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The quick flashes of light in the sky most people call ``shooting stars'' are
meteors---pieces of the rock glowing from friction with the atmosphere as
they plunge toward the surface. Most of the meteors you see are about the size of a
grain of sand.
More meteors are seen after midnight because your local part of the
Earth is facing the direction of its orbital motion around the Sun. Meteoroids
moving at any speed can hit the atmosphere. Before midnight your local part of
the Earth is facing away from the direction of orbital motion, so only the fastest
moving meteoroids can catch up to the Earth and hit the atmosphere. The same
sort of effect explains why an automobile's front windshield will get plastered
with insects while the rear windshield stays clean.
If the little piece of rock makes it to the surface
without burning up, it is called a meteorite.
There are three basic types of meteorites.
- Stones: they are made of silicate material with
a density around 3 (relative to the density of water) and look
like ordinary Earth rocks. This makes them hard to distinguish from Earth rocks so
they don't stand out. About 95 to 97% of the meteorites are of these type. About 85%
of the stones are primitive, unchanged since they first solidified about
4.6 billion years ago. Most of the primitive stones have
chondrules---round glassy structures 0.5 to 5 millimeters across embedded in
the meteorites. They are solified droplets of matter from the
early solar nebula and are the oldest part of a primitive meteorite.
A
meteorite containing chondrules (courtesy of the
Planetary Data Center).
The oldest of the stone meteorites are the carbonaceous meteorites. They
contain silicates, carbon compounds (giving them their dark color), and a
surprisingly large amount of water (about 22%). They are probably chips of C-type
asteroids. Some of the carbonaceous meteorites
have organic molecules called amino acids. Amino acids can be connected together
to form proteins that are used in the biological processes of life. There is the
possibility that meteorites like these may have been the seeds of life on the Earth.
In addition, these type of meteorites may have provided the inner planets with a lot
of water. The terrestrial planets may have been so hot when they formed that most of
the water in them at formation evaporated away to space. The impact of millions to
billions of carbonaceous meteorites in the early solar system may have replenished
the water supply on the terrestrial planets.
About 10 to 12% of the stones are from the crust of differentiated parent
objects. Therefore, they are younger (only 4.4 billion years old). The
lighter-colored stones are chips from the S-type of asteroids.
- Stoney-Irons: only 1% of the meteorites are of this
type. They have a variable mixture of metal (iron and nickel) and rock (silicates)
and have densities ranging from 4 to 6 (relative to the density of water).
They come from a differentiated object at the boundary between the metal core
and the rock crust. They are 4.4 billion years old.
- Irons: although they make up about 40% of the
meteorites found worldwide, only 2 to 3% of the meteorites are these type. They make
up so many of the ones found because they are easily distinguished from Earth rocks.
They are noticeably denser than Earth rocks, they have a density around 7.
They come from the core of a differentiated body and are made of iron and nickel.
They are 4.4 billion years old. Irons
sometimes have large, coarse-grained crystalline patterns
(``Widmanstatten patterns'') that is evidence that they cooled slowly.
An
iron meteorite (courtesy of the
Planetary
Data Center).
The primitive meteorites are probably the most important ones because they hold
clues to the composition and temperature in various parts of the early solar nebula.
Most stoneys look like Earth rocks and so they are hard to spot. The rare irons are
easy to distinguish from Earth rocks and make up most of the ones found worldwide.
Usually the meteorites science museums show off are iron meteorites. Not only does
their high density and metal composition set them apart from ordinary rocks, the
iron meteorites are stronger. This means they will more likely survive the passage
through the atmosphere in one piece to make impressive museum displays. The stone
meteorites are more fragile and will break up into several pieces (less impressive
for museum displays).
To get an accurate number for the proportion of meteorites that fall to the Earth
(an unbiased sample),
meteorite searchers go to a place where all types of rocks will stand out. The best
place to go is Antarctica where the stable, white ice pack makes darker meteorites
easy to find. Meteorites that fell thousands of years ago can still be found in
Antarctica without significant weathering. Since the 1980's thousands of meteorites
have come from here. For further exploration, check out the
Antarctica Meteorite web site at the
Planetary Materials Curation office of NASA and the
ANSMET site
at Case Western Reserve University.
Meteorites stand out against the snow and ice background of
Antarctica.
Most meteorites are pieces of asteroids, but
a few are from the Moon. A select few, the Shergotty-Nakhla-Chassigny (SNC)
meteorites, may be from Mars. The relative abundances of
magnesium and heavy nitrogen (N-15) gases trapped inside the SNC meteorites is
similar to the
martian atmosphere as measured by the Viking landers and unlike any meteorites
from the asteroids or Moon. Also, the isotope ratios of argon and xenon gas trapped
in the meteorites most closely resemble the martian atmosphere and are
different than the typical meteorite. The analysis of the soil and rocks by the
Mars Pathfinder
confirm this.
A meteorite blasted from Mars.
Most SNC meteorites are about 1.4 Gyr old, but the
one with
suggestions of extinct
Martian life is about 4.5 Gyr old. The discovery was published in the August 16,
1996 issue of the journal
Science.
Non-subscribers can find a copy of the article
here.
Recent studies of the meteorite have cast considerable doubt on the
initial claims for fossil microbes in the rock. There is strong evidence of
contamination by organic molecules from Earth, so this meteorite does not
provide the conclusive proof hoped for.
A detailed description of SNC meteorites is given at
JPL's Mars Meteorites web site.
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last updated:
June 7, 2007
Is this page a copy of Strobel's
Astronomy Notes?
Author of original content:
Nick Strobel