Surface history of Mercury: Implications for terrestrial planets

A working hypothesis of Mercury's history is presented. We infer the surface of Mercury to record a sequence of events broadly similar to those recorded on the moon, implying similar histories of impact bombardment. The oldest terrains on Mercury seem to be better preserved from modification by ejecta from subsequently formed impact basins because of higher surface gravity. The large lunarlike impact craters on Mercury can be interpreted as part of a distinct episode of bombardment which may have affected all the terrestrial planets about 4 b.y. ago. The light cratering accumulated on the surfaces of the Mercurian smooth plains is similar in diameter/frequency relationship to that of the lunar maria and of the oldest Martian plains units, consistent with recent interpretations of lunar and Martian flux histories by Wetherill (1974) and Soderblom et al. (1974). A straightforward interpretation of the Mercurian surface record thus supports recent order of magnitude increases in age estimates of many Martian features discovered by Mariner 9 but is not conclusive. The large core inferred for Mercury combined with the lack of recognizable evidence of past atmospheric activity is more easily understood in terms of radially heterogeneous accumulation than in terms of differentiation of a homogeneous planet. Early core cooling may be reflected by widespread evidence of crustal shortening. However, Mercury's surface seems little affected by any tectonic, atmospheric, or volcanic processes for the last 3 b.y. or so, raising questions concerning (1) the relationship of the origin of Mercury's magnetic field to that of earth's and (2) the primary cause of volcanic flooding, which may have begun, and ended, approximately synchronously on Mercury and the moon.