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Assignment 6 (Craters)


Assignment 6 - Craters

Impacted surface type and density. Where the impacted surface is rocky (similar to the Earth) then the vaporisation point is high meaning that for a given impact energy level more of the ejector is retained. Conversely a surface where the material is say, frozen water then much of the ejector is lost as steam. The density of the impacted material also plays a roll for an impact into bedrock will be quite different from one into a sandy surface which again will be quite different into a liquid.

Nature of Impactor. In general terms they come in two flavours, namely Asteroids and Comets. In turn Asteroids and their fragments can be subdivided into classes; Iron, Stony Iron and Stony. The iron meteorites are perhaps the most dense with the highest energy level for a given size and velocity. There is one other class of impactor that needs to be part of this list and has quite different effects on the impacted surface. Comets with their high water content, large size and high velocity (can be in the region of 1.6 million km/h at perigee) also contain huge amounts of kinetic energy and have the ability of causing massive disruption to an impacted surface. It is suspected that most of the water on planet Earth was shipped in aboard comets. In the case of the above images the Moon’s mean density of 3,346.4 kg/m and, from the findings of the Apollo missions, surface rock not unlike the Earth, means that its vaporisation point is very high. The conclusion is that much of the kinetic energy of the impact is held in the liquefied rock and as cooling takes a relatively long period as it radiates back into space and into the surrounding surface, time is available for the rock on the inside of the crater to self level and produce a smooth terrain. A coherent, high density surface will want to resist deformation from the impact an will try to push back. This will cause the ground on the outside of the crater near the rim to compress and elevate, building noticeable slopes away from the rim. Dione on the other hand with a mean density of 1.095 Kg/m2 and a surface of mainly water ice has a low vaporisation point. The implications are that when a surface like this is impacted the kinetic energy is quickly dissipated into space as water vapour leaving little time for the surface to recover and self level. The chaos of the boiling slurry is almost instantly frozen leaving a rough and uneven terrain. The lower density surface also fails to push back and uplift of the surrounding area adjacent to the rim is much reduced.

12) Have you observed any examples of impact craters of any kind? What were they, and how were they formed? (Craters can be formed by a variety of falling objects, not only meteorites!)

These images demonstrate cratering in every day life. Top left is of bullets impacting metal and although the kinetic energy is insufficient to melt the metal for anything but a very short period, it clearly demonstrates push back with the outside surface appreciably rising towards the cater rim. The bottom middle image is of rain drops on a sandy surface. In the realm of geology a question has long been asked about the patterns found in some sedimentary rocks. One of the main contenders to answer this is the fossilisation of ancient rain drop impacts. We may have
many more craters on the earth than we thought! The short lived crater in the bottom left image caused by a water drop into water demonstrates surface rebound, a cause of central peaks in some craters. For the bottom right image cause and effect are well known!

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