Characteristics of impact craters in the Northern Hemisphere of Mars

This study compares results of crater diameter, shape, ejecta morphologies, and interior morphologies in the northern hemisphere of Mars between older results reported from a Viking-derived crater catalog and a new crater catalog obtained from analysis of higher-resolution data sets, primarily Mars Odyssey Thermal Emission Imaging System (THEMIS) daytime infrared and visible data. This report focuses on results from the northern hemisphere, where the new analysis has increased the number of known craters > or =5 km in diameter by 1300 to a total of 14,224. The improved resolution and overall clarity of the new image data sets have improved the classification of ejecta and interior morphologies. This study finds that approximately 3% of the craters display evidence of oblique impact (through elliptical crater shape and/or asymmetric ejecta blankets), and many show an east-west orientation for the major diameter, indicating impact trajectories approximately parallel to the present-day Martian equator. No strong indication of extended periods of high obliquity or polar wander is recorded within the elliptical crater population. Ejecta morphologies are divided into seven classes, with most fresh craters (92%) displaying one of the three layered ejecta morphologies. Diameter and geographic distributions of the different ejecta morphologies are similar to those reported from Viking analysis. A study of the two types of double layer ejecta craters finds that crater diameter and terrain characteristics are the primary factors leading to the different morphologies. Interior morphologies are divided into nine classes, with most Martian impact craters containing some type of floor deposit from eolian, fluvial, glacial, volcanic, or impact processes. Wall terraces, flat floors, and fractured (chaotic) floors are strongly concentrated in highlands regions. Central peak and summit pit craters are found in similar regions, particularly in the highlands, but floor pit craters are distributed across both highlands and plains units. Target strength appears to be an important factor in the formation of many of these interior morphologies.