Evidence from a rock quarry in northern Italy suggests that a shower of comets smacked into the Earth 36 million years ago.
The findings, published recently in Science, help account for huge craters in Siberia and the Chesapeake Bay in Maryland, but also indicate that these were a tiny fraction of the incoming comets that hit over a two or three-million-year period during the late Eocene period.
The work was done by Caltech, the US Geological Survey and the Coldigiocostet Geological Observatory in Italy. The leading astronomer, Gene Shoemaker, who died tragically in a car crash, makes a posthumous return in the paper as a co-author.
The study examines whether the impacting bodies during the Eocene were comets or asteroids, explained Dr Ken Farley of Caltech. Gene Shoemaker suggested that Dr Farley analyse a quarry near Massingnano, Italy were seafloor deposits laid down 36 million years ago would have included debris related to the known large impacts in Siberia and the US.
The scientists were looking for a helium isotope known as 3He, rare on earth but common in materials from outer space. Large heavy objects tend to vaporise as they crash through the earth's atmosphere, releasing 3He which stays in the upper atmosphere before re-entering space. Comet dust and small objects ride through the atmosphere intact, delivering 3He at ground level.
The work showed elevated 3He levels in seafloor sediments from 36 million years ago, when the large impacts were thought to have occurred. This showed, the authors believe, that the Earth's neighbourhood was unusually dusty at that time, possibly because of an abundance of Earth-crossing comets. Some landed, some only passed close by. The record of their passing was there to be seen by those who had the foresight to know how and were to look.