The existence of liquid water on modern Mars is of great significance for the study of interstellar migration and the climate evolution of modern Mars. Liquid water is a prerequisite for shaping the habitable environment and even the existence of life on Mars. Previous studies have proved that there was a large amount of liquid water on Mars in the early days, and then with the escape of the Martian atmosphere in the early days, the climate and environment underwent major changes, and the extremely low air pressure and water vapor content made it difficult for liquid water to exist stably on Mars today and can only exist in a solid or gaseous form. However, the droplets observed on the Phoenix robotic arm prove that salty liquid water can occur in the summer at high latitudes on Mars, and numerical simulations also show that climatic conditions suitable for the presence of liquid water can occur briefly in some parts of present-day Mars. However, there is still a lack of direct observational evidence for the presence of liquid water in the low latitudes of Mars, where the temperature is highest.
In 2021, the Zhurong Mars rover carried by China's Tianwen-1 Mars mission successfully landed on the southern edge of the Utopia Planitia (UP).925°e， 25.066°n）。This area is located in the Late Northern Lowlands unit of the northern hemisphere of Mars, which belongs to the Martian low latitudes. As of the winter hibernation, the Zhurong rover has worked more than 350 Martian days, traveled about 2 kilometers, and obtained a large amount of valuable scientific exploration data.
Qin Xiaoguang, Wang Xu, Wu Haibin, researchers of the Institute of Geology and Geophysics of the Chinese Academy of Sciences, Liu Jianjun and Ren Xin, researchers of the National Astronomical Observatories, and Sun Yong, Ph.D. of the Institute of Atmospheric Physics, used the navigation terrain camera, multispectral camera and Mars surface composition detector carried by the "Zhurong" to study the microscopic morphology characteristics and material composition characteristics of the sand dune surface in the area. At the same time, through spectral data analysis, it was found that the surface of the sand dunes was rich in hydrous sulfate, opal, hydrous iron oxide and other material components. Combining the measured data of the Zhurong Mars meteorological instrument and the surface observation meteorological data of other Mars probes, the researchers determined that these surface features were related to frost or snowfall on the surface of salty sand dunes during cooling after ruling out the possibility of groundwater and CO2. After the brine is dried, the water-containing minerals such as sulfate, opal and iron oxide cement the sand particles to form aeolian sand aggregates and even crusts, and the crusts are further dried to form cracks. Later frost and snowfall further formed traces of liquid water activity on the crust, such as polygonal ridges and banded water marks. At the same time, according to the statistics of craters on the sand dunes, it is determined that the sand dunes were formed about 40-1.4 million years ago, and combined with the three-phase diagram relationship of water, it is inferred that during the great dip of the Martian axis in the Late Amazon, the diffusion and transport of water vapor from the polar ice sheet to the equator led to many humid environments in the low latitudes of Mars. A pattern of formation that causes sand dunes to solidify and leave traces of liquid water activity.
These results advance the study of evidence from ground observations of liquid water in the low latitudes of Mars, revealing that the modern Martian climate can be more humid at the lower latitudes where the surface temperature is relatively warm and suitable. This study is of great significance for exploring the history of Martian climate evolution and finding a habitable environment, and provides a key clue for the search for life in the future. The findings were published in Science Advances.
Water traces on the surface of the dunes. (a) Topographic contour map of the cross-sectional location in Figure E; (b) Banded traces on bright crusts and suspected waterlogged soil fragments and aeolian sand aggregates on dark sand ridges**; c) Enlarged image of bright polygon ridges and cracks; (d) Traces of the annular zone and its banded boundaries**; e) 3D image of the groove between two longitudinal dark sand ridges on the west flank of the bright dune, where the white dotted line is the location of the section of Figure F; (f) Topographic profile of the dotted white line in Figure E.
*: Traces of water activity on the surface of Martian dunes.
*: Voice of the Chinese Academy of Sciences.
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