Eighth International Conference on Mars (2014 ) 1230 · potential desiccation cracks [3–8]. In this study, we summarize and review the global observations of such polygonal patterns

Potential Desiccation Cracks on Mars: A Review of Global Observations using HiRISE M. R. El-Maarry1, W. Watters2, N. K. McKeown3, J. Carter4, E. Noe Dobrea5,6, J. L. Bishop6,7, A. Pommerol1, N. Thomas1. 1Physikalisches Institut, Bern Univeristät, Sidler Str., 5, 3012, Berne, Switzerland ([email protected]). 2Whitin Observatory, Department of Astronomy, Wellesley College, Wellesley, MA, USA. 3Department of Physical Sciences, MacEwan University, Edmonton, Alberta, Canada. 4Institut d'Astro-physique Spatiale, Paris-Sud University, Orsay, France. 5Planetary Science Institute, Tucson, AZ, USA. 6NASA Ames Research Center, Mountain View, CA, USA. 7SETI Institute, Mountain View, CA 94043, USA.

Introduction: Our view of the surface of Mars

and ongoing seasonal processes has been radically improved since the operation of the Mars Reconnais-sance Orbiter High Resolution Imaging Science Exper-iment (HiRISE). The high sub-meter spatial resolution offered by HiRISE has been a key in the identification of small features and surface textures on the surface of Mars such as the seasonal recurring slope lineae [1], and lava coils [2]. In addition to these, HiRISE has aided in the identification of polygonal crack patterns in association with phyllosilicate-bearing terrains in low to mid-latitudes, which have been suggested to be potential desiccation cracks [3–8]. In this study, we summarize and review the global observations of such polygonal patterns and assess their morphology, geo-logic setting and global distribution. HiRISE observa-tions suggest that desiccation cracks and polygons may be more common on the surface of Mars than previ-ously thought and may have profound implications for our understanding of the history of water on Mars, its early climate, and consequently, our choice of candi-date landing sites for future exploration (e.g., ExoMars 2018 and 2020 mission).

The process of desiccation: Desiccation is usual-ly achieved through evaporation from the surface, or diffusion processes either through the migration of liquid water caused by differences in water potential, or via vapor transport due to changes in water vapor pressure. The depth and spacing of any resulting frac-tures (i.e., size of polygonal network) depends on many factors, but mainly on the thickness of the stressed zone. As a result, polygons formed by desic-cation occur on the order of centimeters if the stressed region is a thin surficial layer undergoing evaporation as for example in the case of common mud cracks, or it can be on the order of hundreds of meters if the stressed region is thick enough because of intense evaporation and/or lowering of the water table [9,10]. Generally, the more clay-rich the material is, the more it will shrink with desiccation. In addition, certain clay minerals, smectites, are known for their chemical af-finity to swell and accommodate considerable amounts of water through formation of water interlayers on a molecular level.

On Earth, many dried lakes and playas, in particu-lar in the US states of California and Nevada, display

large

Fig. 1. Potential desiccation polygons (PDPs) on Mars as observed by HiRISE in Mawrth Vallis (a), Libya Montes (b), Margaritifer (c), and chloride-bearing terrains in Sirenum (d).

Fig. 2. Large features on the floor of Mawrth Vallis and the large desiccation features in Lucerne Dry Lake in California, US (34°29'49"N, 116°57'10"W). There is an exceptional similarity between the patterns in terms of morphology, size and crack propagation. Arrows point to the longest and largest sinuous features in both locations. up to 300 m-wide desiccation polygons that maybe analogous to similar features on Mars (Fig. 2). The sediments in these locations can often be more than 50 meters thick and are composed of predominantly silt-

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and clay-rich soils containing clay minerals such as montmorillonite, illite, and vermiculite in addition to carbonates and analcites [11].

Potential desiccation polygons (PDPs) on Mars: PDPs are a common feature in phyllosilicate- and chloride-bearing terrains [8, 12] and have been ob-served with size scales that range from cm- to 10s of meters-wide using images from HiRISE [e.g., 3–8] and currently active rovers [13,14]. The global distribution of PDPs shows that they share certain traits in terms of morphology and geologic setting that can aid in their identification and distinguish them from fracturing patterns caused by other processes (Fig. 3). Most PDPs currently observed attain a size range of 2–30 meter-wide. PDPs are almost exclusively observed in light-toned units with respect to the surrounding terrain. They commonly underlie dark-toned materials, which are often spectrally featureless and display signs of recent exhumation. PDPs are generally flat and usually subdivide extensively to form secondary to multiple generations of cracks in a fractal-like pattern that is embedded within the larger primary polygons and re-quires images with sub-meter spatial resolution to identify. PDPs are mostly associated with sedimentary deposits that display spectral evidence for the presence of Fe/Mg smectites in addition to Al-rich smectites and less commonly kaolinites, sulfates and carbonates. In contrast, PDPs are uncommon in materials that have been heavily modified by erosion, tectonism, or exten-sive reworking (e.g., central-peak materials uplifted by impact cratering). Similarly, they are uncommon in materials of possible geothermal or hydrothermal origin, which is inferred from the presence of high-temperature/pressure mineral phases such as chlorites, prehnite and serpentine.

Implications: PDPs can be excellent markers for paleolacustrine environments and their presence im-plies that the fractured units are rich in smectite miner-als. Together, the following criteria : 1) detection of Fe/Mg smectites along with salts, carbonates, kaolin-ite, and possibly illite, 2) absence of high tempera-ture/pressure phases, and 3) association with polygonal patterns resembling PDPs make a certain location a high candidate for a paleolacustrine site on Mars, which is a top-priority setting for in-situ exploration and search for paleo-organic materials. The presence of PDPs in association with many phyllosilicate expo-sures that are located in natural basins and/or are of sedimentary origin would argue for a more hydrologi-cally active period and warmer conditions than what is observed today. However, the presence of desiccation features is similarly consistent with climatic conditions that display only short intermittent hydrological activi-

ty characterized by ground-water activity in generally arid conditions.

References: [1] McEwen A.S. et al., (2011), Sci-ence 333, 740–743. [2] Ryan, A.J. and Christensen, P.R., (2012), Science 336, 449–452. [3] Ehlmann, B. L., et al. (2009), JGR 114, E00D08. [4] Wray, J. J., et al., (2011), JGR 116, E01001. [5] Erkeling, G., et al., (2012), Icarus 219, 393–413. [6] Bishop, J. L., et al., (2013), JGR 118, 487–513. [7] McKeown, N. K. et al.,(2013), JGR 118, 1245–1256. [8] El Maarry et al., (2013), JGR 118, 2263–2278. [9] El Maarry, et al., (2010), JGR 115, E10006. [10] El Maarry, M. R., et al., (2012), E&PSL 323, 19–26. [11] Neal, J. T., et al., (1968), Geol. Soc. Am. Bull. 79, 69–90. [12] Osterloo M.M. et al., (2010), JGR 115, E10012. [13] Watters, W. A., et al., (2011), Icarus 211, 472–497. [14] Hallet, B., et al, (2013), LPSC XXIV, abstract #3108.

Fig. 3. MOLA-based shaded relief map for the surface of Mars containing locations of PDPs. The dataset includes cracking patterns in smectite-bearing deposits that are found either in horizontal beds (yellow), crustal outcrops (red), or deltas/alluvial fans (light blue). Also included are the crack-ing patterns in chloride-bearing terrains [8,12] (green). PDPs are clustered in certain localities in the southern highlands, which include Mawrth Vallis, Sirenum and Margaritifer Terra, eastern Valles Marineris, circum-Isidis (Nili Fossae and Libya Montes), and northern circum-Hellas.

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