A crater that covers nearly a quarter of the Moon’s surface has revealed new information about how the Earth’s natural satellite formed – and the findings have huge implications, say researchers.
A new analysis of material ejected from the impact of the South Pole-Aitken watershed has allowed scientists to refine the timeline for the development of the lunar mantle and crust, using radioactive thorium to discover the sequence 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 the Moon, which is absolutely covered with impact scars, the South Paul-Aitken watershed really stands out. At 2,500 kilometers (1,550 miles) and up to 8.2 kilometers 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 (currently 4.5 billion years old) was still a baby. At that time, the Moon was still quite warm and malleable, and the impact would have “sprayed” a significant amount of material from below the surface.
Because the watershed is on the far side of the moon, it was not as easy to study as the side of the moon facing us. Researchers have now conducted a new simulation of the spray pattern from the South Pole-Aitken impact and found that where it was supposed to fall, it corresponded to thorium deposits on the Moon’s surface.
One of the unusual things about the Moon is that the near and far sides are very different from each other. The middle side – which is always facing the Earth – is covered with dark rakes. These are the lunar mary, wide plains of dark basalt from ancient volcanic activity inside the moon.
In contrast, the far side is much paler, with fewer basalt patches and many more craters. The bark on the end side is thicker and has a different composition from the close side.
Most of the thorium we have discovered appears on the near side, so its presence is usually interpreted as related to this difference between the two sides. But the connection to the South Pole-Aitken ejaculate tells a different story.
Thorium on the Moon was deposited in a period known as the Moon Ocean Magma. At that time, about 4.5 to 4.4 billion years ago, the Moon was thought to have been covered with molten rock that gradually cooled and hardened.
During this process, thicker minerals sank to the bottom of the molten layer to form a mantle, and lighter elements floated to the top to form a crust. Because thorium is not easily incorporated into mineral structures, it would remain in the molten layer between these two layers, only sinking down to the core during or after the crystallization of the crust and mantle.
According to the new analysis, during the impact of the South Pole-Aitken, he dug up a whole thorium from this layer, spraying it across the Moon from the nearby side.
This means that the impact would occur before the thorium layer sinks. It also suggests that the thorium layer at that time must be distributed globally, rather than being concentrated on the moon near the side.
The South Pole-Aitken impact also melted the rock from a greater depth than the ect. Composite, this is very different from surface-sprayed material with very little thorium. In turn, this suggests that the upper mantle had two compositionally different layers at the time of impact, which were exposed in different ways.
Hence, the spray material has been covered by more than 4 billion years of crater and atmospheric influences 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 lunar missions.
“The formation of the Aitken South Pole basin is one of the oldest and most important events in the history of the Moon. “Not only did it influence the thermal and chemical evolution of the lunar mantle, but it also preserved the heterogeneous materials of the lunar mantle in the form of melting eject and impact,” the researchers wrote in their paper.
“As we enter a new era of international and commercial lunar exploration, these lunar surface coatings must be considered among the highest priority targets for advancing planetary science.”
The research is published in Planets of JGR.