The Moon is a spherical rocky body, probably with a small metallic core, revolving around Earth in a slightly eccentric orbit at a mean distance of about 384,000 km (238,600 miles). Its equatorial radius is 1,738 km (1,080 miles), and its shape is slightly flattened in a such a way that it bulges a little in the direction of Earth. Its mass distribution is not uniform—the centre of mass is displaced about 2 km (1.2 miles) toward Earth relative to the centre of the lunar sphere, and it also has surface mass concentrations, called mas-cons for short, that cause the Moon's gravitational field to increase over local areas. The Moon has no global magnetic field like that of Earth, but some of its surface rocks have remanent magnetism, which indicates one or more periods of magnetic activity in the past. The Moon presently has very slight seismic activity and little heat flow from the interior, indications that most internal activity ceased long ago.
Scientists now believe that more than four billion years ago the Moon was subject to violent heating—probably from its formation—that resulted in its differentiation, or chemical separation, into a less dense crust and a more dense underlying mantle. This was followed hundreds of millions of years later by a second episode of heating—this time from internal radioactivity—that resulted in volcanic outpourings of lava. The Moon's mean density is 3.34 g/cm3 (1.93 oz/in3), close to that of Earth's mantle. Because of the Moon's small size and mass, its surface gravity is only about one-sixth of Earth's; it retains so little atmosphere that the molecules of any gases present on the surface move without collision.
In the absence of an atmospheric shield to protect the surface from bombardment, countless bodies ranging in size from asteroids to tiny particles have struck and cratered the Moon. This has formed a debris layer, or regolith, consisting of rock fragments of all sizes down to the finest dust. In the ancient past the largest impacts made great basins, some of which were later partly filled by the enormous lava floods. These great dark plains, called maria (Latin for "seas"), are clearly visible to the naked eye from Earth. The dark maria and the lighter highlands, whose unchanging patterns many people recognize as the "man in the moon," constitute the two main kinds of lunar territory. Lunar mountains, located mostly along the rims of ancient basins, are tall but not steep or sharp-peaked, because all lunar landforms have been eroded by the unending rain of impacts.
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