Meteors and Meteorites
Meteorite Facts
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Meteorites are bits of the solar system that have fallen to the Earth.
Most come from asteroids, including few are believed to have come specifically
from 4 Vesta (picture 6); a few probably come from comets.
A small number of meteorites have been shown to be of Lunar
(11 finds) or Martian
(12) (picture 5) origin.
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Though they are just boring looking rocks, they are extremely important
in that we can analyze them carefully in our labs. Aside from the few kilos
of moon rocks brought back by the Apollo
and Luna
missions, meteorites are our only physical evidence about the universe
beyond the Earth.
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Meteorites are classified as:
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carbonaceous chondrites: very similar in composition to the Sun
less volatiles;
similar to type C asteroids (picture 4);
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ordinary chondrites: by far the largest number of meteorites fall
into this class; similar in composition to the mantles and crusts of the
terrestrial planets (picture 1, above);
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irons: primarily iron and nickel; similar to type M asteroids (picture
3);
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stony irons: mixtures of iron and stoney material like type S asteroids;
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achondrites: similar to terrestrial basalts; the meteorites believed
to have originated on the
Moon and Mars
are achondrites (picture 2)
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A very large number of meteoroids enter the Earth's atmosphere each day
amounting several hundred tons of material. But they are almost all very
small, just a few milligrams each. Only the largest ones ever reach the
surface to become meteorites.
-
The average meteoroid enters the atmosphere at between 10 and 70 km/sec.
But all but the very largest are quickly decelerated to a few hundred km/hour
by atmospheric friction and hit the Earth's surface with very little fanfare.
However meteoroids larger than a few hundred tons are slowed very little;
only these large (and fortunately rare) ones make craters.
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A good example of what happens when a small asteroid hits the Earth is
Barringer Crater (a.k.a. Meteor Crater) near Winslow, Arizona (picture
7). It was formed about 50,000 years ago by an iron meteor about 50
meters in diameter. The crater is 1200 meters in diameter and 200 meters
deep. About 120 impact craters have been identified on the Earth, so far.
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There are probably at least 1000 asteroids larger than 1 km in diameter
that cross the orbit of Earth.
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The impact of a comet or asteroid about the size of Hephaistos or SL9
hitting the Earth was probably responsible for the extinction of the dinosaurs
65 million years ago.
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Calculations based on the observed number of asteroids suggest that we
should expect about 3 craters 10 km or more across to be formed on the
Earth every million years. This is in good agreement with the geologic
record. It is more difficult to compute the frequency of larger impacts
like the one 180 km in diameter near Chicxulub in the Yucatan Peninsula;
once per 100 million years seems like a reasonable guess.
Pictures
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Chondrite
Meteorite 186k
gif
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Achondrite
Meteorite 178k
gif
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(above) Iron Meteorite 193k
gif
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part of the
Allende fall, a carbonaceous-chondrite 30k
jpg
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Martian meteorite
212k
gif
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a piece of 4 Vesta
78k jpg;
376k gif;
caption
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Barringer Crater
308k
gif; 107k
jpg; 2000k
tif
More about Meteorites
Open Issues
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Was it an small asteroid or something else that caused the Tunguska fireball
over central Siberia in 1908?
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Was it a comet or an asteroid that caused the Chicxulub crater in the Yucatan
(and probably caused the extinction of the dinosaurs)?
... Sun
... Small
Bodies ... Ida
... Meteorites ... Medium
...
Bill
Arnett; last updated: 1995 August 23