Astro 150 - Impact Crater Tutorial |
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Impact craters are the dominate geological feature on the Moon (and
most worlds in the solar system). They are created by the impact of
rocks or ice. The speed of the impact is very large (10 - 70 km/sec).
The crater is formed when the kinetic energy of the impactor is
converted into thermal, mechanical, and acoustic energy that distorts,
fractures and ejects pieces of the target. The result is similar to
an explosion centered a few impactor sizes below the surface.
Although an impactor may strike the ground at any angle, impact
craters are almost always circular unless the impactor hits at a very
shallow angle (<5 degrees).
The characteristics of impact craters depend on the size of the crater, and the gravity of the target planet. As the size of the impact craters grows so does its complexity. Impact craters on the Moon are categorized as follows: |
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Simple Impact Craters are less than 10 - 15 km in diameter. They are characterized by a smooth bowl-shaped interior surrounded by a rim elevated above the surrounding terrain. They are about 1/5 as deep as they are wide (i.e. a 10 km simple crater will have a maximum depth of about 2 km). |
| Simple impact craters were visited by the Apollo astronauts on every mission. The reason they are so important is they expose materials from below the lunar surface. It would take far too much time for the astronauts the dig down to these depths, and the samples from below the lunar regolith are crucial to understanding the structure and history of the Moon. |
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| When an impact happens on the Moon the material immediately below the impactor is vaporized or melted. Material from about 1/3 the crater depth (or about 1/10 the crater diameter) is ejected onto the surface. The material in the lower 2/3 of the crater is crushed and displaced downward forming a breccia lens. |
| When material is ejected from the crater the material from nearest the surface is broken into small pieces and is thrown the furthest. Material from deep within the crater occurs as large blocks and is deposited near the crater rim. Astronauts collecting samples as they approach a crater are in essence collecting a cross-section of the material below their feet. |
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| Complex Impact Craters are larger than about 20 km. They are characterized by 1) a broad floor that is generally level, interrupted by various hills and mounds; 2) a central peak or series of peaks; 3) terraced rim walls that represent wholesale failure of the rim; and 4) are relatively much shallower than simple crater, with depths about 1/5 to 1/40 of their diameter. |
| Since complex craters are larger they evacuated material from deeper within the Moon and deposit it further from the impact. Apollo 12 landed on the ejected material from the complex crater Copernicus that allowed us to determine the age of this very important crater. | |
| Impact Basins are the largest impact features on the Moon having diameters of 300 to over 2000 km. The largest of these basins have concentric multiple rings and lack central peaks. Basins are where mare lavas accumulate and they determine the paths of the faults and folds on the moon. Basin impacts make up the basic geological framework of the Moon. |
| Since impact basins are the largest impact feature it was hoped that they would expose materials from very deep within the moon, possible down to the lunar mantle. Determining the ages of key impact basins was crucial to the Apollo missions since these ages are used to date regions of the Moon not visited by Apollo. | |
| Secondary Impact Craters are created by the impact of debris launched from the creation of simple/complex impact craters and are generally found radially distributed about their primary crater, often occurring in groups aligned in rows. Secondary craters are often irregular in appearance, although if they form a long way from their source they can be circular and hard to distinguish from primary impact craters. |
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Crater Density Diagram: For an airless dead world like the Moon
you can determine the age of a surface from the number of craters on
it. Old surfaces have been exposed to space longer so can accumulate
more craters than younger surfaces. By just counting craters we can
determine the relative ages of the different surfaces of the Moon.
For example the landing site of Apollo 16 has many more craters on it
than the Apollo 12 landing site, so it is older. The samples returned
by the Apollo mission allowed us to put absolute dates on these
surfaces and made it possible to construct the diagram on the right.
With his we can now look and photographs of areas not explored by
Apollo, count the craters of various sized and determine the age of
the surface.
If we are careful we can even use this diagram to determine the ages of surface of other airless worlds in our Solar system. In the absence of returned samples from other worlds, the absolute ages of their surfaces can been estimated from the lunar relation between crater density and the age of returned samples and the likely differences in cratering flux between the Moon and the other world. |
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