The Near Side
The face of the Moonturned toward us is termed the near side (image at right).It is divided into light areas called the Lunar Highlands and darkerareas called Maria (literally, "seas"; thesingular is Mare). TheMaria are lower in altitude than the Highlands, but there is no water on theMoon so they are not literally seas( Recent evidence from the Clementine spacecraft suggests that there may be somewater on the Moon, contrary to previous assumptions). See also here .The dark material filling the Maria isactually dark, solidified lava from earlier periods of Lunar volcanism.Both the Maria and the Highlands exhibit large cratersthat are the result of meteor impacts. There are many more such impact cratersin the Highlands.
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The Far SideThe side of the Moon unseen from the Earth is called the far side. Oneof the discoveries of the first Lunar orbiters is that the far side has a verydifferent appearance than the near side. In particular, there are almost no Maria on the far side, as illustrated in the image shown to the left of aportion of the far side surface. In this figure a number of meteor impactcraters are visible.
Cratering DensityThe amount of cratering is usually an indication of the age of a geologicalsurface: the more craters, the older the surface, because if the surface isyoung there hasn"t been time for many craters to form. Thus, the Earth has arelatively young surface because it has few craters. This is because the Earthis geologically active, with plate tectonics and erosion having obliteratedmost craters from an earlier epoch. In contrast the surface of the Moon ismuch older, with much more cratering. Further, different parts of the surfaceof the Moon exhibit different amounts of cratering and therefore are ofdifferent ages: the maria are younger than the highlands, because they havefewer craters.The oldest surfaces in the Solar System are characterized by maximalcratering density. This means that one cannot increase the density ofcraters because there are so many craters that, on average, any new crater that is formed by a meteor impact will obliterate aprevious crater, leaving the total number unchanged. Some regions of the moonexhibit near maximal cratering density, indicating that they are very old.
The Lunar Surface MaterialThe bulk density of the Moon is 3.4 g/cc, which iscomparable to that of (volcanic) basaltic lavas on the Earth (however, the bulkdensity of the Earth is 5.5 g/cc, because of the dense iron/nickel core).The Moon is coverered with a gently rolling layer of powdery soil withscattered rocks that is called the regolith; it is made from debrisblasted out of the Lunar craters by the meteor impacts that created them. Eachwell-preserved Lunar crater is surrounded by a sheet of ejected material calledthe ejecta blanket.
Geological CompositionOne striking difference between the Lunar surface material and that of Earthconcerns the most common kinds of rocks. On the Earth, the most common rocksare sedimentary, because of atmospheric and water erosion of thesurface. On the Moon there is no atmosphere to speak of and little or nowater, and themost common kind of rock is igneous ("fire-formed rocks").Geologically, the Lunar surface material has the following characteristics:The Maria are mostly composed of dark basalts, which form from rapidcooling of molten rock from massive lava flows.The Highlands rocks are largely Anorthosite, which is a kind ofigneous rock that forms when lava cools more slowly than in the case ofbasalts. This implies that the rocks of the Maria and Highlands cooled atdifferent rates from the molten state and so were formed under differentconditions.Breccias, which are fragments of different rocks compacted and weldedtogether by meteor impacts, are found in the Maria and the Highlands, but aremore common in the latter.
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Lunar Soils contain glassy globules not commonly found on the Earth.These are probably formed from the heat and pressure generated by meteorimpacts.The Anorthosites that are common in the Lunar Highlands are not common on thesurface of the Earth (The Adirondack Mountains and the Canadian Shield areexceptions). They form the ancient cores of continents on the Earth, but thesehave largely been obliterated by overlying sedimentary deposits and by platetectonic activity.