Geoscience Reference
In-Depth Information
H
ELL
ON
E
ARTH
To try to understand whether and how the Alvarez theory might
help to explain the selectivity of the K-T extinction, we need to
know what would happen when a 10-km to 15-km meteorite strikes
the earth. The two halves of the Alvarez theory—that impact
occurred and that it caused the mass extinction—are linked by the
assumption that the resulting effects would be sufficiently lethal to
cause the death of 70 percent of all species. The Alvarez team had
precious little evidence for this assumption; indeed, to gain some
idea of the effect of a global dust cloud, Luis had to rely on the
century-old Krakatoa report of the Royal Society. But over the last
couple of decades, the science of computer modeling of impact ex-
plosions has made great strides, such that it is now possible to say
more confidently what the actual effects would be, though not how
they would all interact with each other and with living organisms.
(The discussion in the next dozen paragraphs is taken largely from
the work of modeler Brian Toon and his colleagues.
62
) To take the
subject from theory to practice, in July 1994 the entire world saw an
actual planetary impact when the fragments of Comet Shoemaker-
Levy 9, some estimated to be 2 km in diameter, collided with Jupiter.
To the delight of Gene Shoemaker, the effects were even more spec-
tacular than the impact modelers had predicted.
According to the Alvarez theory, 65 million years ago a comet or
asteroid 10 km to 15 km in diameter approached the earth (we do
not know which it was, but either would have had the effects I am
about to describe). It was traveling at cosmic speeds somewhere
between 20 km and 70 km per second and for that reason carried
with it an energy on the order of 10
3
1
ergs, or 100 million megatons
of TNT (100,000,000,000,000 tons of TNT), far more energy than
contained in all the world's nuclear weapons at the height of the
Cold War. Once the object struck, that amount of energy had to be
dissipated. An almost irresistible force was about to meet an immov-
able object.
As we saw when Shoemaker-Levy 9 struck Jupiter, a meteorite
entering a planetary atmosphere at cosmic velocities generates a
giant shock wave—a kind of cosmic backfire—that sends a 20,000-
degree jet of flame thousands of kilometers back up the incoming
trajectory. In the largest impacts, the entire atmosphere in the vicin-
ity of the entry point is blasted into space.
The midair explosion of a meteorite at Tunguska in Siberia in
1908 and the eruption of Mount St. Helens in 1980 were strong
