The surface of the Moon can, roughly speaking, be divided into two contrasting types – dark, comparatively smooth areas called MARIA (singular: mare) and brighter, more rugged regions called TERRAE. The maria cover about one-third of the Moon’s near side but are almost completely absent from the far side. Maria is the Latin word for seas and was used because of their sea-like appearance in small telescopes, although it is now known that these low-lying regions are completely waterless. They are of two types:
• Nearly circular regions generally surrounded by circular moun¬tain areas or fault scarps. Examples are Mare Imbrium, Mare Crisium and Mare Moscoviense .
• Regions of irregular outline without bordering mountain walls or fault scarps. Examples are Mare Tranquilitatis, Mare Nubium and Oceanus Procellarum.

The whole lunar surface is dominated by the craters described above although these occur more frequently on the terrae than on the maria; this is particularly true of the large craters. The maria show numerous other irregularities in their apparently smooth surfaces. Lunar DOMES are almost circular surface bulges, and their diameters range up to several kilometers, but they are never more than 100 m or so in height. At least three quarters have a small orator at the top. Only a few have been detected with certainty in the mountainous terrae. Also on the maria are several types of linear features, collectively known as LINEAMENTS. RILLES, or RIMAE are long trench-like features between a half and five kilometers wide, rarely more than 400m deep but extending across the lunar surface for hundreds of kilometers . They usually cross craters, hills and other features with little change in direction or width, showing that they must have formed later than most other lunar features. WRINKLE RIDGES are sinuous, irregular and apparently smooth elevations up to 30km wide, but rarely more than 100m high. FAULTS are dislocations of the surface with either vertical (dip-slip) or essentially horizontal (strike-slip) movement which manifest themselves as discontinuities in various other features or as linear scarps. The best known lunar fault is Rupes Recta (the Straight Wall) in Mare Nubium, a linear scarp about 120km long. It is the surface expression of a normal dip-slip fault with a vertical displacement of more than 300m.

The terrae are mountainous, heavily-cratered regions often known as the lunar HIGHLANDS and, in contrast with the maria were given the Latin name for continents. The craters are of all sizes up to 250km in diameter and are scattered over the highlands in great profusion; frequently they overlap one another. Some craters have walls rising to over 3000m above the surrounding terrain. Most of the principal mountain ranges in the lunar highlands form borders to the maria and should, perhaps, be considered to be part of the maria.

The lunar rocks that have been brought back from the Moon by Apollo astronauts were only taken from the topmost few centimeters . However we can be confident that a proportion of these rocks were originally tens of kilometers deep until they were scattered by the impacts that produced most of the craters. Most of the rocks are composed of what is called ANORTHOSITIC material and so the lunar crust is most probably composed predominantly of this type of material. Anorthositic rocks are mainly made up of one mineral – plagioclase feldspar (CaAl2Si2O8) – and are formed in pools of lava. To have produced a crust 60km thick, the lava must have been about 200km deep over the whole surface of the Moon. All the planets are believed to have formed by accretion of cold particles, but the kinetic energy of this material was sufficient to melt the whole planet. Although most of the heat would have been quickly radiated away, it seems that enough must have been retained by the Moon to melt the outermost 200km. Although the theory behind this idea is not at all certain, there does not appear to be any way to melt the surface after the Moon was formed. Being exposed to space, the molten surface would quite quickly cool down but not before a low density crust of anorthositic rocks had formed.

Another constituent of the lunar rocks is KREP NORITE. This type of rock is named after potassium (K), rare earth elements (REE) and phosphorous (P). Although these elements are only minor constituents of lunar (and terrestrial rocks) they, and others such as barium, uranium and thorium are fifty to a hundred times more abundant in KREEP norite than in other rocks. KREEP norite has a relatively low melting point and will separate out from the lunar crust if there is partial melting of the anorthositic rocks. The rook can be detected from orbiting spacecraft by the gamma rays emitted by the high concentration of the radioactive elements, uranium and thorium, as well as by direct analysis of rock samples. Such observations have shown that it is concentrated mainly in the broad area of Mare Imbrium and Oceanus Procellarum. The source of the heat required to form the KREEP material is a mystery.

The next stage of lunar history was the formation of the main surface features during a period of major impacts about four billion years ago. As the radioactive elements in rocks decay, the pro¬portions of the various decay products change and hence they can be used as a clock to determine the age of the rocks. If rocks are heated sufficiently, the decay products can be released and the clock is reset to zero. The rocks of the lunar highlands are four billion years old, although it is thought the rocks were formed shortly after the Moon – about four and a half billion years ago. They must have been heated four billion years ago, almost certainly by the violent impacts that excavated the basins of the huge circular maria and threw debris out to blanket much of the near side of the Moon. Because these impacts obliterated any older features, we do not know whether they were the only ones of this magnitude or whether they just marked the end of a period of major impacts that started when the Moon was formed. One unexplained aspect of the maria is the way in which they almost all lie on the near side of the Moon. The Earth was presumably responsible, but in what way remains to be discovered.

When these impacts had come to an end, or possibly while they were diminishing, vast quantities of lava erupted onto the lunar surface. This lava flowed into the circular maria basins where it solidified to give them their present relatively smooth appearance. Other low-lying areas were also flooded to produce the irregular, non-circular maria. The solidified lava forms the rock type known as BASALT. Comparison of lunar and terrestrial basalts show that those on the Moon have been systematically depleted of the more volatile elements such as lead, sodium and potassium. This suggests that the Moon was formed from particles in the solar nebula at a somewhat higher temperature than those that went to make up the Earth. Successive lava flows are clearly visible in the maria, and the lava continued to issue from the interior for almost a billion years. The composition of the lava changed with time; for example the oldest samples collected contain more titanium than the younger ones. This appears to be due to a change in the depth at which the lava was generated from 150km initially to 240km or more later on. Although the lunar interior was being heated by radioactive decay at this time, the outer layers were more affected by loss of heat to space and cooled down so that the layers hot enough to generate lava moved inwards. This process has continued during the last three billion years until today we have the situation described above, with a rigid lithosphere about 1000km thick. These three billion years have been a period of quiescence for the Moon. There have been very low major impacts since the maria were formed, and they have kept their .smooth appearance to the present day. They are not completely .smooth, for close-up views have shown them to be pockmarked with .small craters for¬med by minor impacts. One way of determining the relative ages of different parts of the lunar surface is to count the number of craters; the more craters per unit area, the older the surface. Since we do not know how the rate of impacts has changed with time, this method only allows us to put different areas in order of age.

Tracking of spacecraft in orbit around the Moon showed, in 1968, that the pull of gravity above the circular maria was about one per cent stronger than above adjoining regions. It is not possible conclusively to determine the origin of these gravity anomalies but it is highly significant that thin plates one or two kilometers deep that just covered the circular maria would do the job nicely. The pressure at the base of the lithosphere is the same over the whole Moon and can support the same mass per unit area at all points. The situation in which this is the case is called ISOSTASY. Astronomers generally believe that isostasy does not hold below the circular maria and that there is extra mass in their basins. This extra mass is called a MASCON (abbreviated from mass concentration) and it must be sup¬ported by the rigidity of the crust. The origin of mascons is un¬certain but one theory, due to S.K.Runcorn, is shown in figure 10.9. The irregular maria do not contain mascons. Runcorn’s theory will only work if the initial mare basin formed by meteorite impact is deep, about 20km or so. It is reasonable to suppose that the lava in the irregular maria is only a couple of kilometers thick, covering a depression in the original terrain.

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