Lunar and Planetary Science XLVIII (2017) 2852.pdf SURFACE MORPHOLOGIES OF ARCADIA PLANITIA AS AN INDICATOR OF PAST AND PRESENT 1 1 2 2 2 NEAR-SURFACE ICE. N. R. Williams , M. P. Golombek , A. M. Bramson , D. Viola , S. Byrne , A. S. McEw- en2, 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, 2University of Arizona, Tucson, AZ 85721. Introduction: The occurrence of water on Mars Results: Out of the 230 images surveyed, 165 im- holds key records for the planet’s past and present cli- ages contain polygonal terrain, 86 images contain mate. Polar caps are the most obvious large-scale ex- crenulated terrain, and 36 images contain pitted terrain amples of water-ice on Mars today, but a latitude- (Fig. 2). Polygonal terrain is the most widespread, with dependent mantle (LDM) of shallow water-ice has also very few examples at <36°N (in 1 of 34 images, Fig. 2) been remotely observed at 30°N-40°N. Ice is not stable but becoming ubiquitous above 40°N (in 96 of 100 at these mid-latitudes today, but is expected to have images, Fig. 2). Crenulated terrain primarily occurs in a precipitated in the past during different obliquities and narrow latitude band between 38°N and 43°N (median climatic conditions [1] with residual excess ice likely 39.6°N), with a few outlier crenulated areas also found preserved in the subsurface. around the lobate debris aprons on the west side of the Arcadia Planitia, in particular, is a generally flat- surveyed area. Pitted terrain primarily occurs poleward lying region around ~200°E, ~40°N where several of 40°N often in association with polygons, with a few studies suggest a widespread abundance of shallow ice. outlier pits in the lobate debris aprons to the west. Gamma ray spectrometry suggests ~35% ice by weight in the shallow subsurface [2]. On the west side of Ar- cadia (Erebus Montes), lobate debris aprons are inter- preted as dust or regolith-mantled icy flows tens of meters thick [3,4]. Small, fresh impacts eject and ex- pose bright ice from <1 m depth [5]. Ground ice subli- mating 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 re- flectors 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 sur- vey 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 high- resolution images have been acquired and start to re- solve a complex transition of the LDM boundary. These images reveal Arcadia to exhibit much more polygonal terrain (Fig. 1A) than found in the initial Fig. 1: HiRISE images of example morphologies for A) surveys, as well as contain areas with crenulations (Fig. polygonal, B) crenulated, and C) pitted terrains. 1B) and pits (Fig. 1C). Discussion: The presence of polygonal patterned In this project, we map the distribution of these ground over the majority of Arcadia Planitia is con- morphologies and find spatial relationships between sistent with ice in the shallow subsurface. The occur- them. We defined a rectangular survey area between rence of crenulated terrain in a narrow latitude band at longitudes 185°E to 210°E and latitudes 35°N to 44°N, the southern edge of where polygonal terrain is ubiqui- around the approximate edge of the LDM in Arcadia tous suggests that it represents the primary area of sub- Planitia [2,7,10]. A total of 230 red-filter HiRISE im- limating LDM ice in this region; the troughs of the ages at ~25 cm/pixel [11] were downloaded from the crenulated terrain have already undergone sublimation Planetary Data System and individually examined. while the ridges are still ice-rich. Levy et al,. [12] de- Each image was categorized for whether it contained scribed similar “brain terrain” morphologies located in polygonal, crenulated, and/or pitted terrains (Fig. 3). Lunar and Planetary Science XLVIII (2017) 2852.pdf some basin concentric crater fills in Utopia Planitia and 114, E00E06. [10] Levy J. et al. (2009) JGR, 114, proposed it is formed and modified by thermal- E01007. [11] McEwen A. S. et al. (2007) JGR, 112, contraction cracking and differential sublimation. The E05S02. [12] Levy J. S. et al. (2009) Icarus, 202, 462- distribution of similar crenulations in the plains of Ar- 476. cadia agrees with this model. Pitted terrain to the north seems to replace crenulations, and likely is a precurso- ry morphology where pits have not yet sublimated and expanded enough to interconnect making ridge/trough patterns. Polygonal terrain south of ~38°N where cren- ulations are scarce likely represents largely desiccated terrain with relatively little excess ice left, but nonethe- less 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 sublimat- ing 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, 1674- 1676. [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, Fig. 2: Latitudinal distributions of the frequency of 114, E00E02. [9] Mellon, M. T. et al. (2009) JGR, 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. .