mpact craters, from tiny pits to giant basins, are the dominant landforms of the Moon. Just a handful of the largest craters appear nearly unchanged since they formed over a billion years ago. The rest have been reshaped by subsequent events. In fact, modification was common in the first half-billion years of lunar history. Craters large and small were completely destroyed or otherwise altered by other impacts in the earliest lunar epoch. However, between about 3.8 and 1 billion years ago, this degradation slowed as the impact rate rapidly declined. In the last billion years, the mixing of
