The new analysis of material released from the impact of the South Pole-Aitken basin, will allow scientists to fine-tune the timeline of development of the Moon’s mantle and crust, using radioactive thorium to unravel the sequence of events.
“These results have important implications for understanding the formation and evolution of the Moon,” said Daniel Moriarty of NASA’s Goddard Space Flight Center. Science Alert, Thursday (18/2/2021).
The South Pole-Aitken Basin is very prominent on the surface of the Moon. With a width of 2,500 kilometers and a depth of up to 8.2 kilometers, it is one of the largest impact craters in the solar system.
This crater was produced by a giant impact about 4.3 billion years ago, when the solar system was very young.
However, because the basin is on the far side of the moon, it is not as easy to study it as the side of the Moon faces Earth.
Researchers have now run new simulations of the splash pattern of the South Pole-Aitken impact and found that the site of the ejecta supposedly falls according to the deposition of thorium on the lunar surface.
The Thorium of the Moon was deposited during the period known as the Moon Magma Sea.
Today, about 4.5 to 4.4 billion years ago, the Moon is thought to have been covered in molten rock which gradually cooled and solidified.
During this process, denser minerals sink to the bottom of the liquid layer to form a mantle and lighter elements float upwards to form a crust.
Since thorium is not easily incorporated into the mineral structure, it will remain in the liquid layer sandwiched between the two layers and only sink towards the core during or after crystallization of the crust and mantle.
According to the new analysis, when the South Pole-Aitken collision occurred, it excavated large amounts of thorium from this layer and sprinkled it onto the surface of the Moon on the nearest side.
This means that the collision will occur before the thorium layer sinks. The South Pole-Aitken impact also melted rock from deeper depths than ejecta.
This indicates that the upper mantle has two layers that are compositionally different.
The splash material of the collision has been covered by billions of year-old craters and weathering and volcanic activity, but the team managed to find some pristine thorium deposits in the recent impact craters.
Scientists emphasize that the formation of the South Pole-Aitken basin is one of the most ancient and important events in the history of the Moon.
It not only influenced the thermal and chemical evolution of the Moon’s mantle, but also preserved the heterogeneous mantle material on the Moon’s surface in the form of ejecta.
In research published in JGR PlanetsIt is hoped that mantle material on the lunar surface can be considered a high priority target in future human missions to the Moon.