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 IMPACT
CRATERS
Solid surfaces throughout the Solar System are peppered
with round craters formed by meteorite impacts.
Craters form when large meteorites crash into planets with
velocities faster than the speed of sound. The high energy of
the impact causes an explosion, and leaves behind a circular
scar. Formation of a crater is complete within just a few seconds
after the impact
Small craters around 1 kilometer across are simple bowl-shaped
structures. Meteor Crater in Arizona (pictured above) was created
about 30,000 years ago when an iron meteorite the size of a
house crashed into the desert. The crater is more than a kilometer
in diameter. The crater formed within seconds of the meteorite
impact.
Larger craters, tens to hundreds of kilometers across, are
more complicated. The rock from the center of the crater bounces
back from the impact and forms a peak called a "central
uplift." Giant craters, around 1000 km across, form multiple
rings and look like huge geological bulls-eyes. Craters are
about ten times as wide as the meteorites that create them.
| [pictured
above] The highlands of the Moon are covered with
impact craters. The surface of the Moon is ancient and craters
have built up over billions of years. Being so close to
the Moon, the Earth has probably been bombarded at the same
rate. |
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|  CRATERS
EVERYWHERE
It's easy to see with your own eyes that the Moon is covered
with craters. Telescopes and spacecraft have shown us that every
solid surface in the Solar System — each comet, asteroid,
moon and planet — has craters too.
 The
Earth only has about 250 identified impact craters. Our thick
atmosphere protects us from many impacts. More importantly, the
Earth's surface continually renews itself through erosion and
plate tectonics, so there are no very old surfaces that accumulate
craters. By learning about the craters on old surfaces of other
planets, we can understand what was happening on the ancient Earth.
The number of impact craters on a planet's surface relates to
its age. On an old surface, there are many impact craters, and
on a young surface there will only be a few. If we count the number
of craters on different geologic surfaces of planets we can figure
out which surface is older. Crater counting tells us the relative
ages of surfaces, but we can only tell exactly how old a surface
is if we date a rock in a laboratory.
| [pictured
top] The southern hemisphere of Mars has been bombarded
with meteorites for billions of years. This image, taken
by the Viking Orbiters, shows an area of the southern hemisphere
of Mars that is 1250 km across. Photo courtesy of the Lunar
and Planetary Institute.
[pictured
above]Young regions of Mars do not have so many craters.
The volcano in the image is Olympus Mons, an enormous volcano
that is larger than the state of Arizona and three times
as high as Mount Everest. There are very few impact craters
on Olympus Mons which shows that the topmost lava flows
are relatively young. The photo was taken with the Mars
Orbiter Camera in 1998. Courtesy NASA/JPL-Caltech |
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 ESCAPE!
Rocks that are thrown off asteroids and planets in impact
events become meteorites.
The violent force of a large impact can throw rocks upwards with
such speed that they can escape the asteroid or planet's gravity
and become launched into space. We know that pieces of the Moon
and Mars have been launched into space in this way because we
find lunar and martian meteorites on the Earth. It is not possible
to launch a piece of the Earth or Venus into space, because these
planets are so big that it is difficult to accelerate a piece
of rock to escape velocity.
| [pictured
above] Martian meteorite Zagami was launched off the
surface of Mars following an impact, 3 million years ago.
It fell to Earth in Zagami, Nigeria in 1962. Zagami is a basalt
that formed in a lava flow on Mars 180 million years ago. |
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| THE
SHOCKING STORY
Impacts change the minerals inside the target rocks as well
as the rocks themselves.
Mineral grains in impact rocks can be crushed, shattered and
melted. Individual crystals can sometimes show distinctive features,
like thin straight cracks, that can only be caused by shock. If
the pressure is high enough, new minerals can form.
 Broken
fragments of many rocks inside a crater are often mixed back together
to make a new rock. These rocks are called breccias. Some rocks
melt and flow together, creating impact glass and a rock called
suevite. Shatter cones are rocks that have been shaped by
the strong pressures of the shock waves that surge through the
ground around an impact crater.
In a very large crater, rock in the center of the crater is melted.
Some of the molten rock can be splashed out of the crater and
transported hundreds of kilometers away from the impact site.
Melt drops solidify as glasses. Small, millimeter-sized glass
beads from the Chixculub impact site in Mexico are found thousands
of kilometers away. Larger glass drops, called "tektites",
are found hundreds of kilometers away from their original craters.
An example is the tektites known as moldavites that originated
at the Ries crater in Germany and are found in the Czech Republic.
[top left]
Shatter cones from the Wells Creek crater were created in the
aftermath of a large impact. These are original rocks from the
Earth that were shaped by the waves of high pressure resulting
from the impact.
[above left] High temperatures following
a large impact in the Libyan desert melted rocks in the crater.
When molten rock cools down quickly it turns into glass. This
specimen is a solid piece of green colored glass.
[above left] Tektites like this one
from Indochina are droplets of melted rock from a large meteorite
impact. Tektites are flung hundreds of kilometers from the impact
site.
[left] This rock is a jumble of rock
fragments from the Ries impact crater in Germany. It is called
a "suevite" because it contains pieces of impact glass
as well as rock fragments. |
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|  IMPACT
ON LIFE
Large meteorite impacts have caused extinctions of many forms
of life such as the dinosaurs.
The formation of a large impact crater is a violent event. Massive
amounts of dust are thrown into the atmosphere, blocking out heat
from the sun. Heat from the explosion scorches the land for thousands
of kilometers. Plants and animals near the crater are killed immediately,
and very large impacts can force drastic climate changes that
cause the extinction of entire species.
The Chicxulub crater in Mexico was just such a large impact event.
This crater formed 65 million years ago, at the height of the
age of dinosaurs. A meteorite about 10 km in size crashed into
layers of sedimentary rocks. The rocks vaporized, releasing water
vapor, carbon dioxide, and sulfur that caused drastic changes
to the atmosphere. This rapid change caused 60% of the world's
plants and animals, including the dinosaurs, to become extinct.
Fortunately, these large events are rare on the earth, happening
only about once in 100 million years.
| [pictured
above] Debris from the huge Chixculub impact crater
in Mexico is found all around the Earth. It is found in a
thin layer of sedimentary rock at the boundary between the
Cretaceous and Tertiary geological eras (the "K/T boundary").
The photo shows the K/T boundary layer in northern New Mexico,
2000 km away from the impact site. |
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