HR: 15:25h
AN: P23C-08    [Abstracts]
TI: Spatial Patterns of Accumulation and Ablation on the Martian North Polar Cap from Ice Flow Inverse Modeling
AU: * Winebrenner, D P
EM: dpw@apl.washington.edu
AF: Applied Physics Laboratory and Dept. of Earth and Space Sciences, University of Washington, Box 355640, Seattle, WA 98195 United States
AU: Koutnik, M
EM: mkoutnik@u.washington.edu
AF: Dept. of Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195 United States
AU: Waddington, E D
EM: edw@ess.washington.edu
AF: Dept. of Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195 United States
AU: Bamber, J L
EM: j.bamber@bristol.ac.uk
AF: Bristol Glaciology Centre, University of Bristol, University Road, Bristol, BS8 1SS United Kingdom
AU: Pathare, A V
EM: avp@gps.caltech.edu
AF: Planetary Science Institute and California Institute of Technology, MC 150-21, Pasadena, CA 91125 United States
AU: Murray, B C
EM: bcm@caltech.edu
AF: California Institute of Technology, MC 150-21, Pasadena, CA 91125 United States
AB: How ice flow and mass exchange shape the Martian ice caps is a fundamental, open question. This question can be addressed quantitatively by using ice flow inverse modeling to interpret stratigraphic and topographic data from the caps. (Flow inverse modeling estimates ice cap parameters and climatic forcing by assimilating observed ice cap characteristics into an ice-flow model, in contrast to forward modeling, which uses an ice-flow model to predict ice cap characteristics using specified parameters and forcing.) We estimate spatial patterns of accumulation and ablation on Mars? North Polar Cap (NPC) by applying a simple flow inverse model to Mars Orbiting Laser Altimeter (MOLA) observations of surface topography. By highlighting the MOLA data to emphasize regions with flat slopes and high elevations, we identify an apparent ice divide (i.e., a boundary separating regions of ice flow in different directions) on the NPC. By following the surface gradient from the divide to the ice-cap margin, we identify likely flow lines (if flow occurs). Along each flow line, we apply a flow inverse model that assumes uniform, steady-state accumulation from the divide to an equilibrium-line location (which is to be determined), and uniform, steady-state ablation from that location to the cap edge. We combine these elements with assumed basal topography (based on MOLA observations surrounding the cap), and find the model surface profile that best fits the observed topography along each flow line independently. The values of parameters in that fit combine to yield the equilibrium-line location. For each profile, the inverse model places the equilibrium line near the high-elevation side of the highest-elevation trough. The equilibrium-line locations for all profiles taken together trace a smooth boundary between accumulation and ablation on the NPC. This result supports the idea that ablation is key to trough formation, and suggests that ice flow has controlled the surface topography of the cap, at least at some time in the past. Indeed, the cap may have flowed only when higher obliquity caused warmer polar temperatures. Our results can be combined with modeled internal ice cap temperatures to estimate the absolute rates of accumulation and ablation at the time of flow, which range between O(10^-5 mm a^-1) of ice for a depth-averaged temperature of 173K (close to the present mean annual surface temperature), to O(10^-2 mm a^-1) for 213K (a temperature that could be more typical during high-obliquity).
DE: 5416 Glaciation
DE: 5462 Polar regions
DE: 5464 Remote sensing
SC: Planetary Sciences [P]
MN: Fall Meeting 2005