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.
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.