This increase of crystal content occurs most dramatically near the surface, where the slushy magma ocean is cooled, resulting in a hot, well-mixed slushy interior and a slow-moving, crystal-rich lunar ‘lid’. If the crystals remain suspended as a crystal slurry, then when the crystal content of the slurry exceeds a critical threshold, the slurry becomes thick and sticky, and the deformation slow. In the low lunar gravity, the settling of crystal is difficult, particularly when strongly stirred by the convecting magma ocean. Michaut and Neufeld developed a mathematical model to identify this mechanism. “Given the range of ages and compositions of the anorthosites on the Moon, and what we know about how crystals settle in solidifying magma, the lunar crust must have formed through some other mechanism,” said co-author Professor Jerome Neufeld from Cambridge’s Department of Applied Mathematics and Theoretical Physics. The range of anorthosite ages – over 200 million years – is difficult to reconcile with an ocean of essentially liquid magma whose characteristic solidification time is close to 100 million years. Lunar anorthosites appear more heterogeneous in their composition than the original Apollo samples, which contradicts a flotation scenario where the liquid ocean is the common source of all anorthosites. However, since the Apollo missions, many lunar meteorites have been analysed and the surface of the Moon has been extensively studied. “This ‘flotation’ model explains how the lunar Highlands may have formed.” “Since the Apollo era, it has been thought that the lunar crust was formed by light anorthite crystals floating at the surface of the liquid magma ocean, with heavier crystals solidifying at the ocean floor,” said co-author Chloé Michaut from Ecole normale supérieure de Lyon. One of the outcomes of this collision was that the Moon was very hot – so hot that its entire mantle was molten magma, or a magma ocean. The larger of these two protoplanets became the Earth, and the smaller became the Moon. Scientists believe that the Moon formed when two protoplanets, or embryonic worlds, collided.
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