A crater covering nearly a quarter of the Moon’s surface has revealed new information about how the Earth’s natural satellite friend formed – and the discoveries have enormous implications, the researchers say.
A new analysis of the material evacuated from the impact of the South Pole-Aitken Basin allowed scientists to refine the chronology of the development of the lunar mantle and crust, using radioactive thorium to discover the order of events.
“These results,” wrote a team of researchers led by planetary geologist Daniel Moriarty of NASA’s Goddard Space Flight Center, “have important implications for understanding the formation and evolution of the Moon.”
On a Moon that is absolutely covered in impact scars, the South Pole basin of Aitken stands out. At 2,500 kilometers (1,550 miles) and up to 8.2 kilometers (5.1 miles) deep, it is one of the largest impact craters in the solar system.
It was produced by a huge impact about 4.3 billion years ago, when the solar system (now 4.5 billion years old) was still a child. At this time, the Moon was still quite warm and malleable, and the impact would have “splashed” a significant amount of material below the surface.
Because the basin is on the far side of the moon, it has not been as easy to study as the side of the moon facing us. Researchers have now performed a new simulation of the splash pattern from the impact of the South Pole of Aitken and found that the place where the eject should have fallen corresponds to the thorium deposits on the lunar surface.
One of the great things about the Moon is that the near and far sides are very different from each other. The nearby part – which always faces the Earth – is covered in dark spots. These are the lunar maria, the vast dark basalt plains of ancient volcanic activity inside the Moon.
Instead, the far side is much paler, with fewer basalt spots and many more craters. The crust on the far side is also thicker and has a different composition from the near side.
Most of the thorium we detected appears nearby, so its presence is usually interpreted as related to this difference between the two parts. But a connection to the ejection from the impact of the South Aitken Pole tells another story.
The thorium of the Moon was deposited in a period known as the Lunar Magma Ocean. At this time, about 4.5-4.4 billion years ago, the Moon is believed to have been covered by molten rocks that have cooled and gradually solidified.
During this process, denser minerals sank to the bottom of the molten layer to form the mantle, and lighter elements floated to the top to form the crust. Because thorium is not easily incorporated into mineral structures, it would have remained in the molten layer between these two layers, sinking only to the core during or after crystallization of the crust and mantle.
According to the new analysis, when the impact of the South Pole hit Aitken, it excavated a whole pile of thorium from this layer, sprinkling it on the lunar surface on the nearby side.
This means that the impact would have occurred before the thorium layer sank. It also suggests that the thorium layer should be distributed globally at that time, instead of focusing on the near lunar part.
The impact of the South Pole Aitken also melted the rock from depths greater than ejecta. Compositionally, this is very different from the material sprayed on the surface, with very little thorium. In turn, this suggests that the upper mantle had two distinct compositional layers at the time of impact, which were exposed in different ways.
The impact spraying material has been covered by over 4 billion years of cratering and weather and volcanic activity, but the team has managed to locate several clean thorium deposits in recent impact craters. These will be important places to visit in future monthly missions.
“The formation of the South Pole-Aitken basin is one of the oldest and most important events in the lunar history. Not only did it affect the thermal and chemical evolution of the lunar mantle, but it kept heterogeneous mantle materials on the lunar surface in the form of ejection and melting impact, “the researchers wrote in their paper.
“As we enter a new era of international and commercial lunar exploration, these lunar surface mantle materials must be considered among the highest priority targets for the advancement of planetary science.”
The research was published in Planet JGR.