Did a Comet Swarm Kill the Dinosaurs?
by David Tytell
Dec. 2002 Sky & Telescope magazine, p. 24
IN 1991 A MODERN SCIENTIFIC "WHODUNIT" WAS
SOLVED WHEN geologists identified a deeply buried, 180-kilometer-Wide
crater in the
Yucatan peninsula. Now known as Chicxulub, the scar resulted
from the impact of a 10-km asteroid or comet nucleus 65 million
years ago. Geologic evidence indicates that the impact triggered
global tidal waves, worldwide firestorms, and massive earthquakes.
It also left a worldwide layer of extraterrestrial dust. When
Earth finally returned to normal, the dinosaurs and the majority
of all then-living species had gone extinct, opening the way
for mammals to diversify
and dominate Earth.
Now a new study suggests that Chicxulub may not have been
an isolated event. Rather, the dinosaurs may have been the victims
of a one-two punch.
Simon P. Kelley (Open University, United Kingdom) and Eugene
Gurov (National Academy of Ukraine) have reexamined the age
of a much smaller, 24-km-wide crater buried in Ukraine and known
as Boltysh. As recently as 1993, geologists estimated the impact
to be 73 million years old. However, through a number of isotopic
experiments, Kelley and Gurov have refined that date to 65.2 ± 0.6
million years. By comparison, Chicxulub's age is 65.5 ± 0.6
million years.
The overlapping uncertainties suggest that the two impacts
may have occurred simultaneously or nearly so. Kelley believes
a Boltysh-size crater should appear at random every 1.8 to 3.3
million years. So it would be somewhat unlikely for an unrelated
impact to be so close to Chicxuluh's age. "The trouble is that
with only two craters, random impacts are not outside the realm
of possibility," says Kelley. Despite his published range of
errors, "I would be fairly confident that there was only a 250,000-year
difference [between Chicxulub and Boltysh]," he says, adding
that he thinks a link is "highly probable"
What's more, Earth's surface is approximately three-fifths
water. Therefore, if two related objects did hit land, roughly
another three should have splashed down in the oceans. However,
the seafloor bears no obvious trace of these they would have
been subducted into the mantle long ago.
If the dinosaurs did indeed endure multiple hits, scientists
might be able to say something about the nature of the impactors.
Asteroids tend to travel alone (though pairings do exist), while
comets are thought to sometimes arrive in bunches. A gravitational
disturbance in the Oort Cloud or Kuiper Belt the massive comet
reservoirs at the outer reaches of our solar system - could
jostle a swarm of dirty snowballs inward toward the Sun and
Earth. Such impacts could come hundreds of thousands of years
apart. Another possibility is a single comet that broke into
pieces after passing too near a planet (the fate of Comet Shoemaker-Levy
9). Such impacts would happen close together.
Chicxulub was still the big killer, however. The sizes of
the two craters imply that Boltysh hit with only about U as
much energy.
Kelley's next step is to derive isotopic ages for other craters
with roughly comparable ages. Many craters have been dated merely
by stratigraphic evidence, which is less accurate than using
an isotopic chronometer. Perhaps additional, theory-clinching
65-million-year-old craters exist and have simply been assigned
the wrong age. Boltysh's assumed age was in error by some 8
million years; others could be off by that amount or more.
Kelley and Gurov present their findings in the August issue
of Meteoritics and Planetary Science.
A small asteroid or comet nucleus created the 24-kilometer-wide
Boltysh crater in Ukraine 65.2 ± 0.6 million years ago. TOP
| 
