This photograph of the Crescent Moon shows sunlight passing skimming through the strongly marked surface, filling its crater shadows. This is a fairly smooth region of the moon, but elsewhere is high mountains, with some peaks reach about 5000 meters. When illuminated by the Sun, these mountains cast long shadows on the lunar surface. Way back in 1600, Galileo Galilei used these long shadows for determining the height of the peaks. At the poles of the Moon (not seen in this photograph) some permanently shadowed craters are bases and some may not have been illuminated by the Sun for billions of years. Scientists had long suspected that these dark and permanently cold regions of the Moon could harbor water ice, but it was not until late 2009 that found evidence for this.
In a NASA mission called LCROSS (and Sensing Satellite Lunar Crater Observation by its acronym), a booster rocket out to crash into the south pole of the Moon was sent, while the remaining part of the spacecraft It is seeking evidence of water between the ejected debris. The mission was a success and their findings confirmed the presence of water ice in these dark craters. The finding has important implications for the future of human exploration of the moon and elsewhere in the Solar System.