The presence of liquid water is the lodestar which marks the course of any investigation on habitability beyond the Earth. And the assessment of Mars habitability, since the Viking mission was launched in the 70’s, has been a constant concern in the exploration of the planet as a main scientific goal for all the following missions to the planet. Several of them were intended precisely, among others things, to follow the track of water, showing ultimately that Mars surface was once partially covered by great bulks of water for long periods of time (in geological scales), and that presently there is much more water (ice water) than previously thought within the Martian ground.
Finally, a study published in 2015 (Transient liquid water and water activity at Gale crater on Mars, Nature Geoscience, vol. 8, p. 357-361, doi:10.1038/ngeo2412) established that there is a daily water exchange cycle between the soil and the atmosphere, including phases during which this water remains in a metastable liquid state. Considering the environmental conditions of Temperature and Pressure on Mars’ surface, this can happen thanks to the involvement in the process of perchlorates and some chlorides, a kind of highly hygroscopic chlorine salts which are widespread all over Martian soil. They have the capability of absorbing a great amount of water from the surroundings, up to the point of getting dissolved in it forming solutions whose eutectic temperature allows them to reach the liquid state as brines under the mentioned conditions during certain periods of each sol (Martian day).
The aforementioned study was performed from data gathered by three instruments on board Curiosity rover (Mars Science Laboratory -MSL- mission), namely, Sample Analysis at Mars (SAM, which helped assessing the perchlorate content along the rover’s traverse), Dynamic Albedo of Neutron (DAN, intended to measure the water ice present within the ground up to a depth of 60 cm), and Rover Environmental Monitoring Station (REMS, an meteorological station monitoring the atmospheric parameters as well as the UV irradiation and the ground temperature). The combined analysis of these data sets led to the conclusion that, in the landing site (crater Gale), for some periods of every night whose length varies depending on the season, brines stay in a transient liquid state. Curiosity rover is operating close to the Martian equator, the driest region of the planet, so it is estimated that the process might have place to a larger extent in higher latitudes. In addition, subsequent observations performed by means of remote sensing instruments on board the Mars Reconnaissance Orbiter (MRO), have come to support the occurrence of liquid water on different locations.
However, a direct and irrefutable confirmation of the phenomenon would be necessary in order to acquire not only the certainty of its occurrence, but also to go into depth on its understanding, both qualitatively and quantitatively.
HABIT, one of the instruments selected as part of the scientific payload on the Surface Platform module of ExoMars 2020 mission, has been designed to reproduce this natural process and to monitor it as it happens, obtaining this way a temporal and environmental framework of it, while providing a quantitative approach.
These aims can be summarized in the scientific goals to be reached by means of the instrument along the nominal mission (1 Martian year) operations: