MARE

plural Maria, any flat, dark plain of lower elevation on the Moon. The term, which in Latin means sea, was erroneously applied to such a feature of the lunar surface by telescopic observers of the 17th century. In actuality, maria are huge lava flows marked by ridges, graben, rilles, and faults and are devoid of any water. There are 14 surficial areas of this type, and all of them occur on the side of the Moon that always faces the Earth. Maria are the largest topographic features on the Moon and can be seen from the Earth with the unaided eye. (They form the face of the man in the Moon.) Samples of lunar rock and soil brought back by the U.S. Apollo astronauts proved that the maria were composed of basalt formed from surface lava flows that later congealed. The surface, down to approximately 5 metres (16 feet), shows effects of churning, fusing, and fragmenting as a result of several billions of years of bombardment by small meteoroids. This surface material is called regolith. Before Apollo landings, some astronomers feared that the surface would be so pulverized that the astronauts and their machines might sink in. The missions, however, revealed that the regolith was only somewhat compressible and was firm enough to be supportive. The maria were formed from 3,100,000,000 to 3,900,000,000 years ago. This was well after the lunar surface had cooled and solidified. The cooling of the crust had occurred first because heat could more readily radiate from there into space. The heat flow from the interior was not as efficient, and so most of the Moon remained molten for at least the first 1,500,000,000 years after its accretion some 4,600,000,000 years ago. During this early period in the solar system's history, there were many more small asteroid-sized chunks of rock than there are today. Some of these objects collided with the side of the Moon facing the Earth and ruptured through the crust, allowing molten material from below to fill the resultant craters and surrounding low-lying areas. The reason that maria are lacking on the far side of the Moon is because the outer and lighter weight molten surface material of the young Moon tended to flow outward from the Earth side to the far side as the Moon swung around the Earth in its tidally locked position. The crust became thick enough there to sufficiently reduce the chance of crustal rupture by a colliding asteroid.