The Moon has many mysteries. One of the biggest is the Procellarum KREEP Terrain (PKT) (see Fig. 1) in the north west of the lunar nearside (the one you can see from Earth).
This areas main characteristic is that it has high concentrations in iron, thorium and the so called KREEP components (potassium (K), rare earth elements (REE) and phosphorous (P)). It also contains a lot of the lunar maria (the dark patches you can see on the Moon) which are basalts that flooded big impact basins. The KREEP components are incompatible elements, which means, if you have a crystallizing magma they tend to stay in the residual magma. This is because they do not fit very well in the first crystallizing minerals (such as olivine or plagioclase) due to their size and/or electron configuration.
A main idea for the lunar evolution is that the Moon was covered in a global magma ocean after its creation from which the lunar crust and mantle crystallized. In this process the residue containing the KREEP components was trapped between the crust and mantle. It stayed molten because of the radioactive decay of various isotopes of the KREEP components. Due to this molten state the KREEP magmas could penetrate into the crust above.
The question is: Why does that only happened in the PKT?
One possible explanation is, that the crust in that region is (by comparison) thin, which would make it easier for upwelling magmas to penetrate up to the surface. The thinning of the crust could have been caused by a mega impact about the size of the PKT. Though there is no visible sign for such an impact, it is possible that such signs could have been removed by subsequent (smaller but still basin scale) impacts. Another idea is that tidal effects (caused by the Earth’s gravitation) promoted the upwelling of the magma.
While preparing my midterm (which I should rather write right now, instead of this post J) I stumbled across a study from 2011* which proposed a much more fascinating idea about the origin of the PKT. This hypothesis directly connects the PKT with the second biggest feature of the lunar surface – the South Pole-Aitken basin (SPA in Fig. 1). This gigantic crater on the lunar farside, as the name suggests – close to the lunar South Pole, is the oldest known impact basin on the Moon and with a diameter of 2500 km the biggest known impact structure in the solar system. With hyper velocity impact experiments in the lab (see Fig. 2) and computer modelling the study shows that a low angle impact (~30°) of the South Pole-Aitken impact category would have the following effect: The shockwaves caused by the impact would travel through the Moon and reflect on the surface on the other side of the Moon. This would cause extreme stress within the crust and underlying mantle in this region. This stress would cause weakening and cracks which would allow the deep sitting residual magmas to well up. The low angle impact would account for the fact that the PKT and the South Pole-Aitken basin are not sitting directly opposite to each other as the point of impact in a low angle impact can be quite different to the centre of the resulting impact basin.
This hypothesis connects the two most prominent features of the Moon to one gigantic incidence, the mega scale impact that formed the South Pole-Aitken basin. This connection not necessarily elevates this hypothesis over others that try to explain the origin of the PKT – but it definitely adds a (scientific) beauty to it, that the other ideas lack.
*Schultz, P. H. and Crawford, D. A., 2011. Origin of nearside structural and geochemical anomalies on the Moon. The Geological Society of America Special Paper 477, 141-159.