Laboratory studies of the effects of impurities on the flow of icy materials on Mars

(proposal summary)

We propose a laboratory study of the effects of impurities on the flow of icy materials at cool Martian temperatures. Mars exhibits a wide variety of landforms that are indicative of flow, ranging from the viscous creep of ice-rich permafrost, to the glaciation of thick Martian ice sheets, to the surface mobility of thin debris flows. These flow features probably contain some amount of particulates (such as dust and sand), which may be present throughout the impact-disrupted subsurface megaregolith, within dark bands in the Polar Layered Deposits, and inside sublimation lag layers atop ablating near-surface ice. The ground-penetrating orbital radars MARSIS (Mars Subsurface and Ionospheric Sounder) and SHARAD (Shallow Subsurface Radar) have confirmed the presence of large water ice deposits at or near the Martian surface. These include both the North and South Polar Layered Deposits as well as numerous mid-latitude lobate debris apron (LDA) complexes in each hemisphere. A remarkably consistent characteristic of these various icy reservoirs is their relative purity: at least 95% ice in the North Polar Layered Deposits (NPLD) and 90% in the South Polar Layered Deposits (SPLD) and lower-latitude debris aprons.

The main goal of the laboratory work is to characterize and discern the deformation physics of plastic flow of dust-bearing ice when it is deforming in grain size sensitive (GSS) mode, which is particularly relevant to the Martian environment. GSS creep is facilitated by smaller ice grain size, and is expected to operate in the very slow strain rate environment of the Martian regolith. Our preliminary experiments reveal that small amounts of dust (3% by volume) affect GSS creep in unexpected ways. Our goal is to define this behavior by systematic experiment, wherein we vary environmental parameters such as temperature, dust content, and the extent of deformation to see their effect on the basic flow relationship between stress and strain rate. We will make extensive use of cryogenic scanning electron microscopy to search for microstructural clues about the causes of these dependencies on environmental variables.

We propose to carry out two other tasks at a lower level of effort to address current topics in Mars geology that we are uniquely equipped to study.  The first is an intriguing aspect of deformational behavior in frozen sand packs whose composition is sufficiently sand rich that deformation appears to stop entirely. Laboratory and geological time scales are vastly different, however, and a process that appears to stop on the laboratory scale may actually be ongoing, but at too slow a rate to observe directly. We propose to speed up the geologic clock, so to speak, by deforming surrogate frozen sand packs where the sand is not a hard silicate, but instead a slightly more deformable material. Based on previous experience, we expect that methane clathrate, an exceedingly strong icy material, will serve in this role.

The second small task is an investigation of the flow of ice mixed with magnesium perchlorate hydrate, the recent discovery of which near the Martian north pole strongly motivates its investigation as a possible agent of flow enhancement in the NPLD. A preliminary experiment we conducted revealed that a eutectic mixture of ice and perchlorate hydrate is in fact dramatically weaker than pure ice, so the presence of even small amounts of perchlorate may be an explanation for the enhancement. We propose to quantify this observation with tests on ice and perchlorate of varying fractions, from the eutectic to the millimolar level.