Groundwater, lakes and hot fluids discovered at Mars

Discovery of groundwater, lakes, and hot fluids by NASA's Perseverance Mars rover 

Mars was once a much wetter planet than it is today, with plenty of liquid water on its surface. Mars’ water systems were more complex than previously known, according to new findings from the Perseverance rover. Rocks along the former lakeshore of Jezero crater interacted with water in three major episodes, including hot hydrothermal water. NASA’s Perseverance rover has uncovered a surprisingly complicated history of water in Jezero Crater. Scientists expected the crater’s Margin Unit to contain lake sediments, but instead found ancient igneous rocks which were altered by water on at least three separate occasions. Perseverance has revealed a far more complicated history of ancient Mars than scientists expected. 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.

Mars is a dry planet now, but it was once awash in water. This includes Jezero crater, where the Perseverance rover is still exploring. Perseverance has already found ample evidence that the crater used to be a lake with rivers emptying into it. Led by Purdue University in Indiana, the researchers said that a region in the crater called the Margin Unit was shaped by a complex sequence of ancient lakes, groundwater systems and hydrothermal fluids. The Margin Unit is along the shoreline of the ancient lake. The rover explored across about 870 feet (265 meters) of elevation on the Margin Unit. 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 which can support life.

Before Perseverance landed in Jezero crater, scientists had expected to find sedimentary rocks at the landing site. These rocks would be composed of sand, clay and silt. They are great at preserving traces of ancient microbial life. But the researchers were in for a surprise. The rocks were not sedimentary, but igneous, which form from volcanic activity. 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. Igneous rocks can preserve details about minerals, how they formed and interactions with water. And it’s in these rocks that Perseverance found the clues to a complex water system in the past. The rover used its SuperCam instrument to analyze the rocks and determine their composition. Before reaching at the Margin Unit, the main hypothesis, derived from orbital observations, was that the carbonate seen from orbit formed from interaction with the lake which existed in Jezero crater. 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.

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. 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. The rover has found that the rocks interacted with water on three primary occasions. In the first episode of water interaction, CO2-rich groundwater reacted with olivine. This formed ridges of carbonate that run through the fractures in bedrock at low elevations. Those ridges are still there today. The second episode involved water from the former lake. Co-author Eleni Ravanis, a planetary scientist at the University of Hawaii at Manoa, said, Some of the Margin Unit rocks also contain silica. Turning olivine into carbonate can leave silica behind, and we see more of that silica in rocks that sat below the water line. The final water episode created mineral veins at one location in the eastern part of the Margin Unit. They are about 10 inches (25 cm's) thick and contain minerals like calcium sulfate and fluorite. This finding is particularly interesting, since it indicates that hydrothermal fluids, hot water, once flowed through the rocks. As is almost always the case, there are surprises waiting when you are exploring another world. 

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 which certain microbes can use as an energy source. The same reactions can also create carbonate and silica, minerals which are capable of preserving traces left behind by past microbial activity. 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 which 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. 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." Bedford, a research scientist at Purdue University, said.  Another notable discovery was that of olivine. At higher elevations in the Margin Unit, the rocks were course-grained and crystalline. This is n aevidence for the mineral olivine. Plus, it was untouched by water. Instead, the olivine formed in a body of hot magma underground. It then gradually cooled. It was exposed on the surface when the ground above it eroded away. Lower down in the Margin Unit, the olivine grains have silica between them. And interestingly, the carbonate and silica are important clues to possible past microbial life. On Earth, when water interacts with olivine, the reaction can release hydrogen, which can be a food source for some microbes. And it leaves behind carbonate and silica, two minerals that can preserve traces of the ancient microbes.