Meteors and Meteorites
Meteorite Facts
- 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.
- 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.
- Meteorites are classified as:
- carbonaceous chondrites: very similar in composition to the Sun
less volatiles; similar to type C
asteroids (picture 4);
- 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);
- irons: primarily iron and nickel; similar to
type M asteroids (picture 3);
- stony irons: mixtures of iron and stoney material like type S
asteroids;
- achondrites: similar to terrestrial basalts; the meteorites
believed
to have originated on the Moon and
Mars are achondrites
(picture 2)
- 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.
- 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.
- There are probably at least 1000 asteroids larger than 1 km in diameter that
cross the orbit of Earth.
- 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.
- 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
Chondrite Meteorite
186k gif
Achondrite Meteorite
178k gif
- (above) Iron Meteorite
193k gif
part of the Allende fall, a carbonaceous-chondrite
30k jpg
Martian meteorite
212k gif
a piece of 4 Vesta
78k jpg;
376k gif;
caption
Barringer Crater
308k gif;
107k jpg;
2000k tif
More about Meteorites
Open Issues
- Was it an small asteroid or something else that caused
the Tunguska fireball over central Siberia in 1908?
- 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