ANCIENT LOWLANDS ON MARS: BURIED IMPACT BASINS AND AGE OF THE CRUSTAL DICHOTOMY

A very large population of Quasi-Circular Depressions (QCDs) larger than 50 km in diameter has been revealed in the northern lowlands of Mars by high resolution Mars Orbiter Laser Altimeter (MOLA) data. These include about 90 visible impact craters but more than 550 features that are not visible in Viking imagery. Similar QCDs occur in the ancient cratered highlands of Mars. Their wide distribution, lack of association with known volcanic or tectonic regions, subdued and shallow profiles like those expected for buried impacts, and the shape of their cumulative frequency curves all indicate the "MOLA-found" QCDs are most likely buried impact basins. The very large number in northern terrains suggests the lowlands of Mars, below a veneer of young plains-forming units, are very old, and that the lowlands formed very early in martian history. Cumulative frequency curves for the visible and buried QCDs in the lowlands were compared over the same diameter range with similar curves for highland QCDs in the Noachis-Arabia area. The cumulative number of lowland "MOLA-found" basins per unit area is greater than that for visible impact basins in the highlands, but less than that for buried highland basins. The visible highlands in Noachis-Arabia are on average Middle Noachian in age, and the cumulative crater density is consistent with this. The buried lowland surface is therefore older than Middle Noachian and may be as old as the Early Noachian. This implies that the crustal dichotomy on Mars formed very early, and provides a temporal constraint on the processes that could produce the lowlands, favoring those that occur early and quickly.