41st Lunar and Planetary Science Conference (2010) 2268.pdf

PHYLLOSILICATE AND SULFATE LAYERING IN INTERPLAYA DUNES; ANALOGS FOR MARS INTERCRATER DEPOSITS. A. M. Baldridge1, S. J. Hook1, N. T. Bridges2, B. J. Thomson2 and J.D.A. Clarke3 1Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109, 2Applied Physics Laboratory, Laurel, MD 20723, 3Australian Centre for Astrobiology, University of New South Wales, Kensington, NSW 2052 Australia.

Introduction: Data returned from the Compact forms within the larger ones. Small pellets of clay and Reconnaissance Imaging Spectrometer for Mars sand build up during the dry season, are cemented (CRISM) indicate that the central sediment mounds in together by salts during the wet season, and can form Gale Crater contain smectite clays and sulfates [1]. interbedded sand, clay and salt. The clay-bearing units in this and in similar crater Methods: HyMAP data were acquired in deposits commonly occur stratigraphically beneath November and December 2008. These areas contain a sulfates and this stratigraphic relationship is cited for suite of clays, sulfates, and salts formed under variable deposits elsewhere on Mars as evidence of separate pH and salinity similar to what conditions may have and temporally distinct geochemical environments on been like in the martian past. The HyMap sensors have Mars [2, 3]. 126 bands that cover the wavelength range between Acid saline lakes in Western Australia have been 0.45 and 2.5 microns. HyVista Corporation provided recognized as important chemical terrestrial analogs for atmospherically corrected reflectance data at aqueous mineral formation and geochemical variability approximately 3m spatial resolution. on Mars [4]. In these systems, large pH and salinity The first objectives of our study were to identify differences are observed both laterally and vertically of spectrally unique pixels and map their distribution. scales of a few tens of meters [5, 6].The vertical Specifically, our search included kaolinite, smectite variability results in smectite formation at depth and clays, Fe-oxides, opaline silica, jarosite, alunite, sulfate formation at the surface, while the lateral gypsum and Mg-sulfates. Mineral maps were created variability tends to favor kaolinite formation at using a combination of the Mixed Tuned Matched shoreline and sulfate formation in lake interiors [7].The Filtering (MTMF) algorithm provided in ENVI and variability in these lakes has been offered as an relative band depth (RBD) mapping and were overlain alternate formation mechanism for some of the on a georegistered mosaic of all of the flight lines for phyllosilicates and sulfates on Mars, suggesting that each lake (e.g., Fig. 1). Mapped results were validated these different mineral types may be separated by with spectral measurements of field samples, visual chemical gradients rather than temporal boundaries [8]. interpretation, and XRD. Field measurements were To evaluate the co-occurrences of phyllosilicates made when possible and all samples were measured in and sulfates on Earth to better understand similar the laboratory using an Analytical Spectra Device environments on Mars, we have obtained airborne (ASD) Fieldspec spectrometer for comparison to the hyperspectral data from playa lakes in southern HyMAP data. Western Australia. Mineral maps derived from these data indicate that layered phyllosilicates and sulfates are concentrated in the central playa as aeolian features. The sequences of clay and salt minerals may be analogous to the thick sequences of layered, crater- hosted martian sedimentary materials and may be applicable to larger sedimentary deposits on Mars as well. Background: Lake Gilmore consists of salina, gypsum mud flats, and dunes that may be described as a "boinka": playa lake landforms that are primarily influenced by groundwater discharge [9]. The interior of the basin contains multiple aeolian features that resemble crescent dunes. The presence of shallow saline waters with exposed marginal mudflats are

formation requirements for such lunettes [10] or lake Figure 1. Phyllosilicate and sulfate layering in dunes, in addition to strong winds and a marked hot dry dunes/lunettes in Lake Gilmore (Yellow = gypsum, season. These features are derived from lake sediments Green = polyhydrated sulfates, and Cyan= and are arranged in multiple ridges with smaller dune phyllosilicates). 41st Lunar and Planetary Science Conference (2010) 2268.pdf

Results and Discussion: The spectral mapping indicates that the dunes are composed of interbedded layers of phyllosilicate and sulfate minerals (Fig. 1). However, the layering does not reflect general mineral distribution in the lakes (clays primarily occurring stratigraphically below sulfates). Rather, the layers alternate between gypsum, polyhydrated sulfate, and Fe/Mg and Al-OH rich phyllosilicates. Interbedded sulfates and phyllosilicates in the lake dunes may be analogous to intercrater deposits found on Mars in what may have been closed hydrologic basins including Gale Crater, Columbus Crater and Becquerel Crater. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data indicate that the thick sequences of sedimentary deposits found in the central mound in Gale contain smectites and sulfates that occur mixed or interbedded [1](Fig. 2). Figure 3. Shuttle Radar Topography Mission (SRTM) Additionally, the formation hypothesis for the layers in data showing paleo-lunettes in Australia. Gale Crater include both lacustrine and aeolian mechanisms [11, 12]. The terrestrial analog suggests References: [1] R. E. Milliken et al., (2009) LPSC, how interbedded or mixed layers may have formed # 1479. [2] J.-P. Bibring et al., (2006) Science 312, from a combination of these processes; windblown 400. [3] S. M. Wiseman et al., (2008) GRL 35, 19204. sand and clay is cemented by sulfates via groundwater [4] K. C. Benison, D. A. LaClair, (2003) Astrobiology discharge and capillary action in possibly shallow 3, 609. [5] B. B. Bowen, K. C. Benison, (2009) surface waters. Applied Geochemistry 24, 268. [6] G. M. Marion et Although these types of dunes tend to be small on al., (2009) Geochim. Cosmochim. Acta. 73, 3493. [7] Earth, remnants of much larger features are also found D. J. Gray, (2001) Geochemistry: Exploration, in Australia (Fig. 3). The crescent shaped paleo- Environment, Analysis 1, 253. [8] A. Baldridge et al., lunettes can be several tens of kilometers and consist of (2009) GRL 36. [9] P. G. Macumber, (1980) in Aeolian sedimentary sequences of lake derived sediments and Landscapes in the Semi-arid Zone of Southeastern may provide an analog for the formation of the larger Australia. Australian Society of Soil Science, Riverina deposits in Meridiani Planum, which are believed to Branch, pp. 67-85. [10] E. S. Hills, (1940) Australian have formed via a combination of shallow water or Geographer 3, 15. [11] K. S. Edgett, M. C. Malin, playa and aeolian processes [13]. (2001) LPSC, # 1005. [12] K. L. Tanaka, (2000) Icarus 144, 244. [13] S. W. Squyres et al., (2004) Science 306, 1709.

Figure 2. THEMIS B9 mosaic of Gale Crater. Inset shows minerals detected in interior sedimentary deposits (after [1]). Possible remnant of an interlake dune?