![]() They are made of relatively low-density rock that solidified on the cooling Moon like slag floating on the top of a smelter. Most of the crust of the Moon (83%) consists of silicate rocks called anorthosites these regions are known as the lunar highlands. For comparison, as we saw in the chapter on Earth, Moon, and Sky, both Earth and the Moon were formed between 4.5 and 4.6 billion years ago. The solidification ages of the samples ranged from about 3.3 to 4.4 billion years old, substantially older than most of the rocks on Earth. Once lunar samples were brought back by the Apollo astronauts, the radioactive dating techniques that had been developed for Earth were applied to them. To trace the detailed history of the Moon or of any planet, we must be able to estimate the ages of individual rocks. Lunar Highlands: The old, heavily cratered lunar highlands make up 83% of the Moon’s surface. ![]() ![]() The Moon’s relative lack of internal activity, together with the absence of air and water, make most of its geological history unlike anything we know on Earth. Even when they look somewhat similar, the origins of lunar features such as craters and mountains are very different from their terrestrial counterparts. We know today that the resemblance of lunar features to terrestrial ones is superficial. Galileo only has a small crater, however, reflecting his low standing among the Vatican scientists who made some of the first lunar maps. Among the most prominent craters are those named for Plato, Copernicus, Tycho, and Kepler. Thousands of individual craters have been named, however, mostly for great scientists and philosophers (Figure 1). In contrast, the “land” areas between the seas are not named. Their names, Mare Nubium (Sea of Clouds), Mare Tranquillitatis (Sea of Tranquility), and so on, are still in use today. They called the dark areas “seas” ( maria in Latin, or mare in the singular, pronounced “mah ray”). (credit: modification of work by NASA/Goddard/Arizona State University)Ĭenturies ago, early lunar observers thought that the Moon had continents and oceans and that it was a possible abode of life. The central mountain rises 12 kilometers above the crater floor. Sunrise on the Central Mountain Peaks of Tycho Crater, as Imaged by the NASA Lunar Reconnaissance Orbiter: Tycho, about 82 kilometers in diameter, is one of the youngest of the very large lunar craters. ![]()
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