Lunar mare

The lunar maria /ˈmɑːriə/ (singular: mare /ˈmɑːreɪ/) are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for 'seas', by early astronomers who mistook them for actual seas. They are less reflective than the 'highlands' as a result of their iron-rich composition, and hence appear dark to the naked eye. The maria cover about 16% of the lunar surface, mostly on the side visible from Earth. The few maria on the far side are much smaller, residing mostly in very large craters. The traditional nomenclature for the Moon also includes one oceanus (ocean), as well as features with the names lacus (lake), palus (marsh), and sinus (bay). The last three are smaller than maria, but have the same nature and characteristics. The lunar maria /ˈmɑːriə/ (singular: mare /ˈmɑːreɪ/) are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for 'seas', by early astronomers who mistook them for actual seas. They are less reflective than the 'highlands' as a result of their iron-rich composition, and hence appear dark to the naked eye. The maria cover about 16% of the lunar surface, mostly on the side visible from Earth. The few maria on the far side are much smaller, residing mostly in very large craters. The traditional nomenclature for the Moon also includes one oceanus (ocean), as well as features with the names lacus (lake), palus (marsh), and sinus (bay). The last three are smaller than maria, but have the same nature and characteristics. The names of maria refer to sea features (Humorum, Imbrium, Insularum, Nubium, Spumans, Undarum, Vaporum, Procellarum, Frigoris), sea attributes (Australe, Orientale, Cognitum, Marginis), or states of mind (Crisium, Ingenii, Serenitatis, Tranquillitatis). Mare Humboldtianum and Mare Smythii were established before the final nomenclature, that of states of mind, was accepted, and do not follow this pattern. When Mare Moscoviense was discovered by the Luna 3, and the name was proposed by the Soviet Union, it was only accepted by the International Astronomical Union with the justification that Moscow is the state of mind. The ages of the mare basalts have been determined both by direct radiometric dating and by the technique of crater counting. The radiometric ages range from about 3.16 to 4.2 Ga, whereas the youngest ages determined from crater counting are about 1.2 Ga (1 Ga = 1 billion years old). Nevertheless, the majority of mare basalts appear to have erupted between about 3 and 3.5 Ga. The few basaltic eruptions that occurred on the far side are old, whereas the youngest flows are found within Oceanus Procellarum on the nearside. While many of the basalts either erupted within, or flowed into, low-lying impact basins, the largest expanse of volcanic units, Oceanus Procellarum, does not correspond to any known impact basin.