Impact Cratering on the Moon, Mars and Scotland

Tuesday 19th May, 2015

With the very first "Asteroid Day" global event being held at the end of June it was fortuitous that we had arranged for a geologist to come and talk to us about impact craters. Dr Ken Amor is from Oxford University's Department of Earth Sciences and is co-author of a paper concerning Britain's biggest meteorite impact near northwest Scotland.

Dr Amor began his talk, "Impact Cratering on the Moon, Mars and Scotland, by saying that (with a few exceptions) every rocky body in the Solar System is covered in impact craters. The lunar craters were first observed by Galileo in 1610 with the newly invented telescope. Then in 1665 Robert Hooke published his book "Micrographia", which contained his observations using various lenses. In it there was a drawing of the crater called Hipparchus that he erroneously concluded must be volcanic in origin. To try and confirm his theory that the lunar craters resulted from volcanic processes he performed experiments that involved boiling alabaster and then cooling it.

However, as the study of craters progressed there were some dissenting voices to the volcanic theory. For the German physicist Ernst Chladni proposed in 1794 that meteorites were not ejected volcanic rocks but were extra-terrestrial in origin. Also the American geologist Grove Karl Gilbert's studies of lunar craters led him to conclude that some may be the result of impacts. But it was not until the mid-20th century most scientists agreed that lunar craters were due to impacts.

Dr Amor then explained that we now know that the size of a crater depends on the impactor's velocity and mass as well as the properties of the surface it hits. In fact the final diameter of the crater is usually about twenty times that of the impactor and the final crater will be circular unless the impactor approaches the surface at a shallow angle that is less than 15 degrees.

On the Moon the craters are classified into roughly four distinct types depending on their features. The smallest ones, up to 4 km across, are known as "simple" craters and are bowl shaped. Craters larger than 4 km and up to 200 km are called "complex". These have a peak in the centre as well as a flat floor and terraced walls. Above 200 km but less than 300 km across a crater is classified as a "central ring complex" and it will have a flat floor and a series of surrounding rings. The largest craters, over 300 km across, are called "multi-ring basins" but these are not well understood.

On planets or moons where there is little geological activity or weathering craters are clearly visible and somewhat straightforward to study using either Earth-based imaging or visiting spacecraft. However, on Earth past impacts are hidden beneath the ocean or vegetation, eroded away or destroyed by plate tectonics. This means that geologists have to become detectives to look for clues to their whereabouts.

One of the latest impacts to be discovered by Dr Amor and his colleagues is off the coast of northwest Scotland near Ullapool. The tell-tale signs of an impact were deformed and ripped sandstone layers, the presence of a rock called suevite and a form of quartz known as "shocked quartz". The scientists think that the impact occurred about 1.2 billion years ago.