[meteorite-list] Boltysh and the dinosaurs
From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 10:06:15 2004
Best Sunday morning
TYTELL D. (2002) Did a Comet Swarm Kill the Dinosaurs?
(Sky and Telescope, Dec. 2002, 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, world-
dwide 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
Simon P. Kelley (Open University, UK) 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
Chicxulub'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 1/100 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 strati-
graphic evidence, which is less accurate than using an isotopic chrono-
meter. 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.
Received on Sun 10 Nov 2002 06:07:11 AM PST