Craters8

The Earth, the Moon, and every other planet in the solar system is continually bombarded by meteorites and comets, which are probably left-over planetesimals from the formation of the solar system. These objects are usually quite small, and on Earth they almost always burn up in the atmosphere before reaching the surface. However, when they do survive the journey, the results are spectacular. Meteor Crater, Arizona, is thought to have been caused by a meteorite 50 meters in diameter (Figure 1.6).

Comets and asteroids strike the Moon with an average speed of 20 km/sec. These impacts produce craters that are ten to twenty times larger than the impacting object. Thousands of millions of years ago, early in the solar system's history, there were probably more meteors and comets striking all the planets, and they were likely much larger than most of the ones observed today. Because the Earth is geologically active, with volcanoes, plate tectonics, erosion and vegetation, the early craters on Earth have long since been erased.

However, much of the surface of the Moon has remained unchanged for 4,000 million years. Thus, the solar system's early bombardment history is still recorded by relatively intact craters of every size, from thousands of kilometers in diameter (basins) down to "zap pits'' (micrometer-sized craters in lunar soils and rocks that are observed with scanning electron microscopes).

The internal form of the crater depends on its size. Small craters (less than 15 km in diameter) are simply bowl-shaped depressions. Medium-sized craters, with diameters from 20 to 175 km, have flat floors and a central peak. Craters with diameters greater than 175 km have somewhat complex central structures, and rings begin to replace peaks as the diameter increases. See Figures 1.7, 1.8, 1.9, and 1.10.

When material is ejected from a crater, it follows a hyperbolic path, such that most of the material lands nearby, with decreasing amounts landing farther away. If large boulders are ejected, they may create secondary craters when they land. Secondary craters sometimes occur in straight lines that point to the primary crater.

The material that is not ejected may be dramatically altered as well. The surface

8 Much of the material for this section has been paraphrased from the web page "Exploring the Moon - Impact Crater Geology and Structure''. Visit http://www.lpi.usra.edu/expmoon/ sciencejcraterstructure.html

Figure 1.6. Meteor Crater, Arizona.

Figure 1.7. Moltke is an example of the smallest class of craters; a simple bowl shape (from the Lunar and Planetary Institute web page, "Impact Crater Geology and Structure'', http:H www.lpi.usra.edu/expmoon/science/craterstructure.html).

Figure 1.6. Meteor Crater, Arizona.

Figure 1.7. Moltke is an example of the smallest class of craters; a simple bowl shape (from the Lunar and Planetary Institute web page, "Impact Crater Geology and Structure'', http:H www.lpi.usra.edu/expmoon/science/craterstructure.html).

may be broken into rubble, which may then be welded together by the material that has been melted by the energy of the impact. This is one type of breccia. Going deeper below the surface, the rocks are cracked and deformed, but the deformation is less severe at depth.

Figure 1.8. Euler, 29 km in diameter, shows a flattened floor and a central peak (from the Lunar and Planetary Institute web page, "Impact Crater Geology and Structure", http:)! www.lpi.usra.edu/expmoon/science/craterstructure.html).

Figure 1.8. Euler, 29 km in diameter, shows a flattened floor and a central peak (from the Lunar and Planetary Institute web page, "Impact Crater Geology and Structure", http:)! www.lpi.usra.edu/expmoon/science/craterstructure.html).

Figure 1.9. Schrodinger is 320 km in diameter, really large enough to be called a basin rather than a crater. It has an inner ring that is 150 km in diameter. Schrodinger is one of the youngest impact basins on the Moon (from the Lunar and Planetary Institute web page, "Impact Crater Geology and Structure'', http://www.lpi.usra.edu/expmoon/ science/craterstructure.html).

Figure 1.9. Schrodinger is 320 km in diameter, really large enough to be called a basin rather than a crater. It has an inner ring that is 150 km in diameter. Schrodinger is one of the youngest impact basins on the Moon (from the Lunar and Planetary Institute web page, "Impact Crater Geology and Structure'', http://www.lpi.usra.edu/expmoon/ science/craterstructure.html).

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