April 10, 2008

Seawater Holds Clues to Asteroid Impacts

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

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.

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

When an asteroid
, 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
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 (4 to 6 kilometers) in diameter, but previous estimates from models put
it at a whopping 9.3 to 12 miles (15 to 19 kilometers). 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.

hope to find more of these deep ocean impacts," Paquay told SPACE.com.