A crater that covers nearly a quarter of the Moon’s surface has revealed new information about how Earth’s natural satellite companion formed, and the findings have tremendous implications, the researchers say.
A new analysis of material ejected from the impact of the South Pole-Aitken 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.”
On a Moon that is absolutely covered in impact scars, the South Pole-Aitken basin really stands out. At 2,500 kilometers (1,550 miles) wide 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 giant impact about 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 quite hot and malleable, and the impact would have “splashed” a significant amount of material from below the surface.
Because the basin is on the opposite side of the Moon, it has not been as easy to study as the side of the Moon facing us. The researchers have now run a new simulation of the splash pattern of 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 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 spots. These are the lunar marias, wide plains of dark basalt from ancient volcanic activity within the Moon.
In contrast, the far side is much paler, with fewer basalt patches and many more craters. The far side crust is also thicker and has a different composition than the near side.
Most of the thorium that we have detected appears on the near side, so its presence is generally interpreted as related to this difference between the two sides. But a link to the ejection from the South Pole-Aitken impact tells a different story.
Thorium from the Moon was deposited during a period known as the Lunar Magma Ocean. At this time, between 4.5 and 4.4 billion years ago, the Moon is believed to have been covered in 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 the lighter elements floated to the top to form the crust. Since thorium is not readily incorporated into mineral structures, it would have remained in the molten layer sandwiched between these two layers, sinking only toward the core during or after crust and mantle crystallization.
According to the new analysis, when the South Pole-Aitken impact occurred, it excavated a large amount of thorium from this layer, splattering 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 must have been distributed globally at that time, rather than being concentrated on the lunar near side.
The South Pole-Aitken impact also melted rocks deeper than the ejecta. Compositionally, this is very different from material sprayed across the surface, with very little thorium. In turn, this suggests that the upper mantle had two layers of different composition at the time of impact that were exposed in different ways.
Since then, the material from the impact splashes has been covered by more than 4 billion years of craters, 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 on future lunar missions.
“The formation of the South Pole-Aitken 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 ejection and impact melt, ”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 should be considered among the highest priority targets for the advancement of planetary science.”
The research has been published in JGR planets.