RHEOLOGICAL BEHAVIOR OF SALT GLACIERS ON MARS: IMPLICATIONS FOR THE ORIGIN OF VISCOUS FLOW FEATURES

Kilometer scale viscous flow features observed in the mid-latitudes of Mars have been attributed to subsurface ice and rock mixtures. However, many of these observations may be consistent with the presence of salt glaciers and thick deposits of evaporite-rich material. Recent alpha particle x-ray spectroscopy and miniature thermal emission spectroscopy ground measurements from the rovers Spirit at Gusev Crater and Opportunity at Meridiani Planum illustrate the ubiquity of evaporites in martian sedimentary rock composition. To evaluate the potential for evaporitic viscous flow features, we compare the flow strength of ice and rock salt at mid-latitude martian surface conditions. The ice constitutive equation of Goldsby and Kohlstedt (2001) include contributions from dislocation creep, mixed accommodated basal slip and grain boundary sliding, and diffusion creep. Assuming salt deformation mechanisms work in parallel, strain rate contributions for dislocation creep from Wawersik and Zeuch (1986) and pressure solution creep from Spiers (1990) are added together. Under identical conditions, rock salt flows three to seven orders of magnitude slower than ice. However, salt flow is sensitive to grain size to the inverse third power. Finer grained salt can flow faster than ice under stresses up to several MPa, equivalent to the stress at the bottom of an ice glacier approximately 700 meters thick with a grain size of 50 microns. Terrestrial analogues provide evidence that salt glaciers associated with diapirs and thick salt deposits exist and readily flow on a timescale of hundreds to thousands of years. The temperature dependence of salt viscosity shows that martian salt deposits at temperatures of 200 K will flow approximately three orders of magnitude slower than terrestrial salt deposits at 280 K. Salt flows are possible over geologic time and provide one explanation for viscous flow features on Mars.