[meteorite-list] Did a Comet Swarm Kill the Dinosaurs?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:08:24 2004
Message-ID: <200209131816.LAA11910_at_zagami.jpl.nasa.gov>

http://skyandtelescope.com/news/current/article_738_1.asp

Did a Comet Swarm Kill the Dinosaurs?
By David Tytell
Sky & Telescope
September 13, 2002

In 1991 a modern scientific 'whodunit' was solved when
geologists identified a deeply buried, 180-kilometer-wide crater in the
Yucatán peninsula. Now known as Chicxulub, the scar resulted from the impact
of a 10-km asteroid or comet nucleus 65 million years ago that triggered
global tidal waves, worldwide firestorms, and massive earthquakes. When the
planet finally returned to normal, the dinosaurs and the majority of all
then-living species had gone extinct, paving the way for mammals to evolve
and dominate Earth.

Now a new study suggests that Chicxulub might not have been an isolated
event. Rather, it seems the dinosaurs may have been the victims of a one-two
impact punch.

Simon P. Kelley (Open University, United Kingdom) and Eugene Gurov (National
Academy of Ukraine) reexamined the age of a buried 24-km-wide Ukrainian
crater known as Boltysh. As recently as 1993, scientists had determined this
impact to be 73 million years old. However, through a number of isotopic
experiments, Kelley and Gurov refined that date to 65.2 ± 0.6 million years.
By comparison, Chixculub's age is 65.5 ± 0.6 million years.

The overlapping uncertainties strongly suggest, (but don't prove) that these
two impacts occurred simultaneously or nearly so. By extrapolating Earth's
current cratering rate backward in time, Kelley believes a Boltysh-sized
crater should appear every 1.8 to 3.3 million years. To see two unrelated
impacts so close in age is unlikely, though not impossible. "The trouble is
that with only two craters, random impacts are not outside the realm of
possibility, says Kelley. Depsite the range of published errors, "I would be
fairly confident that there was only a 250,000-year difference [between
them]." It's "highly probable" the two craters are linked.

What's more, Earth's surface is approximately 3/5 water. Therefore, if two
related objects hit land, another three should, statistically, have splashed
down in the oceans. However, the seafloor bears no obvious trace of these -
they would have been subducted down 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 found at the outer reaches of our solar
system - could dispatch a swarm of dirty snowballs inward toward the Sun,
and therefore Earth. Such impacts could come hundreds of thousands of years
apart. Another possible cometary multiple-impact source is a comet that
broke into pieces after passing too near a planet (the fate of
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 1/400 as much force.

Kelley's next step is to derive isotopic ages for other craters with roughly
comparable ages. Many craters are dated merely by stratigraphic evidence,
which is a less accurate dating method than using an isotopic chronometer.
Perhaps a third, theory-clinching 65-million-year-old crater exists, and it
has 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.
Received on Fri 13 Sep 2002 02:16:58 PM PDT