New Large Late Heavy Bombardment Impact Basins on Mars Revealed in Crustal Thickness Data: Crater Retention Ages and Implications

Crater retention ages of the 20 largest impact basins previously recognized on Mars suggest most formed in a relatively short time, perhaps in less than 200 million years. New crustal thickness data suggests there are several additional very large basins, some lying outside the areas affected by those already known. Crater retention ages for these newly identified basins are not significantly older than those already known, suggesting either the large diameter Late Heavy Bombardment on Mars was a relatively brief event or the record of previous large impacts was somehow erased, perhaps by the formation of a Borealis Basin. Introduction: Crater retention ages (CRAs) for the 20 largest impact basins on Mars (D> 1000 km) based on superimposed large visible or buried Quasi-Circular Depressions and even more deeply buried impacts revealed as Crustal Thin Areas [1] suggest that most of the basins formed in a relatively short period of time [2,3]. N(300) CRAs for 65% of the large basins lie between 2.5 and 5.0 [3], and 50% of the population have CRAs between 2.5 and 4.0. The oldest known basin has N(300) = 6.67 and to date no large areas of Mars have been shown to be significantly older. Conversion to the Hartmann-Neukum model chronology [4] suggests an absolute age of 4.10 to 4.25 BYA for all but the three youngest basins (Hellas, Argyre and Isidis), with most falling within an even narrower interval of 4.12-4.14 BYA [3]. The sharp peak in likely formation ages for the largest impact basins on Mars and the apparent lack of basins earlier than those known have several important implications. It suggests the possibility of a cataclysmic Late Heavy Bombardment (LHB) on Mars. The short time is consistent the Nice model [5,6] and a “Terminal Lunar Cataclysm” [7,8]. The absolute ages, however, seem wrong: the lunar cataclysm occurred between 4.0 and 3.8 BYA. Martian ages are model ages based a number of assumptions [4], and this difference may well be within the uncertainty of the current Hartmann-Neukum chronology. In fact, if the peak shown in Figure 2 is part of an inner solar system event, it may be that the martian chronology can be corrected by pinning this peak to the ~3.9 BYA cataclysm on the Moon [3,9]. Are there large basins even older than those already known? Although it may be impossible to know if the absolute ages of the large martian basins correspond in time with the Terminal Lunar Cataclysm, it may be possible to determine if there are previously unrecognized large impacts significantly older than the ages already determined. The current inventory of large impacts actually occupies only about 35% of the surface area of Mars, leaving, in principle, a large area in which older basins might exist [9]. Older basins would likely be more subtle, less obvious than those already identified, obscured by both ejecta from the known basins and by other subsequent geologic processes. Perhaps the best hope of finding such ancient large structures is through use of improved crustal thickness data, which is now available in the form of the MarsCrust3 model [10]. Based on new gravity models from MRO, MarsCrust3 has improved signal-tonoise and horizontal resolution and has confirmed the existence of many of the Crustal Thin Areas (CTAs) larger than 300 km diameter which we previously recognized in an earlier crustal thickness model [11] and which were used to help determine the N(300) CRAs for the highlands and lowlands of Mars [1] and the 20 known very large basins [3].