Regional Context of Soil and Rock Chemistry at Gale and Gusev Craters, Mars

46th Lunar and Planetary Science Conference (2015) 2284.pdf

REGIONAL CONTEXT OF SOIL AND ROCK CHEMISTRY AT GALE AND GUSEV CRATERS, MARS. H.E. Newsom1, S. Gordon1, R. Jackson1, R.C. Wiens2, N. Lanza2, A. Cousin2, S. Clegg2, V. Sautter3, J. Bridges4, N. Man- gold5, O. Gasnault6, S. Maurice6, C. D’Uston6, G. Berger6, O. Forni6, J. Lasue6, P.-Y. Meslin7, B. Clark8, R. Anderson9, R. Gellert10, M. Schmidt11, J. Berger11, S. McLennan12, W. Boynton13, M. Fisk14; F. Martin-Torres15, M.-P. Zorzano16, S. Karunatillake17 1Inst. of Meteoritics, Univ. of New Mexico, Albuquerque, NM 87131, USA ([email protected]); 2Los Alamos National Lab., NM, USA; 3MNHN, CNRS Fr; 4Univ. of Leicester, GB; 5Lab. de Planetologie et Geodynamique de Nantes, FR; 6IRAP/CNRS, FR; 7Univ. Paul Sabatier, Toulouse, FR; 8Planetary Science Inst., Tucson, AZ, USA; 9USGS, Flagstaff, AZ, USA; 10Univ. of Guelph, Canada; 11Brock Univ., Canada, 12Stony Brook Univ., NY, USA; 13Univ. of Arizona, AZ, USA, 14Univ. of Oregon, USA. 15Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR), Granada, Spain,16Centro de Astrobiología (INTA-CSIC), Madrid, Spain, 17Louisiana State Univ., LA, USA

Introduction: Geochemical data for both rocks and soils from Gale Crater, and Gusev Crater, are compared here with data from the Gamma Ray Spectrometer (GRS) experiment on the Mars Odyssey Spacecraft [1, 2]. Both Gale and Gusev craters are located near the dichotomy boundary between the Noachian Highlands and the younger volcanics and transitional units to the north (Fig. 1). Element ratios in these samples may provide a link between the regional provinces analyzed by GRS and the materials at the two landing sites. The lith- Example of GRS ophile data may lead to a better understanding of the 10 degree “pixel” origin and evolution of the martian crust in this region of Mars, while the volatile element components SO3, Cl, and water provide information on volcanic aerosols, weathering processes and potentially recent climate [3]. Fig. 2 – Areas with distinctive GRS signatures based on 5 x 5 degree binned data.

Fig. 1 – Portion of the Mars Global Geologic map with arrows showing Gale (West) and Gusev (East) craters on the

dichotomy boundary. See Fig. 2 for scale and reference. Fig. 3 – CaO/SiO2 vs. FeO/SiO2 data for areas near Gale and Gusev Craters (Fig. 2) with distinctive GRS signatures. Geochemical components and normalization: Comparing the chemistry of Gale and Gusev samples Regional trends from GRS: The GRS composi- with other martian data must take into account the dif- tions reflect the integrated abundances to a depth of ~ ferent geochemical components in the samples. The 0.5 m, but the instrument is not collimated, and ~ 50% most important distinction is between the lithophile el- of the received gamma rays come from an area ten de- ements including Al, Si, Fe, Mn, Ca, Na, Mg, etc. that grees in diameter on the surface below the instrument represents the rock component, and the volatile ele- [4]. Thus most of the signal from each 5 degree binned ments including H, C, Cl, S, that represent later external data used in this study comes from outside the nominal input to the soils and rocks. Normalization to SiO2, pro- area represented by the data. Because Gale and Gusev vides a way to correct the lithophile elements for varia- craters are only ~ 3 degrees in diameter, there is no way ble amounts of the mobile element component, mainly to get their unique GRS signatures. Therefore, because sulfur, chlorine and water. the two landing sites are on the dichotomy boundary, 46th Lunar and Planetary Science Conference (2015) 2284.pdf

and due to the low resolution of the GRS data, the area deposits enriched in these elements. For the rocks, the seen in the geological map has been divided into regions lithophile elements including Ca and Fe are more scat- on 5 degree boundaries (Fig. 2) that have distinctive tered, but CaO abundances are lower than the Elysium GRS chemical signatures [1]. An example of the differ- and younger volcanics to the north, probably eliminat- ences in the chemistry of these regions is seen in the ar- ing any relationship with those areas. eas outlined on the CaO/SiO2 versus FeO/SiO2 data The rocks analyzed by Curiosity show a huge range (Fig. 3). Note that the highlands crust south of Gale has in K2O/SiO2 (Fig. 5) that is not reflected in the regional lower FeO/SiO2 and CaO/SiO2 than the nearby Elysium GRS data, suggesting a local phenomenon. For the data volcanic complex and the areas of the Medusa Fossae to from Curiosity, Fisk et al. [6] suggest that addition of a the north of Gale. K-Fe-Mg-rich secondary phase (possibly phyllosilicate) is possible, and/or addition of separate K-rich and Fe- Mg-rich phases. Schmidt et al. [7] call upon mantle met- asomatism and different amounts of partial melting to generate these differences. Volatile elements: The regional water equivalent H content of the areas shown in Fig. 1 from the GRS experiment ranges from ~ 3 to 7 wt%. The water content of materials at Gale ranges from 2 wt% to 9 wt% from MSL ChemCam, SAM and Chemin analyses on Curi- osity [8], and is mostly part of the amorphous component of the soils and rocks. At Gusev, SO3 and Cl in the soils comprise ~ 8 wt% of the total [3], and water in the Columbia Hills ranged up to 17 wt % [3, 9], consistent with a high local GRS water signature, especially for the Fig. 4 – Data from MER Spirit and MSL Curiosity for area of the Medusa Fossae area near Gusev. The lack of enrichment of the highly fluid mobile element lithium CaO/SiO2 vs. FeO/SiO2 compared with regional GRS data. determined by ChemCam [5] in most samples from Gale argues against a hydrothermal component, but does not rule out a volcanic aerosol component of the soil. However, the detection of enriched manganese vein deposits and other diagenetic features in Gale [10], and enriched silicon deposits at Home Plate in Gusev [11] indicate that transport of fluid mobile elements by aqueous or hydrothermal processes is common on Mars. Conclusions: The geochemical data from the GRS experiment on Mars Odyssey provides a regional context for the geochemical data from the Curiosity rover in Gale crater and the data from Spirit in Gusev. These

Fig. 5 – ChemCam data density map for CaO/SiO2 vs. comparisons are providing new insights into the diver- K2O/SiO2 (normalized to APXS, J. Bridges pers. comm.) com- sity of petrologic and aqueous processes on Mars. The pared with the GRS data for the region. The circle at high diversity of compositions at the landing sites may reflect K2O/SiO2 ratios reflects the feldspar-rich end of the basaltic the different regional compositions seen in the GRS trend. Symbol legend in Fig. 3. data. Acknowledgements: Funding for ChemCam Co-investi- gator Newsom provided by NASA/JPL. Comparisons between GRS data, Gale (Curios- References: [1] Gasnault O. et al. (2007) Icarus 207, 226. ity) and Gusev (Spirit): CaO/SiO2 versus FeO/SiO2 [2] Newsom H. (2007) J. Geophys. Res. 112. [3] Clark B. data from the rovers is compared with the GRS data in (2005) EPSL 240, 73. [4] Boynton, W. V., et al. (2007) J. Ge- Fig. 4. In contrast to data from the rocks, the soils at the ophys. Res., 112, E12S99, [5] Boynton, W. V., et al. (2008) two sites are relatively homogeneous. The Gale soil rep- Chapt. 5, in Bell J. ed. The Martian Surface. [6] Fisk M. et al., resented by the Portage analysis is somewhat higher in (2015) this meeting. [7] Schmidt M. et al., (2015) this meeting FeO/SiO2 and CaO/SiO2 (and chlorine) than the soil [8] Leshin, L.A. (2013) Science, 341 (6153):1238937. [9] analyses in the Columbia Hills [5]. Therefore, Gale soil Ming, D. et al., (2008) in Bell ed. [10] Lanza, N.L. et al. (2014) could reflect a contribution from nearby Medusa Fossae Geophys. Res. Lett., 41 (16): 5755-5763. [11] Squyres, S. W., et al. (2008) Science 320.5879: 1063-1067.