Cataclysmic Events on the Moon

NASA recently announced that it has down-selected three New Frontiers mission concepts for additional study. One of these missions, Moonrise, proposes to return rock and soil samples from the floor of the largest impact crater on the Moon, the South Pole-Aitken (SPA) basin, centered on the southern far side. Not only is this the largest [...]

The Imbrium basin immediately after formation, ~3.85 billion years ago. Artwork by Don Davis.
NASA recently announced that it has down-selected three New Frontiers mission concepts for additional study. One of these missions, Moonrise, proposes to return rock and soil samples from the floor of the largest impact crater on the Moon, the South Pole-Aitken (SPA) basin, centered on the southern far side. Not only is this the largest basin on the Moon, it is also the oldest, as evidenced by a high density of impact craters superposed on top of its deposits. But knowing that it is the oldest basin does not tell us exactly when it formed. Samples collected from its floor could potentially determine exactly when, during the early history of the Moon, it was created.
Why is the absolute age of this feature important? When lunar samples are returned to ...