LARGE BURIED AND VISIBLE BASINS ON MARS: TOTAL POPULATION AGES OF THE HIGHLANDS AND LOWLANDS

The large population of buried impact basins found in MOLA data in the northern lowlands of Mars suggests that the crust beneath a relatively thin veneer of lowland plains is extremely old. Cumulative frequency comparison with a large area of Middle Noachian terrain in Arabia and a much smaller area of Early Noachian terrain shows that the buried lowlands have about the same crater retention age as that implied by Early Noachian visible basins. But buried basins in the Early Noachian terrain suggest there is still older crust underlying what is generally considered the oldest surface units on Mars, and the recoverable crater density of these highlands is significantly greater than that of the buried lowland crust. This may indicate a true age for the buried lowlands somewhat younger than for the buried highlands. Alternatively, it could be that burial of the lowlands removed enough of the total population to depress the lowland curve at all diameters. We searched higher resolution global MOLA data for large impact basins (D>200 km diameter) to compare highland and lowland total populations (visible and buried) for those features most difficult to bury. The density of buried highland craters is greater than that in the lowlands for the smaller diameter craters in this sample. We find evidence for several probable buried basins in the lowlands and at least one in the highlands that are similar to Hellas, Argyre, Isidis, Chryse and Utopia in size. All of these buried features must be extremely old because smaller likely buried basins are superimposed on them. We previously used this relationship to suggest Utopia was "Earliest" Noachian in age. The large highland example centered near Aram Chaos appears to have significantly influenced the development of fluvial channels both into (Uzboi-Ladon-Arda Valles and through Margaritifer Chaos) and out of (Ares Vallis) the basin. It is also one of only two very large highland or lowland basins (visible or buried) with significant magnetic anomalies near its center. If the absence of such anomalies within Hellas, Argyre, Isidis, Chryse and Utopia indicates these basins formed after the dynamo died, then the buried highland basin near Aram Chaos may be older than these more obvious basins and may date from before the martian dynamo died.