A pair of 35-million-year-old craters on Earth thought to have been carved by comets now appears to be the result of a broken asteroid that generated a slowly delivered shower of debris over millions of years.
One crater is in Chesapeake Bay off the Maryland coast. The other, called the Popigai crater, is in north-central Siberia. Estimates of their age suggest they were created a mere 10,000 years apart.
Scientists had thought a comet shower of some sort had left the two scars.
Asteroid go platinum
A new study of the Popigai crater finds an abundance of elements from the platinum group, a signature common to asteroids and not thought to suggest comets, which hold more water ice and lower concentrations of metals.
The timing of the two impacts suggests the Chesapeake crater may have been caused by the same parent asteroid, which in this scenario broke apart in space and showered the Earth with many fragments large and small, explained geology professor Phillipe Claeys of Vrije Universiteit Brussel in Belgium.
"The chance to have over a short time scale the impact of an asteroid and that of a comet occurring together is unlikely — not impossible, but very unlikely," Claeys told SPACE.com. He said objects up to 3 miles wide (5 kilometers) — big enough to form the craters in question -- hit Earth once every 25 to 30 million years.
Other studies have found that when asteroids collide and break apart, their fragments can move from the asteroid belt between Mars and Jupiter to reach Earth's vicinity over long time periods. Earth can then be hit by successive pieces on its continual trip around the Sun.
Other evidence reveals extra space dust particles, perhaps the product of the asteroid's breakup in space, falling on Earth for 2.5-million-years around the same time as the two large impacts, Claeys said.
That suggests a slow rain of debris created increased rates of dramatic fireballs in the sky and possibly other large impacts. The drama might have spanned at least 10,000 years, he said, and other than the Chesapeake and Popigai craters, there would be little evidence today.
The asteroid that broke up to produce the asteroid shower "was much bigger" and "should have produced many more fragments than the two that came on an Earth collision course," Claeys said. "The Earth is two-thirds water, and we know little about craters in the deep ocean, but perhaps there were also one or two other projectiles that fell in the ocean during the period."
The uptick in activity would not have been noticeable over the course of the typical human lifetime.
The study, detailed Friday in the journal Science, was led by Roald Tagle of the Museum fuer Naturkunde in Berlin. The researchers plan to look for similar hard evidence that an asteroid built Chesapeake Bay.
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