Why are impact craters almost always round? (or polygonal?)
The long answer (for being almost always round) from Scientific American:
Why are impact craters always round? Most incoming objects must strike at some angle from vertical, so why don’t the majority of impact sites have elongated, teardrop shapes?
When geologists and astronomers first recognized that lunar and terrestrial craters were produced by impacts, they surmised that much of the impacting body might be found still buried beneath the surface of the crater floor. (Much wasted effort was expended to locate a huge, buried nickel-iron meteorite believed to rest under the famous Barringer meteor crater near Winslow, Ariz.) Much later, however, scientists realized that at typical solar system velocities–several to tens of kilometers per second–any impacting body must be completely vaporized when it hits.
At the moment an asteroid collides with a planet, there is an explosive release of the asteroid’s huge kinetic energy. The energy is very abruptly deposited at what amounts to a single point in the planet’s crust. This sudden, focused release resembles more than anything else the detonation of an extremely powerful bomb. As in the case of a bomb explosion, the shape of the resulting crater is round: ejecta is thrown equally in all directions regardless of the direction from which the bomb may have arrived.
This behavior may seem at odds with our daily experience of throwing rocks into a sandbox or mud, because in those cases the shape and size of the ‘crater’ is dominated by the physical dimensions of the rigid impactor. In the case of astronomical impacts, though, the physical shape and direction of approach of the meteorite is insignificant compared with the tremendous kinetic energy that it carries.
Non-circular Impact Craters
An exception to this “almost always round” rule occurs only if the impact occurs at an extremely shallow, grazing angle. If the angle of impact is quite close to horizontal, the bottom, middle and top parts of the impacting asteroid will strike the surface at separate points spread out along a line. In this case, instead of the energy being deposited at a point, it will be released in an elongated zone–as if our ‘bomb’ had the shape of a long rod.
Hence, a crater will end up having an elongated or elliptical appearance only if the angle of impact is so shallow that different parts of the impactor strike the surface over a range of distances that is appreciable in comparison with the final size of the crater as a whole. Because the final crater may be as much as 100 times greater than the diameter of the impactor, this requires an impact at an angle of no more than a few degrees from horizontal. For this reason, the vast majority of impacts produce round or nearly round craters, just as is observed.
Gregory A. Lyzenga, associate professor of physics at Harvey Mudd College, SCIENTIFIC AMERICAN October 21, 1999
Craters generally tend to be round, but this image shows some craters that are more square than round. The likely reason for this is that Eros had some fractures (known as structural features) at its surface that were present before the impact events that formed the craters. These fractures can be seen in the lower part of the top crater. When the impacts that formed these craters occurred, the resulting crater cavities were influenced by the fractures. The result is that the crater rims that run parallel to the fractures are straighter than craters located in nonfractured areas. Geologists call this kind of effect on the craters “structural control”.
Note also the boulder perched just beyond the right hand rim of the top crater that looks like a bright speck in the image. The shape of the boulder can be seen by its shadow, which is cast onto the crater floor. The shadow shows that the boulder is diamond shaped, and it appears to be standing on one tip.
A similar occurrence to the “square” craters on Eros can be seen on Earth at Arizona’s Barringer Impact crater, which also has a slightly squared-off shape to its rim.
Dr. Simon Hanmer, Ottawa Centre RASC
….. most impact craters do indeed have a circular plan view immediately after impact, but … very rapidly after the initial impact – the circular shape evolves and in many cases becomes polygonal.
Van Dujk Th., Kistemaker, JA
1. The possibility of impacts of large meteorites on the thin crust of the early moon accounting for the formation of the hexagonal lunar craters is discussed. Solidified basalts comprising a lunar crust of thickness 10 to 50 km characteristic of the earliest stage in lunar evolution are shown to have a large-scale hexagonal pillar structure, due to the effects of shrinkage. Results of experimental simulations of the propagation in this hexagonal pillar structure of the shock wave generated by the impact of a meteorite of diameter 10 km and mass 10 to the 15th kg on the lunar crust are then presented which demonstrate the pushing away from a central circular shock of pillars resting on a low-friction surface in a hexagonal pattern.
2. If a mechanical shock-wave expands symmetrically in a two-dimensional non-isotropic medium, then the amplitude of the radial displacements is directionally dependent. This has been demonstrated by a system of 1400 hexagonal plastic pillars, simulating the impact of a 10 km size meteorite on a frozen 50 km thick basalt crust in the earliest phase of the Moon’s life.