Geoscience Reference
In-Depth Information
would have made it through. At the other extreme, tiny shooting
stars burn up in the atmosphere and thus have no effect. To be
credible, the size of the putative impactor would have to be much
smaller than a planet and much larger than a shooting star.
The size of the alleged K-T meteorite could no longer be mea-
sured directly—the impact explosion would have blasted it to
pieces. But what of the residue it might have left behind, the iridium
in the Gubbio clay layer? If the impact event had worldwide effects,
approximately the same amount of iridium as found at Gubbio
would have been deposited in a layer that extended all around the
earth, coating its entire surface. Knowing both the amount of irid-
ium in the Gubbio clays and the size of the surface area of the earth,
the Alvarez team calculated that about 200,000 tons of iridium had
been emplaced. Since they knew the average iridium content of
meteorites, they were then able to figure out how large a meteorite
would have been required to deliver that much iridium. Using rea-
sonable assumptions as to density and shape, the answer was a mete-
orite about 6.6 km in diameter. Applying the same technique to the
Danish clays gave about 14 km. That the two estimates agreed
within about a factor of 2 was encouraging at this rough level of cal-
culation. Averaging them gave 10 ± 4 km, neither as large as a planet
nor as small as the pip-squeaks that produce shooting stars, and well
within the credible range. The figure of 10 km has become accepted
as the diameter of the Alvarez impactor. That happens to be about
the elevation of Mt. Everest, the earth's highest mountain. Imagine
that Everest, instead of standing above the already lofty Himalayan
plateau, rose straight from the sea to its height of over 29,000 feet.
Now imagine a mountain of that size approaching the earth at a
speed of 100,000 miles per hour. No thanks!
The Alvarezes next compared a meteorite 10 km in diameter
with three observational facts:
I . Asteroids (solid rocklike meteorites] and comets (balls of dirty
ice), either of which could have produced the impact, in the
range of 5 km to 10 km in diameter are relatively plentiful in
space and are routinely observed through telescopes.
2
. Estimates based only on astronomical observations show that
an asteroid or a comet 10 km in diameter should strike the
earth about every 100 million years, so having one hit 65 mil-
lion years ago but none since would fit the observations (see
Table 1).
3.
Over 150 terrestrial impact craters are known; from their size
and frequency, crater experts estimate that a 10-km object
