Signs of Transport of Chemical Elements and Soil-Forming Processes in Surface Soils at Gale Crater, Mars E.M

47th Lunar and Planetary Science Conference (2016) 2493.pdf

SIGNS OF TRANSPORT OF CHEMICAL ELEMENTS AND SOIL-FORMING PROCESSES IN SURFACE SOILS AT GALE CRATER, MARS E.M. Hausrath1, W. Goetz2, A. Cousin3, R.C. Wiens4, P.-Y. Mes- lin3, W. Rapin3 1UNLV, Las Vegas, NV [email protected] 2MPS, Göttingen, Germany 3IRAP, CNRS/UPS, Toulouse, France 4LANL, Los Alamos, NM

Introduction: Soils provide an integrated record of martian surface processes. Recorded in soils is information about parent material, the effect of impacts, aeolian and fluvial transport, chemical weathering from both ancient and modern Mars, and addition and loss of material. Analysis of soils can therefore provide important information about the history of Mars. Soil-forming processes in desert soils on Earth are preserved in the chemistry and morphology of those soils. Desert soils often contain desert pavement, an interlocking layer of rocks on the surface of the soil, and a dust-rich layer termed the Av horizon beneath the desert pavement [1]. Desert pavement can indicate age, with a more interlocking pavement indicating greater age [2]. Increasing age also results in a thicker dust-rich layer (Av horizon) beneath the desert pavement [2]. This dust-rich layer preserves not only the composition of the deposited aeolian dust, but also

evidence of any interactions with liquid water. The Figure 1. NavCam image of the target Soda (indicated dust-rich layer also strongly affects how liquid water with a circle) Image credit: NASA/JPL-Caltech interacts with soils. Soils on Earth, including desert soils, can be stud- ied by analyzing depth profiles such as in soil pits or augered samples. Changes with depth on Earth have been used to interpret profiles dominated by immobili- ty of elements, depletion of elements, depletion fol- lowed by enrichment of elements, addition of elements and biogenic impacts [3]. Depth profiles can also indicate information about soils such as the age of the soils, characteristics of interaction with liquid water, and composition of deposited dust. ChemCam measurements at Gale Crater, Mars form depth profiles over different scales, including fine scale profiles formed by repeated shots in one location, as well as across transects of vertically exposed material, such as the example shown in Figures 1 and 2. Previ- ous work on depth profiles resulting from repeated ChemCam shots in the same location has shown the presence of Mn-rich weathering rinds interpreted to indicate oxidation conditions [4], as well as, in one case, weathering indicated by mobility of Li [5]. Here, in order to examine potential soil-forming processes at Gale Crater, we examine transects at the millimeter scale measured by ChemCam through vertical soil exposures. Figure 2. Remote Microscopic Imager mosaic of the target Methods: First, NavCam images of ChemCam tar- Soda Image credit: NASA/JPL- gets were examined through sol 1062. From these im- Caltech/CNES/CNRS/LANL/IRAP/IAS/LPGN ages, steep to nearly vertical soils were identified visu- Ten targets contained a nearly vertical soil face, ei- ally, such as the example shown in Figures 1 and 2. ther naturally exposed or disturbed by the rover wheel. 47th Lunar and Planetary Science Conference (2016) 2493.pdf

For these ten targets, elemental concentrations across movement and chemical weathering [9]. Further exam- the soil face were examined from ChemCam measure- ination of soils on Mars therefore has potential to simi- ments available on the Planetary Data System (PDS) larly help interpret the past history of that planet. (https://pds.nasa.gov/). Many of the soil profiles show concentrations that Results and discussion: Multiple profiles show ev- are relatively consistent from top to bottom, indicative idence of slight Fe enrichment closer to the surface of of relatively homogeneous soil compositions at that the depth profile than deeper in the depth profile. Alt- scale. In addition, as previously shown [10, 11], mul- hough slight, the multiple occurrences suggest the tiple measurements also indicate grains of different presence of a process that should be investigated fur- compositions, indicating the importance of original ther. Iron concentration versus depth for the target parent material on soil formation on Mars. Soda, which is one of the targets that shows this fea- Conclusions and future work: In this review of ture, is shown in Figure 3. We propose that this Fe variation in chemistry with depth at ten sites, similar enrichment closer to the top of the profile may result trends were observed at multiple sites, suggesting that from Fe-rich material at the surface such as nanophase this approach may be a valuable application of Chem- Fe-oxides found in martian dust [6], transported Cam data in interpreting soils at Gale Crater. Slight downward. Dust deposition could also result in finer enrichment in Fe nearer the soil surface is observed in material observed beneath pebble surfaces at Rocknest multiple profiles, suggesting it may result from a pro- [7, 8] and other locations at Gale Crater. cess that should be investigated further. It is potential-

ly consistent with Fe-rich material at the surface such as nanophase Fe-oxides found in martian dust [6], 128

130 transported downward. On Earth, deposition of dust in

132 desert soils forms dust-rich horizons that record the age

134 of the soil, the composition of the deposited dust, and 136 potential water: rock interactions and chemical weath- 138 ering [9]. Examination of soils on Mars therefore has

140 potential to similarly help interpret its history. 142 More work is needed to further interpret soil- 144 Estimateddepth (mm) forming processes on Mars. Future work will include 146

148 analysis of additional measurements from the MSL

150 ChemCam instrument and comparison of analyzed soil 15 16 17 18 19 20 21 22 23 24 25 surfaces and measured dust with soil profiles, as well FeO T as an investigation of soil variability. Examination of Figure 3. Plot of Fe concentration versus estimated possible Fe enrichment may also help better understand depth for the Soda target. Error bars indicate one secondary materials previously observed in soils at standard deviation as reported in the PDS, and depth Gale Crater [12, 7, 10, 13-16], thereby helping further was estimated from image analysis. These data cor- interpret the past history of Mars. respond to the images in Figures 1 and 2, with the References: 1.Wells, S.G., et al., (1995) Geology, 23 deepest depth corresponding to ChemCam point 1, and 613-616. 2. Easterbrook, D., Quaternary Geology of the the shallowest depth corresponding to point 5. The United States 2003: Geological Society of America. 3. slightly higher concentration of Fe nearer the surface Brantley, S.L., et al. in Fundamentals of Geobiology, A.H. is characteristic of multiple profiles examined, and Knoll et al. , Editors. 2012, Blackwell Publishing Limited. p. may reflect transport of Fe-rich material from the sur- 205-227. 4. Lanza, N.L., et al., (2014) GRL 41 face downward. 2014GL060329. 5. Ollila, A.M., et al., (2014) JGRP, 119 This possible process could be similar to that 2013JE004517. 6. Bell, J.F., et al., (2000) JGRP, 105 1721- which, in desert soils on Earth, causes a dust-rich soil 1755. 7. Blake, D.F., et al., (2013) Science, 341. 8. Goetz, layer (the Av horizon) beneath desert pavement [1]. W., et al., LPSC 2013: Abstract # 1222. 9.McFadden, L.D., In these terrestrial desert soils, dust is trapped beneath et al., (1998) Geomorphology, 24 101-145.10. Meslin, P.-Y., pebbles on the surface, causing the gradual inflation of et al., (2013) Science, 341. 11. Cousin, A., et al., (2015) the landscape. Importantly, on Earth such dust-rich Icarus, 249 22-42. 12. Bish, D.L., et al., (2013) Science, 341. horizons record not just the accumulated dust and age 13. Dehouck, E., et al., (2014) JGRP119 2014JE004716. 14. of the soil, but also show chemical alteration of the Schröder, S., et al., Eighth International Conference on dust, with changes such as translocated salts deeper in Mars. 2014. Abstract #1214. 15. Vaniman, D., et al., (2014) the profiles indicating the rates and amount of water Science, 343 1243480. 16. Schröder, S., et al., (2015) Icarus, 249 43-61.