june: impact crater

The Moon's history of bombardment is obvious to anyone who looks at it through binoculars. Its surface is covered in craters ranging from impact basins the size of a country down to microscopic pits.

As the Earth is larger, logically it has received at least as many hits, but whilst the airless Moon will preserve the evidence for millennia, the Earth's atmosphere will annihilate the smallest meteoroids before impact, and erosion and geological activity will progressively erase the craters left by those which get through.

Nevertheless, some 150 craters have been identified on the Earth. Some, like Meteor Crater in Arizona, are easily seen. Others are much more difficult to find.

Mjølnir, named after the hammer of the Norse god Thor, is actually on the floor of the Barents Sea. The image above was created from seismic data, and uses false colour to reveal relief. The 39 km (24 mile) wide structure was probably produced some 150 million years ago by the impact of an asteroid in excess of 1.5 km or 1 mile wide. The energy released may have been the equivalent of a million megatons of TNT.

Image: F. Tsikalas, S.T. Gudlaugsson, J.I. Faleide, O. Eldholm (Geology Department, University of Oslo).