Laboratory Studies of Surface-Atmosphere Water Exchange Processes on Mars

The recent discoveries of highly deliquescent perchlorate salts and strongly adsorbing porous minerals on the surface of Mars are intriguing as liquid brine solutions and adsorbed water could potentially provide a habitable environment for living organisms on present day Mars. A deeper understanding of these processes would also aid in the interpretation of data from past and present Mars missions, provide insight into the geological history of water on Mars, and assist in constraining the current water inventory on Mars.

A Raman scattering lidar (light detection and ranging) instrument and environmental simulation chamber were developed to investigate the exchange of water between the atmosphere and surface of Mars. Raman scattering was applied to detect water uptake by samples of magnesium perchlorate hexahydrate. When exposed to the water vapour pressure and temperatures found at the landing site of the NASA Phoenix mission, magnesium perchlorate samples of the size found on Mars began to take up water from the atmosphere (deliquescence) at temperatures above the frost point temperature for pure water ice. Significant water uptake from the atmosphere began to occur within minutes, indicating that bulk deliquescence is likely to occur on present-day Mars. This demonstrates that perchlorate in the surface material can contribute to the hydrological cycle on Mars by absorbing water directly from the atmosphere and forming liquid water brine.

Chamber experiments were also conducted to study the adsorption of water on regolith grains. Raman spectroscopy was applied to study the adsorption properties of zeolites under conditions found at the Mars Phoenix site. Experiments demonstrated that zeolites on the surface of Mars are capable of adsorbing water from the atmosphere on diurnal time scales and that Raman spectroscopy provides a promising method for detecting this process during a landed mission. When the water vapour pressure and temperature were low enough, the zeolite sample also adsorbed carbon dioxide, resulting in the simultaneous adsorption of water and carbon dioxide on the mineral grains. Additional experiments were carried out using a mixture of magnesium perchlorate and chabazite. The sample of mixed surface material remained visually unchanged during water adsorption, but darkened during deliquescence.