How long would evidence of an extraterrestrial visit survive: an educational exercise

With the discovery of objects entering the solar system from interstellar environments, the discussion of the possibility of extraterrestrial visitors to Earth has resumed. We examine the question of how long the evidence for such visits could be expected to last on Earth and on the Moon. Using geological estimates and our current knowledge of the lunar regolith, we conclude that evidence for visits to Earth more than 100,000 years ago would not survive to the present. Radiocarbon dating of some organic material (assuming large C-14 abundance) is of the same order of magnitude. Similarly, evidence of visits to the lunar surface would not survive for more than 100 million years.