This work contributes to the understanding of liquid water stability, with an emphasis on the role that dissolved solutes may have had on liquid water formation on Mars, past and present.

In chapter 2, the stability of liquid water under martian conditions is explored through experiments on ferric sulfate brines. First, it is demonstrated that such brines can be formed starting from typical martian mineralogy. Ferric sulfates are quite soluble, up to 48 wt%, and can form solutions which remain liquid down to 205 ±1 K at the eutectic. As a result of low water activities, these solutions exhibit evaporation rates 20 times lower than pure water. The combination of a low eutectic point and low evaporation rates allow subsurface liquids to be stable at high martian latitudes, where the majority of gullies and viscous flow features are located. Thus, the characteristics of ferric sulfate brines were further investigated in chapter 3, where the viscous properties of such solutions were measured, with respect to changing temperature and concentration. Using these results, the viscosity of these solutions on the formation of gullies was considered, where calculated fluid flow velocities were found to be in accordance with some estimates from image analyses of gully formations.

In chapter 4, other Mars-relevant brines were studied and characterized under martian surface conditions. Magnesium and ferrous sulfate, and magnesium and ferric chloride brines were found to stabilize water, through lower evaporation rates and freezing point depression, much like the ferric sulfate brines. For these sulfate brines, it was found that the thermodynamic process of phase change, i.e. ice formation and/or salt crystallization, can affect the kinetic process of evaporation, through very low water activities in solution. Furthermore, in chapter 5 these studies were extended to recent results from the Phoenix mission, by examining the stability of perchlorate brines under conditions measured at the landing site. It was found that both Na and Mg perchlorate brines can indeed form, and in addition, Mg perchlorate solutions may even remain stable for a few hours during the martian day.

Thus, this work has demonstrated that the stability of liquid water has important implications for the mineralogical evolution of the surface, as well as for the potential habitability of Mars. The outcome of this research may also provide necessary information with which future results from the surface of Mars may be interpreted.