The Origin and Evolution of the Solar System

24 The structure of the Solar System(a)(b)Figure 1.11. A Moon-globe showing (a) the near-side (b) the far-side.1.4.6 The Earth–Moon systemThe Moon is the fifth most massive satellite in the Solar System. With a massof ¿ ¢ ½¼ ¾¾ kg and a diameter 3476 km it slots between Io and Europa, theinnermost Galilean satellites, in both mass and density. However, while in itscharacteristics it is a normal large satellite, its association with a terrestrial planetclearly makes it anomalous and an explanation of the existence of the Earth–Moonsystem is a requirement of any well-developed cosmogonic theory.1.4.6.1 Surface features of the MoonThe Moon has been examined in more detail than any body, other than the Earth,in the Solar System. It has been studied by telescopes from Earth for nearly400 years, has been the subject of manned exploration, in the Apollo missions,and also exploration by automated vehicles designed to collect particular kinds ofinformation.The side of the Moon facing the Earth shows the full range of lunar features(figure 1.11(a)). There are two general types of terrain—the highlands and themare basins. The highland regions consist of low-density heavily-cratered oldcrust. The mare basins are the result of large projectiles having struck the Moonand excavated large basins. These then were filled up from below by molten materialby successive bouts of volcanism lasting over several hundred million years.Eventually the molten material retreated into the interior of the Moon and was nolonger able to reach the surface. From radioactive dating of the mare basalts itappears that the main episodes of volcanism were between about ¿ ¢ ½¼ and¿½ ¢ ½¼ years ago. The mare regions show comparatively few craters, sincethey were excavated after the period of early bombardment by smaller bodies,