EVIDENCE FOR BURIED CRUST OLDER THAN THE OLDEST OBSERVED SURFACE UNITS ON MARS

Buried impact basins revealed by MOLA elevation data in both the lowlands and highlands of Mars provide evidence for older subsurface rocks having a significant and recoverable history of cratering. We find similar evidence in the very oldest geologic units on Mars, which suggests that the surface materials in even the most densely cratered areas must be younger than 4.6Gya (or whenever the original crust of Mars formed). The base of the Early Noachian was not definable until now, because it was not known whether a recoverable earlier history did exist on Mars. To address this problem, we searched high-resolution MOLA data for buried craters in Hellas rim material (mapped as unit Nh1), which on the east side of Hellas comprises over 1.2 million square kilometers and is the largest occurrence of Early Noachian material on Mars. We found clear evidence for Quasi-Circular Depressions (QCDs) larger than 25 km diameter that are not visible on Viking imagery and which we believe may be buried impact basins. This is supported not only by their topographic characteristics but also by their cumulative frequency distributions: both visible and hidden QCDs have similar curves that follow -2 power-law slopes on a log-log plot. Contrary to our experience for larger QCDs in Middle Noachian and younger terrains elsewhere on Mars, the cumulative number of hidden QCDs at smaller diameters is less than that for the visible basins. But the contribution of hidden QCDs is significant-the total cumulative population (hidden plus visible) in unit Nh1 is roughly 1.6 times the visible population. This is clear evidence for buried rocks somewhat older than those at the surface. Comparison with studies done in several Middle Noachian terrains suggests that the total populations of QCDs (visible and hidden) are nearly the same in both Early and Middle Noachian units, perhaps suggesting a common-age, primordial basement. The total populations may represent either a true crater retention age for basement rocks or a saturation of craters which limits how far back into martian history we can see. We propose to more strictly define the Early Noachian in terms of the ages of the exposed surface rocks, and adopt the informal term "pre-Noachian" for the oldest cratered basement unit which underlies and predates unit Nh1 and other Noachian surface units.