CRATER DENSITY AS AN AID TO MAPPING THE CRATERED SURFACE AT GALE CRATER, MARS

Using orbiter images, the Curiosity landing ellipse was mapped as six distinct units based mainly on geomorphic characteristics. These units are the alluvial fan material (ALF), fractured light-toned surface (FLT), cratered plains/surface (CS), smooth hummocky plains (SH), rugged unit (RU), and striated light-toned outcrops (SLT) (Grotzinger et al., 2014; DOI: 10.1126/science.1242777). Stratigraphically, the CS is the youngest unit, despite the fact that it has the greatest crater density (# craters per km²). Clearly in this case, crater density is not a proxy for relative age, but indicates relative crater retention, which is, in part, a product of rock resistance to erosion and burial/exhumation history. The relationships between these processes are particularly relevant for sub-km diameter craters (Edgett, 2009; Bull. Am. Astron. Soc. 41(3), 1116). Our study focuses on the CS unit, which we subdivided into four geomorphic sub-units. Crater density provides a quantifiable parameter that supports this subdivision and may aid in understanding how the sub-units differ lithologically. The results provide a relative scale of variations in erosional resistance among the four CS sub-units.

In Gale, some of these mapping results can be examined using data from the Mastcam and ChemCam instruments. For instance, ChemCam data show that rocks interpreted to be part of the CS have higher concentrations of Al, Na, and K than the SH and RU units, although how these elemental differences translate to specific mineralogies and lithologies is not yet understood. Potential CS targets analyzed by ChemCam near the Shaler outcrop showed no signs of chemical alteration. This is consistent with CS material being well-cemented and impermeable to water that was present post-deposition. Mastcam images show that the CS is comprised of material that is much darker in color than any other unit in the landing ellipse. The images also show that the CS weathers into boulders of various sizes, which is a much different weathering pattern than other units. All of these characteristics suggest the CS is harder and more resistant to erosion than the SH and RU units. Crater densities enable us to more confidently link the physical characteristics seen in satellite images to the relative pattern of erosional resistance and rock types present on Mars.