NASA researchers is now reporting in the May 21st issue of Nature that water could remain liquid at sub-freezing temperatures if made stable against freezing by containing dissolved minerals. From the abstract:
Many features of the Martian landscape are thought to have been formed by liquid water flow, and water-related mineralogies on the surface of Mars are widespread and abundant. Several lines of evidence, however, suggest that Mars has been cold with mean global temperatures well below the freezing point of pure water. Martian climate modellers, considering a combination of greenhouse gases at a range of partial pressures find it challenging to simulate global mean Martian surface temperatures above 273 K, and local thermal sources cannot account for the widespread distribution of hydrated and evaporitic minerals throughout the Martian landscape. Solutes could depress the melting point of water in a frozen Martian environment, providing a plausible solution to the early Mars climate paradox. Here we model the freezing and evaporation processes of Martian fluids with a composition resulting from the weathering of basalts, as reflected in the chemical compositions at Mars landing sites. Our results show that a significant fraction of weathering fluids loaded with [various minerals] remain in the liquid state at temperatures well below 273 K. …
273 K is the freezing point of water in Kelvin, the preferred temperature scale for astro-scientoid types.
The paper says little more for the average reader than what is in the abstract, but NASA has a press release:
“We found that the salts in water solutions can reduce the melting point of water, which may help explain how liquid water existed in a frozen Martian environment,” said Alberto Fairén, a space scientist at NASA Ames Research Center, Moffett Field, Calif. and the lead author of the study.
“Our goal was to learn how a combination of different processes of evaporation and freezing affect the freezing point of a hypothetical Martian solution. We also wanted to see how the liquid phases formed and destabilized over the evolution of different solutions, added Alfonso Davila, a co-author of the paper at NASA Ames Research Center, Moffett Field, Calif.
Fairén, A., Davila, A., Gago-Duport, L., Amils, R., & McKay, C. (2009). Stability against freezing of aqueous solutions on early Mars Nature, 459 (7245), 401-404 DOI: 10.1038/nature07978