The First Automatic Survey of Impact Craters on Mars: Global Maps of Depth/diameter Ratio

The new, global catalog of 75,919 impact craters on Mars has been compiled entirely by a computer algorithm. Using crater depths listed by the new catalog global maps of depth/diameter ratio are created. The global pattern of crater depths is consistent with the existence of the cryosphere at depths that decrease toward the poles. Introduction: If left to traditional manual surveys, the fraction of cataloged craters to the craters actually present in the available and forthcoming data will continue to drop precipitously. The only practical solution to a comprehensive surveying of craters is to automate the process of their detection. Using the topographybased crater detection algorithm (CDA) [1,2] we have generated the catalog of Martian craters having diameters D ≥ 3 km. This is the first ever catalog of craters due entirely to a computer algorithm. The limit on the size of detected craters is set by the resolution of topographic data used for crater detection. Presently, the highest resolution global topographic coverage is provided by the 128 pixels/degree MOLA Mission Experiment Gridded Data Record (MEGDR) [3]. Methods. Our CDA is a two stage system that first identifies round depression on Mars and then uses machine learning technique to separate true craters from false positives. The application of our CDA to the entire surface of Mars requires subdividing the surface into 356 overlapping tiles. The craters are identified and measured at each tile separately and the results from individual tiles are concatenated into a single catalog from which duplicate detections are eliminated. In its present version the catalog lists coordinates of the center of each crater, its diameter, depth, and an underlying geological unit. Results. The catalog lists 75,919 craters ranging in size from 1.36 km to 347 km. Fig.1 shows the exceedance probability of crater diameter, D, for all craters in the catalog. Exceedance probability, P(D>X), is a probability that a randomly chosen crater has diameter larger than X. It represent a convenient way of displaying distribution of crater sizes. For comparison, Fig. 1 also shows exceedance probability of crater diameter in manually collected Barlow catalog [4] that lists 42,283 craters. The exceedance probability curve levels off for craters with D < 3 km setting a limit on statistical completeness of the catalog; the completeness limit of the Barlow catalog is about 5 km. Fig. 2 shows the density of craters in the new catalog. The density is calculated using a moving circular window of radius 250 km. It reflects the combined effect of true distribution of craters and the existence of some biases in the methodology due to different rate of automatic detection between different surfaces. Figure 1: Exceedance probability of crater diameter D in the new catalog (blue) and the Barlow catalog (red). The curves level off at small values of D due to declining fraction of detected craters. Figure 2: Crater density derived from our catalog. Craters of all sizes are included. The numbers given in the legend are the number of craters in a circular window having radius of 250 km. The new catalog makes possible mapping the relative depths of craters over the entire Martian surface. For 1117.pdf 40th Lunar and Planetary Science Conference (2009)