Rolling moon boulder photographed by Nasa The image, captured by Nasa's Lunar Reconnaissance Orbiter Camera, shows a boulder which has tumbled down an incline on the Schiller crater, leaving behind it a pristine track. But far from being a recent event, the impact craters along the rim of the track suggest the rock moved as long as 100 million years ago. Lunar scientist James Ashley wrote: "A casual glance might suggest that it happened last week, or even that its rolling might resume at any moment. However, closer inspection will detect a few craters that clearly superpose and therefore post-date the track, showing that this 9-metre diameter boulder stopped rolling some time ago. Experts use density of craters to approximately date events on the moon's surface. They assume a roughly steady number of meteorite impacts over time, with smaller impacts being more common than large ones. Mr Ashley added: "Though long ago to humans, however, this boulder's journey was made in geologically recent times. Studies suggest that regolith development from micrometeorite impacts will erase tracks like these over time intervals of tens of millions of years. If rate estimates are accurate, this boulder track might not be older than 50-100 million years. Eventually its track will be erased completely. He suggested the boulder started rolling after being struck by a meteorite, or after being dislodged by the tremors from a nearby impact.