METEORITE OR METEORWRONG?
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Meteoroids* enter the atmosphere at speeds of many miles per
second. At those tremendous speeds, the air in the path of the meteorite
is severely compressed. When air is compressed rapidly, its temperature
increases (like air in a bicycle tire pump). This hot air causes
the exterior of stony meteoroids to melt. The melted portion is
so hot and fluid that it immediately ablates (sloughs off) and new material
is melted underneath. A meteoroid can lose most of its mass as it passes
through the atmosphere. When it slows down to the point where no melting
occurs, the last melt to form cools to make a thin, glassy coating called
a fusion crust. On stony meteorites, fusion crusts are seldom more than
1 or 2 mm thick. Except for some lunar meteorites (less that 1 in 1000
of all meteorites), fusion crusts are not distinctly vesicular
- there are no bubbles. Some fusion crusts will show flow features; others
may cover regmaglypts.
* Before it enters the atmosphere, it is a meteoroid -
a small rock orbiting the sun. The visible light seen as it passes through
the atmosphere is a meteor. After the rock lands, it is
One of atmospheric entry is that any corners,
edges, or protuberances are the first parts to ablate away. The
result is that a meteorite is rounded and aerodynamic in shape.
Unlike many stones found on a beach or in a river, meteorites
seldom have symmetrical or spheroidal (oblate, prolate) shapes.
One of the
Camel Donga stones from Australia.
(Photo courtesy of Jim Strope)
These two meteorites (left and above) are from Antarctica.
Both stones are fragments of larger meteorites.
The shiny fusion crust is evident in both.
Some meteoroids break apart as they pass through the atmosphere
or when they hit the Earth's surface. Stones from such meteoroids
might have sharp edges and corners, but usually one side is still
smooth and glassy. The interior of a meteoroid that breaks apart
after passing through the atmosphere will not have a fusion
Above and below:
On these two meteorites, both ordinary chondrites
from the Sahara desert, some of the fusion crust has flaked away.
Note that the fusion crust is darker than the underlying material.
Even though the meteorites
in these photos have been on Earth for hundreds or thousands of
years, the fusion crusts are still shiny. For meteorites found in
temperate environments where it rains more often, however, fusion
crusts may not be so shiny and black (see, e.g., Dimmit and
Meteorite fusions crusts consist of glass, but the underlying
material is crystalline and sometimes weaker than the crust. As
a consequence, the fusion crust sometimes flakes off if a meteorite
has been on Earth a long time. Most terrestrial weathering crusts,
varnishes, and rinds do not flake like this, so the "flakiness"
characteristic is an important characteristic by which to recognize