MORPHOLOGIC CHARACTERISTICS OF PEBBLE AND COBBLE-SIZED CLASTS ALONG THE CURIOSITY ROVER TRAVERSE

Clast characteristics record primary and secondary processes as a function of time, distance, or both. Documenting variation in clast characteristics from location to location is fundamental to defining the effects of clast transport, and to deconvolving the effects of clast emplacement from those of secondary modification. Regular measurements with consistent, well-modeled parameters are required to document the changes along the rover traverse. Utilizing an imaging campaign designed to systematically examine clast characteristics as they varied along a traverse, we have assessed the characteristics of clasts along Curiosity’s traverse to shed light on the processes important in the genesis, modification and transportation of surface materials.

Each Mastcam clast survey image pair was used to determine the range and distribution of clast sizes. These data were then used to calculate the degree of sorting. We also estimated clast size, shape (sphericity, or how closely a clast profile resembles a sphere), roundness (a measure of corner sharpness), and texture (qualitative variation from a flat surface at scales smaller than the corners). Elements of texture also include the size, morphology and dispersion of grains within an individual clast, where the term “grain” is limited to features observed within clasts.

Pebble- to cobble-sized clasts at Bradbury Landing, and subsequently along Curiosity’s traverse to Yellowknife Bay, reflect a mixing of two endmember transport mechanisms. The general clast population likely represents material deposited via impact processes, including meteorite fragments, ejecta from distant craters, and impactites consisting of shocked and shock melted materials from within Gale Crater, which resulted predominantly in larger, angular clasts. A subset of rounded pebble-sized clasts has likely been modified by intermittent alluvial or fluvial processes. The morphology of this rounded clast population indicates that water was a more important transporting agent here than at other Mars sites that have been studied in situ. Finally, we identified populations of basalt clasts and porphyritic clasts of undetermined composition by their morphologic and textural characteristics; basalts are confirmed by geochemical data provided by ChemCam.