Tuesday, February 7. 2012 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 mil