Sunday, February 22, 2004
Collision with comet may have hastened first plague epidemic
News:By Steve Connor, Science Editor 04 February 2004

A collision between Earth and a passing comet in the 6th century AD may have caused the collapse of agriculture, mass famine and indirectly led to the bubonic plague in Europe, a study has suggested.

Scientists have calculated that a relatively small comet, or fragment of a comet, could have caused huge amounts of dust and debris to be ejected into the atmosphere, blocking the sun for months at a time.

The resulting crop failures and famine would have allowed bubonic plague to spread easily among a physically weakened population.

Studies of tree rings - from preserved oaks retrieved from Irish bogs to ancient American pine trees - have shown that plant growth around the world almost stopped between about 536AD to 545AD. Chinese records from this time refer to a "dust veil" obscuring the skies. Mediterranean historians record a "dry fog" that blocked out much of the sun's heat for more than a year.

Scientists have suggested two causes, both involving the ejection of dust or debris into the atmosphere to block the sun and so prevent photosynthesis.

One idea is that a super-volcano erupted, but neither the volcano nor its acidic deposits have been identified, Derek Ward-Thompson, who carried out the latest study at Cardiff University, said. The other proposal involved a collision with a big asteroid or comet, but there was no direct evidence such as a crater.

However, Dr Ward-Thompson and his colleagues Mel Symonds and Emma Rigby believe a much smaller comet which exploded in the atmosphere could easily have generated the dust and debris in the 6th century catastrophe. "The surprising result of these calculations is just how small a comet fragment we have estimated was needed to cause the observed effects," Dr Ward-Thompson said.

"A comet less than 1km in diameter has not been previously considered to represent a global hazard - as opposed to a local hazard - let alone one 0.5km across," he said.

Using information gathered from the impact of Comet Shoemaker-Levy with Jupiter in 1994, the scientists have produced a model of how comet fragments would behave if they collided with Earth. "The comet plunges into the upper atmosphere leaving an effectively hollow tube behind it, where it has been, and into which the surrounding air has not yet had time to diffuse," the scientists write in the journal Astronomy and Physics.

"This tube then acts rather like a gun barrel, focusing much of the energy of the airburst explosion along the tube and carrying with it much of the comet debris," they write.

As a result, the plume would have spread around the world in a massive fountain of debris. "This period coincides with a mass population decrease in Europe. This is commonly known as the Justinian plague, and is believed to be the first appearance of the Black Death in Europe," the scientists say.

They said that if such an event happened today, a large percentage of the population could face starvation."

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