Though there may be different angle at which impactors hit the planetary surfaces, target lithology may be different, impactors lithology and shapes may be different. Then why most of the impact craters are circular to near circular in shape?
@Luisa Elena Iatan ,Thank you ma'am for the answer , definitely this is helpful. But my confusion still not get very clear because the ejected material is not uniform in all the directions.
Hello!
For the Earth one Russian geologist considered, that part of craters are from explosion of hydrogen. Inorganic oil genesis theory and so on.
BR,
Edgars
Vertical incidence meteorite impacts are probably vanishingly rare in the solar system; the most likely impact angle for a randomly incident impactor can be shown to be around 45 degrees for terrestrial bodies over a large range of sizes, with 20-30% of impactors striking at angles of less than 30 degrees. The ubiquity of crater circularity suggests the underlying physics of crater formation is largely independent of impact angle.
Because the ratio of an interplanetary impactor's kinetic energy to its size is very large, it was recognized in the 1960's that the impact site effectively behaves as a point source energy radiator, the shock waves from which spread (hemi)spherically through the terrestrial half-space, irrespective of impact angle. This determines the resulting symmetry of the transient cavity and the symmetrical gravitational collapse of the cavity walls and proximal ejecta. The resulting post-impact morphology has a high degree of radial symmetry.
The advent of numerical modelling and hydrocode simulation confirmed the insensitivity of impact angle on final crater shape and suggests that impact angles need to fall below 10 degrees (some sources suggest less than 6) before shock wave asymmetry becomes significant and the resulting crater departs from circularity.
Non-circularity (ellipticity) is a characteristic feature of secondary craters, where the ratio of kinetic energy to impactor size is dramatically reduced, enabling the influence of oblique impact to produce a non-spherical shock front.
Given the apparently universal mechanism of circular crater formation, observed departures from circularity then become significant. Target rock structural inhomogeneity (jointing, faulting), and post-impact deformation (regional extension or compression) have been invoked to explain non-circular, and polygonal crater shapes which are not uncommon on terrestrial, lunar and other planetary surfaces.
@Graham Howes , Sir, I am thankful to you for such a nice explanation.
Dear Satyendra,
I concur with Graham Howe's assessment. Only a small fraction of hypervelocity impacts, at very shallow impact angles ~10-15° from the horizontal surface and smaller, seem to create non-circular (oval-shaped, elliptical) impact craters and also alter the distribution of impact ejecta. See also, for example, the paper by Herrick & Forsberg-Taylor (2010):
Article The shape and appearance of craters formed by oblique impact...
.In most impact scenarios the projectile penetrates the target rock down to a depth approximately its own diameter (contact/compression stage) before the impacting asteroid 'explodes' and the excavation stage of the cratering process begins - this is, in most cases, pretty much a point source resulting in radial crater symmetry. Square-shaped (in plan view) impact craters, such as Meteor Crater (Arizona, USA), or polygonal craters, e.g., Söderfjärden (Finland), demonstrate how the pre-existing fabric within the target rock (joints, fractures, faults, etc.) also influences the final crater shape.
Best,
Martin
Already lots of good answers here. The crater shape is not so much controlled by the initial passage of the projectile into the target, which is aligned with the trajectory, but the subsequent subsequent release of energy from projectile and target as the projectile tends to be radial.
Not a result of the impactor having a circular shape as we know that almost no impactors are spherical when the reach surface of earth. I guess it is more of the impact side material moving in all directions as a result of impact.
Most documentation on oblique impacts indicate that for hyper sonic impacts of less than 30 degrees in sand and 15 degrees in granite the crater begins to become oblique. See Pierazzo-Melosh 2000a, p146
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