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 as a result of descending
through the Earth's 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 to find meteorites is in Antarctica
because they are easy to spot against the white landscape.
Thousands of meteorites have been recovered from Antarctica.
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.
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