Craters produced by hypervelocity impacts all develop according to similar laws. Indeed, similar craters are observed from any explosive release of energy such as underground tests of nuclear bombs or explosive release of energy from volcanoes. The shapes of impact features are nearly independent of the nature of their impactors and nearly independent of the approach angle, if that angle is more than 25° from the horizontal. Some differences due to impactor composition are introduced by entry through an atmosphere, but that is not relevant to the Moon.
There are three primary divisions of the features of an impact feature: the transient crater, the apparent crater, and the ejecta.
The transient crater forms quickly, within seconds or minutes, depending on size. It is roughly hemispherical below the surface and produces a turbulent zone of fractured, melted, and pulverized material in the target. Its diameter relative to that of the impactor depends on the gravity of the planet, but for Earth, estimates range from 5 to 10 times the diameter of the impactor. It follows, of course, that the impactor contributes less than 1% of the material that remains in the transient crater.
The apparent crater is the cavity that is in the visible surface after the event. It results from the material that is removed when the expanding energy released by the impact reaches the surface and, not meeting further resistance, ejects a layer of the material there. The apparent crater is much shallower than the transient crater for large craters and basins.
Ejecta is the material thrown out from the apparent crater. Some of this material forms a rim around the crater whose depth is about 20% of the depth of the apparent crater, measured from the original target surface. Ejected material that falls beyond this rim, out to about twice the rim radius from the center of the feature, forms a continuous deposit called the ejecta blanket. The depth of ejecta falls off rapidly beyond the rim. This ejecta blanket often has ridges and troughs radial to the center of the apparent crater. Beyond the ejecta blanket, the ejected debris consists of powdery material that may produce plains and rays. Boulders that are ejected produce, upon landing, their own secondary craters. These "secondaries" sometimes land in radial strings or chains, called catena.
For a given type of target surface, crater morphology undergoes a progression from a simple symmetrical cavity to a cavity with a central peak, and then to a disrupted surface that forms an internal ring. Beyond a certain size, multiple rings appear both inside and outside of the apparent depression. Such impact features are called ringed basins. Lunar impact features larger than 300 km in diameter usually exhibit rings (or arcs of rings) and those smaller than 300 km usually do not. Therefore, the term basin is used for features with diameters larger than 300 m and the term crater is used for features that are smaller.
Throughout this book, attention is drawn to the examples re of these structures and how the structures of neighboring atu impact features interact. The reader is encouraged to form Fe personal views of the patterns of these structures. Its
3.7. Mare Basalt I
Some deep craters and basins on the far side, as well as most such features on the near side, are filled with mare material, he lava extruded from below. As we have mentioned, there are £
' o fewer and smaller areas of mare on the far side, relative to y r that on the near side. The largest, in the central floor of the tor South Pole-Aitken Basin, is about the size of Mare Imbrium £ (1,023 km in diameter). The Imbrium Basin is filled all the «> way to its rim but the larger South Pole-Atken Basin just has Th concentrations of mare in the deeper craters and basins of its central floor. All other areas of mare on the far side are much smaller; the largest of them is Mare Moscoviense, 277 km in diameter.
Mare material is basaltic, like our ocean floor. It originates from the heavier material in the mantle, which is richer in iron and magnesium and lacking in aluminum and silicates, in relation to the crust. As the Magma Ocean cooled, the mantle material settled because of its higher density. However, radioactive elements of the uranium series liberated heat energy, remelting parts of the mantle. The molten material expanded, became less dense, and rose to the surface, where it cooled. The radioactive elements leave their signature: high concentrations of the products of radioactive decay of uranium, like thorium. A related set of elements is known as KREEP, for potassium (chemical symbol K), rare earth elements (REE), and phosphorus (chemical symbol P). They tend to stick together, and are enriched when mantle rock is remelted, because they do not crystallize easily but they do melt easily. These two types of tracers, both detected in the maria, are telltales of the history of basalt that has come to the surface. The KREEP tells us that the material was remelted and the thorium tells us what remelted it.
The high concentrations of thorium in the mare material of the South Pole-Aitken Basin tells us that uranium, normally low in the mantle, has been stirred up to be nearer the surface by the large transient crater of that basin. The same sort of turbulent lifting of heavy elements probably occurred beneath the floor of the Near Side Megabasin, which has an even stronger thorium and KREEP signature on its surface.
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