Article from the February 2005 issue of
ON THE SURFACE
Impact craters are one of the most obvious landforms on the terrestrial planets and their satellites in the Solar System. On Earth, despite the presence of active erosional and tectonic recycling processes, about 150 impact craters are known.
Impact craters form as a result of a hypervelocity collision of a planet with a much smaller body, such as asteriods, planetisimals, comet nuclei and meteoroids. Although the formation process is thought to be essentially the same everywhere and over a wide range of energies, and most fresh impact craters can be described as “circular rimmed depressions”, the detailed morphologies show a lot of variation. Craters can differ systematically in morphology by planet, location on a planet, size and age. This probably reflects the influence of external factors on the impact process, such as the density, velocity and angle of the impactor; strength and structure of the target material, the planet’s gravitational acceleration as well as the effects of post- impact processes such as erosion, sedimentation, isostasy and magmatism.
This photograph shows King Crater on the Moon's far side. King Crater is 77 kilometers in diameter and more than 5 kilometers deep. This is a complex crater with a central peak. The inside of the crater rim contains a series of terraces and slump blocks. (credit: NASA) http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=794
The interior morphology of impact craters becomes more complicated with increasing crater size. However, this size/morphology relationship is not continuous, but shows marked transitions. The two most important discontinuities are the transition between simple and complex craters, and between large craters and small basins.
It is believed that these morphological differences are not a direct result of the crater excavation process, but develop after most of the material has been expelled from the crater.
The initial product of an impact is thought to be a circular, bowl-shaped cavity with a depth to diameter ratio between 1:4 and 1:3. The shape of this initial crater is independent of its diameter, the impact velocity, impact angle (except for very small angles), gravitational acceleration, and the properties of the target and impactor. This “transient” crater is subsequently modified by gravitational collapse, giving rise to a final morphology which is determined by the conditions of the target site.
Crater collapse depends on the stength of the material surrounding the crater, which is shattered, heated and shaken by the impact. However, if these rocks retained their static strength properties, impact craters would not collapse at all. This indicates some rock weakening processes must be active during crater formation, resulting in a much lower dynamic strenth of the material; acoustic fluidisation and shock weakening have been proposed as possible mechanisms.
Simple craters are circular, bowl-shaped depressions with elevated rims and an approximately parabolic interior profile. They have no major internal topographic features.
One of the most important characteristics of impact craters is their ratio of rim-to-floor depth to the rim-to-rim diameter (d/D), which has a value of about 1:5 for most simple craters on the terrestrial planets as well as on the Moon. The upper rim of a simple crater often shows stratification and evidence of mass-wasting. Most lunar craters smaller than about 15 km in diameter are simple, while on Earth the upper limit is about 4 km.
The floor of simple craters is underlain by a breccia lens consisting of rock debris and shock-melted rock with a thickness of about half the rim-to-floor depth of the crater. This lens in turn lies in a bowl of fractured country rock. It is thought that the final shape of a simple crater is formed mainly by the gravitational collapse of the rim of the transient crater immediately after it forms.
Complex craters range in diameter from a few kilometers on Earth to a huge 460 km diameter crater observed on asteroid 4 Vesta. The 15 Ma Ries crater in Bavaria, which was visited during the last AGM in Munich, is one of the best known middle-sized complex craters on Earth, with a diameter of about 24 km.
Complex craters have a d/D ratio which varies widely fom 1:5 for small fresh complex craters to 1:150 for large craters, depending upon the planet. On Earth they exhibit central stuctural uplifts, rim synclines, and outer concentric zones of normal faulting. Extraterrestrial craters have been observed with multiple central peaks and terraced rims. The central uplift consists of strata which have been uplifted above the pre-impact level, and is surrounded by a ring depression (or rim syncline) filled with fragmented material and impact melt. The uplift of the transient cavity’s floor — accompanied by subsidence of the crater rim — is thought to be the main modification mechanism for complex craters.
The transition between simple and complex craters occurs over a narrow diameter range on a particular solar system body and is thought to scale as the inverse power of the surface gravity, g. The transition occurs around 15 km diameter on the Moon, 7 km on Mars and about 4 km on Earth. As the crater size increases further, the central peak complex in a complex crater begins to break up and form an inner ring of mountains. In large craters the ring is about half the rim-to-rim diameter, and these craters are called “peak-ring” craters.
However, not all of the complex crater features appear at the same diameter. Therefore the transition diameter is often expressed as the geometric mean, Dt, of several diameter values at which particular morphological features, such as central uplifts and terraced walls, appear. Studies on Martian craters indicate that the first complex features to appear are a flat floor, a central peak and a low d/D ratio.
Interestingly, the Dt values for Earth (3.1 km) and the Moon (18.7 km) differ by a factor of six, which is exactly the ratio of their values of g. In addition, complex craters on the Moon are on average six times deeper than on Earth.
Despite the importance of gravity, the lithology of the target area influences the value of Dt as well. This is best established for terrestrial craters of course, giving a simple-to-complex crater transition diameter of 2.25 km for sedimentary rocks and 4.75 km for crystalline rocks. It is thought that at least three (interrelated) target properties influence the shape of the final crater: rock strength, stratification and volatile content. An impact of a given energy will excavate a larger cavity in soft rocks, while there is evidence that complex craters develop more readily in stratified rocks.
The simple-to-complex transition coincides with a change in the texture of the ejecta surrounding fresh martian impact craters (this is of course more difficult to observe on Earth), indicating the mechanism of ejecta emplacement is dependent on crater size.
Craters less than 4 km in diameter show typical ballistic ejecta characteristics, while between 4 and 80 km emplacement seems to occur at least partly by surface flow, and larger craters again have ballistic ejecta emplacement. This might be explained by the incorporation of subsurface volatiles in the ejecta for impacts of intermediate size, while small impacts do not excavate deep enough to tap these volatiles, and large impacts completely vaporise them.
The largest impact craters form small basins containing a series of multiple concentric ridges, and they seem to come in two basic types. The classic basin is Orientale on the Moon, which features at least five circular rings that form inward-facing scarps up to 6 km high. The second type is the Valhalla structure on the Jovian moon Callisto. This multi-ring basin consists of a bright central patch surrounded by a system of concentric ridges. These ridges are surrounded by dozens of grabens that may extend thousands of kilometers from the impact point. On Earth, the 65 Ma, 180 km diameter Chicxulub crater in Mexico is one of the few examples positively identified as a multi-ring basin. These basins are thought to form as a tectonic response of the target’s lithosphere the the crater formed by the impact, and indicates the presence of a low-viscosity or low-strength layer below the surface. The transition between peak or peak ring craters and rimmed basins on different bodies does not relate to the surface gravity of the body, but seems to depend a great deal on the rheological properties near the surface, in particular the presence of a weak subsurface layer which can flow on the timescale of the crater collapse. However, the formation of these basins is still not very well understood.
H. J. Melosh and B. A. Ivanov Impact Crater Collapse, Annu. Rev. Earth Planet Sci. 27 (1999) pp. 385-415
R. J. Pike Control of crater morphology by gravity and target type: Mars, Earth, Moon, Proc. Lunar Planet. Sci. Conf. 11th (1980) pp. 2159-2189
By Paul De Schutter
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