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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 (SiO2, TiO2, Al2O3, FeOT, MgO, CaO, Na2O, K2O) 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 rover's instrument suite. In the end, we present a bird's-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. [ABSTRACT FROM AUTHOR]

• Using the novel "long distance" capability of the Remote Micro Imager of ChemCam. • Amalgamated eolian deposits compose the Light-Toned Yardang Unit of Mt Sharp. • Exposed yardang forms of Mt Sharp are paleo geomorphic features. • A likely global climate change at the Early-Late Hesperian Transition on Mars. • Exhumed yardangs are relevant supports to study paleo-climatic conditions on Mars. Gale crater is a large impact crater with a ca 5 km thick sequence of stratified rocks in it, expressed today as a central eroded mound (i.e., Aeolis Mons informally named Mt. Sharp). A goal of the current Mars Science Laboratory mission in Gale crater is to investigate the processes that deposited, lithified, and eroded this fill. The Light-Toned Yardang Unit (LTYu) unit, the subject of the present research, is one of the geological units of Mt Sharp. Our specific purpose here is to refine and interpret the imaging documentation of the morphologic and stratal components of the LTYu, at large outcrop scale. In combination with established orbital images, we use the Remote Micro-Imager (RMI) of ChemCam , a remote sensing instrument currently operated onboard Curiosity rover, which provides several types of context imaging. RMI capabilities now include "Long Distance" acquisitions of targets several kilometers away. In these new acquisitions, substantial differences are visible in LTYu yardang attitudes from lowest to uppermost elevations allowing tentative subdivision of the LTYu into subunits. Bedding geometries in the lower LTYu are consistent with eolian dune foresets which collectively prograde towards an average N134° direction. Based on stratal architectures, the LTYu is viewed as an amalgamated stack of at least two, and possibly four, ancient erg systems bounded by large deflationary "supersurfaces". Observations point to a multistory generation of yardangs interpreted to have been successively buried during the stratigraphic building of Mt Sharp. We conclude that the successive sequences of eolian deposition-erosion recorded by the LTYu have been generated by cyclic changes from semi-arid to arid conditions, coupled in climatic cycles, including wind regime change. The regional unconformity that tops the Lower mound formation of Mt Sharp, and the subsequent emplacement of the LTYu, collectively express a clear tendency toward sustained arid environments for this region of Mars around the Early – Late Hesperian transition. Given the large time scale involved (i.e., a few tens of million years as a minimum), we consider it likely that the local evidence for increased aridity in the Mt Sharp stratigraphy is a manifestation of climate change affecting the whole planet. [ABSTRACT FROM AUTHOR]

The Curiosity rover is exploring Hesperian-aged stratigraphy in Gale crater, Mars, where a transition from clay-bearing units to a layered sulfate-bearing unit has been interpreted to represent a major environmental transition of unknown character. We present the first description of key facies in the sulfate-bearing unit, recently observed in the distance by the rover, and propose a model for changes in depositional environments. Our results indicate a transition from lacustrine mudstones into thick aeolian deposits, topped by a major deflation surface, above which strata show architectures likely diagnostic of a subaqueous environment. This model offers a reference example of a depositional sequence for layered sulfate-bearing strata, which have been identified from orbit in other locations globally. It differs from the idea of a monotonic Hesperian climate change into long-term aridity on Mars and instead implies a period characterized by multiple transitions between sustained drier and wetter climates. [ABSTRACT FROM AUTHOR]

This study provides the first systematic comparison of orbital facies maps with detailed ground-based geology observations from the Mars Science Laboratory (MSL) Curiosity rover to examine the validity of geologic interpretations derived from orbital image data. Orbital facies maps were constructed for the Darwin, Cooperstown, and Kimberley waypoints visited by the Curiosity rover using High Resolution Imaging Science Experiment (HiRISE) images. These maps, which represent the most detailed orbital analysis of these areas to date, were compared with rover image-based geologic maps and stratigraphic columns derived from Curiosity's Mast Camera (Mastcam) and Mars Hand Lens Imager (MAHLI). Results show that bedrock outcrops can generally be distinguished from unconsolidated surficial deposits in high-resolution orbital images and that orbital facies mapping can be used to recognize geologic contacts between well-exposed bedrock units. However, process-based interpretations derived from orbital image mapping are difficult to infer without known regional context or observable paleogeomorphic indicators, and layer-cake models of stratigraphy derived from orbital maps oversimplify depositional relationships as revealed from a rover perspective. This study also shows that fine-scale orbital image-based mapping of current and future Mars landing sites is essential for optimizing the efficiency and science return of rover surface operations. [ABSTRACT FROM AUTHOR]