Ratios of Impact Crater Depth to Diameter in Mercury's North Polar Region from MESSENGER Laser Altimeter Observations

Earth-based radar images dating back two decades show that the floors of polar craters on Mercury host radar-bright deposits having radar characteristics best matched by water ice. A 2005 study of the depths of north polar craters determined with photoclinometric techniques applied to Mariner 10 images by Vilas and others yielded the conclusion that the mean ratio of crater depth d to rim-crest diameter D for craters hosting polar deposits is two-thirds that of the mean d/D of a comparable population of neighboring craters lacking such deposits. Here we use altimetric profiles and topographic maps obtained by the Mercury Laser Altimeter (MLA) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, now in orbit about Mercury, to repeat this experiment. From MLA profiles we have created a regional digital elevation map (DEM) of high northern latitudes southward of 85° N. The depth and diameter of 14 craters hosting polar deposits and ranging in diameter from 3.0 to 50.9 km, and 24 neighboring craters that lack such deposits between 75.0° N and 83.1° N, have been determined from this data set. For each crater, 50 radial profiles that extend from the crater center past its rim have been extracted from the DEM to allow for a robust assessment of depth and diameter. The original altimetry tracks that pass near the center of each crater have also been examined to validate the measurements from the elevation maps. Overall, both methods yield similar values for depth and diameter: d/D values range from 0.052 to 0.116 with a mean of 0.080 and a standard deviation of 0.018 for the population of craters hosting polar deposits, and 0.042 to 0.102 with a mean of 0.066 and a standard deviation of 0.014 for the neighboring 24 craters that lack such deposits. As the MLA continues to collect data, measurements of additional craters are underway, as well as an error analysis of the measurements and statistical tests to evaluate the similarities and differences of the two crater populations.