Scientists decided to find out the exact timeline of the Moon's development, paying special attention to the South Pole basin — Aitken
The oldest, deepest and largest crater on the Moonformed as a result of a powerful impact about 4.3 billion years ago, at the beginning of the history of the Earth's satellite. Its shape and composition can give researchers an idea of what the moon was like at the time.
Modeling shows that the impactthat caused the SPA was strong enough to eject materials from the moon's early mantle. The simulated burst pattern corresponds to thorium-rich regions of the lunar surface. Scientists hypothesize that this ejected material represents the "waste" of the early molten mantle. When the Moon's early magma ocean cooled, minerals and elements crystallized and either rose to become part of the crust or sank to become part of the lower mantle. Later in this process, thorium and other dense, incompatible elements remained in the sediments sandwiched between the crust and mantle.
To explain the sharp difference between near andthe far side of the moon, early theories suggested that thorium scum was found only on the near side. However, new results show that these substances were ejected by a blow on its back. This observation implies that during the collision, the thorium-rich material was distributed throughout the satellite and had not yet migrated to the near side or descended to the core. This usually occurs at the end of the mantle crystallization process due to its high density.
Shown here are the thorium concentrations measuredLunar Prospector in the vast South Pole-Aitken Basin on the far side of the Moon, which shows how mantle material ejected by an impact event about 4.3 billion years ago is now spreading over the surface. Warmer colors represent higher concentrations; contours are plotted at 0.5 ppm intervals. Photo: Daniel P. Moriarty III.
Unlike discarded materials, the breed,melted as a result of exposure to SPA, does not contain a large amount of thorium. Its lack in the melt indicates that the early mantle had separate layers: a thorium-rich layer just below the crust and another layer below it.
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