 SURFACE MORPHOLOGIES OF ARCADIA PLANITIA AS AN INDICATOR OF PAST AND PRESENT NEAR-SURFACE ICE.  N. R. Williams1, M. P. Golombek1, A. M. Bramson2, D. Viola2, S. Byrne2, A. S. McEwen2, 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, 2University of Arizona, Tucson, AZ 85721.  Introduction:  The occurrence of water on Mars holds key records for the planet's past and present climate. Polar caps are the most obvious large-scale examples of water-ice on Mars today, but a latitudedependent mantle (LDM) of shallow water-ice has also been remotely observed at 30°N-40°N. Ice is not stable at these mid-latitudes today, but is expected to have precipitated in the past during different obliquities and climatic conditions [1] with residual excess ice likely preserved in the subsurface. Arcadia Planitia, in particular, is a generally flatlying region around ~200°E, ~40°N where several studies suggest a widespread abundance of shallow ice. Gamma ray spectrometry suggests ~35% ice by weight in the shallow subsurface [2]. On the west side of Arcadia (Erebus Montes), lobate debris aprons are interpreted as dust or regolith-mantled icy flows tens of meters thick [3,4]. Small, fresh impacts eject and expose bright ice from <1 m depth [5]. Ground ice sublimating around secondary craters diffusively expands crater diameters, with average removed thicknesses of ~1.9-4.7 m [6]. Terraced craters suggest a mechanical discontinuity at the base of an ice layer with a mean 51 m thickness [7]. Ground-penetrating radar shows reflectors at the same tens of meters depth with overlying dielectric constants similar to ice [4,7]. Near-surface ice was also identified in situ farther northeast (68°N, 234°E) at the Phoenix landing site [8,9]. A global survey by Levy et al. [10] identified a few examples of a polygonal surface pattern in Arcadia similar to many other high-latitude sites including those at Phoenix and have been interpreted as due to cryoturbation in ice [9,10]. Since that study, many additional highresolution images have been acquired and start to resolve a complex transition of the LDM boundary. These images reveal Arcadia to exhibit much more polygonal terrain (Fig. 1A) than found in the initial surveys, as well as contain areas with crenulations (Fig. 1B) and pits (Fig. 1C). In this project, we map the distribution of these morphologies and find spatial relationships between them. We defined a rectangular survey area between longitudes 185°E to 210°E and latitudes 35°N to 44°N, around the approximate edge of the LDM in Arcadia Planitia [2,7,10]. A total of 230 red-filter HiRISE images at ~25 cm/pixel [11] were downloaded from the Planetary Data System and individually examined. Each image was categorized for whether it contained polygonal, crenulated, and/or pitted terrains (Fig. 3). Results:  Out of the 230 images surveyed, 165 images contain polygonal terrain, 86 images contain crenulated terrain, and 36 images contain pitted terrain (Fig. 2). Polygonal terrain is the most widespread, with very few examples at <36°N (in 1 of 34 images, Fig. 2) but becoming ubiquitous above 40°N (in 96 of 100 images, Fig. 2). Crenulated terrain primarily occurs in a narrow latitude band between 38°N and 43°N (median 39.6°N), with a few outlier crenulated areas also found around the lobate debris aprons on the west side of the surveyed area. Pitted terrain primarily occurs poleward of 40°N often in association with polygons, with a few outlier pits in the lobate debris aprons to the west.  Fig. 1: HiRISE images of example morphologies for A) polygonal, B) crenulated, and C) pitted terrains. Discussion:  The presence of polygonal patterned ground over the majority of Arcadia Planitia is consistent with ice in the shallow subsurface. The occurrence of crenulated terrain in a narrow latitude band at the southern edge of where polygonal terrain is ubiquitous suggests that it represents the primary area of sublimating LDM ice in this region; the troughs of the crenulated terrain have already undergone sublimation while the ridges are still ice-rich. Levy et al,. [12] described similar "brain terrain" morphologies located in some basin concentric crater fills in Utopia Planitia and proposed it is formed and modified by thermalcontraction cracking and differential sublimation. The distribution of similar crenulations in the plains of Arcadia agrees with this model. Pitted terrain to the north seems to replace crenulations, and likely is a precursory morphology where pits have not yet sublimated and expanded enough to interconnect making ridge/trough patterns. Polygonal terrain south of ~38°N where crenulations are scarce likely represents largely desiccated terrain with relatively little excess ice left, but nonetheless indicates where ice deposits were present. This collective distribution of textures spanning Arcadia Planitia supports a transitional environment for the receding latitude-dependent mantle (LDM) where ice is still present in the shallow subsurface but is sublimating with increasing sparsity from 40°N to 35°N. References:  [1] Head J. W. et al. (2003) Nature, 426, 797-802. [2] Boynton W. V. et al. (2002) Science, 297, 81-85. [3] Mangold N. (2003) JGR, 108, 1885. [4] Plaut J. J. et al. (2009) Geophys. Res. Lett., 36, L02203. [5] Byrne S. et al. (2009) Science, 325, 16741676. [6] Viola D. et al. (2015) Icarus, 248, 190-204. [7] Bramson A. M. et al. (2015) Geophys. Res. Lett., 42, 6566-6574. [8] Arvidson R. E. et al. (2009) JGR, 114, E00E02. [9] Mellon, M. T. et al. (2009) JGR,  114, E00E06. [10] Levy J. et al. (2009) JGR, 114, E01007. [11] McEwen A. S. et al. (2007) JGR, 112, E05S02. [12] Levy J. S. et al. (2009) Icarus, 202, 462476.  Fig. 2: Latitudinal distributions of the frequency of terrain morphologies in Fig. 1 within HiRISE images.  Fig. 3: Map showing locations of HiRISE images surveyed, color coded by selected terrain morphologies present.  No HiRISE images had only pits (dark blue) without also  having either polygons or crenulations.  
