Icarus 194 (2008) 513–518 www.elsevier.com/locate/icarus

Ice grain size and the rheology of the martian polar deposits Amy C. Barr a,∗ , Sarah M. Milkovich b a Department of Space Studies, Southwest Research Institute, 1050 Walnut St., Suite 300, Boulder, CO 80302, USA b Jet Propulsion Laboratory, California Institute of Technology, M/S 183-501, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

Received 31 July 2007; revised 28 September 2007 Available online 16 January 2008

Abstract The history and dynamics of the martian polar deposits (MPD), the largest known water reservoirs on Mars, are of great interest, but estimates of ice grain size are required before detailed modeling can be performed. We clarify the microphysical processes that may control grain size in the MPD. If the MPD are ∼2% dust by mass, the maximum ice grain size is ∼1 mm due to grain boundary pinning by silicate microparticles. Relatively dusty layers in the MPD will have smaller grain sizes. If MPD ice has a very low impurity content and has experienced a significant amount of strain, grains may reach a steady state size of ∼1.5 to 3 mm due to dynamic recrystallization, wherein a steady state grain size is maintained due to the balance of grain growth and destruction during flow. If the near-bed ice in the MPD is warmed close to its melting point and has been extensively sheared, grain sizes at its base may be between 10 and 40 mm, by analogy with warm, dirty, near-bed ice in terrestrial ice sheets. © 2007 Elsevier Inc. All rights reserved. Keywords: Mars; Ices

1. Introduction The martian polar deposits (MPD) (Fig. 1) are composed of layers of dusty water ice totaling several kilometers thick covered by a layer of residual ice (H2 O ice in the north, CO2 and H2 O in the south). The history and dynamics of these deposits are of great interest because they are the largest known water reservoirs on the planet, and because their layers are thought to contain the record of recent climate variations (e.g., Thomas et al., 1992). Interpretations of the morphology of the MPD, their formation and evolution over time, and the link between climatic history and individual layers, depend in part on how large a role ice flow has played in their geologic history. Current efforts to model flow in the MPD are hampered by uncertainties in the physical properties of the deposits, chiefly, the rheology and dust content. Recent laboratory experiments seeking to clarify the mechanisms that accommodate flow in solid ice suggest that deformation in ice is accommodated by three mechanisms: diffusion creep which occurs by both vol* Corresponding author. Fax: +1 303 546 9687.

E-mail address: [email protected] (A.C. Barr). 0019-1035/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2007.11.018

ume diffusion and grain boundary diffusion (see, e.g., Goodman et al., 1981), grain-size-sensitive creep (Goldsby and Kohlstedt, 2001; Durham et al., 2001; Durham and Stern, 2001) which may occur by grain boundary sliding and easy slip (Goldsby and Kohlstedt, 2001), and dislocation creep. At the modest stresses associated with topography-driven flow in the MPD, deformation is accommodated primarily by diffusion creep and grain size-sensitive creep, leading to a viscosity that is grain size-dependent. Thus, knowledge of the grain size in the MPD is required for detailed modeling of their flow over time. Consideration of ice grain sizes is also important for the interpretation of high-resolution imaging datasets. Small variations in albedo of pure ice at visible wavelengths may be due to changes in grain size (e.g., Nolin and Dozier, 2000); variations in ice grain size with depth may be a contribution to varying albedo of individual MPD layers (Milkovich and Head, 2006). Spectral observations by Mars Express’ OMEGA are consistent with grain sizes at the surface of d ∼ 1 mm (Langevin et al., 2005). Close to the surface of the deposits, ice grains grow by sintering, and grain sizes increase with depth, or equivalently, time (Kieffer, 1990; Colbeck, 1997). Detailed modeling of this process suggests grains between 200 µm and 1 mm at a

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A.C. Barr, S.M. Milkovich / Icarus 194 (2008) 513–518

Fig. 1. Martian north polar deposits. Left: Residual ice cap on top of layered deposits. Right: Polar layered deposits (MOC M00/02100).

depth of 60 m (Kieffer, 1990), consistent with OMEGA observations. Although spectral studies can constrain grain size in the top few microns of the MPD, they cannot address the grain size at depth, which is a key control on rheology. Our first glimpse into the composition of the deposits at depth came from the radar sounder onboard Mars Express, MARSIS, whose data suggest that the deposits are relatively pure ice (