Humans have walked on the Moon, collecting samples which were thoroughly analyzed, and some became travelling exhibits. I still recall the emotion of watching a thin section of a moonrock under my microscope while still a university student [too!] many years ago. And what I saw so nicely matched the model I have already assimilated from my Profs: the Moon is probably a chunk of the primitive Earth ripped off during the late accretion stage of the solar system because of the impact with a massive body (possibly a proto planet). And because it was much smaller than the earth, the Moon cooled faster, its surface the frozen image of the molten inferno it was in the beginning. Once the surface solidified, it started preserving the scars of numerous impacts, which were filled with flat-laying basalt flows—maria—a landmark of our moon (or should we call them a ‘moonmark’?). Furthermore, because the cooling took place before a full gravitational separation of all chemical elements (with the accompanying gravitation friction heat) took place, there was not enough time for plate tectonics to be initialized. Plate tectonics on Earth are believed to be a recycling mechanism of rocks which keeps the mantle active and hot-enough to drive it.

It soon became dogma that moonrocks are essentially mafic i.e. rich in magnesium and iron. It therefore came as a surprise when Apollo 14 brought back some fragments of genuine granite. As these granites have been radiometrically dated at nearly 4 billion years, it is now believed to be evidence of water having been present in the infancy of the Moon, before its ‘thermal death’.

NASA has recently brought new evidence that is putting all that neatly assembled interpretation of field data into question. On the dark (or ‘far’ depending on one’s taste) side of our moon, evidence of silicic volcanism was revealed by the successive data provided by the Lunar Prospector in 1998 and Lunar Reconnaissance Orbiter (2011). In order to understand why this is so important, a little petrographic ‘excursion’ is needed….

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