Halley's Comet from 1986 (NASA photo)According to a new study, a chunk of Halley's comet may have hit the Earth in the 6th century, causing a brief climate shift that cooled the planet, caused drought and famine, and led to the first known outbreak of the Black Death in Europe.
From both historical records and scientific research, it's clear that something strange happened to our planet around the year 536 A.D. Accounts tell of the sun dimming as if it was in eclipse, unusual weather such as snow and frost in summer, drought, and famine due to crop failures. Modern studies have revealed very little growth in tree rings during that year, and ice cores from Greenland and Antarctica show a substantial layer of dust from that time. It's thought that this change in the climate and the resulting famines made the people of Europe more vulnerable to sickness, and that the pandemic known as the 'Plague of Justinian' in 541-542 was a direct result of it.
There's some thought that it was a massive volcanic eruption that triggered the event. However, based on a study from 2009 and a recent talk she gave at a meeting of the American Geological Union, Dallas Abbott, a research Scientist at Columbia University's Lamont-Doherty Earth Observatory, believes that a volcanic eruption isn't enough to account for it.
"There was, I think, a small volcanic effect," she said, according to LiveScience. "But I think the major thing is that something hit the ocean."
In a study of the dust layer in the Greenland ice cores, she and her co-authors found high levels of tin, as well as nickel and iron oxide spherules, indicating that the dust came from a comet. They also found samples of algae and plankton that are normally only found in tropical ocean waters. An impact in the ocean could have thrown these organisms far away, or possibly caused a tsunami that delivered them from the tropics to Greenland.
As for which comet dealt us the blow, the timing of the dust suggests that the event happened during spring in the Northern Hemisphere. That points to either Comet Thatcher, which is the source of the Lyrid meteor shower in April, or Comet Halley, which causes two meteor showers, the Eta Aquariids in April/May and the Orionids in late October. Turning to historical records again, it appears as though Halley might be the culprit.
"Of the two brightest apparitions of Comet Halley, one of them is in 530," Abbott told LiveScience. "Comets are normally these dirty snowballs, but when they're breaking up or they're shedding lots of debris, then that outer layer of dark stuff goes away, and so the comet looks brighter."
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If a chunk of Halley's comet struck the planet back in 536 A.D., where did it hit and would we see evidence of an impact crater? Not necessarily. We've already seen examples of powerful atmospheric airbursts, with the 1908 Tunguska Event and the Chelyabinsk meteorite explosion over Russia in February of this year. An airburst of a large comet fragment could have put enough dust into the atmosphere to account for the climate shift and caused a big enough shock wave to blast tiny sea creatures from the tropics to Greenland.
Does this definitively settle the question of what caused the climate shift of the period? Not really. The evidence is compelling, but it's still circumstantial. More study of the event will likely shed more light on it, and it could give scientists insight into other similar events in Earth's past.
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