Evidence for Buried “pre-noachian” Crust Pre-dating the Oldest Observed Surface Units on Mars

MOLA gridded data shows clear evidence for Quasi-Circular Depressions not visible on images in Early Noachian (EN) terrain units on Mars. We suggest these are buried impact basins that pre-date the superimposed craters whose high density makes these EN units the oldest visible at the surface of Mars. There is crust older than the oldest visible terrain units on Mars, and these EN units cannot date from 4.6 BYA. These and other Noa-chian units have similar total (visible + buried) crater retention ages, suggesting a common " pre-Noachian " crustal age OR crater saturation beyond which we cannot see. Introduction: Even though the " Early Noachian " (EN) used in geologic mapping [1,2,3] is undefined at the early end, it is often assumed in absolute chronologies [4,5] to extend back to 4.6 BYA. We explored this assumption by searching for evidence of buried impact basins, [6,7] in the largest occurrences of Early Noachian terrain. The hypothesis is that if such basins exist, they indicate crust which must pre-date the surface units mapped as the oldest on Mars, and those units must then be less than 4.6 BY old. Alternatively, if no such buried features are seen, then the surface units may represent crust of the same age below, which could in principle be as old as Mars. Here we show the former alternative is true. There must be crust older than the oldest mapped surface units. We also show that a number of Noa-chian terrains on Mars appear to have a common total (visible + buried) crater retention age. This might be either the age of the original (planet-wide?) crust of Mars, or may indicate crater saturation. Buried Basins near Hellas and Isidis: The two largest occurrences of EN materials on Mars are the basin rim materials of Hellas (Nh 1 , about 1.2 million sq. km) and Isidis (Npl 1 , about 0.6 million sq km). Figure 1 shows these units along with Middle Noa-chian (MN) units near Hellas (Npl 1) and a very large area we call " Arabia " , which on average is MN based on its visible crater retention age. We systematically searched 64 pixel/degree MOLA data for Quasi-Circular Depressions (QCDs) >25 km diameter in the Hellas [8] and >15 km in the Isidis [9] areas, and found a significant population of QCDs not visible on images that we assume are buried impact basins. Cumulative …