A crater covering nearly a quarter of the Moon’s surface has revealed new information about how the Earth’s natural satellite companion formed, and the findings have huge implications, researchers say.
A new analysis of the material ejected from the impact of the South Aitken Pole Basin has allowed scientists to refine the timeline 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.”
In a Moon absolutely covered in impact scars, the South Pole-Aitken basin really stands out. With a depth 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.
It was produced by a giant impact about 4.3 billion years ago, when the Solar System (currently 4.5 billion years) was still a baby. At that 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 lunar side, it has not been as easy to study as the side of the Moon facing us. Researchers have now conducted a new simulation of the splash pattern of the impact of the South Pole-Aitken and have found that where the ejecta should have fallen corresponds to thorium deposits on the lunar surface.
One of the peculiar things about the Moon is that the near side and the far side are very different from each other. The near side, which always looks at the Earth, is covered in dark spots. This is the lunar mary, wide plains of dark basalt from ancient volcanic activity inside the Moon.
In contrast, the farther 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 than the one on the near side.
Most of the thorium we have detected appears on the near side, so its presence is often interpreted to be related to this difference between the two sides. But a link to the expulsion of the impact of the South Pole-Aitken tells a different story.
The thorium of the Moon was deposited during a period known as the lunar ocean magma. At this time, about 4.5 to 4.4 billion years ago, the Moon is believed to have been covered by molten rock that gradually cooled and solidified.
During this process, the 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. Since thorium is not easily incorporated into mineral structures, it would have remained in the molten layer sandwiched between these two layers, only sinking into the core during or after crystallization of the crust and mantle.
According to the new analysis, when the impact of the South Pole-Aitken impacted, he excavated a whole gang of thorium from this layer, splashing it across the lunar surface on the near side.
This means that the impact would have occurred before the thorium layer sank. It also suggests that the thorium layer should have been distributed globally rather than concentrating on the near lunar side.
The impact of the South Aitken pole also melted the rock from greater depths than the ejected ones. 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 compositionally different layers at the time of impact that were exposed in different ways.
Since then, the impact splash material has been covered for more than 4 billion years of cratering and weathering and volcanic activity, but the team managed to locate several immaculate thorium deposits in recent impact craters. These will be important places to visit in future lunar missions.
“The formation of the South Aitken Pole Basin is one of the oldest and most important events in lunar history. It not only affected the thermal and chemical evolution of the lunar mantle, but also preserved heterogeneous mantle materials on the lunar surface. in the form of expulsion and impact merge, ”the researchers wrote in their article.
“As we enter a new era of international and commercial lunar exploration, these mantle materials on the lunar surface must be considered among the highest priority goals for the advancement of planetary science.”
The research has been published in Planets JGR.