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 ≥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 ~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.

Central pit craters are common on ice-rich bodies, such as Mars, Ganymede, and Callisto. Mars and Ganymede represent the two end members regarding target characteristics (mixed ice and soil for Mars vs. almost pure ice for Ganymede). Comparisons of central pit craters on these two bodies can provide insights into the environmental conditions under which these craters form and provide constraints on the proposed formation models. This analysis includes 1604 central pit craters on Mars and 471 central pit craters on Ganymede. Martian central pit craters are divided into floor pits and summit pits, whereas all central pit craters on Ganymede are floor pits. Central pit craters form in similar-diameter ranges on both Mars and Ganymede when gravity differences are considered, and both bodies show no regional variations in pit crater distribution within the ±60° latitude zone. Martian floor pits are larger relative to their parent crater than summit pits, but the Ganymede pit/crater diameter ratio is larger than for either central pit type on Mars. Central pits have formed over the entire history of both bodies, and there is no indication that excavation depths have varied over time. Lack of crater floor updoming in Martian floor pit craters indicates that low concentrations of ice (estimated at ~20%) still allow production of central pits. The results of this study argue against central peak collapse as the formation mechanism for central pit craters. Excavation into a subsurface liquid layer cannot be ruled out but is difficult to support based on the distributions and consistencies in excavation depth on both bodies. These results support the model of vaporization and gas escape for central pit formation on both Mars and Ganymede.

Impact cratering has affected the surfaces of all bodies in our Solar System. These short-duration but energetic events can drastically affect the regional and occasionally the global environment of a planet. The cratering record is better preserved on Mars than on Earth due to longer-term stability of the Martian crust and lower degradation rates. Impact cratering had its greatest effect early in Solar System history when bombardment rates were higher than today and the sizes of the impacting objects were larger. The record from this period of time is largely lost on Earth. High bombardment rates early in Solar System history may have eroded the Martian atmosphere to its present thin state, causing dramatic climate change. The regolith covering much of the Martian surface and the large quantities of dust seen in the atmosphere and covering much of the ground have been attributed to fragmentation of target material by impacts. Heating associated with crater formation may have contributed volatiles to the Martian atmosphere and initiated some of the outflow channels. The effects of an impact event extend far beyond the crater rim, and the planet’s volatile-rich environment likely contributes to the greater ejecta extents seen on Mars than on the Moon. The cratering record of Mars thus holds important implications for how impacts may have affected the geologic evolution of Earth.

Most fresh Martian impact craters are surrounded by layered (“fluidized”) ejecta which were emplaced as flow deposits. Observational data, laboratory experiments, and numerical modeling strongly suggest that particle size, particle density, atmospheric density and pressure, and the presence of subsurface volatiles all contribute to the features observed in these craters and their ejecta blankets. In this contribution, we review the evidence that both subsurface volatiles and the thin Martian atmosphere contribute to the morphologic, morphometric, and thermophysical characteristics of Martian impact craters and their ejecta deposits.

The cratering records on the Moon, Mercury, and Mars are studied to provide constraints on (1) terrestrial conditions prior to about 3.8 Ga, (2) why biology was not extensively established prior to 3.5 Ga, (3) whether impact-induced volcanism can explain some feature of the Cretaceous/Tertiary (K/T) boundary event, and (4) how common large single-impact events are in the inner Solar System. Earth underwent a period of high impact rates and large basin-forming events early in its history, based on the cratering record retained in the lunar, mercurian, and martian highlands. The widespread occurrence of life around 3.5 Ga is linked to the cessation of high impact rates. Impact of a 10-km-diameter object into terrestrial oceans could excavate through crustal material and into mantle reservoirs, creating extended basaltic volcanic activity. Scaling laws, coupled with the record retained on lunar and martian plains, indicate that between one and seven craters of ≥90 km diameter could have formed on Earth in the past 65 m.y.