INCREDIBLE SOLAR SYSTEM CRATERS

Why these spectacular craters carved out by past impact events are among the most widespread and distinctive geological features in the Solar System

- ANDREW MAY

The planets and moons of the Solar System travel in neat, almostcircular orbits, which means they never run the risk of crashing into each other. But that's not true of smaller bodies like asteroids and comets, which are essentially chunks of material left over from the Solar System's formation. These move on much more eccentric orbits that can and do - collide with each other and with the larger members of the Solar System. Most of these collisions occur at very high speeds, typically many thousands of miles per hour, producing enough energy to vaporise the impactor and melt the surrounding rock. When the dust clears, all that's left is a big hole in the ground - an impact crater.

Impacts have been happening ever since the dawn of the Solar System. In fact, they reached their peak a few billion years ago, when there was far more rocky debris around. We still see evidence of these primordial impacts in places like the Moon and Mercury, where there's never been any atmosphere to erode the crater-covered terrain. On the other hand, our own planet lies at the opposite extreme. Not only do we have plenty of weather to wear the craters away, but they also get destroyed by volcanic activity and the gradual movement of tectonic plates. So finding traces of past impact events here on Earth often involves quite a bit of geological detective work.

The usefulness of craters for planetary scientists goes well beyond the impacts that formed them. Craters allow different terrains to be dated simply by counting them - the more craters, the older the surface. And because impacts often eject deeply buried material onto the surface, the composition of planetary interiors can be studied without the need for any digging or drilling.

WOLFE CREEK

LOCATION: EARTH

IMPACT SITE AGE: 120,000 YEARS