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Dove, Adrienne ¹ ; Toon, Brian ² ; Heldmann, Jennifer ³

¹ University of ����ֱ��, LASP
² University of ����ֱ��, LASP
³ NASA Ames Research Center 

Gully-like features observed on the surface of Mars are likely the result of erosion by aqueous flow. The exact mechanism responsible for this flow is uncertain; however, one proposed explanation is that gullies are a result of basal melting of snowpacks, or “pasted-on” deposits found along the edges of craters (Christensen, 2003). By studying terrestrial snowpacks as analogs for those on Mars, we gain a better understanding of the physical processes that drive snow accumulation and melt. We utilize observations recorded over three field seasons at Lassen National Park, CA, which provide representative profiles of temperature, moisture, and light within the snowpacks, as well as amount and timing of runoff each season. We have used a widely distributed terrestrial snowpack model, SNTHERM (Jordan, 1991), to model the energy- and mass-balance within the snowpack during the third field season.

Our numerical simulations were run to model the snowpack growth and ablation, as compared to the observed burial and ablation at given measurement heights. In order to reproduce these observations, we found it useful to simultaneously vary multiple parameters within SNTHERM. The density of newly fallen snow and the albedo of the top snow layer can either be varied within the numerical model or held constant throughout the simulation; changing these parameters had the greatest effect on the depth, lifetime, and ablation rate of the snowpack. Air temperature was also varied within a few degrees for different simulations, and this had a significant effect on the slope of the ablation curve. Our best-fit model utilized a constant albedo of 0.62 and a constant density of 325 kg/m³ for newly fallen snow, and all temperatures were shifted by -1.7 K from the observations initially used. Model ablation of the snowpack occurred 7 days before the observed ablation at the highest sensor and 7 days after that observed at the lowest sensor (1.96 and 0.02 m above the ground, respectively). This yields a linear ablation rate of about 5.4 cm/day, slightly slower than the observed rate of about 8.6 cm/day. Depth variation of the best-fit model throughout the season is shown compared with the observed depths in Figure 1. Temperature profiles produced by this model are compared with the observational data in Figure 2, and the contribution of radiative and turbulent energy terms throughout the season are shown in Figure 3.

While we are able to reproduce a reasonable fit to the observations, we find that there are a large number of parameters whose variations have a considerable effect on the energy balance of the snowpack, so that more detailed in situ data collected throughout the lifetime of the snowpack is required to further constrain the its physical metamorphosis. As a result, further constraint of models of Martian snowpacks, such as that of Williams et al. (2008), is beyond our current capabilities.

Christensen, P., 2003, Formation of recent gullies through melting of extensive water-rich snow deposits: Nature, 422, 45-48.

Jordan, R., 1991, A one-dimensional temperature model for a snow cover: U.S. Army Cold Reg. Res. and Eng. Lab., Special report 91-16.

Williams, K., et al., 2008, Stability of mid-latitude snowpacks on Mars: Icarus, 196, 565–577.