BURIED AND VISIBLE IMPACT BASIN DISTRIBUTION ON MARS: COMPARISION WITH MAGNETIZATION, GRAVITY AND CRUSTAL THICKNESS MODELS

The large population of buried impact basins found in MOLA elevation data on Mars provides compelling evidence for a “pre-Noachian” crust below the oldest visible Early Noachian surface units, and lowland crust below the younger plains that is Early Noachian in age, older than much of the visible highlands, but not as old as the buried “pre-Noachian” highlands. The large (D>200 km) buried basins are suggested by “Quasi-Circular Depressions” (QCDs) that are not apparent in image data, and include features up to 3000 km diameter in both the lowlands (Utopia) and highlands (a newly found “Ares Basin”). There are about a dozen QCDs larger than 1000 km diameter. We have placed these large features in a relative age sequence based on superimposed smaller QCD. Only the youngest and most obvious of these (Hellas, Argyre, Isidis) lack magnetic anomlies within their main rings. These all have an N(200) cumulative crater density of < 2.5. Somewhat older “lowland-making basins” (Utopia, Chryse, Acidalia) with an N(200) age of ~3.0, have weak magnetic anomalies, and the oldest, most subdued basins (including Ares) with N(200) > 3.5 have many strong magnetic anomalies within their main ring. These older basins likely formed before the main magnetic field died. We have compared our inventory of large QCDs with the distribution of gravity anomalies and with a crustal thickness model which shows many roughly circular areas of thinner crust completely or partly surrounded by narrow regions of thicker crust. These have the structure expected for impact basins, and many of them do correspond to the visible or buried QCDs we previously identified. But there are cases where the crustal thickness feature is offset from the QCD found in topography alone, and there are also several, sometimes large examples of such features which do not coincide with QCDs previously identified. For example, we find several likely buried basins revealed in the crustal thickness data in the Arcadia and Amazonis regions which we did not previously identify, including several features in the 600-1200 km diameter range.