Date: 2018-02-12 14:27
The rock on the surface of the planet or moon is bent backward, upward, and outward so the amount of material ejected is much larger than the projectile. Large craters will have a central peak formed by the rock beneath the impact point rebounding upward and they may also have terracing of the inner walls of the crater from the collapsing of the crater rim inward. The size of the craters having central peaks depends on the gravity of the planet or moon: on the Moon craters larger than about 65 kilometers in diameter have central peaks while the crater diameter on the Earth needs to be larger than just 6 to 8 kilometers. Impact cratering was especially prevalent for the first several hundred million years after the planets formed as the planets swept up left-over material. The last stage of that sweeping up , called the late heavy bombardment, occurred from about to billion years ago. Impacts as large as the one that led to the demise of the dinosaurs in much more recent history were happening about once a month. Most of the impact basins---craters measured in hundreds of kilometers---were made during this time. It is noteworthy that about the time the heavy bombardment ended, life took hold. The oldest fossil evidence of ancient organisms dates back to billion years ago and evidence for biological activity based on isotopic ratios of carbon may date back to about , even up to billion years ago, though the carbon isotope ratio evidence is controversial. The number of craters per unit area on a surface can be used to determine an approximate age for the planet or moon surface if there is no erosion. The longer the surface has been exposed to space, the more craters it will have. If you know how frequently craters of a given size are created on a planet or moon, you can just count up the number of craters per unit area. This assumes, of course, that the cratering rate has been fairly constant for the last few billion years. The heavy bombardment of about billion years ago must be taken into account when using the crater age dating technique. For example, the highland regions on the Moon have ten times the number of craters as the maria, but radioactive dating (explained in the next chapter ) shows that the highlands are approximately 555 million years older than the maria, not ten times older. At a minimum crater-age dating can tell you the relative ages of surfaces (which surface is older than another). Careful studies of how the craters overlap other craters and other features can be used to develop a history or sequence of the bombardment on the moons and planets. All bodies with hard surfaces have impact craters. Worlds with less volcanism or erosion or tectonic activity in their history will retain more impact craters since the planet formed. Worlds with more geological or erosional activity will have newer surfaces or craters that have been so worn away as to be unrecognizable. Earth has over 675 impact craters on its continents with the -km diameter Meteor Crater in northern Arizona being one great, well-preserved example. Even Venus with its thick atmosphere has impact craters, though they all have diameters measured in kilometers because smaller projecticles burn up in its atmosphere.
Thermoluminescence, or TL, was first used in the 6955s for the measurement of radiation exposure, and underwent a period of difficulties before being applied to dating the first dates it produced being too young. Upon resolution of the technical problems the method was used for dating pottery and burnt flints from archaeological sites with a precision of about 7-65 per cent. Subsequently it has been used in the investigation of recent geological formations reaching back to half a million years. In its most common form it may shed light on the age of fired clay and quartz based materials but approaching the present no closer than about a thousand years. A modern variant on the technique is able to date far more recent fired clay material.
TL depends upon minute levels of background radiation in the clay matrix, a tiny fraction of which is absorbed and stored as a charge at imperfections in the crystal lattice of quartz inclusions. The firing of pottery removes the inherited geological TL and sets the dating clock to zero. In the laboratory grains of quartz are extracted from the pottery and heated in light-tight apparatus at a constant rate to around 955° C. Superimposed upon the red-hot glow, a tiny flash of light is produced as the stored energy is released (hence 'thermoluminescence') and the flash is recorded by computer. The quantity of light produced is proportional to the length of time since it was last fired. Unfortunately, problems remain since all samples do not have the same sensitivity to the radiation and background radiation levels vary. Furthermore, the results are sensitive to water content. Thus many measurements must be made in order to obtain a date.
Recently this method has been improved. The flash of light is released by scanning the sample with an energetic green laser beam and light-emitting diodes are used as detectors. This form of the method, known as 'optically stimulated luminescence dating', enables objects which are not more than a few hundred years old to be dated to within a few decades. Hence it is far more useful than the original TL technique in dating buildings. The requirement remains that the sample should have undergone some heating event to set the clock to zero. It also requires that a dosimeter be left undisturbed in situ at the site for some months in order to discover the natural radioactivity permeating the samples. These must be inorganic and contain some light transmitting materials. Pottery artefacts and certain bricks might be suitable specimens, and often TL provides the only way to distinguish medieval or Tudor bricks and chimney pots from Victorian reproductions.
There are several laboratories capable of this sort of measurement in this country which include the Geology Department, Aberystwyth the British Museum the Godwin Institute, Cambridge the Department of Archaeology, Durham Environmental Sciences the Institute of Archaeology, University College, London and the Research Laboratory for Archaeology and the History of Art, Oxford.