Geochemistry of Martian soil and bedrock in mantled and less mantled terrains with gamma ray data from Mars Odyssey

Abstract

Surficial materials, including soil and dust, are abundant in the upper tens of centimeters of the Martian surface sensed by the Mars Odyssey Gamma Ray Spectrometer (GRS). Seven large areas (14% of the Martian surface) that represent possible compositional end‐members were selected, including three regions heavily mantled with surficial materials. The selection process included mapping the ratio of exposed rocky terrain to surficial materials using high‐resolution imagery. GRS data for H, Cl, Fe, Si, K, and Th were obtained for each area. The areas are chemically homogeneous within each area, given the spatial resolution and analytical uncertainty of the GRS data. However, substantial chemical differences exist among the areas, including the different mantled terrains, contrary to earlier assumptions that surficial materials are globally homogeneous due to aeolian mixing. The observed chemical differences among the areas may be due to variations in the protolith compositions, extent of alteration of the protolith regions, or post soil formation processes. The abundances of Cl, K, and Th in rockier (but still soil‐rich) areas such as Syrtis Major Planum can be explained by mixing between a soil with higher concentrations of Cl, K, and Th, similar to the abundances in the mantled terrains (and some of the landing sites), and crustal rocks containing lower abundances of these elements, similar to Martian meteorites.

Soil Diversity and Hydration as Observed by ChemCam at Gale Crater, Mars

“…It could reflect the widespread presence of regions with similar basaltic composition, possibly characterized by lower resistivity to physical and chemical weathering. On the other hand, investigations at other landing sites also found relatively little influence from local bedrock composition (21), rock compositions found at Gale crater differ from other sites (38), and Mars Odyssey GRS found large provinces characterized by different compositions (22,53); these findings could indicate that the mafic soil component in fact represents an average of different compositions as a result of large-scale homogenization processes. It was suggested, for instance, that this ubiquitous material was made of two unrelated components: a component derived from relatively young olivine-rich basalts degraded under relatively anhydrous conditions and a component containing clay minerals, amorphous silica, and sulfur-and chlorine-rich nanophase ferric oxides resulting from the alteration of ancient rocks under hydrous conditions (2).…”

GRS evidence and the possibility of paleooceans on Mars

“…This includes extreme variations in endogenic and exogenic conditions fram ancient to present Mars , such as: (1) crustaljlithospheric and heat flow conditions (e.g., thraugh time, crustaljlithospheric thickness increases, tectonism decreases but becomes concentrated near the large shield volcanoes, and heat flow diminishes, all of which have a significant effect on ancient vs. present-day topographies; e.g., Schubert et aL (1992 ), Seott and Dohm (1997 ), Dohm et al (200la ), Anderson et al (2001), McGovern et al (2002, Fairén et aL (2003), Ruiz (2003), Ruiz et aL (2004)); (2) impact cratering (Neukum et aL, 2001 ), especially with respect to the period of reported impact catastraphism (Stram et al, 2005 ); (3) surface and near-surface conditions (sratigraphic, tectonic, geomorphic, mineralogic, geochemical, geophysical, and topographic, etc. ;e.g., see Scott and Tanaka (1986 ), Dohm et al (2001c), Fairén et al (2004, , Connerney et aL (2005), Tanaka et aL (2005), Taylor et aL (2006a, b ), Karunatillake et aL (2006), , Hahn et aL (2007), Newsom et aL (2007), Boynton et aL (2007 )); (4) atmospheric and enviranmental conditions (e.g., Baker et al, 1991 ), such that aqueous conditions, which may have been less acidic to form clays early on in the weathering micraenviranment, gave way to more acidic long-lasting conditions to form sulfates (Bibring et al , 2006 ) andjor acidic clay minerals such as kaolinite and halloysite, largely related to the grawth of the dominant, yet enduring development of the Tharsis superplume pulsating fram the Noachian to the Amazonian (Dohm et al 200la, 2007b ) and to lesser magma tic complexes such as Elysium (e.g., Tanaka et al, 2005 ); these heat engines were episodically active to interrupt the long-persistent, cold-dry state for relatively short transient periods; (5) clay genesis of (4) …”

“…Analysis of the data acquired fram the GRS instrument suite has mapped the presence of significant subsurface ice deposits in regions poleward of appraximately 60" north and south latitudes Feldman et aL, 2002 ;Mitrafanov et aL, 2002 ;Litvak et aL, 2006 ), the distribution of H in low and mid latitudes Mitrafanov et aL, 2004;Fialips et aL, 2005 ), distinct elevated chlorine (CI) regions , and other elements such as K, Th, Fe, and silicon (Si) . These six elements of the Martian surface mapped thraugh GRS have also formed the basis for evaluating the variations in K/Th (Taylor et al, 2006a ), bulk composition (Taylor et al, 2006b ), composition of northern low-albedo regions (Karunatillake et aL, 2006 ), elemental abundances with respect to relative surface age (Hahn et aL, 2007 ), geochemistry of Martian soil and bedrack in mantled and less mantled terrains (Newsom et aL, 2007), and whether a giant ancient basin exists in Arabia Terra (Dohm et aL, 2007a). In places, depletion of Si might well relate to Fe accumulation in ancient surface weathering enviranments (e.g., Noachian surface exposures).…”

The loss of K, REE, Th, and U from a Martian and a terrestrial basalt by acidic leaching

“…The average concentration is 0.63 ppm in the southern basalts and 0.94 ppm in the northern, possibly andesitic, basalts (Karunatillake et al 2006). GRS data of individual areas indicate an enrichment of Th in dust and soil relative to the local bedrock (Newsom et al 2007). The differences in the concentrations of Th, as for K, were attributed principally to differing compositions of the parent magmas, with later modest modifications by aqueous alteration or aeolian erosion.…”

“…Evaluations for individual areas yield an enrichment of K in dust and soil relative to local bedrock (Newsom et al 2007). The differences in concentrations of K generally were attributed to the compositions of the parent magmas, with later modest modifications by aqueous alteration or aeolian erosion.…”