CRATER DEGRADATION OF KILOMETER-SIZED CRATERS ON THE LUNAR MARIA : INITIAL OBSERVATIONS AND MODELING

Introduction: Impact cratering at all scales has dominated the landscape evolution of the Moon in the period following the emplacement of the lunar maria. As a result, the degradation of craters on the maria has long been thought to be diffusional and controlled primarily by the accumulation of smaller later impacts [1-4]. Recently-acquired topographic data from the Lunar Orbiter Laser Altimeter (LOLA) [5], as well as stereo imaging from the Kaguya Terrain Camera [6] and the Lunar Reconnaissance Orbiter Camera (LROC) [7, 8] provide an excellent basis for improved characterization of the degradation states of craters in a quantitative manner. In this preliminary study, craters in the D=1–3 km size range are explored. These data have a number of applications. If the most degraded craters of a given size in a locale are the most ancient, a local sequence of craters in a given location can be established. In addition, examining crater degradation states in the context of models for the impact flux allow inferences of absolute degradation rates. Methods: Lunar Observations: A portion of the maria totaling ~2.5x10 km was examined (approximately 5% of the maria). Craters were mapped with ArcMap down to ~700 m to assure completeness of the crater datasets at 1 km. Following this initial mapping, a database of LOLA shot data was queried to determine which craters were crossed by profiles; a total of 515 D=1-3 km craters had sufficient data to be assessed. Each crater’s mapped position was then redetermined based on the LOLA data by finding the location that maximizes the radial symmetry of the crater profile inside the crater’s rim. This step is required because the co-registration of LROC WAC and LOLA shots is imperfect: these data suggest an average offset of ~180 m (~1.8 pixels). Model Fitting: To analyze these data, a forward model of topographic diffusion was created (explicit FTCS), with the initial conditions representing the fresh crater form based on direct observations of rayed craters. Results of the diffusion model were then used to establish a lookup database of crater profiles at a range of initial crater sizes (D=0.8–5 km) and diffusion times (κT=0–60000). The resulting crater profiles change most significantly in the curvature and position of the rim and overall crater depth as κT changes. Using this database, best-fit degradation states were determined by minimizing the difference between the lookup profiles and LOLA observations of every crater with sufficient data. Initial Observations and Results. Example of a sequence of 1.2 km craters with different derived degradation states is shown in Fig. 1. For both the craters shown here and the population as a whole, the fits to data appear to be qualitatively reasonable, which bolsters the idea that degradation is primarily diffusional. The 1σuncertainty in fitting κT is estimated to be 2.5% based on the bootstrap method with replacement [9]. Primary sources of misfits are: nearby large craters, deviations from the the initial crater shape model (i.e., if some craters are secondaries), inaccuracy in the spatial location of the crater, and inaccuracy in the determination of the initial elevation of the crater relative to its surrounding surface.