The main features of the lunar surface were observed for the first time by Galileo in 1609, but over the centuries observations have become more and more precise and has led to the identification of three main structures: seas, upland plains and craters. The so-called seas, which have nothing to do with the Earth's oceans but were called in this way because their dark and smooth surface resembles a liquid expanse, are the youngest regions on the Moon and consist of basalt which is similar to what is found at the bottom of Earth's oceans. They are the result of gigantic eruptions which covered vast regions of the lunar surface. The largest seas, some of which have a width of even more than a thousand kilometers, are almost all located on the side that faces the Earth and were "christened" with fanciful names: Sea of Serenity, Sea of Storms, Sea of Tranquillity and the Sea of Rains. Instead, the upland plains are the oldest regions on the Moon and consist of rocks which formed as a result of cooling of the molten mass that originally formed the surface of the satellite. More than three and half billion years old, they include large mountain chains with names that are similar to those found on the Earth: Alps, Appennines, Caucasus, etc. Some of the peaks in these chains reach considerable heights, such as the 7 500 meters of Mount Doerfel, in the area of the southern lunar pole and the more than 5 000 meter tall peaks of the Appennines. However, the most outstanding elements of the lunar surface are the large and small craters that cover our satellite. Their dimensions vary from about 800 kilometers of the Eastern Basin to the dozens of centimeters of the cavities produced by the myriad of "pebbles" that have rained down from the sky. The bottom of the oldest craters sometimes appears smooth and was probably filled by lava that flowed in later periods. More recent craters have well-defined walls, with slopes with rather uniform gradients, and even terraced in some cases. Sometimes they contain isolated central peaks, especially when the diameter is greater than 40 kilometers. Identifying the nature of craters, which appear to be much more numerous on the far side of the Moon, is an arduous task. In fact, to distinguish an impact crater from another made from volcanic origin is almost impossible from Earth: first because they are very similar and second because the impacts often triggered volcanic activity. The lunar space missions were developed to answer these and other questions.
The main features of the lunar surface were observed for the first time by Galileo in 1609, but over the centuries observations have become more and more precise and has led to the identification of three main structures: seas, upland plains and craters. The so-called seas, which have nothing to do with the Earth's oceans but were called in this way because their dark and smooth surface resembles a liquid expanse, are the youngest regions on the Moon and consist of basalt which is similar to what is found at the bottom of Earth's oceans. They are the result of gigantic eruptions which covered vast regions of the lunar surface. The largest seas, some of which have a width of even more than a thousand kilometers, are almost all located on the side that faces the Earth and were "christened" with fanciful names: Sea of Serenity, Sea of Storms, Sea of Tranquillity and the Sea of Rains. Instead, the upland plains are the oldest regions on the Moon and consist of rocks which formed as a result of cooling of the molten mass that originally formed the surface of the satellite. More than three and half billion years old, they include large mountain chains with names that are similar to those found on the Earth: Alps, Appennines, Caucasus, etc. Some of the peaks in these chains reach considerable heights, such as the 7 500 meters of Mount Doerfel, in the area of the southern lunar pole and the more than 5 000 meter tall peaks of the Appennines. However, the most outstanding elements of the lunar surface are the large and small craters that cover our satellite. Their dimensions vary from about 800 kilometers of the Eastern Basin to the dozens of centimeters of the cavities produced by the myriad of "pebbles" that have rained down from the sky. The bottom of the oldest craters sometimes appears smooth and was probably filled by lava that flowed in later periods. More recent craters have well-defined walls, with slopes with rather uniform gradients, and even terraced in some cases. Sometimes they contain isolated central peaks, especially when the diameter is greater than 40 kilometers. Identifying the nature of craters, which appear to be much more numerous on the far side of the Moon, is an arduous task. In fact, to distinguish an impact crater from another made from volcanic origin is almost impossible from Earth: first because they are very similar and second because the impacts often triggered volcanic activity. The lunar space missions were developed to answer these and other questions.
