Evidence for biggest UK impact crater unearthed
BY EMILY BALDWIN
ASTRONOMY NOW
Posted: March 26, 2008

Evidence for the biggest asteroid ever to hit the United Kingdom has been unveiled by a team of British scientists.

Researchers from the universities of Oxford and Aberdeen think a large object hit north-west Scotland about 1.2 billion years ago, near the present-day town of Ullapool. The research has centred around the ejected debris of the impact crater that was flung out of the crater cavity as it was formed, and has been preserved in an almost pristine condition. “The exposed outcrop forms a thin strip of rock along the northwest Scottish coast which we interpret as a cross sectional slice through the ejecta blanket,” says Ken Amor of Oxford University, lead author of the research which is published in the latest issue of the journal Geology.

The researchers believe that the deposit was emplaced as a single fluidised flow that formed as a result of an impact into water-saturated sediments. Interaction with the ground water is thought to have played a dominating role in the morphology of the ejecta, and interaction with the Earth’s atmosphere is also evident. "If there had been human observers in Scotland 1.2 billion years ago they would have seen quite a show," says Amor. "The massive impact would have melted rocks and thrown up an enormous cloud of vapour that scattered material over a large part of the region around Ullapool.”

A Martian impact crater, exhibiting the characteristic feature of an apparently fluidised ejecta blanket. The conditions under which the Scottish impact crater formed may have been similar to conditions on Mars. Image: NASA/JPL/Arizona State University

Unusual rock formations in the area were previously thought to have been formed by volcanic activity, but the lack of volcanic vents or volcanic sediments was posing a problem. Now unequivocal evidence has been presented that confirms an extra-terrestrial origin for these deposits. “Chemical testing of the rocks found the characteristic signature of meteoritic material, which has high levels of the key element iridium, normally only found in low concentrations in surface rocks on Earth,” explains Amor. “We found more evidence when we examined the rocks under a microscope; tell-tale microscopic parallel fractures that also imply a meteorite strike.”

Impact craters are rarely preserved on Earth due to rapid erosion or burial; the crater at the focus of this research is thought to lie beneath the Minch, the waterway that separates Lewis in the Outer Hebrides from the northwest highlands of Scotland. “Our best way of locating the crater will be by geophysical surveys, such as seismic reflection and gravity,” says Amor.  “Drilling would be great but prohibitively expensive.”

The continental setting, and the presence of groundwater, make this crater a potential analogue for cratering processes that have occurred on Mars.