| At
the moment of impact, an asteroid essentially explodes,
excavating a crater. The size and structure of the crater
depend mostly on the kinetic energy of the object that
created it, and to some extent on the angle of impact
and the gravity and composition of the target body.
An object's kinetic energy depends on its mass
and speed. A typical speed for asteroids approaching
Earth and the moon is around 17 km/s. Mass is calculated
from the asteroid's diameter and density. Typical diameters
are up to a few hundred kilometers, and densities range
from 3 g/cm3 (rock) to 8 g/cm3
(iron). The angle of impact ranges from 90 degrees
(a direct hit) to 0 degrees (a glancing blow).
! Examine the images and information, and answer question
6 below.
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Moon Craters |
Earth Craters |
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NASA
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Simple Crater
Moltke Crater
Diameter 7 km |
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D. Roddy, USGS
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Simple Crater
Barringer Crater, Arizona
Diameter 1.3 km |
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NASA
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Complex
- Central Peak
Euler Crater
Diameter 28 km
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NASA
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Complex
- Central Peak
Mistastin Crater, Canada
Diameter 28 km
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NASA
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Complex
- Central Ring
Schrodinger Crater
Diameter 320 km
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NASA
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Complex
- Central Ring
Clearwater Lakes, Canada
Diameter 32km
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NASA
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Multi-Ring
Basin
Mare Orientale
Diameter 970 km
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VL Sharpton, LPI
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Multi-Ring Basin
Chicxulub Impact Basin, Mexico
Diameter >170 km
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These examples show a range of size and form for impact
craters.
Simple craters have raised rims and simple bowl shapes. As the crater size increases, loose
debris in the crater walls slumps inward to partially fill the crater floor.
Complex craters have a central peak or
ring, depending on their size. These features
are caused by the elastic rebound of the surface
material following the impact. Slumping fills
the floor with debris, making complex craters
relatively shallow.
Larger complex craters have multiple rings, and
the largest, called impact basins, are
hundreds of kilometers in diameter.
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| 6.
The acceleration of gravity (g) on Earth is 9.8 meters/sec2,
but on the moon, it is only 1.6 meters/sec2. How might
this relate to the sizes of craters produced by impacts
with identical amounts of kinetic energy on the two bodies? |
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