Maximum Impact
Impact Craters in the Solar System by Walter S. Kiefer
Lunar and Planetary Institute
The Earth, the Moon, and the other planets are the targets of a continuing bombardment of asteroids and comets from outer space. The meteorites or "shooting stars" that are commonly seen in the night sky are mostly dust-sized objects striking the Earth's atmosphere. Although much rarer, larger objects sometimes strike the Earth or Moon, producing holes in the ground known as craters.
Meteor Crater in Arizona is one of the best known examples of an impact crater on Earth. The crater is 1.2 kilometers in diameter and 200 meters deep. It formed approximately 49,000 years ago when an iron meteorite that was roughly the size of a school bus struck the Arizona desert east of what is now Flagstaff.
As of 2002, fewer than 200 impact structures are recognized on Earth. Like the Moon (see below), the Earth must have been struck innumerable times by asteroids and comets over its history. Most craters on Earth have been destroyed by erosion. A particularly large crater formed near Chicxulub, Mexico, about 65 million years ago. This impact event is thought by many scientists to be responsible for the extinction of the dinosaurs.
Impact Crater Structure
Comets and asteroids strike the Earth and 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.
Impact Craters on the Moon
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 rough, rocky 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. The object in the right center of the photograph is an experiment boom attached to the Apollo spacecraft. (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 photograph looks obliquely across the crater and 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, Lunar and Planetary Institute.)
Impact Craters as a Measure of Planetary Age
The density of impact craters on a planetary surface can be used as a measure of the age of that surface. Surfaces with relatively few craters are young, while surfaces with many craters are old.
A simple thought experiment may help to clarify this concept. Imagine throwing darts at a painted wall. After a period of time, half of the wall is painted over, simulating covering by lava flows or destruction of craters by erosion. Additional darts are then thrown at the wall. Even if you did not see the wall get painted, you could use the distribution of dart holes to map out where the fresh paint was emplaced. Scientists use this same principle in mapping the geology of planetary surfaces.
This photograph looks obliquely across the central part of the Moon's farside. This region is virtually saturated with craters. This type of intensely cratered surface is typical of most of the Moon's farside and of those parts of the nearside that have not been flooded by lava flows. The average age of this region is probably 4 billion years. (Apollo 16 Metric photograph AS16-0728.)
This photograph looks obliquely to the south across Mare Imbrium, which is the smooth region at the lower right. Mare Serenitatis is the smooth region in the upper left, and Sinus Medii is the smooth region in the upper right. The Apennine Mountains, which form part of the main rim of the Imbrium impact basin, are prominent in the center of the photograph. In places, the Apennines are more than 4 kilometers higher than Mare Imbrium.
The smooth surface in Mare Imbrium has relatively few impact craters, indicating that it is much younger than the cratered surface shown in the previous image. The Apollo 15 mission returned samples from both Mare Imbrium and from the Apennine Mountains. These samples show that the Imbrium basin formed in a large impact 3.84 billion years ago. The smooth material on the basin floor is basalt, formed in volcanic eruptions 3.3 billion years ago. (Apollo 17 Metric photograph AS17-2432.)
Impact Craters in the Rest of the Solar System
Yuty crater on Mars is 18 kilometers in diameter. Its ejecta deposits consist of many overlapping lobes of material. This type of ejecta morphology is characteristic of many craters at equatorial and mid-latitudes on Mars but is unlike that seen around small craters on the Moon (compare with Euler crater, shown above). This style of ejecta deposit is believed to form when an impacting object rapidly melts ice in the subsurface. The presence of liquid water in the ejected material allows it to flow along the surface, giving the ejecta blanket its characteristic, fluidized appearance. (Viking 1 Orbiter image 3A07.)
The circular structure in this image is the Tyre impact basin on Europa, a moon of Jupiter. At least 5 basin rings can be distinguished (compare with the Moon's Schrodinger impact basin, shown above). The general absence of other impact craters in this image indicates that Europa has a very young surface and remains geologically active. For a discussion of how impact crater geology helps to constrain the possible presence of a buried liquid ocean on Europa, see the article "Europa and Titan: Oceans in the Outer Solar System?", included elsewhere on this CD-ROM. The image is 424 km across. (NASA Galileo image.)
This image of Saturn's moon Tethys shows numerous impact craters as small as 5 kilometers across. Much of this moon is very heavily cratered, indicating an ancient surface. In the lower right, the crater density is somewhat reduced, indicating that this part of Tethys was resurfaced by volcanic activity early in its history. NASA's Cassini spacecraft will arrive at Saturn in July 2004 and conduct a four year study of Saturn's atmosphere, ring system, and many moons. (NASA Voyager 2 image.)
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