Meteors and Meteorites
- Meteors are tiny fragments of rock (usually millimeters
in size or smaller) which strike the Earth's atmosphere at high speed
and are incinerated
before reaching the ground.
- The Earth collects hundreds of tons of this material every day!
- Occasionally the fragments are large enough
to survive the fiery passage through the atmosphere. Fragments
which reach the ground are called meteorites.
- Meteorites are pieces of asteroids (and occasionally of the Moon and Mars) produced by impacts. The pieces eventually cross paths with the
- Since some asteroids are large enough to have melted
and become differentiated
their fragments can even consist of chunks of pure iron broken from an exposed iron core.
- Types of meteorites:
- Stones: 93% of falls
- Mainly rocky in composition with some small metallic
- The rocky component is largely made up of fused
small glassy beads called chondrules thus the name chondrites.
- Irons: 6% of falls (but the majority of ``finds"
because they are so recognizable as meteorites (i.e. they
don't look like rocks!).
- Solid chuncks of refined iron and nickel.
- The metal shows a crystal pattern indicative of
slow cooling over millions of years -- consistent
with this material once being inside a large (and thus slowly
- Carbonaceous Chondrites: 1%
- The carbonaceous chondrites represent some of the most
unprocessed, and thus original, material from the
asteroid belt. They appear to come from asteroids that
never became hot enough to melt and differentiate.
- Some carbonaceous chondrites contain amino
acids -- the primary building blocks of life.
- < 0.1% - "Special" falls - pieces of Mars, the Moon, the asteroid Vesta, and ???
- Meteorites are found in two ways
- Directly associated with the fall of meteorites from
a bright fireball (i.e. fresh).
- Recognized as different when seen lying on the ground
due to their distinct appearance.
- Irons are particularly recognizable and, although they
are rare as falls, they are the majority of "finds".
- Meteorites are easiest to find in places where rocks are
rare -- the Sahara Desert and Antarctica for example.
- Meteors and meteorites span a range of sizes.
- Millions of tiny ones -- the size of a grain of sand --
hit the Earth each day.
- Large ones -- kilometers in size -- are rare. They
hit the Earth every few millions of years.
- Many known asteroids have orbits which cross inside
the orbit of Mars (Amor class) and Earth (Apollo class).
- Many of these will eventually impact the Earth or Mars.
- Meteorites are of fundamental importance to astronomers because they
represent material largely preserved from the epoch of the formation of
the solar system.
- Meteorites uniformly have an age of 4.6 billion years.
Revised May 9, 2001