Lunar Impact Crater Geology and Structure
Both the Earth and the Moon are the targets of a continuing bombardment of meteorites, asteroids, and comets from outer space. The "shooting stars" that are commonly seen in the night sky are mostly dust-sized objects striking the Earth's atmosphere. Much rarer, larger objects sometimes strike the Earth or Moon, producing holes known as craters. Meteor Crater in Arizona (1.2 kilometers in diameter) is a well-known terrestrial example. Over its history, the Moon has had countless millions of craters form on its surface.
Comets and asteroids strike the Moon at a wide range of impact speeds, with 20 kilometers per second being typical. Such a high-speed impact will produce a crater that is 10 to 20 times larger in diameter than the impacting object. The detailed form of the crater depends on its size. This figure shows idealized cross-sections of the structure of small, simple craters (top) and of larger, more complex craters (bottom). Simple craters have bowl-shaped depressions and are the typical crater form for structures on the Moon with rim diameters (D in the figure) of less than about 15 kilometers. Craters on the Moon with diameters larger than about 15 kilometers have more complex forms, including shallow, relatively flat floors, central uplifts, and slump blocks and terraces on the inner wall of the crater rim. In craters on the Moon with diameters between about 20 and 175 kilometers, the central uplift is typically a single peak or small group of peaks. Craters on the Moon with diameters larger than about 175 kilometers can have complex, ring-shaped uplifts. When impact structures exceed 300 kilometers in diameter, they are termed impact basins rather than craters. More than 40 such basins are known on the Moon, and they have an important control on the regional geology of the Moon.
Much of the material ejected from the crater is deposited in the area surrounding the crater. Close to the crater, the ejecta typically forms a thick, continuous layer. At larger distances, the ejecta may occur as discontinuous clumps of material. Some material that is ejected is large enough to create a new crater when it comes back down. These new craters are termed secondary craters and frequently occur as lines of craters that point back to the original crater.
Material below the surface of the crater is significantly disrupted by the shock of the impact event. Near the surface is a layer of breccia (a type of rock composed of coarse, angular fragments of broken-up, older rocks). Rocks at deeper depths remain in place (and are termed bedrock) but are highly fractured by the impact. The amount of fracturing decreases as the depth below the surface increases. The energy of the impact typically causes some material to melt. In small craters, this impact melt occurs as small blobs of material within the breccia layer. In larger craters, the impact melt may occur as sheets of material.
The following photographs illustrate how crater morphology changes with increasing crater size on the Moon.
Moltke Crater, 7 kilometers in diameter, is an excellent example of a simple crater with a bowl-shaped interior and smooth walls. Such craters typically have depths that are about 20 percent of their diameters. The hummocky material surrounding the crater is Moltke's ejecta deposit. (Apollo 10 photograph AS10-29-4324.)
Bessel Crater, 16 kilometers in diameter and 2 kilometers deep, is an example of a transitional crater between simple and complex craters. Slumping of material from the inner part of the crater rim destroyed the bowl-shaped structure seen in smaller craters and produced a flatter, shallower floor. However, wall terraces and a central peak have not developed. (Part of Apollo 15 Panoramic photograph AS15-9328.)
Euler Crater, 28 kilometers in diameter and about 2.5 kilometers deep, is a good example of complex crater morphology. It has a flattened floor, a small central peak, and material that has slumped off the inner crater rim. The blanket of ejecta surrounding the crater is quite clear. (Part of Apollo 17 Metric photograph AS17-2923.)
King Crater, on the Moon's farside, is 77 kilometers in diameter and more than 5 kilometers deep. The terraces and slump blocks on the inside of the crater rim and the relatively flat floor are both typical of large lunar craters. However, the central peak is much larger at King Crater than at other lunar craters of similar size, such as Copernicus or Tycho. (Apollo 16 Metric photograph AS16-1580.)
Copernicus Crater, 93 kilometers in diameter, is one of the youngest and freshest impact craters on the nearside of the Moon. Like King Crater, Copernicus is a well-developed complex crater, with a prominent central peak and a relatively flat floor. This oblique photograph clearly shows the terracing and slump blocks on the inside of the crater rim and the rough ejecta deposit outside the crater. (Apollo 17 photograph AS17-151-23260.)
Schrodinger is 320 kilometers in diameter, large enough to be considered an impact basin rather than a crater. In addition to the main, outer rim, Schrodinger also has an inner ring that is 150 kilometers in diameter and about 75 percent complete. Schrodinger is one of the youngest, freshest impact basins on the Moon. (Mosaic of Clementine images. Image processing by Ben Bussey, LPI.)
Terrestrial Impact Craters slide set
Craters in Three Dimensions
Photographs taken while looking down from great heights, such as from an airplane or an orbiting spacecraft, often have a two-dimensional quality to them, with little or no indication of how high the features shown in the image actually are. If a region is photographed from two different perspectives, the differences in appearance of the two photos can be used to determine the heights of features in the images. This is known as stereo photography and is similar to the process the human brain uses to merge the images from the left and right eyes into a single image that provides information about the distances to various objects.
The images shown here have been digitally processed to illustrate this stereo effect. The images should be viewed with special red-blue stereo glasses. The red lens goes over the left eye and the blue (or green) lens goes over the right eye. These stereo images were processed by Paul Schenk, Lunar and Planetary Institute. (Stereo images copyright © Lunar and Planetary Institute, 1997.)
Lambert Crater, 30 kilometers in diameter, is a typical example of a small, complex crater, similar to Euler Crater. In this image, slumping and terracing of the inner rim of the crater, formation of a central peak, and the ejecta blanket are all prominently seen. Lambert is about 2.4 kilometers deep and the rim is about 800 meters higher than the surrounding plains. The heights in this image are vertically exaggerated by a factor of 4.2. (Based on Apollo 15 Metric photographs AS15-260 and AS15-265.)
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