ANCIENT LOWLAND BASEMENT ON MARS: EVEN OLDER THAN WE THOUGHT

At the surface the lowlands of Mars appear very different from the highlands. The obvious physiographic and especially crater retention age differences were major drivers in promoting endogenic theories for the origin of the lowlands, even though suggestions of ancient impact basins were also reported. Below the surface, the lowland basement may in fact be very similar to the highlands, at least in age. Quasi-Circular Depressions (QCDs) revealed by MOLA data showed hundreds of features that appear to be buried impact craters and which implied a crater retention age for the lowland basement that was substantially older than previously thought, perhaps as old as the exposed and visible highlands. The highlands themselves also have a population of buried impact basins, so based only on topographic expressions of impact basins, the lowlands appeared to be still somewhat younger than the highlands. But crater retention ages based on QCDs alone are likely minimum ages, as there could well be impact basins buried so deeply that no topographic expression remains. Crustal thickness data derived from topography and gravity data reveal Circular Thin Areas (CTAs) which may be deeply buried impact basins. Support for this comes from (a) many of the CTAs are in fact coincident with previously observed QCDs, (b) the ratio of non-QCD CTAs to QCDs is greatest in areas of thickest burial (the lowlands, Tharsis), and (c) cumulative frequency curves including non-QCD CTAs along with QCDs have a similar character to those for QCDs alone. These same curves indicate both the highlands and lowlands are even older than previously thought (based on QCDs alone) and that the lowland basement may be as old as the highland basement. While this has important implications for possible endogenic origins for the lowlands, the same crustal thickness data reveal the presence of more large basins in the lowlands, indicating that impact played a major role in creating the topographic character of the northern third of Mars. Dating of these basins indicates all those in the lowlands, and most of those in the highlands, formed in a brief interval of time, perhaps analogous to the Late Heavy Bombardment on the Moon.