The surface of Mars today is a hyperarid desert, yet in many places has features apparently formed by liquid water. It is now generally accepted that liquid water was present on early Mars, when the planet had a very different atmosphere. However, that early atmosphere has subsequently disappeared, and it is now assumed that the atmospheric pressure on Mars is now to low for liquid water to form.
This being the case, it came as a great surprise to scientists when, in March 2009, droplets of liquid were observed on one of the robotic arms of NASA's Phoenix Rover. Studies of the data recovered by Phoenix eventually concluded that the conditions for hypersaline water could exist on Mars at high latitudes during the summer, when the temperature rose above the melting point of such solutions (which is significantly lower than 0°C), resulting in a freeze-thaw cycle that might help to explain some of the features seen in these regions.
However, the wider presence of apparent water features, such as slope lineae and gullies, is harder to explain. This had led scientists do develop alternative explanations, under which such features could have developed under dry conditions, since it is difficult to understand how sufficient liquid water could be present on the surface of Mars to explain features such as slope flows hundreds of metres in length. This disparity between a theoretical presence of small amounts of water on Mars, and a necessity to invoke dry formation of features we would typically associate with the presence of large amounts of water on Earth, makes it necessary to study smaller (less than one metre) features on Mars's surface in order to understand the planet's hydrological conditions properly. This is particularly true of features at lower (i.e non-polar) latitudes, where milder conditions make a freeze-thaw cycle unlikely, and there is a higher potential for the presence of some form of microbial life.
The China National Space Administration's Zhurong Rover landed on the southern Utopia Planitia on 15 May 2021, and spent nine months exploring the Late Hesperian northern lowlands, including studying the microstructure and chemical composition of the dune features observed there.
In a paper published in the journal Science Advances on 28 April 2023, Xiaoguang Qin of the Key Laboratory of Cenozoic Geology and Environment at the Institute of Geology andGeophysics of the Chinese Academy of Sciences, Xin Ren of the Key Laboratory of Lunarand Deep Space Exploration at the Chinese National Astronomical Observatories, Xu Wang, also of the Key Laboratory of Cenozoic Geology and Environment at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, Jianjun Liu, also of the Key Laboratory of Lunar and Deep Space Exploration at the Chinese National Astronomical Observatories, Haibin Wu, again of the Key Laboratory of Cenozoic Geology and Environment at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, and of the College of Earth and Planetary Sciences at the University of Chinese Academy of Science, Yong Sun of the Institute of AtmosphericPhysics of the Chinese Academy of Sciences, Zhaopeng Chen, again of the Key Laboratory of Lunar and Deep Space Exploration at the Chinese National Astronomical Observatories, Shihao Zhang, also of the Key Laboratory of Cenozoic Geology and Environment at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, Yizhong Zhang, Wangli Chen, Bin Liu, Dawei Liu, and Lin Guo, again of the Key Laboratory of Lunar and Deep Space Exploration at the Chinese National Astronomical Observatories, Kangkang Li, again of the Key Laboratory of Cenozoic Geology and Environment at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, Xiangzhao Zeng, Hai Huang, Qing Zhang, Songzheng Yu, and Chunlai Li, again of the Key Laboratory of Lunar and Deep Space Exploration at the Chinese National Astronomical Observatories, and Zhengtang Guo, once again of the Key Laboratory of Cenozoic Geology and Environment at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, present the results of a study of the surficial microstructure, morphology, and chemical compositions of dunes studied by the Zhurong Rover, and the implications of these results for the possibility of liquid water having existed on the surface of Mars at low latitudes in the recent past.
A series of detached barchan (crescent-shaped) dunes with sinuous profiles are present in the Zhurong Rover landing area, each completely detached from its neighbour. The Zhurong Rover encountered four of these as it made a north-to-south transect of the area in the first four months after it landed.
The rover discovered that the dunes were covered by two very different types of sand, one light and one dark, with the dark sand overlying the lighter dunes, implying a second generation of deposition. The barchan dunes are composed of the lighter sand, and are 15-30 m long and 3-10 m wide. The darker sand matches the surrounding soils, and forms longitudinal dunes and ridges running over the barchans, most commonly at northwest orientated longitudinal dunes crossing the western flank of the barchan. These longitudinal dunes appear more recent, and are likely to have formed under the current modern conditions.
Close examination of the surface of these dunes shows more that one form of cementation holding the particles together, with a continuous crust having formed on the light dues and the particles of the darker ridges held together in agglomerated clusters. Examination of the agglomerated particles suggests the presence of hydrated sulphates, hydrated silica (particularly opal), iron oxide minerals, and possibly chlorides. The hydrated sulphates and hydrated silica are most likely to be forming the cements holding these particles together.
Compositionally, both the light and dark sands have high iron and magnesium contents, although silica remains the most abundant material, comprising between about 52% and about 90% of all samples. Oxides make up about 15% by weight of the light sand and about 10% by weight of the dark sand. The instrumentation used is known to be incapable of detecting volatile elements such as sulphur, chlorine, and phosphorus, as well as hydrogen and hydroxide ions.
The second and third dunes encountered are covered with polygonal cracks, although these are seen only on the underlying bright sand dunes, not the darker sand ridges running across them. The polygons formed by the cracks have an average area of 55.2 cm², and an average side length of 4.8 cm, far smaller than cracks previously observed on Mars by remote sensing. Assuming that the cracks have a depth to width ratio of between 1/3 and 1.4, this would equate to a depth of 1.25-1.7 cm. The majority of the polygons are pentagons, though they range from triangular to heptagonal in shape. The average internal angle of the polygons is 120°, and intersections between cracks are typically Y-shaped.
A light-toned, strip-like trace, over 40 cm long and about 1.5 cm wide was observed within the interdune depression of the second barchan dune. This ran along the lowest part of the trough depression, and separates light and dark bands of sand, with a dark sand slope to the north and a light sand slope to the south, with abundant polygonal cracks. The shape of this trace appears to be exactly what would be expected by pooled water, should this be able to exist here, and the underlying crust be impermeable to water.
Based upon the superposition of the features, Qin et al. conclude that the light-coloured barchan dunes were formed first, then became encrusted with sulphates, and possibly chlorides, during a more humid climatic phase.
In order to determine the age of these dunes, Qin et al. looked at the density of craters on the land-surface they form part of (the rate at which asteroids randomly impact Mars is considered to be approximately constant, so that parts of the Martian surface can be dated by the density of impact craters), concluding that this surface was between 400 000 and 1.4 million years old.
The polygonal cracks which have formed on the surface of some of these cracks are believed to have been caused by a loss of moisture, either through drying or desiccation, with the dark, longitudinal dunes forming after this, and finally the sand in the longitudinal dunes becoming agglutinated into clumps. The cementing of the sands requires a liquid or gas which was able to fill the pore spaces between them, then transform into a solid state. Such substances would include carbon dioxide gas turning into dry ice, liquid water freezing into ice, or various salts and other hydrated chemicals precipitating out of solution as the water in which they were dissolved evaporates. The conditions around the Zhurong landing site make the formation of dry ice and/or water ice highly improbable, and both of these would be detectable by the laser-induced breakdown spectrometer on the Zhurong Lander, which has found no evidence of their presence. However, hydrous sulfates, opaline silica, ferric oxides, and probably chlorides, have bee detected, and this mixture would provide a suitable cement for the sand grains.
The formation of a cement from a mixture of salts and hydrated minerals requires the presence of liquid water. This could have originated from rain, snow, or frost, or have upwelled from a subterranean source, although the evaporation of groundwater drawn upwards by capillary action seems unlikely, as there are cracked evaporation surfaces on the raised dunes, but not the surrounding flatlands, which makes the precipitation of water, either as rain or frost/snow which then thawed before evaporation, the most likely explanation.
The saturated vapor pressure (point at which the atmosphere can hold no more evaporated water, and it begins to precipitate out) is unrelated to the atmospheric pressure, although the temperature must be above 0°C for liquid rain to fall. At 0°C on Mars the saturated vapor pressure would be 611 pascals, while the atmospheric pressure observed on Mars by the Zhurong Lander is between 786 and 834 pascals, meaning that the atmosphere would need to be about 72% water for rain to fall. Since the modern Martian atmosphere is about 95% carbon dioxide, liquid precipitation on Mars is currently impossible.
Several different landers have now taken atmospheric readings on Mars, giving a range of surface temperatures between -105°C and -5°C, a range of atmospheric pressures between 683 and 849 pascals, and a vapor pressure of 0.27 pascals. Under these conditions, the frost point (point at which the temperature drops so low that water absorbed into the atmosphere precipitates out as frost) would be about -74°C, which means frost would be possible at the Zhurong landing area. More widely, it is assumed that frost and snow are relatively common on Mars.
Mixing water ice, from frost or snow, with salts could potentially lead to its melting point being lowered sufficiently for highly saline liquid water to form. Any subsequent raise in temperature could subsequently lead to water evaporation, with seasonal or even daily cycles of frost formation, melting, and evaporation enabling the formation of cements.
The temperature on Mars is thought to rise rapidly between 5.00 and 6.00 am, local true solar time, providing an interval in which frost can sublimate, and potentially also in which it could melt and then evaporate in a hyper-saline environment. This happens seasonally, with steeper rises and higher temperatures achieved in local summer.
The orbital obliquity of Mars (i.e. the angle at which it is turned to the Sun, which determines the severity of the seasons) is thought to have been equal to or greater than it is now throughout the past 1.4 million years, which would mean that the climate of Mars has been comparable to or more humid than Today throughout this interval. This would imply that the formation of liquid water at low latitudes on Mars has remained at least as possible as it is today over this period.
Such a process of repeatedly forming hypersaline solutions would facilitate the dissolution of silica from sand grains to form opal, as well as attacking other minerals, enabling hydrated sulphates and iron oxides to form.
The polygonal cracks on the surface Mars are also almost certainly the result of either freeze-thaw thermal contraction or desiccation, in response to seasonal or daily changers in temperature, with their general shape suggesting the later is more likely. Meteorite impact effects and carbon dioxide freeze/sublimate cycles have been suggested as an origin for similar cracks elsewhere on Mats, but there are no signs of any meteor impacts large enough to have caused these cracks near the Zhurong landing site, and carbon dioxide is unable to freeze out of the Martian atmosphere this far from the Martian poles.
Qin et al.'s study is the first small-scale study of such cracks at low latitudes on Mars. They believe that these features are almost certainly the result of desiccation, but cannot rule out an alternative hypothesis, in which the cracks are formed by the freezing of hypersaline water, causing cracks to form in the crust under tensile stress.
All of the features seen in the Zhurong landing area point towards the presence of saline water, providing evidence that liquid water can form at low latitudes on Mars. Qin et al. propose that water accumulated on top of the dunes as frost or snow after the atmospheric temperature dropped below the frost point, then melted due to a combination of rising temperatures and contact with salt within the sands. which would in turn facilitate the formation of hydrated silica (opal) and iron oxides. This water would then evaporate away, at fairly low temperatures due to the low atmospheric pressure on Mars, leaving the salts to precipitate out and form a cement between the sand grains, forming cracks on the dune surface as they dried out. This cycle would likely repeat numerous times.
If this hypothesis is correct, then it suggests that the amount of liquid water available on the surface of Mars in the recent past is considerably higher that previously suspected. It has previously been suggested that transient films of water might have formed on the surface of rocks in the recent past due to acid weathering, and that small amounts of water might have formed duricrusts and rock surface coatings over geological timescales. The situation at the Zhurong landing site appears quite different, with apparently mobile sands unlikely to have become cemented together by any process operating on a geological timescale. Rather this appears to be the result of an evaporative process operating over a relatively short period. less than 1.4 million years ago, and possibly less than 400 000 years ago.
This recent presence of water at 'tropical' latitudes on Mars becomes less unreasonable when it is remembered that Mars is thought to have undergone a significant change in the obliquity of its orbit about 5 million years ago, and only to have reached its current, low-obliquity orbital configuration about 3 million years ago. This may suggest that the thick ice caps present at the current Martian poles are a relatively modern feature, a result of a fairly recent transfer of water from lower latitudes, something which may well have still being occurring 1.4 million years ago. This provides further support for the theory that high-obliquity excursions in the Martian orbit might well have provided enough water for gully formation. Thus the presence of sufficient saline water at low latitudes on Mars for evaporite formation in the recent past is in fact in accord with our current understanding of the planet's recent geological past.
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