Date of Award
Doctor of Philosophy (PhD)
Margaret A. Tolbert
Raina V. Gough
Pure liquid water is not stable on the surface of Mars because of low pressures and temperatures. The possibility of liquid water seemed more achievable since 2008 when the Phoenix lander detected ~0.5-1% perchlorate (ClO4-) in the Martian soil. Perchlorate and chloride (Cl-) salts are of interest because they readily absorb water vapor and transition into aqueous solutions, a process called deliquescence. This thesis explores the interaction of several Martian surface analogs with water. To study the phase transitions of the Mars-relevant surface analogs, we use a Raman microscope and an environmental cell. Different combinations of salts and soil analogs were studied: pure Mg(ClO4)2, MgCl2, CaCl2, Ca(ClO4)2, and also Mg(ClO4)2 mixed with Mojave Mars Simulant (MMS) and montmorillonite clay.
We found that Mg(ClO4)2 and MgCl2 solutions can supercool, forming ice only at higher relative humidity than the theoretical ice saturation conditions. It was also found that MgCl2 has a large hysteresis between deliquescence and recrystallization and can also form liquids below the eutectic temperature. These results suggest that brines can persist longer than predicted by thermodynamics alone, and longer than previously predicted in the literature. It was also found that the phase transitions of magnesium perchlorate were not affected by the presence of insoluble, Mars-relevant soil analogs. Using data from past and current spacecraft, it was found that aqueous solutions are predicted in the shallow subsurface of the Phoenix landing site, but not where the Curiosity rover has traversed. We also showed that MgCl2, CaCl2, Ca(ClO4)2 can hydrate (incorporate H2O into the crystal structure) without passing through a liquid phase. This helps shed light on the potential formation of chemical signatures found in recurring slope lineae, a current Martian mystery.
Our previous studies have measured RH with respect to liquid (RHL), but data from Martian landers is collected as RH with respect to ice (RHi) This difference can drastically alter the interpretation of potential brine formation and some previously stated brine stability in the literature may be incorrect. However, the results in this thesis support liquid solutions existing in the subsurface at the northern regions of Mars (Phoenix landing site).
Primm, Katherine M., "Exploring the Water Uptake and Release of Mars-Relevant Salt and Surface Analogs Through Raman Microscopy" (2018). Chemistry & Biochemistry Graduate Theses & Dissertations. 259.