The quest for liquid water is the driving force for many current scientific and economic enterprises. On Earth, climate change goes along with a redistribution of water – while the total amount of non-frozen water appears to increase, its availability for agricultural purposes is becoming more and more of an issue, manifested by decreasing ground water level and salinization. Outside Earth, the presence of liquid water is commonly considered a prerequisite for life.
Mars is the closest object where liquid water is suspected to exist in larger amounts. Merely taking into account atmospheric pressure and temperature, Mars appears to be located just outside Sun’s “Goldilocks zone”; however, the effects of salt and mesoporous environments reduce the melting point of water significantly while protecting it from evaporation, making the existence of liquid (diffusing, flowing) water possible. In the NMR language, “liquid” properties can be summarized by sufficiently long transverse relaxation times and sizeable diffusion coefficients. The power of NMR must therefore be sought not in the identification of water per se, but by quantifying its mobility.
We investigate which are critical hardware parameters that enable the measurement of water relaxation and diffusion on Mars, and whether these experiments can be carried out today even with existing apparatus such as the NMR-MOUSE. We hereby consider typical surface temperatures and soil compositions, in particular the very high iron oxide content and the presence of water in perchlorate eutectics which feature the most pronounced freezing point depression of all salts. Employing Mars simulant soil we come to the conclusion that the determination of microscopic (rotational) water dynamics from relaxation times measurements is straightforward, while the measurement of diffusion coefficients, their time-dependence and the translation propagators is feasible with current hardware at maximum iron and salt concentration down to temperatures of at least 230-240 K.