Scientists have found evidence of water on Mars 757 million years ago
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- Scientists have found evidence of water on Mars 757 million years ago
Water could persist on the surface of Mars much longer than previously thought. This is indicated by large crystals of selenite discovered by the Chinese Mars rover Zhuzhong on the Utopia Plain. They could have formed about 757 million years ago in a shallow reservoir with concentrated saline solution. This was reported by the journal Nature Astronomy.
What traces of water did "Zhuzhong" find?
For most of the last 3 billion years, Mars has remained a predominantly cold and ice-covered planet. It was assumed that the conditions favorable for the existence of open reservoirs disappeared at the beginning of the so-called Amazonian period. However, new data indicate that individual accumulations of liquid water could have appeared on the surface much later — hundreds of millions of years ago.
This conclusion was reached by an international group of researchers led by Jiacheng Liu, an employee of the University of Hong Kong. The scientists re-examined the images and spectral data that Zhuzhong collected in the southern part of the Utopia Plain. The Chinese rover arrived on Mars in May 2021 as part of the Tianwen-1 mission. He became the first Chinese spacecraft to successfully operate on the surface of another planet.
Initially, the Zhuzhong mission was designed for 90 Martian days, or about 93 Earth days. In fact, the device worked for about a year and covered almost 2 km. In May 2022, it was put into hibernation mode due to the onset of the Martian winter and worsening weather conditions. He could not return to work, presumably because of the large amount of dust on the solar panels.
The authors of the new work focused on the internal structure of flat stones. In the images, they found clusters of lamellar crystals up to 5-10 cm long. Some of them curved upwards and resembled cabbage leaves in shape, while others formed branched structures similar to Christmas trees or fish tails.
Such forms are typical for selenite, a coarse—crystalline type of gypsum. On Earth, it forms when water saturated with salts gradually freezes or evaporates, and the substances dissolved in it precipitate out and form transparent or light crystals.
How scientists have determined the composition of Martian rocks
The researchers compared the photos with data from the MarSCoDe device installed on the Zhuzhong. It combines laser and short-wave infrared spectroscopy to determine the chemical and mineralogical composition of the Martian surface. Infrared spectra of flat rocks showed absorption lines associated with water and hydroxyl groups. They matched the characteristics of the Earth's gypsum. Laser analysis also revealed sulfur, calcium, and hydrogen in the samples, which are the main components of hydrated calcium sulfate.
The combination of chemical composition, crystal sizes, and their characteristic shape allowed the researchers to conclude that the rocks are predominantly composed of selenite. However, scientists have not ruled out the presence of a small amount of other hydrated substances.
The authors consider the discovered layer to be a primary evaporite, a mineral sediment formed directly from a concentrated aqueous solution. Previously, gypsum on Mars was found mainly inside cracks, in the form of veins, cement, or small grains formed as a result of later exposure of groundwater to pre-existing rocks.
The selenite discovered by Zhuzhong has a different structure. Large and relatively regular crystals, according to the researchers, should have grown directly in stagnant salt water. The authors call them the first extended layer of primary large-crystalline evaporites of Amazonian age known on Mars.
How could water have appeared on young Mars
The surface in the Nakhodka area has an estimated age of about 757 million years with an error of about 66 million years. This estimate is based on the number and size of the surrounding impact craters. It is not a direct dating of the crystals themselves, but scientists believe that the formation of the mineral layer could be related to the surface renewal that was taking place at that time.
For comparison, the more ancient geological base of the Utopia Plain was formed about 3.2 billion years ago. The location of the selenite almost at the surface, without traces of multi-kilometer burial and subsequent uplift, speaks in favor of a much later origin of the layer.
According to the model proposed by the authors, the water source could be underground ice and salty groundwater. Magmatic activity or the introduction of hot rocks into the Martian crust contributed to the melting of ice and the displacement of a mineral-rich solution to the surface.
In conditions of low temperature and a thin atmosphere, the reservoir quickly froze from above. At the same time, a shallow layer of liquid brine could remain under the ice crust, heated from below. The ice on the surface gradually evaporated, and the remaining water became more and more salty. As a result, gypsum fell out at the bottom, from which large crystals of selenite grew. When the underground spring dried up, the remaining brine froze, and the surface ice gradually evaporated. The mineral layer was exposed and subsequently exposed to wind erosion.
How much water could be on the surface of Mars
Calculations show that the formation of the selenite layer required an amount of water equivalent to a column from 6.25 to 25 m high. However, this does not mean that there was a lake of such depth at the landing site of the Zhuzhong at the same time. We are talking about the total volume of water that could come to the surface gradually — during one long or several separate episodes. According to the authors of the work, the actual depth of the liquid layer at each individual moment was most likely about 1 m or less. Most of the reservoir could have been covered with ice.
Thus, the find does not indicate the existence of a warm open lake on relatively young Mars. It indicates the possibility of the temporary appearance of shallow subglacial reservoirs with very salty water associated with local geological activity.
Could the crystals have retained traces of life
Large crystals of selenite are of interest not only for studying the climate of Mars. During growth, such minerals are able to capture small volumes of the surrounding liquid. Microscopic inclusions of ancient brine may remain inside them. If the researchers' interpretation is correct, these inclusions may contain information about the chemical composition of the Martian groundwater during the Amazonian period. Theoretically, they may also contain organic compounds or other signs of an environment potentially suitable for microorganisms.
At the same time, scientists do not declare the discovery of life or its traces. The composition of the alleged liquid inclusions cannot be determined from images and remote spectral measurements. This will require examining the crystals directly on Mars or delivering samples to Earth.
The authors consider the Nakhodka area to be a promising target for future missions. A lander or rover with a microscope and more precise chemical instruments could check the structure of the crystals, detect the brine particles contained in them and determine which substances were in the Martian water about 757 million years ago.
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