A life detection mission should explore it.
A team of Russian and ESA scientists, led by Igor Mitrofanov from the Russian Academy of Sciences, published a new paper in which they reported the results from the Fine-Resolution Epithermal Neutron Detector onboard of ESA's ExoMars Trace Gas Orbiter. The instrument can measure the hydrogen content in soil up to a depth of about 1 meter. The expected amount of water in the shallow subsurface of Mars can be calculated from it. Valles Marineris is a vast canyon system in the equatorial region of Mars, which is about 4000 kilometers long and up to 7 kilometers deep (the Grand Canyon in Arizona is rather small in comparison, it's about 800 km long and up to 1.6 km deep). In one central area of Valles Marineris, the team measured an astounding 40.3 (weight) % of water!
This discovery is very significant because large quantities of water were previously detected only in the polar and subpolar regions on Mars, not in the equatorial region. The detection from the instrument itself determines the amount of hydrogen, but since there is no common molecule that contains hydrogen on Mars other than water, it is assumed that all (or nearly all) of the hydrogen occurs in the form of water. The same assumptions have previously been made when using this type of instrument for both the Moon and Mars. However, the instrument does not tell us in what form the detected water is on Mars.
In principle, it does not need to be freely available water; it could also be adsorbed water or chemically bound water, such as in a crystal structure. For example, sulfate minerals such as gypsum can contain a huge amount of water. Indeed, some of the detected 40.3% of water in the shallow subsurface of the central part of Valles Marineris is likely to be mineral water. However, since the value is so high, Mitrofanov concluded that some part of the detected water - likely the largest part - would be in the form of water ice.
The area where the high-water content was measured is huge and about 41,000 square kilometers (240 x 360 km) in extension. Interestingly, that area, referred to as LANS-C, is at the lowest mean elevation of the measured sections of Valles Marineris. It has a mean elevation of 2561 meters below the average topographic elevation of Mars. That also means that the atmospheric pressure in these lower parts of the canyon is well above the triple point of water, making it likely that some of the detected water is liquid.
Liquid water is, of course, essential for life, and thus I think that this part of Valles Marineris should be considered a potential habitat for life and explored when we search for life on Mars. The problem with sending a rover to this location is the challenging terrain. A lander or rover may be smashed inside the canyon walls of Valles Marineris during touchdown. A different mission design would be needed. A few years ago, a solution was proposed by a team of scientists that included me. We suggested that a better mission design would be to use penetrators instead of a rover. The goal was that the penetrators, including a life detection package, would have a landing accuracy of several meters and use existing orbiters to relay information from the life detection experiments. Hopefully, this or a similar approach will be developed further and employed in the future so that we can reach the canyon floor of Valles Marineris and search for life.