Geoscience Reference
In-Depth Information
which travel through space at around 45 km/sec to 60 km/sec, cause
a significant fraction of all impacts (see Table 1, page 51). Asteroids
move at slower velocities, averaging about 20 km/sec. Since kinetic
energy is proportional to velocity squared, this three-fold difference
in speed means that a crater of a given size can be produced by a
comet one-ninth the size of the asteroid required to produce that
same crater. Thus an impact crater produced by a comet would leave
no more than one-ninth the iridium to be found in a crater of the
same size formed by an asteroid. But even this is an upper limit.
Comets, being as much as 50 percent ice, carry much less iridium to
start with (calculations that combine crater size and composition
show that the amount of iridium left by a comet might be as low as
1 percent of that left by an asteroid, too little to be detected). As
Table 1 shows, larger craters are successively more apt to have been
formed by comets (one of the reason most specialists now believe
that Chicxulub was formed by the impact of a comet). Thus it is
ironic but true that the larger the crater, the less likely it is to leave
iridium behind. To carry matters further, in the largest impacts,
regardless of impactor type, almost all the ejecta is blasted back out
into space, escaping the earth's gravity field altogether and leaving no
trace behind.
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Still another complication is that in smaller impacts,
extraterrestrial material composes only about 10 percent of the
ejecta, so that if the impactor happened to be an asteroid relatively
low in iridium, which some are, little would be left to find. Finally,
even when iridium was present initially, reworking and bioturbation
could have smeared it out, or acid leaching could have removed it.
All in all, iridium is a kind of one-way indicator: Its presence is strong
evidence of impact; its absence is not evidence of no impact.
Impact by either comets or asteroids, however, would leave be-
hind shocked minerals and possibly spherules, maybe even spinel and
diamond. Since these markers are less subject to alteration or removal
by chemical and geologic processes, they make a better bet as indica-
tors of impact than iridium. Most geologists continue to be most im-
pressed by shocked quartz, the indicator that they discovered.
Finding a crater that dates to the time of a geologic boundary is
fraught with the same difficulties that we encountered in the search
for Chicxulub. Erosion will have erased most impact craters; others
will have disappeared down subduction zones. The older the crater,
the more likely one of these fates. Most crater ages are not known
with precision, making it difficult to assign them to a given geologic
boundary with much confidence. The Manson crater in Iowa is a
good example. For years its age was known only roughly, then the
first measurement gave 65 million years, and finally more precise
