The Moon, like its companion the Earth, was born about 4 600 000 000 years ago. Up to the beginning of the Seventies there were many different theories about why and how this celestial body appeared on the Earth's horizon. One hypothesis stated that the Moon was the "daughter of the Earth", a fragment that detached from our planet and moved into an orbit around it. Then there was the totally opposite alternative: the Moon had been formed somewhere far from the solar system and had been captured by the Earth only when, during its wandering, it came close enough to our world with the right speed to enter its orbit and avoid a collision. Both theories had their good points and bad points, but neither of the two could be completely refuted. Only the analysis of the moon rocks brought to the Earth's laboratories made it possible to reject both. In fact, the first thing that was discovered was that the moon rocks, even if they belong to known categories on the Earth, are so different from those on our planet that the possibility that the Moon had once been a part of our world had to be ruled out. On the other hand, the lunar rocks, though different from those on the Earth, do contain "signs" that indicate that the Moon could not have been formed very far from the Earth. One of these "signs" consists of the oxygen in the composition of the rocks. Even on the Earth, the atmosphere, the water or the rocks do not all have the same amount of oxygen: the nucleus of the most widely diffused oxygen atom contains 16 particles - 8 neutrons and 8 protons - and for this reason is called oxygen 16. There are two other types of oxygen, which contain, respectively, one or two additional neutrons. They are called oxygen 17 and oxygen 18. It is the "dosage" of these three types (or isotopes) of oxygen, i.e. the mutual quantities present, which for the Earth creates a sort of factory seal that was impressed at the time of its origin. In fact, by studying meteorites (the rock fragments which occasionally fall on our planet from other regions of the solar system) we know that the "recipe" changes in relation to the place in which the rocks were formed. If the Moon had been created far from the Earth, then it should have a recipe that is quite different than the planet. Instead, the ratio between the different types of oxygen in the moon rocks is practically identical to what is found in the Earth rocks. Therefore, the Moon must have been formed in a region not very far from the Earth, and probably in an orbit that is not very different from the present one: fragments and particles from the huge cloud of dust and gas that formed the solar system at that time condensed into two nuclei in the area of the third planet, a smaller one, the Moon, and a larger one, the Earth. Therefore, our satellite is an excellent witness of the entire history of the Earth and has even shared a good part of it. But then, on the Moon, time came to a grinding halt.
The Moon, like its companion the Earth, was born about 4 600 000 000 years ago. Up to the beginning of the Seventies there were many different theories about why and how this celestial body appeared on the Earth's horizon. One hypothesis stated that the Moon was the "daughter of the Earth", a fragment that detached from our planet and moved into an orbit around it. Then there was the totally opposite alternative: the Moon had been formed somewhere far from the solar system and had been captured by the Earth only when, during its wandering, it came close enough to our world with the right speed to enter its orbit and avoid a collision. Both theories had their good points and bad points, but neither of the two could be completely refuted. Only the analysis of the moon rocks brought to the Earth's laboratories made it possible to reject both. In fact, the first thing that was discovered was that the moon rocks, even if they belong to known categories on the Earth, are so different from those on our planet that the possibility that the Moon had once been a part of our world had to be ruled out. On the other hand, the lunar rocks, though different from those on the Earth, do contain "signs" that indicate that the Moon could not have been formed very far from the Earth. One of these "signs" consists of the oxygen in the composition of the rocks. Even on the Earth, the atmosphere, the water or the rocks do not all have the same amount of oxygen: the nucleus of the most widely diffused oxygen atom contains 16 particles - 8 neutrons and 8 protons - and for this reason is called oxygen 16. There are two other types of oxygen, which contain, respectively, one or two additional neutrons. They are called oxygen 17 and oxygen 18. It is the "dosage" of these three types (or isotopes) of oxygen, i.e. the mutual quantities present, which for the Earth creates a sort of factory seal that was impressed at the time of its origin. In fact, by studying meteorites (the rock fragments which occasionally fall on our planet from other regions of the solar system) we know that the "recipe" changes in relation to the place in which the rocks were formed. If the Moon had been created far from the Earth, then it should have a recipe that is quite different than the planet. Instead, the ratio between the different types of oxygen in the moon rocks is practically identical to what is found in the Earth rocks. Therefore, the Moon must have been formed in a region not very far from the Earth, and probably in an orbit that is not very different from the present one: fragments and particles from the huge cloud of dust and gas that formed the solar system at that time condensed into two nuclei in the area of the third planet, a smaller one, the Moon, and a larger one, the Earth. Therefore, our satellite is an excellent witness of the entire history of the Earth and has even shared a good part of it. But then, on the Moon, time came to a grinding halt.
