The
Barringer Crater shown in the adjacent image
is only the best-preserved of large meteor impacts. There is evidence for many
more. Here is a link to some views of terrestrial
impact craters. Although in many instances
these impact structures have been partially obliterated by erosion and
tectonics, there is rather strong evidence that most are fossil impact craters.
Meteorite Impact Velocities
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The average velocity of meteoroids entering our atmosphere is 10-70 km/second. The smaller ones that survive the trip to the Earth's surface are quickly slowed by atmospheric friction to speeds of a few hundred kilometers per hour, and so hit the Earth with no more speed than if they had been dropped from a tall building. For meteorites larger than a few hundred tons (which fortunately are quite rare), atmospheric friction has little effect on the velocity and they hit the Earth with the enormous speeds characteristic of their entry into our atmosphere. Thus, for example, it is estimated that the meteorite that produced the Barringer Crater was still travelling at 11 km/second when it struck what is now the Arizona desert 49,000 years ago. Such objects do enormous damage, because the kinetic energy carried by the meteorite is the product of the mass and the square of the velocity.
There is no documented record of a human being killed by a meteorite, but in
recent years meteorites have crashed into bedrooms in Alabama, dining rooms in
Connecticut, and a car in New York.
The Peekskill Meteorite and other Fireballs
On October 9, 1992, a fireball was seen streaking
across the sky from Kentucky to New York. At least 14 people captured
part of the fireball on videotape; here is a spectacular MPEG
movie (1 MB MPEG) of the fireball. A 12-kilogram stony meteorite (chondrite)
from the fireball fell in Peekskill, New York, smashed the trunk of a
parked automobile, and came to rest beneath it. These are the first motion
picture recordings of a fireball with an associated meteorite fall. Here
is a short MPEG movie of a -4 magnitude Delta Capricornid fireball (253
kB MPEG) recorded near Hannover, Germany, on January 5, 1995.
The K-T Event and the Extinction of the Dinosaurs
Sixty-five million
years ago, about 70 percent of all species then living on Earth disappeared
within a very short period in what is termed the Cretaceous-Tertiary Mass
Extinction---commonly known as the K-T Event (K is used to denote the
Cretaceous period rather than C to avoid confusion with other periods such as
the Cambrian). Among the species that disappeared were the last of the
dinosaurs. The cause of this and other sudden species extinctions has long been
an important and controversial topic.
In 1980, physicist Luis Alvarez and coworkers reported finding a very high concentration of the element iridium in the sedimentary clay layer laid down at the time of the K-T extinction. On Earth, iridium is very rare in the crust because it was concentrated in Earth's core when it was largely molten. However, chondritic meteorites often still have the primordial solar system abundances of these elements. This led Alvarez et al to suggest that a chondritic asteroid 10 kilometers in diameter that struck the Earth in the K-T period would contain enough iridium to account for the worldwide clay layer iridium enhancement, and that this meteor impact could also have triggered dramatic climatic changes that produced the K-T extinction.
The Crater Associated with the K-T Extinction?
Since the original proposal, additional evidence has accumulated that strongly supports the impact theory. However, the theory remains contentious, and opponents had long argued that there was no evidence for the requisite crater associated with the impact. In 1990, it was realized that geophysical data taken 10 years earlier in a search for oil in the Yucatan region of Mexico indicated a 180-kilometer diameter ring structure centered on the present coastline and lying beneath several hundred meters of sedimentary deposits called Chicxulub that had the requisite characteristics.The Chicxulub structure has been age dated at 65 million years, and additional studies support its interpretation as a relic impact crater Although the original crater has been eroded and covered with sediment, models suggest that shortly after formation the Chicxulub crater may have resembled the crater Strindberg on Mercury.
Summary: The Asteroid and the Dinosaur
Thus, it has been proposed that 65 million years ago in what is now Yucatan the impact at a velocity of 11 km/second of a 10-kilometer wide asteroid threw huge amounts of matter into the atmosphere (in addition to local phenomena like generating 2000 foot waves that may have emptied the Gulf of Mexico!). This created months of darkness (interfering with photosynthesis) and much cooler temperatures globally, and the resulting harsh conditions led to the extinction of many species, including the last of the dinosaurs.Although this is a compelling hypothesis with substantial support, it remains controversial and has broad but not yet uniform acceptance within the scientific community. It is estimated that impacts of asteroids as large as the one thought responsible for the K-T extinction occur about once every hundred million years .
Information supplied by: http://csep10.phys.utk.edu