Sands on Meridiani Planum, Mars

The analyses of 184 sediment targets and more than 70,000 individual grains revealed that along the Opportunity rover traverse there are four distinct fractions of deposits related to different geomorphological settings: (a) dust mixed with very fine sand is common behind topographical obstacles, (b) fine sands are deposited in depressions, (c) very coarse sands occur in coarse-grained ripple fields, and (d) gravel dominates at the rims of craters and on bedrock as lag deposits. Medium size sands were not observed on the plains, but they can be trapped in relatively large craters, where they form dunes and are within coarse-grained ripples and transverse aeolian ridges (TARs). The fine sands show no regional variations in chemical composition and granulometry, as these sands are easily transported by wind. The very coarse sands vary in composition and shape between the plains and the Endeavour crater rim as their sources are local and their transport distances are short. On the plains, the gravel and the coarse sands are enriched in iron and characterized by higher roundness than the grains from the Endeavour crater rim. The source of iron-rich, rounded grains on the plains are hematite spherules that are eroded out of Burns formation rocks. The smallest, the best sorted, and the least rounded coarse sand samples are found on coarse-grained ripple crests. They are mainly composed of spherule fragments and their low roundness indicates shorter transport path lengths than those of grains transported in the past when coarse-grained ripples migrated. The analyses of more than 70,000 individual grains revealed that there are four distinct types of sediments on Meridiani Planum: (a) dust mixed with very fine sand grains (diameter below 0.10 mm) are common behind topographical obstacles, (b) fine sand grains (diameters 0.10-0.25 mm) are deposited in depressions, (c) very coarse sand grains (diameters 1-2 mm) occur in coarse-grained ripple fields, and (d) gravel (diameters above 2 mm) dominates at the rims of craters and on bedrock. Deposits composed of medium sand grains (diameter 0.25-0.50 mm) were not observed on the surface of the plains, but they can be trapped in relatively large craters, where they form dunes and are within coarse-grained ripples and transverse aeolian ridges formed by wind. The fine sands show no regional variations, indicating that these sands are mixed on a regional scale by sand-driving winds. The very coarse sands vary between the plains and the Endeavour crater rim, as their sources are local, and they can be moved by wind only over short distances. Sediments on the plains are different from those at the Endeavour crater rim in terms of the shape and chemical composition of grainsMedium size sands are not present on the plains but are trapped in craters, where they can form dunesVery coarse sand grains are currently transported by wind but have shorter transport path lengths than grains transported in the past