Background Information on Meteorites
(by Horton and Joan Newsom)
A meteorite is a piece of rock or metal from outer space that has survived its descent through
the Earth’s atmosphere. Most meteorites are from the asteroid belt, which is between Mars and
Jupiter. In contrast, a comet is a dirty snowball of ice and dust. Most comets orbit around the Sun
and spend most of their time far away from the sun. Asteroids and comets are left over from the
formation of the solar system 4.56 billion years ago, from a cloud of gas and dust called the solar
nebula. Asteroids formed between Mars and Jupiter, but comets formed farther from the sun, where
ices were stable. Scientists believe that the gravitational pull of Jupiter prevented the asteroids from
combining to form a planet. A few meteorites are from the Moon and Mars, and were ejected from
these planets by large impact events. The meteorites from the Moon are called lunar meteorites, and
those from Mars are called Martian meteorites, or sometimes they are called by the abbreviation
“SNCs” named after some of the meteorite types. They are very interesting because they provide
information about the composition of the Moon and Mars.
Small (dust to pea-sized) pieces of rock and metal that enter the atmosphere appear as
meteors (shooting stars) and meteor showers. After floating through space, the rocks enter the
Earth’s atmosphere and the outsides of the rocks get very hot and melt because they are traveling
over 25,000 miles per hour. Meteors can form from asteroid or comet debris. Larger pieces, which
survive their descent through the atmosphere, and land on Earth, are called meteorites. Just as
marshmallows get burnt on the outside, meteorites develop a black fusion crust from the friction of
descending through the Earth atmosphere. Meteoroids or comets, large enough to form impact
craters the size of a house or larger, are rare. Meteor crater in Arizona formed 20-50,000 years ago.
The diameter of Meteor Crater is 1.2 kilometers in diameters and more than 180 meters deep.
Meteorites fall everywhere on Earth. There might even be one in your backyard! One of the
best places, however, to find meteorites are in Antarctica because they are easy to spot against the
white background. Thousands of meteorites have been recovered from Antarctica.
Types of meteorites - Traditionally, meteorites are classified into three broad categories
based on their composition:
Stones
Stony meteorites or stones contain largely silicate minerals, which are minerals that contain
silicon (Si) and oxygen, O. Stony meteorites can be very similar to Earth rocks, which also often
contain silicate minerals. However, stones also contain some metallic iron-nickel and are thus
unique. Most stones are chondrites, pronounced “kondrites”. Most chondrites contain chondrules,
which are small, spherical silicate objects on the order of a millimeter in size. Scientists are
interested in chondrites because their chemical composition reflects that of the solar nebula, which is
the cloud of gas and dust that condensed and accumulated to form the sun and the planets. The
chondrules were formed in the nebula by a rapid heating process, possibly lightning, or other
processes.
Another type of stony meteorite is the achondrites (meteorites without chondrules). These
are actually igneous rocks from magmas that formed on asteroids. These asteroids became hot
enough to melt from radioactive elements just after the solar system formed.
Irons
Iron meteorites or irons are composed mainly of iron alloyed with nickel and cobalt, and they
usually contain a sulfide mineral (like fools gold) troilite (FeS). Irons are the meteorites that are most
easy to identify because they are heavier than most rocks, are magnetic, and are made of iron and
nickel metal. Just like the Earth which has an iron-nickel core, the composition of irons indicates that
they represent the cores of their parent asteroids. The irons also come from asteroids that melted.
Stony-Irons
The stony-iron meteorites or stony-irons are composed of approximately equal amounts of
rocky material and metallic iron-nickel. What is the difference between stones, irons, and stony-irons.
If an asteroid, like the Earth, contains a core, mantle, and crust then the irons would be from the core,
the stones from the crust and the stony-irons from in between the core and mantle.
Why Study Meteorites?
Meteorites provide information about the early history of the solar system. The primitive
meteorites contain clues about the composition of the solar nebula and how the solar system formed
from the solar nebula. Meteorites from the Moon and Mars provide material from those planets
without a space mission.
Impact Craters
During the early history of the solar system, the Earth was covered with impact craters and
probably looked like the Moon. Over the last few billion years, erosion and mountain building have
erased most of these original craters. Because there is less erosion and mountain building on the
Moon, Mars, asteroids, and other moons, these craters remain. Large impact craters known on the
Earth range up to several hundred miles in diameter. During the formation of an impact crater, large
amounts of rock can be melted, and some melted material, called tektites, can be thrown completely
out of the atmosphere before reentering and falling back to Earth, far from the crater.
Vocabulary
Solar System — The sun, the nine planets, asteroids, and comets that revolve around the sun.
Meteor— The flash of light that we see in the night sky caused by the friction of a meteoroid passing
through the atmosphere.
Meteoroid— An interplanetary chunk of matter that is smaller than a kilometer in diameter and most
frequently measured in millimeters.
Meteorite— Any part of a meteoroid that that survives its fall through the atmosphere and lands on
the Earth.
Asteroid— A small solar system object composed mostly of rock. Many of these objects orbit the sun
between Mars and Jupiter. Their size can range anywhere from 10 meters in diameter to less than
1000 kilometers.
Comet— A small solar system object consisting of ice and other compounds that form a coma and
sometimes a visible tail whenever they orbit close to the Sun.
Comet Debris — Clumps of dust that are released from comets as they approach the sun.
Chondrite— A meteorite containing chondrules, which are small, spherical silicate objects on the
order of a millimeter in size that formed in the solar nebula before the asteroids were formed.
Achondrite— A meteorite consisting of igneous minerals and no chondrules.
Meteorites.doc Rev1, 6/04