Margaret A. Tolbert
Atmospheric chemistry; heterogeneous reactions, polar stratospheric clouds and cirrus clouds, nucleation, and planetary atmospheres.
Water uptake and loss by salt particles: Application to Mars
Research in the Tolbert Group is focused on heterogeneous atmospheric chemistry, specifically determining the chemical, physical, and optical properties of atmospheric particulate. Along with fundamental studies of particles, we are exploring how atmospheric particulate impacts current problems such as stratospheric ozone depletion, global climate change, urban smog, and visibility degradation. As well as studies of atmospheric aerosols on Earth, we are also probing particles present on other worlds including Titan and Mars. Here we highlight recent results relevant to possible liquid water on present-day Mars.
A surprising discovery made by the Phoenix Lander’s Wet Chemistry Laboratory (WCL) in the Martian arctic plains was the presence of 0.5 percent by weight perchlorate (ClO4 -). Perchlorate’s presence is interesting due to its impact on the soil’s ability to retain water, thus influencing the water cycle and habitability. Perchlorate salts are known to readily absorb water vapor from the atmosphere and deliquesce into aqueous solution. Perchlorates, like most salts, also tend to remain in a supersaturated aqueous phase instead of efflorescing back into a solid crystal. This hysteresis behavior allows liquid brine solutions to exist at low relative humidity (RH) values. Such brines may explain observations of residual slope lineae, resembling running water, that appear on Mars today. Work in our group recently reported the deliquescence and efflorescence RH values of Ca(ClO4)2 as a function of temperature and applied the results to Martian surface and subsurface conditions. To characterize the deliquescence and efflorescence of Ca(ClO4)2, we studied changes in salt phase and hydration state, using Raman and optical microscopy. To probe phase state and morphology of individual Ca(ClO4)2 droplets free from substrate, we levitated droplets in an optical trap recently developed in our lab. The figure shows optical images of a Ca(ClO4)2 particle during a deliquescence experiment in which RH was raised. It shows the initially dry particle at 0 percent RH underwent a hydration phase transition at 15 percent RH, followed by deliquescence at 26 percent RH, and growth at higher RH values. Experiments with increasing and decreasing RH, combined with martian temperatures and RH values, allow us to suggest that calcium perchlorate could be aqueous for several hours per day on Mars’ surface, and for more than half a day in the subsurface.
Honors and Awards
- CU Distinguished Professor, 2010 Award for Creative Advances in Environmental Science and Technology
- 2009 Hazel Barnes Prize
- 2007 Guggenheim Fellowship
- 2005 National Academy of Sciences Member
- 2004 NASA Group Achievement Award
- Crystal-Face Science Team
- 2003 NASA Group Achievement Award SOLVE Experiment
- 2001 BFA Award for Excellence in Research
- Scholarly and Creative Work, 2001