The moon's stores of water-ice, hidden in permanently shadowed craters at the lunar south pole, probably arrived on the surface of our nearest neighbor gradually rather than in one big event, according to new research.
Furthermore, these 'permanently' shadowed craters are not as permanently shadowed as we thought, since changes in the moon's tilt with respect to Earth and the Sun means that the angle of illumination has also changed over the course of billions of years. Craters that were engulfed in cold shadows and capable of hosting water-ice 3 billion years ago are not necessarily in shadow now, and vice versa.
