlier evidence. Ancient rocks can preserve several chapters of planetary history in the same location.

And at Jezero Crater, Perseverance has found evidence that those chapters were written long after the first lake appeared.

FAQ

1. What did NASA's Perseverance rover discover at Jezero Crater?
Perseverance found that rocks in the Margin Unit were altered by water during at least three different episodes. The evidence points to groundwater, the ancient lake and a later period of hot underground fluids.

2. Was Jezero Crater once a lake?
Yes. Geological evidence shows that Jezero contained an ancient lake. Perseverance has also found evidence of an inlet and outlet, indicating that water entered and later drained from the crater.

3. Does the discovery prove there was life on Mars?
No. The findings do not provide evidence that life existed. However, some of the water-rock reactions identified by scientists could have created chemical energy sources relevant to studies of ancient habitability.

4. Why are the rocks important to Mars research?
The rocks preserve evidence of several water-related processes and contain minerals such as olivine, carbonate, silica and fluorite. Studying them can help scientists reconstruct how Mars' environment and water systems changed over time.