FREY, Herbert V., Planetary Geodynamics Lab, NASA/Goddard Space Flight Center, Code 698, Goddard Space Flight Center, Greenbelt, MD 20771, Herbert.V.Frey@nasa.gov|
The discovery of a large population of previously unrecognized large impact basins on the Moon and Mars, revealed as both Quasi-Circular Depressions (QCDs) in gridded topography and as Circular Thin Areas (CTAs) in crustal thickness model data, makes it clear the early impact rate (for at least large objects) and likely its effects were underestimated. Current estimates are that the minimum cumulative population of impact basins > 300 km diameter on the Moon is at least a factor 2 greater than thought, and in the highlands of Mars it seems there may be a cumulative factor 12 increase over what was originally believed. The existence of this previously unknown population of large impact basins suggest several things: the early cratering rate on both bodies was substantially higher than we thought, it is almost certain that we will never know the true total impact population (there were likely more large impacts than we can possibly detect), and, because it is likely the previously unknown population extends to even smaller diameters, all current crater retention ages based on visible craters alone are probably too low. The Late Heavy Bombardment (LHB) in the inner solar system was probably much more severe than previously thought. The largest impact basins on Mars, all > 1000 km diameter, appear to have formed in a relatively short period of time, and may have played a role in the demise of the martian global magnetic field, which disappeared near the end of this Late Heavy Bombardment. Recent data for eight new candidate basins found in the most recent crustal thickness models supports a relatively brief but intense LHB on Mars. The duration and especially the concentration of the very largest basins (D>2500 km) in the middle of this bombardment are at least consistent with a Nice-type model. This may allow for a period of several hundred million years prior to a Terminal Lunar (and Planetary) Cataclysm at ~ 4 BYA during which the Earth, Moon, Mars and the rest of the inner solar system experienced little in the way of large diameter impact bombardment.
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