JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, E12S08, doi:10.1029/2006JE002728, 2006 Nature and origin of the hematite-bearing plains of Terra Meridiani based on analyses of orbital and Mars Exploration rover data sets R. E. Arvidson,1 F. Poulet,2 R. V. Morris,3 J.-P. Bibring,2 J. F. Bell III,4 S. W. Squyres,4 P. R. Christensen,5 G. Bellucci,6 B. Gondet,2 B. L. Ehlmann,7 W. H. Farrand,8 R. L. Fergason,5 M. Golombek,9 J. L. Griffes,1 J. Grotzinger,10 E. A. Guinness,1 K. E. Herkenhoff,11 J. R. Johnson,11 G. Klingelho¨fer,12 Y. Langevin,2 D. Ming,3 K. Seelos,1 R. J. Sullivan,4 J. G. Ward,1 S. M. Wiseman,1 and M. Wolff 8 Received 7 April 2006; revised 10 July 2006; accepted 13 July 2006; published 22 November 2006. [1] The 5 km of traverses and observations completed by the Opportunity rover from Endurance crater to the Fruitbasket outcrop show that the Meridiani plains consist of sulfate-rich sedimentary rocks that are largely covered by poorly-sorted basaltic aeolian sands and a lag of granule-sized hematitic concretions. Orbital reflectance spectra obtained by Mars Express OMEGA over this region are dominated by pyroxene, plagioclase feldspar, crystalline hematite (i.e., concretions), and nano-phase iron oxide dust signatures, consistent with Pancam and Mini-TES observations. Mo¨ssbauer Spectrometer observations indicate more olivine than observed with the other instruments, consistent with preferential optical obscuration of olivine features in mixtures with pyroxene and dust. Orbital data covering bright plains located several kilometers to the south of the landing site expose a smaller areal abundance of hematite, more dust, and a larger areal extent of outcrop compared to plains proximal to the landing site. Low-albedo, low-thermal-inertia, windswept plains located several hundred kilometers to the south of the landing site are predicted from OMEGA data to have more hematite and fine-grained olivine grains exposed as compared to the landing site. Low calcium pyroxene dominates spectral signatures from the cratered highlands to the south of Opportunity. A regional-scale model is presented for the formation of the plains explored by Opportunity, based on a rising ground water table late in the Noachian Era that trapped and altered local materials and aeolian basaltic sands. Cessation of this aqueous process led to dominance of aeolian processes and formation of the current configuration of the plains. Citation: Arvidson, R. E., et al. (2006), Nature and origin of the hematite-bearing plains of Terra Meridiani based on analyses of orbital and Mars Exploration rover data sets, J. Geophys. Res., 111, E12S08, doi:10.1029/2006JE002728. 1. Introduction 1 Department of Earth and Planetary Sciences, Washington University, [2] The Mars Exploration rover, Opportunity, touched St. Louis, Missouri, USA. down on Meridiani Planum on 24 January 2004 and started 2Institut d’Astrophysique Spatiale, Universite´ Paris-Sud, Orsay, France. 3NASA Johnson Space Center, Houston, Texas, USA. an extensive campaign to explore the plains and craters in 4Department of Astronomy, Cornell University, Ithaca, New York, the vicinity of the landing site [Squyres et al., 2004a]. At USA. approximately the same time, the Mars Express Orbiter 5 Department of Geological Sciences, Arizona State University, Tempe, began observing Mars, including coverage of the Spirit and Arizona, USA. 6Istituto di Fisica dello Spazio Interplanetario, Istituto Nazionale di Opportunity landing sites with the Mars Express Observ- Astrofisica, Rome, Italy. atoire pour la Mine´ralogie, l’Eau, les Glaces et l’Activite´ 7School of Geography and Environment, University of Oxford, Oxford, (OMEGA) hyperspectral imaging system (0.35 to 5.08 mm) UK. 8 [Bibring et al., 2005]. In addition, the two orbiters Mars Space Science Institute, Boulder, Colorado, USA. Global Surveyor (MGS) [Albee et al., 2001] and Mars 9Jet Propulsion Laboratory, Pasadena, California, USA. 10Geological and Planetary Sciences, California Institute of Technology, Odyssey [Saunders et al., 2004] continued to observe Mars. Pasadena, California, USA. The purpose of this paper is to report results from exami- 11U.S. Geological Survey, Flagstaff, Arizona, USA. nation and analyses of orbital data along with data from the 12 Institut fu¨r Anorganische und Analytische Chemie, Johannes Athena Payload on Opportunity [Squyres et al., 2003], Gutenberg-Universita¨t, Mainz, Germany. focusing on the nature and origin of soils and outcrops at Copyright 2006 by the American Geophysical Union. Meridiani Planum. Specific emphasis is placed on analyses 0148-0227/06/2006JE002728 E12S08 1of19 E12S08 ARVIDSON ET AL.: NATURE OF TERRA MERIDIANI PLAINS E12S08 Figure 1. Geologic map of the Opportunity landing site and surrounding areas superimposed on a mosaic compiled from THEMIS daytime IR images. Footprints for OMEGA coverage for orbits 171-1 and 1183-5 are also shown. The oldest units are the cratered highlands (Hct). Hematite-bearing plains (Ph) sit unconformably on the cratered highlands. Etched terrain materials beneath the Ph units and discussed in the text are not exposed in the mapped area. Note channel systems buried by Ph materials, the crater with its ejecta deposit overlying the Ph unit and covered by data for orbit 171-1, and partially buried crater to southeast of Opportunity and labeled CS. Image is a Mars Equirectangular map projection. of OMEGA data and implications for the presence and operations during this time period) are then used as ‘‘ground abundances of minerals that are exposed on the plains. truth’’ to compare to the mineral abundances and grain sizes [3] The first part of this paper provides a regional-scale retrieved from OMEGA data for specific pixels covering the overview of the geomorphology, stratigraphy, and ages of traverses. Similarities and differences are highlighted and units exposed in Terra Meridiani, including the hematite- discussed. Finally, a synthesis is presented that shows how bearing plains of Meridiani Planum. The overview takes the plains formed and evolved to the present distribution of advantage of published work as well as analyses presented deposits and landforms. in this paper using Mars Odyssey Thermal Emission Imaging System (THEMIS) [Christensen et al., 2004a] and MGS Mars 2. Regional-Scale Geologic Context Orbiter Camera (MOC) [Malin et al., 1992] data. OMEGA hyperspectral reflectance data covering the plains (including [4] The hematite-bearing plains of Meridiani Planum the landing site) and the cratered terrains to the south of the were identified and mapped from MGS Thermal Emission plains are placed in this regional context. The extensive Spectrometer (TES) data based on comparisons with labo- Panoramic Camera (Pancam) [Bell et al., 2003], Miniature ratory spectra of crystalline hematite [Christensen et al., Thermal Emission Spectrometer (Mini-TES) [Christensen et 2000, 2001] (Figure 1). Hematite was inferred to cover al., 2003], Navigation Camera (Navcam) [Maki et al., 2003], 15% of the surface. Opportunity confirmed the surface Hazard Avoidance Camera (Hazcam) [Maki et al., 2003], coverage and discovered that the hematite signature is Microscopic Imager (MI) [Herkenhoff et al., 2003], Alpha associated with 1- to 5-mm-diameter concretions that Particle X-Ray Spectrometer (APXS) [Rieder et al., 2003], have concentrated as an aeolian lag deposit weathered and Mo¨ssbauer (MB) [Klingelho¨fer et al., 2003] data from sulfate-rich outcrops [Christensen et al., 2004b; acquired by Opportunity along its 5 km of traverses from Squyres et al., 2004b]. Other minerals present on the the landing site to the Fruitbasket outcrop (sols 1 to 560; plains as inferred from TES observations include feldspar, see S. W. Squyres et al. (Overview of the Opportunity Mars pyroxene, and weathering products [Arvidson et al., 2003]. Exploration rover mission to Meridiani Planum: Eagle crater [5] From orbit the hematite-bearing plains exhibit var- to Purgatory ripple, submitted to Journal of Geophysical iable spatial patterns, ranging from bland, low-albedo Research, 2006) (hereinafter referred to as Squyres et al., surfaces (0.15 Lambert albedo) interspersed with bright submitted manuscript, 2006) for an overview of mission patches, to low-albedo dune fields [Arvidson et al., 2003; 2of19 E12S08 ARVIDSON ET AL.: NATURE OF TERRA MERIDIANI PLAINS E12S08 [6] The etched terrain materials sit unconformably on the Noachian cratered terrain materials in some places, and in others may be part of the cratered terrain section [e.g., Hynek et al., 2002; Arvidson et al., 2003; Edgett, 2005]. Several hundred kilometers to the south of the landing site the cratered terrain exhibits channel systems that disappear beneath or merge into the hematite-bearing plains, implying an age for the plains materials in this locality that is younger than and in place contemporaneous with the channeling events (Figure 1). This inference is confirmed by examina- tion of crater size frequency distributions derived from the cratered terrain and plains units (Figure 2). Specifically, the size frequency distributions for the hematite-bearing plains and cratered terrain overlap for sizes smaller than 2km across. This size range corresponds to a population of fresh- looking impact craters on the cratered terrain (i.e., formed after or during waning stage of fluvial erosion) and to craters clearly superimposed on the plains materials. Larger craters for both units show a complex size distribution associated with either the impactor size distribution and/or Figure 2. Cumulative crater size frequency distributions a period of crater erosion and burial that preferentially are shown for the cratered highlands and hematite-bearing removed smaller craters before the population of fresh plains, using the mosaic shown in Figure 1 as the base map craters began forming and being retained [e.g., Chapman for counts. The Hartmann production function and iso- and Jones, 1977; Neukum and Ivanov, 1996]. The craters in chrons as defined by Hartmann and Neukum [2001] are also the cratered terrain in this larger size range appear degraded, shown.