Geoscience Reference
In-Depth Information
TABLE
I
Number of Craters of Different Diameters Produced During Last 100
Million Years, Based on Astronomical Observations.
Crater diameter
>20km
>30km
>50km
>60km
>100km
>150km
Asteroid source
190
58
8
3.2
0.3
0
Comet source
60
24
8
5
1.6
1
Total
250
82
16
8
2
1
noted in Chapter 1, is that by observing comets and asteroids
astronomers can calculate how often one of a certain size is apt to
hit the earth. Note from Table 1, from the work of Shoemaker and
his colleagues, that the larger the crater, the more likely that it was
formed by a comet. Astronomers and crater scientists usually ex-
press the frequency of crater formation as the number of craters
larger than 20 km that form during a 100-million-year period on
each 10 million km
2
of earth surface. Based on his astronomical ob-
servations, Shoemaker estimates the rate (for asteroids and comets
combined) at 4.9 ± 2.9. In other words, Shoemaker reckons that
during the last 100 million years, for each 10 million km
2
of earth
surface (the earth has a total surface area of 500 km
2
), 4.9 ± 2.9 (or
roughly between 2 and 8) craters larger than 20 km have formed.
From his observations of terrestrial craters, Richard Grieve of the
Geological Survey of Canada obtains a rate of 5.5 ± 2.7, basically
the same number as Shoemaker's. In recent years, Shoemaker and
his wife Carolyn, when not searching for comets, were apt to be
found camped in the Australian outback, mapping ancient impact
craters. It was on such a field trip in July 1997 that he lost his life in
a tragic automobile accident. Based on his most recent mapping and
age dating, Shoemaker's last estimate of the frequency of impact for
the old Australian craters, presented at a conference at the Geologi-
cal Society of London in February 1997, was 3.8 ± 1.9. For impacts
on the moon, including the lunar farside, it is 3.7 ± 0.4. It is simply
astounding that all these rates, measured from the back of the moon
to the outback, give the same answer.
Reason requires that we acknowledge that meteorite impact has
been an inexorable fact of life in our solar system and on our planet.
But even if we admit that, it does not tell us why meteorites are so
terribly destructive. How can a ball of ice—even one much larger
than the snowballs we hurled during our childhood with little ef-
fect—create a crater as large as those excavated at sites of nuclear
