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Pyroclastic Activity at Home Plate in Gusev Crater, Mars

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Pyroclastic Activity at Home Plate in Gusev Crater, Mars REPORTS 5. R. T. T. Forman, R. D. Deblinger, Conserv. Biol. 14,36 14. H. John Heinz III Center for Science, Economics, and the 20. The movie of RV change for the Colorado Front Range is (2000). Environment, The State of the Nation's Ecosystems: available as supporting material on Science Online. 6. K. H. Riitters, J. D. Wickham, Front. Ecol. Environ. 1, 125 Measuring the Lands, Waters, and Living Resources of the 21. W. R. Tobler, Econ. Geogr. 46, 234 (1970). (2003). United States (Cambridge Univ. Press, Cambridge, 2002). 22. We thank the Southern Rockies Ecosystem Project for 7. R. T. T. Forman, L. E. Alexander, Annu. Rev. Ecol. Syst. 15. C. Homer, M. Coan, C. Huang, L. Yang, B. Wylie, introducing us to large-area DTR maps nearly a decade 29, 207 (1998). Photogramm. Eng. Remote Sens. 70, 829 (2004). ago. Support for the development and analysis of 8. J. L. Gelbard, J. Belnap, Conserv. Biol. 17, 420 (2003). 16. Materials and methods, including a table of county values national DTR data was provided by the USGS Geographic 9. R. L. DeVelice, J. R. Martin, Ecol. Appl. 11,1008 of mean DTR and RV, are available as supporting Analysis and Monitoring Program. (2001). material on Science Online. 10. I. F. Spellerberg, Ecological Effects of Roads, Land 17. Because calculations were done with a 30-m grid, the Supporting Online Material Reconstruction and Management (Science Publishers, accuracy of the calculations is low where roads are dense. www.sciencemag.org/cgi/content/full/316/5825/736/DC1 Materials and Methods Enfield, NH, 2002). The approximate RV per-capita value for Kings County in 11. A. C. Yost, R. G. Wright, Arctic 54, 41 (2001). New York is provided to illustrate the great contrast with SOM Text 12. J. A. Sethian, Level Set Methods and Fast Marching the value for Hinsdale County in Colorado. Tables S1 and S2 References Methods: Evolving Interfaces in Computational Geometry, 18. Bureau of the Census, in National Atlas of the United Fluid Mechanics, Computer Vision, and Materials Science States, 2006 (www.nationalatlas.gov). Movie S1 (Cambridge Univ. Press, Cambridge, ed. 2, 1999). 19. Bureau of Economic Analysis, in National Atlas of the 29 November 2006; accepted 22 March 2007 13. E. W. Dijkstra, Numer. Math. 1, 269 (1959). United States, 2006 (www.nationalatlas.gov). 10.1126/science.1138141 thickly laminated. Cross-strata also preserve evi- Pyroclastic Activity at Home Plate in dence of reactivation surfaces, cut at variable angles and generally backfilled by cross-strata Gusev Crater, Mars concordant with the scour surface. Such geom- etries typically form during reconfiguration of the bed in response to scouring during flow S. W. Squyres,1 O. Aharonson,2 B.C. Clark,3 B. A. Cohen,4 L. Crumpler,5 P. A. de Souza,6 bursting, migration of three-dimensional bedforms W. H. Farrand,7 R. Gellert,8 J. Grant,9 J. P. Grotzinger,2 A. F. C. Haldemann,10 J. R. Johnson,11 with frontal scour pits, and at times when the G. Klingelhöfer,12 K. W. Lewis,2 R. Li,13 T. McCoy,14 A. S. McEwen,15 H. Y. McSween,16 sediment concentration of a flow is decreased. D. W. Ming,17 J. M. Moore,18 R. V. Morris,17 T. J. Parker,10 J. W. Rice Jr.,19 S. Ruff,19 We used planar fits to bedding seen in M. Schmidt,14 C. Schröder,12 L. A. Soderblom,11 A. Yen10 Pancam images to estimate the structural attitudes of beds in the upper unit of Home Plate, at four Home Plate is a layered plateau in Gusev crater on Mars. It is composed of clastic rocks of locations that sample roughly a third of the moderately altered alkali basalt composition, enriched in some highly volatile elements. A coarse- plateau’s perimeter (fig. S4). At all four locations, grained lower unit lies under a finer-grained upper unit. Textural observations indicate that the the beds dip inward toward the center of Home lower strata were emplaced in an explosive event, and geochemical considerations favor an Plate. The 1s range of derived dips is 5° to explosive volcanic origin over an impact origin. The lower unit likely represents accumulation of 20°, with occasional values up to 30°. Dips are pyroclastic materials, whereas the upper unit may represent eolian reworking of the same consistent within each outcrop, suggesting that pyroclastic materials. the measurements reflect a true structural trend and are not greatly influenced by low-angle ome Plate is a light-toned plateau ~90 m Bomb sags are found in volcaniclastic deposits cross-bedding. in diameter and 2 to 3 m high within the on Earth, where outsized clasts ejected from an Chemical compositions analyzed by the HInner Basin of the Columbia Hills, at explosive vent are emplaced ballistically into Alpha Particle X-ray Spectrometer (APXS) are Spirit’s landing site in Gusev crater (1–3). Home deformable materials, causing downward deflec- Plate appears prominent from orbit, and was tion of layering. 1Department of Astronomy, Space Sciences Building, Cornell identified after landing as a high-priority target. In contrast to the lower unit, the upper unit is University, Ithaca, NY 14853, USA. 2Division of Geological It is the most extensive exposure of layered fine grained, well sorted, and finely laminated, and Planetary Sciences, California Institute of Technology, 3 bedrock encountered by Spirit at Gusev to date. and it exhibits cross-stratification. On the Pasadena,CA91125,USA. Lockheed Martin Corporation, Littleton, CO 80127, USA. 4Institute of Meteoritics, University Spirit arrived at the northern edge of Home northern edge of Home Plate, the upper unit of New Mexico, Albuquerque, NM 87131, USA. 5New Mexico Plate on sol 744 (4), following the path shown in exposes a facies that is well bedded and char- Museum of Natural History and Science, Albuquerque, NM Fig. 1. Images of the plateau show a thick stack acterized by ubiquitous fine lamination that is 87104, USA. 6Vallourec Research Center, F-59260 Aulnoye- Aymeries, France. 7Space Science Institute, Boulder, CO of layered rocks, with a lower coarse-grained unit arranged in bed sets with planar to low-angle 8 and an upper finer-grained unit (Fig. 2). The cross-stratification (Fig. 2C). Other features of 80301, USA. Department of Physics, University of Guelph, Guelph, ON, N1G 2W1, Canada. 9Center for Earth and lower unit is characterized by prominent parallel this facies include gently dipping curved or Planetary Studies, Smithsonian Institution, Washington, DC layering with low apparent dips and a coarse irregular surfaces of erosion, small-scale cut- 20560, USA. 10Jet Propulsion Laboratory, California Institute granular texture (Fig. 2B). Individual granules and-fill structures, convex-upward laminations, of Technology, Pasadena, CA 91109, USA. 11United States 12 are roughly equant, and typically 0.5 to 3 mm in and occasional intercalation of thin beds of Geological Survey, Flagstaff, AZ 86001, USA. Institut für Anorganische und Analytische Chemie, Johannes Gutenberg- size. It is difficult to determine in Microscopic high-angle cross-bedding. In MI images, this Universität, Mainz, Germany. 13Department of Civil and Imager (MI) images whether the granules are facies exhibits a distinctly clastic texture, with Environmental Engineering and Geodetic Science, Ohio State original clasts, such as accretionary lapilli, or grains 200 to 400 mmindiameterthatare University, Columbus, OH 43210, USA. 14Department of textures reflecting postdepositional cementation exceptionally well rounded and sorted (fig. S3). Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA. (fig. S1). Toward the top of the lower unit is a A second facies in the upper unit that is par- 15Lunar and Planetary Laboratory, University of Arizona, massive section roughly 10 cm thick where ticularly well developed at the eastern edge of Tucson,AZ85721,USA.16Department of Earth and Planetary layering becomes indistinct (Fig. 3) and grains Home Plate exhibits high-angle cross-bedding Sciences, University of Tennessee, Knoxville, TN 37996, USA. 17 are difficult to identify in MI images (fig. S2). (Fig. 4). Here, the geometry is expressed as wedge Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, TX 77058, USA. 18NASA A particularly notable feature in the lower sets up to several tens of centimeters thick of Ames Research Center, Moffett Field, CA 94035, USA. unit is a ~4-cm clast with deformed layers distinctly trough-shaped cross-strata. Internal strat- 19Department of Geological Sciences, Arizona State Uni- beneathit,interpretedtobeabombsag(Fig.3). ification ranges from finely laminated to more versity, Tempe, AZ 85287, USA. EMBARGOED UNTIL 2PM U.S. EASTERN TIME ON THE THURSDAY BEFORE THIS DATE: 738 4 MAY 2007 VOL 316 SCIENCE www.sciencemag.org REPORTS given in Table 1. The outcrops Posey and Cool compared directly because of uncertainties in netite, whereas MB yields abundant Fe from Papa Bell (5) from the upper unit were brushed the Fe content of both phases. Mini-TES decon- magnetite (26 to 32%) and nanophase oxide with the Rock Abrasion Tool (RAT) before volution yields ~45% basaltic glass and no mag- (npOx) (27 to 29%). This apparent difference analysis. Barnhill, from the lower unit, could not be brushed; Pancam color images were used to Fig. 1. Home Plate as select relatively dust-free regions. Fuzzy Smith, viewed from orbit. North a small (~10-cm) loose rock on the plateau, also is at the top. The path could not be brushed. The lack of strong SO3 followed by the rover is and Cl enrichments for unbrushed surfaces rela- shown, and the locations tive to brushed ones suggests that surface dust of rocks discussed in the contamination was modest. text are shown. [High- Most Home Plate outcrop rocks are fairly Resolution Image Sci- uniform in elemental composition, except for ence Experiment image Barnhill Fastball, which has lower Al2O3 and PSP_001513_1655, Na2O and higher MgO.
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