A new study suggests that Jezero Crater’s mysterious “Margin Unit” records a far more complicated history of water than scientists expected, involving ancient lakes, groundwater, and later hydrothermal activity.
When NASA’s Perseverance rover arrived at the inner rim of Mars’ Jezero Crater in September 2023, researchers expected to encounter sedimentary rocks along what was once the shoreline of a Martian lake. Such rocks form as layers of material accumulate over long periods of time. On Earth, sedimentary rocks made from clay and silt can be especially valuable because they are capable of preserving evidence of ancient microbial life.
Scientists were also interested in the area because Mars orbiters had detected strong signatures of carbonate minerals there. On Earth, carbonates commonly develop in shallow lakes and oceans, including environments that can support life.
Instead of the expected sedimentary deposits, Perseverance encountered igneous rock. These rocks can form when magma cools underground or when volcanic material solidifies at the surface. Because the minerals inside igneous rocks can preserve information about the conditions present when they formed, they can provide exceptionally detailed geological records.
In the Margin Unit, those rocks revealed an unexpectedly complicated history. The evidence indicates that they interacted with water on at least three separate occasions, and each episode changed their chemistry and physical appearance in different ways. The findings were published in the journal Communications Earth & Environment.
SuperCam Reveals Mars’ Hidden Water History
Much of this evidence came from SuperCam, an instrument mounted high on Perseverance’s mast. SuperCam can identify the mineral composition of geological features by analyzing the light they reflect.
When mission scientists identify a promising target, they can direct SuperCam to fire its laser from as far as 21 feet (6.5 meters) away. The laser creates a small burst of plasma, and the spectrum of that plasma reveals the chemical makeup of the rock. Using this technique, Perseverance has studied more than 185 bedrock targets across the Margin Unit.
“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,” said Candice Bedford, a research scientist at Purdue University in West Lafayette, Indiana, and the study’s lead author. “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.”
Ancient Magma Beneath Jezero Crater
Perseverance examined the Margin Unit over approximately 870 feet (265 meters) of elevation. At the higher parts of the area, the rover encountered coarse, crystalline rock rich in olivine. These rocks showed almost no evidence of having been altered by water.
Olivine contains magnesium and iron. The researchers concluded that this olivine-rich unit originally formed inside a mass of magma deep beneath the Martian surface. The magma cooled slowly enough for large mineral grains to develop. Much later, erosion removed the material above it and exposed the rock at the surface.
The situation was very different farther down the Margin Unit, near the ancient lakebed. There, the olivine appears heavily altered. Its grains are fractured, and silica fills spaces between them.
Carbonate and silica are especially interesting to scientists searching for signs of ancient habitability. On Earth, when water reacts with olivine, the process can produce hydrogen that certain microbes can use as an energy source. The same reactions can also create carbonate and silica, minerals that are capable of preserving traces left behind by past microbial activity.
Three Separate Encounters With Water
Researchers can reconstruct the order in which water altered the Margin Unit, although they cannot yet determine exactly when each episode happened.
The first known episode involved groundwater rich in carbon dioxide. As this water moved through the rock, it reacted with olivine and produced carbonate inside fractures at lower elevations. Over time, erosion removed some of the softer surrounding material, leaving the harder carbonate-filled fractures standing out as ridges.
A second period of water activity may have been connected to the ancient lake that once filled Jezero Crater.
“Some of the Margin Unit rocks also contain silica,” said Eleni Ravanis, a planetary scientist at the University of Hawaii at Manoa and a coauthor of the study. “Turning olivine into carbonate can leave silica behind, and we see more of that silica in rocks that sat below the water line.”
Hot Water Came Later
A third and later episode appears to have involved heated water moving underground.
At one site in the eastern Margin Unit, Perseverance found mineral veins roughly 10 inches (25 centimeters) thick. These veins contain minerals including calcium sulfate and fluorite.
The presence of fluorite provides an important clue about the conditions that produced them. Fluorite commonly forms when hot water circulates through volcanic rock, indicating that Jezero Crater experienced a later period of heated groundwater activity after the earlier interactions with groundwater and the lake.
Together, the observations show that the Margin Unit was not shaped by a single lake environment. Instead, it became a meeting point for several different water systems that altered the same rocks at different stages of Mars’ history.
“If there is one thing I have learned after 10 years working with Mars rovers, it is that Mars constantly throws surprises at you,” said Bedford. “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.”