  Special Regions on Mars are defined as environments able to host liquid water
that meets certain temperature and water activity requirements that allow known
terrestrial organisms to replicate, and therefore could be habitable. Such
regions would be a concern for planetary protection policies owing to the
potential for forward contamination (biological contamination from Earth). Pure
liquid water is unstable on the Martian surface, but brines may be present.
Experimental work has shown that brines persist beyond their predicted
stability region, leading to metastable liquids. Here we show that (meta)stable
brines can form and persist from the equator to high latitudes on the surface
of Mars for a few percent of the year for up to six consecutive hours, a
broader range than previously thought. However, only the lowest eutectic
solutions can form, leading to brines with temperatures of less than 225 K. Our
results indicate that (meta)stable brines on the Martian surface and shallow
subsurface (a few centimeters deep) are not habitable because their water
activities and temperatures fall outside the known tolerances for terrestrial
life. Furthermore, (meta)stable brines do not meet the Special Regions
requirements, reducing the risk for forward contamination and easing threats
related to the exploration of the Martian surface.
