The Little Heavy Bombardment: Decoupling the time history of impact basins from smaller craters leads to alternative ways of interpreting the ancient crater records of the inner solar system

The cratered terrains of the Earth’s Moon provide the most comprehensive sources of data used for measuring the ages of cratered surfaces across the Solar System. The combination of ages derived from returned samples from a variety of lunar sites obtained by the Apollo, Luna, and Cheng’E programs and high quality imagery, topography, and gravity measurements of cratered surfaces is used build a relationship between the absolute age of a planetary surface and the population of craters observed on its surface. Because the lunar impact bombardment record is so well calibrated with returned samples, it has served as one of the most important tools for estimating ages of surfaces across the Moon and beyond. However, the use of the lunar bombardment record as a general planetary surface chronometer is not without its limitations. To estimate the age of a planetary surface, one must use a crater production function, which is a model that relates the rate of crater formation to crater size and time. The most commonly used lunar crater production functions, such as the Neukum Production Function, are well calibrated for some terrains, but poorly calibrated for others. In particular, it is usually assumed that the production rate of large craters, such as D>300 km impact basins, follows a similar curve as smaller craters. This assumption must be made because the production function curve in time is best constrained by samples from lunar maria terrains, and individual flows with directly dated samples contain craters only as large as a few km in diameter. Individual impact basins, including Imbrium, are only indirectly dated from samples, and therefore the production curve in both the time and crater size becomes much more uncertain on terrains both older than the oldest sampled lunar mare and for surfaces with crater populations larger than a few km in diameter. Here we reassess the assumptions that underpin lunar crater chronology for the most ancient lunar terrain, in particular we challenge the assumption that lunar basins followed the same production curve in time as smaller craters, and propose new models for the early bombardment history of the inner Solar System.