Concentration of H, Si, Cl, K, Fe, and Th in the low‐ and mid‐latitude regions of Mars

Abstract

We report maps of the concentrations of H, Si, Cl, K, Fe, and Th as determined by the Gamma Ray Spectrometer (GRS) on board the 2001 Mars Odyssey Mission for ±∼45° latitudes. The procedures by which the spectra are processed to yield quantitative concentrations are described in detail. The concentrations of elements determined over the locations of the various Mars landers generally agree well with the lander values except for Fe, although the mean of the GRS Fe data agrees well with that of Martian meteorites. The water‐equivalent concentration of hydrogen by mass varies from about 1.5% to 7.5% (by mass) with the most enriched areas being near Apollinaris Patera and Arabia Terra. Cl shows a distribution similar to H over the surface except that the Cl content over Medusae Fossae is much greater than elsewhere. The map of Fe shows enrichment in the northern lowlands versus the southern highlands. Silicon shows only very modest variation over the surface with mass fractions ranging from 19% to 22% over most of the planet, though a significant depletion in Si is noted in a region west of Tharsis Montes and Olympus Mons where the Si content is as low as 18%. K and Th show a very similar pattern with depletions associated with young volcanic deposits and enrichments associated with the TES Surface Type‐2 material. It is noted that there appears to be no evidence of significant globally distributed thick dust deposits of uniform composition.

Martian surface heat production and crustal heat flow from Mars Odyssey Gamma-Ray spectrometry

“…Although it has been suggested, there is no definitive evidence for current or past plate tectonic processes, nor have orbital remote sensing instruments detected any major provinces of the highly-evolved crustal products that we see on Earth or on the Moon [Jolliff et al, 2000]. GRS data show no suggestion that ejecta from the largest (and deepest) impact basins have compositions that differ from surrounding areas [Boynton et al, 2007]. Although there are localized regions of quartz-bearing igneous rocks [Bandfield et al, 2004;Bandfield, 2006;Christensen et al, 2005], previously observed regions of relatively high-silica, igneous "andesite" (Surface Type 2) have been more recently reinterpreted to reflect surface coatings of a silicabearing secondary mineral phase [Wyatt and McSween, 2002;Rogers and Christensen, 2007].…”

“…Normally these elements are preferentially sequestered into a planet's crust during differentiation [Taylor and McLennan, 2009], and this is especially true for Mars, which possesses a thick and mostly ancient crust that is proportionally large with respect to the planet's total volume compared to Earth. The Gamma-Ray Spectrometer (GRS) instrument on board the 2001 Mars Odyssey spacecraft has been used to map the K and Th abundances across nearly the entire Martian surface [Boynton et al, 2007]. Here we present the first detailed Martian surface heat production and crustal heat flow maps based on unambiguous orbital geochemical measurements that show significant geographic variation in the crustal thermal reservoir.…”

“…Fifty percent of incident gamma rays collected by the GRS are from a footprint of 440 km diameter. The penetration depth is on the order of a few tens of centimeters, depending on the density of the underlying material (see Boynton et al [2007] for technical specifications). Data were collected and summed over 2 mapping periods from June 2002 through April 2005 and from April 2005 through March 2006.…”

“…Elemental abundance data for K and Th have been compiled into smoothed maps with 5°× 5°per pixel resolution [Boynton et al, 2007;Taylor et al, 2006aTaylor et al, , 2006b. At this time, it is not possible to generate a useful map of U abundances at comparable resolution (due to U concentrations too low to provide the necessary gamma-ray detection counting statistics).…”

“…Also, H concentrations have a strong influence on the correction techniques used for determining other elemental abundances. To compensate for these complexities, we have excluded data pole-ward of ∼45°-60°latitude in both hemispheres using a cut-off based upon water equivalent hydrogen concentration -the H-mask described by Boynton et al [2007].…”