The rainfall over very long periods of time is not constant. From boring relative position to the nearby star, to the activity of said star, to more exotic http

The rainfall over very long periods of time is not constant. From boring relative position to the nearby star, to the activity of said star, to more exotic https://en.wikipedia.org/wiki/Gamma-ray_burst https://en.wikipedia.org/wiki/Gamma-ray_burst and God knows what else. A more honest statement: "We measured Ne-21 in meteorite minerals in higher concentration than on similar Earth minerals. We think it's caused by cosmic rays. We have no clue where the cosmic rays came from or when."