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Decades of speculation about a warmer, wetter Mars climate in the planet's first billion years postulate a denser C[O.sub.2]-rich atmosphere than at present. Such an atmosphere should have led to the formation of outcrops rich in carbonate minerals, for which evidence has been sparse. Using the Mars Exploration Rover Spirit, we have now identified outcrops rich in magnesium-iron carbonate (16 to 34 weight percent) in the Columbia Hills of Gusev crater. Its composition approximates the average composition of the carbonate globules in martian meteorite ALH 84001. The Gusev carbonate probably precipitated from carbonate-bearing solutions under hydrothermal conditions at near-neutral pH in association with volcanic activity during the Noachian era. 10.1126/science.1189667

Author(s): Walter Goetz (corresponding author) [1]; Preben Bertelsen [2]; Charlotte S. Binau [2]; Haraldur P. Gunnlaugsson [3]; Stubbe F. Hviid [1]; Kjartan M. Kinch [3]; Daniel E. Madsen [2]; Morten [...]

Author(s): Larry A. Haskin [1, 18]; Alian Wang (corresponding author) [1]; Bradley L. Jolliff [1]; Harry Y. McSween [2]; Benton C. Clark [3]; David J. Des Marais [4]; Scott M. [...]

Author(s): Albert S. Yen (corresponding author) [1]; Ralf Gellert [2]; Christian Schröder [3]; Richard V. Morris [4]; James F. Bell, III [5]; Amy T. Knudson [6]; Benton C. Clark [7]; [...]

We discuss the use of Mossbauer (MB) spectroscopy to study Fe-bearing minerals on Venus' surface. At present, there is no direct information about the mineralogy of Venus' surface, although in situ chemical analyses by X-ray fluorescence (XRF) spectroscopy have been done by the Venera 13, 14, and Vega 2 spacecraft at three landing sites. The XRF elemental analyses are sensitive to major rock-forming elements heavier than sodium and show the presence of several mass percent iron. Normative mineralogical calculations model the Fe mineralogy at the Venera 13, 14, and Vega 2 landing sites, but the actual Fe minerals present are unknown. We calculate synthetic MB spectra for the normative Fe minerals at the Venera 14 and Vega 2 landing sites. Some calculations include several mass percent of the Fe oxides (magnetite, hematite) and sulfides (pyrite, pyrrhotite) that are frequently discussed in the literature as being present on Venus' surface. Our results indicate that the normative iron minerals at the different landing sites can be identified, quantitatively measured, and distinguished from each other using MB spectroscopy. We also find that about 1 mass percent of any of the different iron oxides and sulfides considered in our modeling should be detectable using MB spectroscopy. This is significant because the loaded dielectric model for low radar emissivity regions on Venus requires several mass percent of Fe-bearing phases. The temperature dependence of the MB spectra is also calculated. Finally, the implications of these results for future spacecraft missions to Venus using MB spectroscopy are discussed. [C] 2000 Academic Press Key Words: Venus; Mossbauer spectroscopy; mineralogy; geo-chemistry; pyrrhotite; pyrite; olivine; fayalite; ferrosilite; clinopy-roxene; orthopyroxene; magnetite; hematite; Venera 13 and 14; Vega 2.

Author(s): Albert S. Yen; Ralf Gellert; Christian Schröder; Richard V. Morris; James F. Bell, III; Amy T. Knudson; Benton C. Clark; Douglas W. Ming; Joy A. Crisp; Raymond E. Arvidson; [...]