High Habitability Potentials Generated by Subsurface Salt Mixtures at DLT Playa on Tibet Plateau

Abstract:

What is the habitability of a salty subsurface environment in a hyperarid region, where the salts are mixtures of chlorides, chloride hydrates, and hydrous sulfates?

We report a new concept developed on the basis of observations on Mars, field investigations at DLT playa in Qaidam basin on Tibet Plateau, and the laboratory studies of chlorides and sulfates.

The water activities maintained in a closed environment by either chlorides or by hydrous sulfates have been studied separately in laboratory. It was found that hydrous sulfates can keep high water activities (i.e., high relative humidity RH levels) in a closed environment (e.g., subsurface salty layers), and also require high RH for their deliquescence in order to generate liquid brine to support the essential functions of life form. In contrast, the deliquescence of chlorides and chloride hydrates can happen at mid-low RH levels, but an environment filled with chlorides and chloride hydrates would not provide a suitable RH range to induce their own deliquescence even with a considerable temperature (T) change, because their capability of maintaining a water activity (i.e., a RH level) in an enclosure is not a strong dependent of temperature.

The coexistence of chlorides and hydrous sulfates was found by the in situ measurements of all landed missions to Mars (rovers and landers). This coexistence was also found in the subsurface of DLT playa on Tibet Plateau by a detailed mineralogy study (Laser Raman spectroscopy and XRD). In addition, microbial life at DLT was evidenced by four types of analyses.

Within DLT subsurface, the highly hydrated sulfates (MgSO₄.6-7H₂O) would provide a suitable RH range (90-97%) for the deliquescence of co-existing chlorides (MgCl₂.6H₂O, KMgCl₃.6H₂O, NaCl, RH need to be > 33, 65, 75%) to start, which will form thin films of liquid brine at the surface of salt grains. Furthermore, we have found that the rates of deliquescence of chlorides are strong dependents of temperature. Thus a higher T during day time at DLT would generate a thicker brine film at grain surface. These parameters, sunlight, warmer T, and more aqueous brine, would favor life, i.e., the halophiles found at DLT.

This analog study shows that a salty subsurface (mixtures of chlorides, hydrated sulfates, and even perchlorates) in hyperarid region can have high habitability potentials.