Sixth Mars Polar Science Conference (2016) 6036.pdf

INVESTIGATING CONCENTRIC CRATER FILL ON MARS WITH AN ICE FLOW MODEL. N. Weitz1, G. R. Osinski1,2, J. L. Fastook3 and M. Zanetti1, 1Department of Earth Sciences / Centre for Planetary Science and Exploration, University of Western Ontario, London, Canada, 2Department of Physics and Astronomy, University of Western Ontario, London, Canada, 3Department of Computer Sciences, University of Maine, Orono, USA.

Introduction: Concentric crater fill (CCF) is across the crater. This obliquity scenario gives a cyclic- thought to be the result of thick accumulations of ice al pattern of growing and decaying ice layers filling the and dust infilling craters in the mid-latitudes of Mars crater over time. To date, the development of CCF in [1,2,3] and may behave similarly to terrestrial ice- both simple and complex craters between 5 and 45 km and/or rock- glaciers. Satellite imagery of CCF show at 5 km intervals has been modeled. flow features, such as lobes and parallel ridges on crater floors, interpreted to be related to glaciation [2,4]. Recent global climate modeling suggests that concentric crater fill develops over many cycles in which thin layers of snow and ice gradually accumu- late in craters during global changes in the planet's ob- liquity [1,5]. To date, little effort has been made in un- derstanding the geophysics and mechanics of glaciers in impact craters (essentially enclosed basins) and the question of how glaciers modify craters remains open. We use finite element ice flow modeling to understand the process of concentric crater fill and subsequent ice- flow on Mars and determine if there is potential for remnant ice buried in the subsurface. Modeling the temporal formation of CCF can help validate climate models that aim to reproduce past climates on Mars.

Methods: Ideal crater bed profiles are calculated based on the crater morphometric properties determ- ined by Garvin [6]. With a finite element ice flow line model developed by Fastook [1,3] based on the Uni- versity of Maine Ice Sheet Model (UMISM), we in- vestigate the evolution of CCF over time. The model uses the Shallow Ice Approximation (SIA) and in- cludes an Arrhenius temperature-dependent ice rhe- ology and Glen's flow law. The ice flow model is coupled with an advection-based model of surface debris transport [3]. Debris is incorporated into the model as deposits on the ice near the crater walls when Figure 1: A 5 km diameter ideal crater is filled with ice and the ice surface falls below the crater rim. The debris is surface debris. This figure illustrates the early stage filling then transported crater-inwards with the movement of process (only one half of the symmetric crater is shown the ice flow. here). Upper panel: Filling level of ice (dashed lines) and The model surface mass balance is obliquity-driven covering debris layer (solid lines) for the initial 0.4, 0.6 and using the scenario 301003_BIN_A_P001_N from Las- 0.7 Ma. There is distinct lobate ice flow from the walls to- kar [7]. In this scenario the obliquity is relatively high wards the crater center. Lower panel: Respective ice velocity for the last 40 to 5 million years; after that it gradually for the profiles above. Velocities are highest for steep ice sur- decreases to its current value. We use an obliquity face gradients, and (for this early stage) increase with ice threshold of 35 degrees. Above this threshold ice accu- thickness. mulates with a rate of 1mm/year (typical GCM results for high obliquity) [1], and below the threshold ice Results: In the case reported here, we use the model sublimates with a rate depending on the amount of to fill a 5 km diameter ideal crater profile with ice and debris armoring the surface [3]. Ice accumulation and debris, beginning in the late Amazonian. Depending on sublimation are currently set to be spatially uniform the obliquity, the ice accumulates and sublimates in a Sixth Mars Polar Science Conference (2016) 6036.pdf

'cyclic' behavior. At the onset of ice layer formation, ice flow velocities are highest on the steep slopes just under the crater rim. Figure 1 shows ice thickness (dashed lines), debris layer thickness (solid lines) and accompanying flow velocities (lower panel) at inter- vals of 0.4, 0.6, and 0.7 Ma of crater fill. At 0.4 Ma, after some initial ice accumulation throughout the crater of up to 36 m, the majority of the ice has sublim- ated such that no ice is left in the crater center (blue). However, ice is preserved at the upper crater walls where it is protected by surface debris. At the 0.4 Ma stage, ice velocities are highest in the middle of the crater wall, where ice surface slope is steep and ice thickness is largest, (ice velocities are always zero in the center point). At 0.6 Ma more ice has been depos- ited in the crater and ice is flowing from the walls to- wards the center, forming distinctive lobes (green). The surface debris layer is also transported further inwards. Ice velocities are now highest near the end of the lobes, closer to the crater center. At 0.7 Ma more ice has been deposited and the ice flow lobes have touched in the center (red), although the debris cover has not reached the center yet. Velocities reach a maximum about 1.5 km from the crater center. Flow velocities in these early stages of crater fill can be several m/yr, whereas they later decrease to a few mm/yr as the ice surface slope flattens with increased ice deposition. In our model runs, it is possible to fill the whole crater with ice and debris in about 2 million years. However, this scenario depends on an adequate debris layer covering the whole ice surface. The debris layer protects the underlying ice and prevents excessive sub- limation during times of low obliquity. Figure 2b shows a MOLA surface profile (blue, el- evation shifted) and the ideal crater geometry (brown) of a sample crater in Figure 2a, located in Utopia Figure 2: (a) Section of CTX image P16_007423_2190 Planitia. The crater has a diameter of ~4.5 km and the showing a crater with concentric fill at 38.1N, 101.8E, with profiles indicate a current crater fill of more than 500 the MOLA profile across crater (blue) shown in (b). (b) meters at the center. Our ice flow model (orange: ice, MOLA surface elevation profile (blue, shifted), ideal bed red: debris) is able to recreate the measured MOLA (brown) and modeled crater fill (red). The model can repro- surface elevation very well, independent of the model duce the present crater fill level very well. start time. The modeled debris layer (red) shows 'ripples' due to the start and stop motion of the ice flow References: [1] Fastook J. L. and Head J. W. (2014) and hence debris transport, which can be observed as Planetary and Space Sciences, 91, 60-76. [2] Pearce G. concentric rings on many CCF surfaces [2,5]. R., Osinski G. R. and Soare R. J. (2011) Icarus, 212, 86-95. [3] Fastook J. L., Head J. W. and Marchant D. Future Work: We will use SHARAD radar data to R. (2014) Icarus, 228, 54-63. [4] Levy J. S. et al. obtain information about the thickness and structure of (2014) Journal of Geophysical Research, 119, 10, subsurface ice layers in craters, and use this informa- 2188–2196.[5] Dickson J. L., Head J. W. and Fassett tion to validate model parameter settings. Further, we C. I. (2012) Icarus, 219, 723-732. [6] Garvin J. B., will continue to investigate CCF for different crater Sakimoto S. E. H., Frawley J. J. and Schnetzler C. sizes and complex crater geometries. Including a mass (2002) LPSC XXXIII, abstract #1255.[7] Laskar J. et balance with a spatial dependency across the crater al. (2004) Icarus, 170, 343-364. profile is also desirable.