So far it is unclear how long the habitable period lasted on Mars and how long there was still liquid water. The Chinese Mars rover Zhurong is now providing exciting data in this regard. Because his analyzes of rock samples in the south of the great impact depression Utopia Planitia suggest that there was still water at the surface or just below in the youngest epoch of the planet. Researchers see hydrated silicates and sulfates as evidence of this, which must have formed within the Amazonian age, which began around 1.8 billion years ago. In their view, this could indicate that liquid water has existed, at least intermittently, on the red planet for longer than previously thought.
Our neighboring planet Mars was probably warmer and more livable in its early days than it is today. Common assumptions back then were that it had a denser atmosphere, lakes, rivers and, according to some researchers, maybe even an ocean. So far, however, it is disputed how much liquid water actually existed at that time and also how long it remained. Because after the milder early period, the climate of the red planet changed, it lost large parts of its atmosphere and it became colder and drier. At the latest at the beginning of the Amazonian period around 1.8 billion years ago, but probably already at the beginning of the Hesperian Martian Age 3.5 billion years ago, there were no longer any permanent occurrences of liquid water on the surface of Mars. Today, water on Mars only exists in the form of water ice, which is found, among other things, in the polar ice caps and in the remains of underground glaciers. It is disputed whether there is still water in the form of liquid, highly saline brine under the surface of Mars.
Rover examines rocks in Utopia Planitia
But what about at least a temporary appearance of liquid water? According to some hypotheses, even in the cold, dry Martian modern era, water from hydrothermal vents or other warmed groundwater sources could have surfaced at least occasionally. “Recent analyzes have identified fluvial landforms that intersect Amazonian terrain,” explain Yang Liu of the National Space Research Center in Beijing and his colleagues. “This indicates that even the most recent epoch of Mars still showed watery activity that shaped the surface.” , was to deliver, among other things, the Chinese Tianwen-1 Mars mission, which dropped the rover Zhurong in May 2021 in the south of the Utopia Planitia plain.
Equipped with six different scientific instruments, the rover has spent the 92 days of its mission studying the morphology, mineralogy, surface structure and ice distribution around its landing site in the largest impact basin on Mars. The data now analyzed by Liu and his team show that there are two different rock types in this part of Utopia Planitia. The first includes dark, basaltic boulders thought to come from older, deeper strata, the team explains. The second type of rock, on the other hand, is predominantly light in color, with the light-colored material sometimes forming a coating on a darker interior. “These chunks are dust-covered and often show flaking or scaly surfaces that indicate physical weathering from thermal stress and wind,” the researchers report. From the investigations and the geological conditions, they conclude that these chunks must come from the Amazonian period.
Hydrated mineral crusts indicate pore water
Closer analysis of the bright rocks using various spectrometers revealed that these chunks contain hydrated minerals — minerals that have water bound in their structure and are normally formed on Earth in the presence of liquid water. The spectral signatures of Martian rocks could match those of hydrated silicates and sulfates, as well as gypsum, according to Liu and his colleagues. Given the light layers partially formed around dark cores, they hypothesize that it is a type of duric crust — hard crusts formed by the precipitation of minerals from the pore water of rocks. In the case of the Martian crusts, the researchers suspect that they formed when saline brine rose underground during phases of higher groundwater levels. The saline mineral crusts then formed where evaporation from the nearby surface further increased the salinity of this pore water.
According to the scientists, the existence of these mineral crusts therefore suggests that there was still sufficient groundwater in the Amazonian period to temporarily soak the near-surface subsoil and lead to the formation of the duric crusts. “These observations suggest that aqueous activity on Mars has lasted far longer than previously thought,” Liu and his team write. They see volcanic activity and the oscillation of the Martian axis as possible causes for the episodic occurrence of liquid water underground. Because this has repeatedly changed its orientation by more than a dozen degrees in the course of Martian history, a somewhat milder climate may have prevailed in the southern Utopia Planitia and other areas, at least temporarily.
However, the new results also have potential implications for future manned Mars missions: “The Zhurong landing site and other areas of the northern plains may contain significant amounts of usable water in the form of hydrated minerals and subsurface ice, which future Mars exploration will use as a pro… -place resource,” emphasizes the research team.
Source: Yang Liu (National Space Science Center, Chinese Academy of Sciences, Beijing) et al., Science Advances, doi: 10.1126/sciadv.abn8555