By John von Radowitz, Science Correspondent, PA News
People living in southern Germany during Roman times may have witnessed a comet impact 5,000 times more destructive than the Hiroshima atom bomb, researchers say.
Scientists believe a field of craters around Lake Chiemsee, in south-east Bavaria, was caused by fragments of a huge comet that broke up in the Earth’s atmosphere.
Celtic artefacts found at the site, including a number of coins, appear to have been strongly heated on one side.
This discovery, together with evidence from ancient tree rings and Roman reports of “stones falling from the sky”, has led researchers to conclude that the impact happened in about 200BC.
However the claim still needs to be verified by other experts.
The crater field was uncovered after amateur archaeologists working in the area found pieces of metal containing unusual minerals.
A team of geologists led by Kord Ernston, from the University of Wurzburg in Germany, went to the site and discovered evidence of a cataclysm that would have left the region devastated for decades.
Not only would trees and homes have been flattened for many miles by the blast, but the local climate would have changed for years afterwards.
Tree rings show that vegetation growth slowed down in around 207BC, possibly because of the “nuclear winter” effect of dust blotting out the sun.
More than 80 craters were found in an elliptical area 36 miles long and 17 wide, ranging in size from 10 to 1,215 feet across. The largest, filled with water, now formed Lake Tuttensee.
Around the site the team found clues that suggested an impact from space, including rock heated into glass and minerals associated with meteorites.
The most likely cause was a low-density comet, 0.7 miles (1.1 kilometres) wide, that broke up at an altitude of 43 miles and fell in pieces to Earth, the scientists reported in Astronomy Magazine.
They wrote: “The main mass of the projectile struck the ground at 2,200 miles per hour, releasing an amount of energy equivalent to 106 million tons of TNT.”
The bomb that destroyed Hiroshima at the end of the Second World War had an explosive force of just 20,000 tons of TNT.
The scientists gave a graphic description of what it might have been like to experience the impact.
“About two seconds after the strike, people six miles away (10 kilometres) would have felt the ground shake as it would in a magnitude six earthquake. The air blast, arriving 30 seconds after impact, would have swept through at a speed of 500 miles per hour and produced a peak pressure of about 1.4 atmospheres, easily collapsing buildings, especially wooden ones.
“Even from 10 kilometres away, sound from the impact would have reached 103 decibels – loud enough to cause strong ear pain. Up to 90% of the trees would have blown over; the rest would have lost their branches.”
Forest beneath the blast would have ignited suddenly, and continued to burn until the shock wave blew the fire out, said the scientists.
The conflagration had left a thin layer of ash in and between the craters.
Roman authors at the time wrote about showers of stones falling from the sky and terrifying the local population.
Because of these events, the Senate in 205BC ordered that a conical meteorite known as the Needle of Cybele, which had been worshipped in Asia Minor, be brought to Rome.
“The impact undoubtedly had a major effect on the environment and people then living in the vicinity of Altoetting-Chiemgau,” wrote Ernston’s team.
“The region must have been devastated for decades. We are currently looking for gaps in the historical and archaeological records during the time we propose for the impact to better understand both the event itself and its cultural effects.”
Dr Benny Peiser, a leading expert on impact events from Liverpool John Moore’s University, said the report should be treated with caution until more was known.
He said the date was speculative, and pointed out that asteroids or comets a kilometre wide struck the Earth on average only once every 500,000 years. Generally such a large impact would cause much more severe and obviously traceable damage.
“In short, this is an an intriguing find, but I remain sceptical for the time being,” said Dr Peiser. “The impact cratering research community has not assessed these claims yet. That’s what needs to be done next.”