The Moon

Early telescopic observations showed that the near side of the Moon is composed of two types of surfaces: topographically low, dark areas-referred to as maria (or seas) and topographically elevated, light areas known as highlands. The highlands are more heavily cratered and are therefore

presumed to be older because the flux of meteorities is known to have decreased with time. Because of its synchronous rotation with respect to the Earth, the far side of the Moon was first observed from spacecraft in lunar. Full hemispheric image of the Moon taken by the Galileo space- craft on December 7, 1992, on its way to explore the Jupiter system in 1995–97. The distinct bright ray crater at the bottom of the image is the Tycho impact basin. The dark areas are basaltic-rock-filled impact basins: Oceanus rocellarum (on the left), Mare Imbrium (center left), Mare Seren- itatis and Mare Tranquillitatis (center), and Mare Crisium (near the right edge) (NASA Image PIA00405). orbit. Highland terrain dominates the far side of the Moon; there are no extensive maria on the farside lunar surface. The first manned landing on the Moon took place on July 20, 1969. This Apollo 11 mission as well as the subsequent Apollo 12, 15, and 17 missions landed on the lunar maria. Chemical studies of the rocks returned on thesemissions showed that the maria are composed of basaltic rocks similar in major element chemistry to the basalts of the oceanic crust. Radioactive dating of these rocks gives ages of 3.16 to 3.9 Ga. The Apollo 14 and 16 missions returned samples from the lunar highlands. These rocks have a much more complex chemical history than the mare rocks. They have been extensively shocked and melted by meteorite bombardment. Detailed chemical studies have shown, however, that these rocks are highly fractionated igneous rocks. Radioactive dating of the highland rocks indicates that they crystallized about 4.5 Ga ago, close to theestimated age of the solar system.