The meteor that streaked over the city of Chelyabinsk in Russia’s Ural Mountains on Friday 15 February 2013 was a reminder of the fragility of life on Earth. If dinosaurs can be wiped out, so can human beings.
The collision of a meteor or asteroid with another celestial object is called an impact event. Many such events have occurred throughout Earth’s history, disrupting the environment and likely causing mass extinctions. Impact craters on the Moon are clearly visible; less obvious today are those on Earth both on land and beneath the sea.
One of the best-known recorded impacts in modern times was the Tunguska event, which occurred in Siberia in 1908. It involved an explosion probably caused by the airburst of an asteroid or comet 5 to 10 km above the Earth’s surface. It felled an estimated 80 million trees over 2,150 square kilometres. Photographic evidence exists of the devastation.
On 10 August 1972 a meteor which became known as the “Great Daylight Fireball” was seen moving north over the Rocky Mountains from the USA to Canada. The object was the size of a car or small house and could have ended in a Hiroshima-size blast, but there was never any explosion. Analysis of the trajectory indicated that it had grazed Earth’s atmosphere for about a minute and a half before skipping off the atmosphere to return to its orbit around the Sun.
Several huge meteorites – among hundreds of small ones – are known to have struck the Earth. Buried underneath the Yucatán Peninsula in Mexico, near Chicxulub, is a prehistoric impact crater 170km in diameter. A comet or asteroid the size of a small city struck there roughly 65 million years ago causing an explosion estimated to be equal to 100 million megatons of TNT – roughly 8 billion times stronger than each of the bombs that hit Hiroshima and Nagasaki. In a split second it would have annihilated everything within a thousand miles of the impact and is widely believed to have led to the extinction of dinosaurs.
About 30 km south-east of Kumasi, Ghana, lies Lake Bosumtwi (right), the country’s only natural lake. Some 1.3 million years ago the impact of a meteorite opened up a hole with a 10.5 km diameter. The crater gradually filled with water to form the lake we see today. Elsewhere in Africa, in northern Chad, Aorounga is an eroded meteorite impact crater that formed 2-300 million years ago when a comet or asteroid 1.6 km in diameter hit the Earth.
Canada bears several scars from meteor impacts. Deep Bay, near the south-western tip of Reindeer Lake in Saskatchewan, is strikingly circular and very deep in an otherwise irregular and shallow lake. It was formed 100 to 140 million years ago when a large meteorite hit the area.
Two circular craters on the Canadian Shield in Quebec were formed simultaneously from the impact of a pair of asteroids which crashed to Earth approximately 290 million years ago, near the eastern shore of Hudson Bay. The larger of the two craters is West Clearwater Lake with a 32km diameter, and the smaller, East Clearwater Lake,with a 22km diameter. Both are clearly visible from Space.
To the west of the Clearwater Lakes is an anomaly. A map of Hudson Bay reveals part of a huge semi-circle known as the Nastapoka Arc, which looks like the mother of all meteor impacts. Geologists disagree about what created it. Some believe it resulted from the movement of tectonic plates. Others argue that it is related to a Precambrian extraterrestrial impact, comparing it to the Mare Crisium on the Moon, which is 555km in diameter.
No credible evidence has been found by geological studies for such an impact, but it too would have obliterated much of life on Earth. Undoubtedly, the Chelyabinsk meteor will not be the last visitor from deep-space. As the Roman poet Horace reminded us, Carpe diem quam minimum credula postero – “Seize the day, and put no trust in tomorrow.”