updated 4/10/2008 3:37:11 PM ET 2008-04-10T19:37:11

Asteroids that strike Earth have cosmic origins, but clues to the size of ancient impactors now have come from a decidedly Earth-bound source: the chemistry of ancient seawater.

Asteroids and other extraterrestrial objects have struck Earth countless times over its multi-billion-year history, but most have left little visible trace. Though these cosmic projectiles pack a wallop when they collide with our planet, they often vaporize on impact or fall into the ocean, making it hard to find any resulting craters and estimate their size.

However, the impactors may leave behind certain chemical traces in ancient ocean-floor sediments that can act as a telltale sign of their impact and record what was floating around in the seawater in the distant geologic past.

The new study, detailed in the April 11 issue of the journal Science, has found higher levels of a particular isotope of the element osmium in ocean sediment layers that correspond to the timing of known impacts.

"So it's like a label in the ocean," said study leader Francois Paquay of the University of Hawaii.

Slideshow: Space Shots When an asteroid hits, it vaporizes into miniscule particles "and everything rains down in the ocean," Paquay explained. By examining the ratios of two osmium isotopes in ocean sediment cores, scientists can identify points in our planet's history when there was an impact.

They can also use the isotope levels to estimate the size of impactors. Interestingly, Paquay's estimates are at odds with those from model projections taken from known crater sizes, such as the Chicxulub crater thought to be made by an asteroid impact that led to the extinction of the dinosaurs. Paquay's study estimates this asteroid was about 2.5 to 3.7 miles in diameter, but previous estimates from models put it at a whopping 9.3 to 12 miles. Paquay says he is confident in estimates made by his method and that further research will eventually square the estimates made with different methods.

The new method could also be used to find the signatures of unknown crater impacts.

"We hope to find more of these deep ocean impacts," Paquay told

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