any of our features and didn't believe in them, and that this was a poor way to start a scientific career. Urey had his own idea of the Imbrium basin, that it had been formed by an impact not at its center, but off to one side, and that it had not radial symmetry but axial symmetry. It was amazing that he couldn't see the concentric patterns; his Imbrium idea has been abandoned.)

    There are several dozen of these large impact basins on the moon and probably many more formed so long ago that they have been destroyed by later impacts. These are believed to mark the impacts of fairly large scale ‘planetesimals,’ or asteroid-like debris left over from the formation of the planets. This painting shows and early such example, as the moon was still moving away from Earth, soon after the Earth-moon system formed. Impact experiments and computer modeling suggest formation of huge conical curtains of incandescent ejected debris, much of which falls back onto the surrounding countryside, forming rim structures on the basin. In the distance, Earth at the time was spinning somewhat faster than it does today (due to angular momentum constraints on the Earth-moon system), and might have had more band-shaped cloud systems than it does today.

William K. Hartmann
2003, January 1