L. Le Deit

Mineralogical composition, structure, morphology, and geological history of Aram Chaos crater fill on Mars derived from OMEGA Mars Express data

[1]xa0Aram Chaos is a crater 280 km in diameter centered at 2.5°N, 338.5°E. It is filled by chaotic terrains overlain by a dome-shaped, layered 900 m thick formation displaying spectral signatures of ferric oxides on Thermal Emission Spectrometer (TES) and Observatoire pour la Mineralogie, LEau, les Glaces et LActivite (OMEGA) medium spatial resolution data. We describe in detail the mineralogical composition, structure, and morphology of this crater fill using high-resolution data (OMEGA, Mars Orbiter Laser Altimeter, Mars Orbiter Camera, TES, Thermal Emission Imaging System, and High-Resolution Imaging Science Experiment). We infer the following formation scenario: the crater was first filled by a geological formation, the composition of which remains unclear because it is covered by dust. Widespread fracturing of this formation led to the development of chaotic terrains. Later, a second layered formation, presently dome shaped, was emplaced unconformably on the chaotic terrains. This younger unit is composed of a bright, poorly consolidated material that contains both monohydrated sulfates and ferric oxides according to OMEGA data. The surface of this formation is partially covered by dust and displays landforms indicating that the bright material has been mobilized by wind during or after its deposition. After its emplacement, this formation has been grooved down to various depths by large eolian erosion corridors. In these corridors, eolian removal of the bright material with a sulfate-rich matrix has left debris fans, sand sheets, and dunes, which display some of the strongest spectral signatures of ferric oxides on Mars. Similar residual deposits enriched in ferric oxides, overlying a layered formation containing both ferric oxides and sulfates, have been observed by the Opportunity rover in Meridiani Planum, suggesting a common formation process.

ChemCam activities and discoveries during the nominal mission of the Mars Science Laboratory in Gale crater, Mars

At Gale crater, Mars, ChemCam acquired its first laser-induced breakdown spectroscopy (LIBS) target on Sol 13 of the landed portion of the mission (a Sol is a Mars day). Up to Sol 800, more than 188 000 LIBS spectra were acquired on more than 5800 points distributed over about 650 individual targets. We present a comprehensive review of ChemCam scientific accomplishments during that period, together with a focus on the lessons learned from the first use of LIBS in space. For data processing, we describe new tools that had to be developed to account for the uniqueness of Mars data. With regard to chemistry, we present a summary of the composition range measured on Mars for major-element oxides (SiO_2, TiO_2, Al_2O_3, FeO_T, MgO, CaO, Na_2O, K_2O) based on various multivariate models, with associated precisions. ChemCam also observed H, and the non-metallic elements C, O, P, and S, which are usually difficult to quantify with LIBS. F and Cl are observed through their molecular lines. We discuss the most relevant LIBS lines for detection of minor and trace elements (Li, Rb, Sr, Ba, Cr, Mn, Ni, and Zn). These results were obtained thanks to comprehensive ground reference datasets, which are set to mimic the expected mineralogy and chemistry on Mars. With regard to the first use of LIBS in space, we analyze and quantify, often for the first time, each of the advantages of using stand-off LIBS in space: no sample preparation, analysis within its petrological context, dust removal, sub-millimeter scale investigation, multi-point analysis, the ability to carry out statistical surveys and whole-rock analyses, and rapid data acquisition. We conclude with a discussion of ChemCam performance to survey the geochemistry of Mars, and its valuable support of decisions about selecting where and whether to make observations with more time and resource-intensive tools in the rovers instrument suite. In the end, we present a birds-eye view of the many scientific results: discovery of felsic Noachian crust, first observation of hydrated soil, discovery of manganese-rich coatings and fracture fills indicating strong oxidation potential in Mars early atmosphere, characterization of soils by grain size, and wide scale mapping of sedimentary strata, conglomerates, and diagenetic materials.

The potassic sedimentary rocks in Gale Crater, Mars, as seen by ChemCam on board Curiosity

Key Points: n• Mean K2O abundance in sedimentary rocks >5 times higher than that of the average Martian crust n• Presence of alkali feldspars and K-phyllosilicates in basaltic sedimentary rocks along the traverse n• The K-bearing minerals likely have a detrital origin

Interior layered deposits within a perched basin, southern Coprates Chasma, Mars: Evidence for their formation, alteration, and erosion

A basinA¢Â�Â�like area containing three interior layer deposits (ILDs) on the southern nmargin of Coprates Chasma was studied. We interpret the area as an ancestral basin and ndemonstrate that ILD deposition postdates the formation of the current wall rock slopes. nThe geometry of the ILD and the wall rock spurs form a catchment area between each nILD and the plateau to the south. Erosional remnants of extensive ash or dust layers ndeposited on the plateau south of Valles Marineris also crop out on the southern plateau of nCoprates Chasma. A mass balance calculation suggests that the volume of each ILD is ncompatible with the volume of the ash or dust that would have been deposited within each ncatchment area. We propose that the ILDs likely formed by episodically washing such naerially deposited material down from chasma walls. Rifting of the IusA¢Â�Â�MelasA¢Â�Â�Coprates ngraben opened the enclosed basin and removed any standing water. Faults within the nILDs are compatible with this chasm opening. Sulfates are associated with the ILDs and nlightA¢Â�Â�toned material on the basin floor. We suggest that they result from water alteration of npreexisting deposits, though the timing of that alteration may predate or postdate the nbreaching of the basin. Scours within one ILD are similar to terrestrial glacial scours. nDuring a period of high obliquity ice would accumulate in this region; hence we argue the nscours are Martian glacial scours. A late deposited layer marks the end of the active nlocal geological history between 100 My and 1 Gy.

Composition of conglomerates analyzed by the Curiosity rover: Implications for Gale Crater crust and sediment sources

The Curiosity rover has analyzed various detrital sedimentary rocks at Gale Crater, among which fluvial and lacustrine rocks are predominant. Conglomerates correspond both to the coarsest sediments analyzed and the least modified by chemical alteration, enabling us to link their chemistry to that of source rocks on the Gale Crater rims. In this study, we report the results of six conglomerate targets analyzed by Alpha-Particle X-ray Spectrometer and 40 analyzed by ChemCam. The bulk chemistry derived by both instruments suggests two distinct end-members for the conglomerate compositions. The first group (Darwin type) is typical of conglomerates analyzed before sol 540; it has a felsic alkali-rich composition, with a Na2O/K2Ou2009>u20095. The second group (Kimberley type) is typical of conglomerates analyzed between sols 540 and 670 in the vicinity of the Kimberley waypoint; it has an alkali-rich potassic composition with Na2O/K2Ou2009<u20092. The variety of chemistry and igneous textures (when identifiable) of individual clasts suggest that each conglomerate type is a mixture of multiple source rocks. Conglomerate compositions are in agreement with most of the felsic alkali-rich float rock compositions analyzed in the hummocky plains. The average composition of conglomerates can be taken as a proxy of the average igneous crust composition at Gale Crater. Differences between the composition of conglomerates and that of finer-grained detrital sediments analyzed by the rover suggest modifications by diagenetic processes (especially for Mg enrichments in fine-grained rocks), physical sorting, and mixing with finer-grained material of different composition.

Observation of > 5 wt % zinc at the Kimberley outcrop, Gale crater, Mars: ZN DETECTION AT KIMBERLEY WITH CHEMCAM

Zinc-enriched targets have been detected at the Kimberley formation, Gale crater, Mars, using the Chemistry Camera (ChemCam) instrument. The Zn content is analyzed with a univariate calibration based on the 481.2u2009nm emission line. The limit of quantification for ZnO is 3u2009wt % (at 95% confidence level) and 1u2009wt % (at 68% confidence level). The limit of detection is shown to be around 0.5u2009wt %. As of sol 950, 12 targets on Mars present high ZnO content ranging from 1.0u2009wt % to 8.4u2009wt % (Yarrada, sol 628). Those Zn-enriched targets are almost entirely located at the Dillinger member of the Kimberley formation, where high Mn and alkali contents were also detected, probably in different phases. Zn enrichment does not depend on the textures of the rocks (coarse-grained sandstones, pebbly conglomerates, and resistant fins). The lack of sulfur enhancement suggests that Zn is not present in the sphalerite phase. Zn appears somewhat correlated with Na2O and the ChemCam hydration index, suggesting that it could be in an amorphous clay phase (such as sauconite). On Earth, such an enrichment would be consistent with a supergene alteration of a sphalerite gossan cap in a primary siliciclastic bedrock or a possible hypogene nonsulfide zinc deposition where Zn, Fe, Mn would have been transported in a reduced sulfur-poor fluid and precipitated rapidly in the form of oxides.