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Thorium concentrations on the Moon. (NASA)

The Moon's Biggest Crater Is Revealing Lunar Formation Secrets We Never Knew

17 FEBRUARY 2021

A crater that covers nearly a quarter of the Moon's surface has revealed new information on how Earth's natural satellite buddy formed - and the findings have tremendous implications, researchers say.

 

A new analysis of the material ejected from the South Pole-Aitken basin impact has allowed scientists to refine the timeline of the development of the lunar mantle and crust, using radioactive thorium to uncover 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's absolutely covered with impact scars, the South Pole-Aitken basin really stands out. At 2,500 kilometres (1,550 miles) across and up to 8.2 kilometres (5.1 miles) deep, it's one of the biggest impact craters in the Solar System.

It was produced by a giant impact around 4.3 billion years ago, when the Solar System (currently 4.5 billion years old) was still a baby. At this time, the Moon was still pretty warm and malleable, and the impact would have "splashed" a significant amount of material from below the surface.

Because the basin is on the lunar far side, it hasn't been as easy to study as the side of the Moon that faces us. Researchers have now run a new simulation of the splash pattern from the South Pole-Aitken impact, and discovered that where the ejecta should have fallen corresponds with 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 faces Earth - is covered in dark splotches. These are the lunar maria, wide plains of dark basalt from ancient volcanic activity inside the Moon.

By contrast, the far side is far paler, with fewer basalt patches, and a lot more craters. The crust on the far side is thicker, too, and it has a different composition from the near side.

Most of the thorium we've detected appears on the near side, so its presence is usually interpreted as related to this difference between the two sides. But a link to ejecta from the South Pole-Aitken impact tells a different story.

The Moon's thorium 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 have been covered by molten rock that 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 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 down towards the core during or after crystallisation of the crust and mantle.

 

According to the new analysis, when the South Pole-Aitken impact hit, it excavated a whole bunch of thorium from this layer, splashing it across the lunar surface on the near side.

This means the impact would have occurred before the thorium layer sank. It also suggests that the thorium layer must at that time have been globally distributed, instead of being concentrated on the lunar near side.

The South Pole-Aitken impact also melted rock from greater depths than the ejecta. Compositionally, this is very different from the material sprayed across the surface, with very little thorium. In turn, this suggests the upper mantle had two compositionally distinct layers at the time of the impact that were exposed in different ways.

The impact splash material has since been covered up by over 4 billion years of cratering and weathering and volcanic activity, but the team managed to locate several pristine thorium deposits in recent impact craters. These will be important sites to visit in future lunar missions.

"Formation of the South Pole-Aitken Basin is among the most ancient and important events in lunar history. Not only did it affect the thermal and chemical evolution of the lunar mantle, but it preserved heterogeneous mantle materials on the lunar surface in the form of ejecta and impact melt," the researchers wrote in their paper.

"As we enter into a new age of international and commercial lunar exploration, these mantle materials at the lunar surface must be considered amongst the highest-priority targets for the advancement of planetary science."

The research has been published in JGR Planets.