Mathematics – Logic
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p32a..04v&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P32A-04
Mathematics
Logic
[3672] Mineralogy And Petrology / Planetary Mineralogy And Petrology, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
Gale Crater, field site for the 2011 Mars Science Laboratory rover Curiosity, is located in one of the most chlorine- and sulfur-rich areas detected by Odyssey gamma-ray spectroscopy (GRS) along the dichotomy boundary between Mars' southern cratered highlands and the northern plains. Comparably elevated chlorine and sulfur abundances occur in Arabia Terra, where modeling and MER Opportunity observations suggest that groundwater upwelling led to evaporite salt precipitation. In a deep crater such as Gale upwelling may have produced playa systems. High GRS sulfur at Gale Crater is supported by visible-infrared spectra in Gale's central mound consistent with Mg-sulfates (kieserite and polyhydrated), interstratified with smectite. There is a significant (~20 m) section with abundant nontronite and little or no evidence of hydrated sulfate in the lower mound. Mineral stratification at Gale promises the possibility of capturing a transition in sedimentary environment between phyllosilicate- and sulfate-dominated deposition. Sedimentary fabrics, textures, and mineralogy all bear on determining sedimentary history and Gale will provide an exceptionally detailed history through Curiosity's instrument suite. Should the clay minerals at Gale be limited to nontronite and other smectites, with little evidence of chlorite or mixed layer chlorite/smectite, limits can be placed on the extent of interaction with Mg-sulfate solutions. Gale has evidence of being at one time completely filled and a lack of chloritic forms would be particularly limiting on occurrence and composition of paleo-groundwater. Complete infilling of Gale would likely result in past burial temperatures, at the deep elevation of the landing site, favoring chloritization of smectite if concentrated Mg-sulfate groundwater were present. Sedimentary history at Gale may be further constrained by absence or presence of Ca-sulfates, which should form by cation exchange between smectite and Mg-enriched brines if the two were in communication. Since Mg-sulfate solutions are implicated by presence of Mg-sulfates at Gale, absence or presence of Ca-sulfates may be a measure of hydrologic isolation or interaction, respectively, within and between stratigraphic horizons. The forms in which Ca-sulfates may occur (gypsum, bassanite, or anhydrite) would provide further constraints on Gale's history of deposition and alteration. The high solubility of Mg-sulfates can also place limits on interpretations of post-depositional alteration by groundwater, wherein preservation of primary depositional structure, fabric, and mineralogy will be an especially sensitive marker of deposits undisturbed by such alteration. Groundwater, snowmelt, and surface water are all capable of attacking Mg-sulfate (and potential halide) deposits at Gale but so too will elevated water vapor pressure that may lead to deliquescence or efflorescence, effects that may be expected if episodes of high obliquity allowed prolonged frost or ice accumulation in Gale Crater. Among the concerns of Curiosity will be identification and sampling of sedimentary strata that are as pristine as possible. The sensitivity of phyllosilicate-sulfate associations to transformation promises that evidence of alteration will be identifiable.
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