Effect of Concentration, Cooling, and Warming Rates on Glass Transition Temperatures for NaClO₄, Ca(ClO₄)₂, and...

The hygroscopic and supercooling properties of perchlorates make them potentially important for sustaining liquid water on Mars. To understand the possibility for supercooled liquids and glasses on Mars and other cold bodies, we have characterized the supercooling and vitrification features using differential scanning calorimetry for Na, Ca, and Mg perchlorate brines in a temperature range relevant to Mars. Results show that the glass transition temperature (T_g) depends on the salt composition, concentration, and cooling or warming rate. The difference in T_g may be significant even in a single composition, producing glass transitions with over 40 K difference. A new model was developed to describe these T_g dependencies, with the warmest T_g values found for high concentrations and fast cooling rates. These results emphasize the importance of considering T_g as a range rather than a discrete temperature. For all perchlorates measured, the degree of supercooling was extensive at high concentrations, exceeding 100 K from the liquidus. With a highly reduced glass temperature (T_g/liquidus temperature) and low critical rate of temperature change to avoid crystallization, concentrated perchlorate brines are strong glass formers when compared to other glass-forming materials. The consideration of cooling rates in the context of cellular cryopreservation suggests that cooling and warming rates may be an important astrobiological factors in a diverse set of planetary environments. These findings provide additional constraints on the possibility of liquid water on Mars in terms of concentration, different latitudes, seasons, and times of day.