A Rolling Stone on the Moon: Image of the Month

Once, in complete silence, a large boulder rolled down a slope on the central peak of the MoonòÀÙs Schiller crater. This rock is about 9m (30ft) across, meaning that it would fit neatly inside Armagh PlanetariumòÀÙs 11 m wide Digital Theatre. The MoonòÀÙs surface has been to all intents and purposes dead for nearly 4 billion years, so why did it start moving? Probably the rockòÀÙs journey began when a nearby meteoroid impact shook it from where it had lain since it was deposited there by the impact that excavated Schiller itself.

A roll’n’rock can be seen in this image from NASA’s Lunar Reconnaissance Orbiter. The image shows an area 500m wide. (Image credit: NASA/GSFC/Arizona State University )

This rolling stone gathered no moss but left its track in the regolith until the slope levelled off and it slowed to a halt. Planetary scientists have found small impact craters superimposed on the track it left as it moved and then, based on an assumed rate cratering (based on observations from across the Solar System), used these craters to estimate when this stone stopped rolling. The answer they found suggests this boulder track is a mere 50-100 million years old.

Compared to how long the Universe has existed, this is a miniscule age, but compared to even the ages of species it is vast. Say the lonely boulder trekked downhill 50 million years ago. Then the Moon shone down on an Earth thatˆà looked superficially familiar to that of today but was oddly different. This was a world where whales still walked on land and horses would have been small enough to fit in a shopping trolley. Somewhere in the treetops scampered and played little squirrel-like creatures whose glittering eyes suggested a potential for intelligence realised 50 million years later.

The slow rain of micrometeorite impacts will eventually erase this track over many millions of years. When the trail is finally gone, obliterated by time, what will our Earth be like?