A crater covering almost a quarter of the moon’s surface has revealed new information about how the Earth’s natural satellite formed – and the findings have far-reaching consequences, researchers say.
A new analysis of the material ejected from the impact of the South Pole-Aitken Basin has enabled scientists to refine the timeline for the evolution of the lunar mantle and crust by using radioactive thorium to sequence the to discover 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-Aitken basin really stands out. It is 2,500 kilometers (1,550 miles) wide and up to 8.2 kilometers deep and is one of the largest impact craters in the solar system.
It was produced about 4.3 billion years ago by a giant impact, when the solar system (currently 4.5 billion years old) was still a baby. At this point, the moon was still fairly warm and malleable, and the impact would have ‘splashed’ a significant amount of material from below the surface.
Because the bowl is on the other side of the moon, it was not as easy to study as the side of the moon looking at us. Researchers have now performed a new simulation of the splash pattern from the South Pole-Aitken impact and discovered that where the ejection should have fallen corresponds to thorium deposits on the lunar surface.
One of the strange things about the Moon is that the near and the other side are very different from each other. The near side – which always looks at the earth – is covered with dark spots. This is the lunar maria, wide plains of dark basalt from ancient volcanic activities in the moon.
In contrast, the far side is lighter, with fewer basalt stains and many more craters. The crust on the other side is also thicker and has a different composition from the near side.
Most of the thorium we have detected appears on the near side, and its appearance is usually interpreted as related to this difference between the two sides. But a link to ejection from the South Pole-Aitken impact tells a different story.
Thorium of the moon was deposited during a period known as the Lunar Magma Ocean. At this time, about 4.5 to 4.4 billion years ago, the moon is thought to be covered by molten rock that has gradually cooled and solidified.
During this process, denser minerals sank to the bottom of the molten layer to form the mantle, and lighter elements floated upward to form the crust. Since thorium could not be easily absorbed into mineral structures, it would have remained in the molten layer between these two layers, sinking just to the core during or after crystallization of the crust and mantle.
According to the new analysis, when the South Pole-Aitken impact hit, it dug up a whole bunch of thorium from this layer and splashed it on the near side over the lunar surface.
This means the impact would have occurred before the thorium layer sank. It also indicates that the thorium layer had to be distributed worldwide at that time, instead of being concentrated near the moon.
The South Pole-Aitken impact also melts rock of greater depths than the ejection. In composition, it differs greatly from the material sprayed over the surface, with very little thorium. In turn, this indicates that the upper mantle had two constituent different layers at the time of the impact that were exposed in different ways.
The impact splash material has since been covered by more than 4 billion years of cratering and weathering and volcanic activity, but the team has managed to detect several pristine thorium deposits in recent impact craters. These will be important sites to visit in future lunar missions.
“The formation of the South Pole-Aitken basin is one of the oldest and most important events in lunar history. Not only did it affect the thermal and chemical evolution of the lunar mantle, but it preserved heterogeneous mantle material on the lunar surface in the form of ejection and impact melt, ‘the researchers wrote in their paper.
“As we enter a new era of international and commercial lunar exploration, this mantle material on the lunar surface must be considered one of the highest priority objectives for the advancement of planetary science.”
The research was published in JGR Planete.