Hypothesis Testing for Subfreezing Mass Movements, 20–R8407
Donald M. Hooper
Cynthia L. Dinwiddie
Inclusive Dates: 07/01/13 – Current
Background — Debris flows, some seasonally recurring, are known to be present on dune slopes in several mid- to high-latitude dune fields on Mars. Mass movement signatures are among the best records of historical and ongoing geologic activity on Mars, and suggest the erosive action of water and present habitability — a major investigative theme of the modern Mars Exploration Program. Sand dunes are young, transient features on Earth and Mars; debris flows on Martian sand dunes therefore imply recent alluvial activity and seasonally recurring debris flows imply modern, ongoing alluvial activity. Analogous debris flows consisting of a sand and liquid water mixture that cascaded down leeward dune slopes under subfreezing conditions were observed in March 2010 at the Great Kobuk Sand Dunes, Alaska. Similar mechanisms may be responsible for generating Martian debris flows on dune slopes.
Approach — During winter 2013–2014, SwRI will conduct a field survey of subfreezing debris flow development on sand dunes at the Great Sand Dunes National Park and Preserve (GSDNPP) in Colorado. The hypothesis is that relatively dark sand lying on bright snow may cause local hot spots to form where solar radiation can be absorbed by the sand and conducted into the snow, enabling meltwater to form at subfreezing air temperatures and sand to mobilize through alluvial processes. Measurement of the areally distributed and multilevel subsurface environmental conditions that are associated with initiation of subfreezing debris flows will be acquired to better understand the fundamental processes that may lead to similar alluvial mass wasting events on Mars. Surveys of slope angles, solar radiation, areal and temporal surface temperature distributions, moisture profiles, timing of flow initiation, debris flow velocities, water-to-sediment ratio, and debris flow morphologies will be performed to validate SwRI’s conceptual model of the processes that control subfreezing debris flow initiation and development.
Accomplishments — Meteorological data was downloaded from the GSDNPP Remote Automatic Weather Stations (RAWS) site. For the initial analysis, the mean daily values for air temperature and solar radiation were examined. The maximum mean daily total solar radiation was plotted as a measure of the maximum potential total solar radiation that could fall on the dunes at this latitude and time of year. For the temperatures, the focus was on the maximum temperature during the course of the day. During field work, debris flows may be observed when the maximum daily air temperature is subfreezing, but solar radiation is strong. In October 2011, GSDNPP was mapped by LiDAR (Light Detection and Ranging) at 1 laser return per meter to provide a high resolution digital elevation model (DEM). The National Park Service provided this data set at no cost to the project. The LiDAR-based DEM is fundamental for geomorphologic evaluation because it provides topographic detail and the ability to resolve spatial derivatives of elevation, such as slope and aspect. These data were used to identify on an hourly basis the maximum winter solar illumination across dune slopes. Identifying the diurnal maximum total solar radiation levels and the maximum air temperature levels can determine which dune slopes are strongly illuminated during a specific portion of the day and then the most likely period of the day when meltwater-induced debris flows will form during the winter can be estimated.