It is the rocks brought back to the Earth that allow us to reconstruct the evolution of our satellite over time. The astronauts from the six "Apollo" missions basically brought three different types of rocks: a whitish rock, which is typical of the upland plains, with a similar composition to an Earth rock called anorthosite: a dark rock, which is typical of the lunar seas, similar to our basalt; and a third type, similar to Earth norite but very rich in potassium, phosphorous and rare Earth elements and therefore called norite Kreep (where K and P stand for the chemical symbols of potassium and phosphorous and "ree" stands for rare earth elements). Around four billion years ago, the young lunar crust was bombarded by a real shower of meteorites of all shapes and sizes which, as we have already seen, left deep scars on its surface. The anorthosite and norite were fragmented, broken apart and partially melted by the violence of the impacts, forming large and small craters all over the surface. This phase in the Moon's history is similar to what occurred on the planet Mercury. It is likely that these were not separate events, but that there was a moment in the history of the solar system in which huge swarms of meteorites, traveling freely through space, crashed into all the inner planets, at least beginning with Mars. If this is true, the moon rocks have told us when this took place: between four and three and half million years ago. The lunar crust broke in many points: enormous basins were formed almost everywhere and basaltic lava began to flow from the bottom of these formations. Then the flow diminished and the lava slowly cooled: this created the lunar seas. From this moment on the Moon was an inert and passive body. Its crust by now had become too thick to allow the internal forces, coming from its partially molten core, to cause volcanic eruptions or create mountain chains. There is no atmosphere or running water to change its surface. Every so often the weak tremor of a "moonquake" still shakes this almost dead body, along with the dull thud of a few lost meteorites which are attracted by its force of gravity and fall on the surface projecting dust and fragments. For the last three billion years these have been the only events on the most familiar of the celestial bodies that surround us.
It is the rocks brought back to the Earth that allow us to reconstruct the evolution of our satellite over time. The astronauts from the six "Apollo" missions basically brought three different types of rocks: a whitish rock, which is typical of the upland plains, with a similar composition to an Earth rock called anorthosite: a dark rock, which is typical of the lunar seas, similar to our basalt; and a third type, similar to Earth norite but very rich in potassium, phosphorous and rare Earth elements and therefore called norite Kreep (where K and P stand for the chemical symbols of potassium and phosphorous and "ree" stands for rare earth elements). Around four billion years ago, the young lunar crust was bombarded by a real shower of meteorites of all shapes and sizes which, as we have already seen, left deep scars on its surface. The anorthosite and norite were fragmented, broken apart and partially melted by the violence of the impacts, forming large and small craters all over the surface. This phase in the Moon's history is similar to what occurred on the planet Mercury. It is likely that these were not separate events, but that there was a moment in the history of the solar system in which huge swarms of meteorites, traveling freely through space, crashed into all the inner planets, at least beginning with Mars. If this is true, the moon rocks have told us when this took place: between four and three and half million years ago. The lunar crust broke in many points: enormous basins were formed almost everywhere and basaltic lava began to flow from the bottom of these formations. Then the flow diminished and the lava slowly cooled: this created the lunar seas. From this moment on the Moon was an inert and passive body. Its crust by now had become too thick to allow the internal forces, coming from its partially molten core, to cause volcanic eruptions or create mountain chains. There is no atmosphere or running water to change its surface. Every so often the weak tremor of a "moonquake" still shakes this almost dead body, along with the dull thud of a few lost meteorites which are attracted by its force of gravity and fall on the surface projecting dust and fragments. For the last three billion years these have been the only events on the most familiar of the celestial bodies that surround us.
