A Recent Journey – LROC News System
The lonely journey of this large boulder is apparent from its track in a sloping regolith surface. 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-meter diameter boulder stopped rolling some time ago. Impacts are used in this way to provide a relative sense for the timing of events on planetary surfaces across the solar system. The procedure assumes a steady flux of impacting bodies in each size range, with smaller impacts being much more frequent than large impacts.
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. What might have caused the rock to roll so recently? Perhaps this boulder was sent on its way by ground-shaking caused by the violence of a nearby impact. Perhaps a direct hit by a small meteoroid did the job.