Impact constraints on, and a chronology for, major events in early Mars history

Large‐diameter visible and buried impact basins, seen as “quasi‐circular depressions” (QCDs) in MOLA gridded data, provide a self‐consistent chronology for major events on early Mars in terms of N(200) crater retention ages. On the basis of a conversion to model absolute ages, this chronology extends back hundreds of millions of years into a previously unknown “pre‐Noachian” epoch during which a now buried highlands surface was established, in which several very large impact basins formed while the global magnetic field was still present. A cluster of very large “lowland‐making” basins occurred at a model age of about 4.13 GY (or earlier), forming the fundamental topography of the Mars crustal dichotomy at a time prior to the age of the oldest visible highland crust. This early event in Martian history marks the transition from the “pre‐Noachian” to the Early Noachian when the well‐preserved Hellas, Argyre, and Isidis basins formed, all after the global magnetic field died.