september: the largest crater

The Moon is an almost-dead world. Whilst some other moons spew sulphur from volcanoes or have slowly shifting frozen oceans, Earth's companion can claim only disputed sightings of subtle changes.

But this lack of activity is scientifically a good thing. Because the surface does not significantly shift, weather or become buried, it displays a record of impacts stretching back over four billion years.

In general, the larger impacts belong to the earlier phases in the Moon's history, and barely detectable through the many more recent craters is the largest of them all: the South Pole-Aitken Basin. It is revealed in this false-colour topographic image (red for high elevation, purple for low, each colour covers a 500m band) and marked by the dotted line. Although very indistinct (it was only discovered by the Galileo spacecraft and confirmed by the recent Clementine spacecraft) it is actually the largest impact crater in the entire solar system, being 1300 miles (2100 km) wide and some 7.5 miles (12 km) deeper than its surroundings.

It was the result of the impact of a comet or asteroid which penetrated right through the Moon's rocky crust and into the mantle. It is principally a farside feature: the lunar south pole is just inside bottom edge of the basin in this photograph, and the prominent crater above and to the left of the pole is Scrödinger.

Image: Clementine Project.