The Two-Faced Moon
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In 1959, the Soviet Luna 3 space probe snapped the first picture of the Moon's far side. It proved to be very different from the hemisphere that faces Earth. The lunar far side is almost devoid of maria (a word meaning "seas," although these areas are, in fact, giant basins that long ago filled with lava and are now carpeted with dark, basaltic rock). Yet maria are quite obvious on the side we see. Measurements collected since Luna 3 have revealed many other manifestations of the Moon's hemispheric asymmetry. Explanations for how the Moon came to have two such different sides benefit from study of lunar impact basins, these structures providing a unique window on the nature and evolution of the deep lunar crust.