May 20, 2020
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Water on Mars: very cold and very salty, not suitable for life

Recent observations have suggested that liquid may exist on the surface of Mars, at least in the form of saline solutions. Calculations show that such solutions, if formed, will be stable on part of the surface and only for a short period of time in a year, and their conditions (temperature and salinity) are outside the range in which our usual life is possible.

Due to the low temperature, pressure and dry atmosphere on Mars, a drop of fresh water, formed on its surface should instantly freeze, boil or evaporate. The same fate awaits water ice on most of the surface - it must quickly sublimate (evaporate without melting). If water contains dissolved salts, its freezing temperature will be lower, and physical conditions in certain areas of the planet may be favorable for maintaining the liquid phase for some time. Since there are salts on Mars, salt solutions (brines) are theoretically possible there. Article published in May 2010 in Nature Astronomy , describes the results of modeling phase diagrams of salt solutions under Martian conditions and the consequences in terms of the range of possible life.

Water in a liquid state could exist even on the surface of Mars in the distant geological past. So, on Mars, relief forms formed at one time by the fluid medium — river valleys and river deltas — have long been discovered and are being studied. The Perseverance rover in the framework of the Mars project - 2014 will investigate one of the craters just in the vicinity of the material removal cone from paleorecs. In addition, it was in the area of ​​his future landing with the help of the orbital station that carbonate minerals were first identified (for example, ordinary limestone). Such materials on Earth often indicate sedimentation, so the region has attracted attention as a suitable place for the study of biosignatures (for more details see the note). However, such greenhouse conditions ended on Mars about 2-3 billion years ago, when the Earth was in the Archean or early Proterozoic era.

“Seasonal stripes on the slopes”, or RSL, in Newton’s crater on Mars. NASA / JPL-Caltech / Univ. of Arizona .

Now atmospheric conditions on Mars are believed to prevent the existence of liquid water on the surface. However, there are some indications that, from time to time, water may appear on the surface or underground close to it. In the beginning 1830 - s in the pictures of the Mars Reconnaissance Orbiter station, dark stripes up to several wide were found meters, the size of which increased during the warm seasons and decreased in the cold. Such relief microforms can be associated with places of seepage of water to the surface or even temporary streams in the Martian summer. They were called Recurring Slope Lineae (RSL) (“seasonal stripes on the slopes”, there is no established term in the Russian language yet.)

These discovered structures an obstacle for the Curiosity rover, which must avoid them on its way, so as not to accidentally introduce microorganisms from the Earth into a humid environment. In such an environment, they have some chance to survive and then be “open” as “life on Mars”. These are the general requirements of planetary protection measures. The same problem is encountered when studying deep under-ice lakes in Antarctica, which are important not to infect biota from the surface. Therefore, in 1241 - 2014 The NASA research groups NASA determined the conditions under which studies on Mars should be carried out with extreme caution as part of planetary quarantine. So, the so-called “Special Regions” were defined: the conditions in which liquid water can exist at temperatures above - 23 ° C and with “water activity” above 0.5 (here is a measure of water salinity: 1 - distilled water, 0 - pure salt , at sea water this indicator is about 0, 40). It is believed that these are the limits within which known terrestrial organisms can survive.

Landing Martian Phoenix module and an ice covered surface of Mars. Researchers were attracted by drops of water or saline solution on the apparatus bracket in the background.

Subsequently, astronomers admitted that with “water currents” on Mars, they may have gotten excited, and RSL structures can be caused by flows of dry material (such as sand screes), then there are no longer any of these areas under the category of “special areas”. However, conditions on Mars may be favorable for the existence of concentrated saline solutions. So, in the photograph of the Phoenix lander 1241, years in the polar region of Mars are supposedly visible on its bracket drops of liquid. Therefore, research continued on the conditions for the existence of salt solutions on Mars and their capabilities for maintaining the life of microorganisms.

The astrophysicists of the Lunar and Planetary Institute, LPI, simulated physical conditions on the surface of Mars and studied the phase diagrams of the stability of solutions of different salts depending on the region and season. . As reference input parameters, climate data obtained from Martian orbital stations and rovers, models of the Martian atmosphere and earth experiments with solutions of concentrated salts - calcium and magnesium perchlorates were used. Salts in terrestrial conditions at certain ratios of temperature and humidity can be hygroscopic, that is, take moisture from the atmosphere and turn into solutions. Theoretically, such a mechanism is possible on Mars, and it was taken into account during modeling, although the Martian atmosphere is much drier and colder than the earth.

Zones of possible existence of salt solutions on the surface of Mars (perchlorate calcium). The color indicates the proportion of the season during which the brine can be on the surface in liquid form. Rivera-Valentín et al., Nat. Astronomy (2010) .

The results indicate the possibility of the formation and existence of saline solutions on 22% of the surface of Mars. Brines can form at various Martian latitudes from the equator to its polar regions and remain in a liquid state for up to six hours. As can be seen from the example of a consolidated map of the stability of a solution for calcium perchlorate, salt solutions can indeed exist on the planet, gravitating to moderate Martian latitudes. By the way, similar patterns of distribution have water ice deposits below the surface according to satellite radar data. However, brines can form only in certain seasons and exist for only two percent or less of the duration of the Martian year. The highest temperature of existence of such a brine is 23 ° C. This is a wider range than previously thought. But the temperatures of such brines at different points are still significantly lower than the temperature limit at which, as it is believed, life in the forms known to us on Earth is possible.

The stability of the liquid phase only for a limited period of time in a year is not enough to maintain and reproduce life. However, hypothetical salt solutions on the surface of Mars are unsuitable for life, falling into another fork of limitations. The stability temperature of such a brine will be either too low for life, or a solution that does not disappear at a higher temperature will have to have too high salinity, which also excludes the existence of organics.

More favorable conditions for the existence of water in the liquid or solid phase are found at depths of several centimeters below the surface (under the regolith layer). Also, taking into account the metastable states of solutions somewhat expands the window of opportunity for liquid water, but so far there is no need to talk about the existence of “Martian lakes”.