The crater, which covers almost a quarter of the Moon’s surface, has revealed new information about how the Earth’s natural satellite friend was formed – and the findings have huge implications, researchers say.
A new analysis of the material ejected from the impact in the South Pole-Aitken basin enabled scientists to specify the time frame 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 shock scars, the South Pole-Aitken basin really stands out. With 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 created 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 that time, the Moon was still quite warm and supple, and the blow would “splash” a significant amount of material below the surface.
Since the basin is on the far lunar side, it was not as easy to study as the one facing the Moon. Researchers have now performed a new simulation of the spray pattern from the South Pole-Aitken impact and found that the place where the ejection was supposed to fall corresponds to thorium deposits on the lunar surface.
One of the unusual things about the Moon is that the near and far sides are very different from each other. The near side – which is always facing the Earth – is covered with dark spots. These are the moons, the wide plains of dark basalt of ancient volcanic activity within the moon.
In contrast, the far side is far paler, with fewer basalt spots and many more craters. The bark on the far side is also thicker and has a different composition than the closer one.
Most of the thorium we have discovered appears on the close side, so its presence is usually interpreted as related to this difference between the two sides. But the link to the South Pole-Aitken strike is a different story.
The lunar thorium was deposited during a period known as the lunar ocean of magma. At this time, about 4.5 to 4.4 billion years ago, it is believed that the Moon was covered with a melted rock that gradually cooled and solidified.
During this process, dense minerals sank to the bottom of the molten layer to form a mantle, and lighter elements floated to the top to form a crust. Since thorium cannot be easily incorporated into mineral structures, it would remain in the molten layer between the two layers, only to descend towards the core during or after crystallization of the crust and mantle.
According to a new analysis, when the South Pole-Aitken impact struck, it excavated a whole bunch of thorium from this layer, spraying it on the lunar surface on the near side.
This means that the impact could occur before the thorium layer has sunk. It also suggests that the thorium layer must have been globally distributed at the time, rather than concentrating on the lunar near side.
The impact of the South Pole-Aitken also melted the rocks from greater depths than the ejection. In general, this is very different from the material scattered 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.
The impact-resistant material has been obscured by over 4 billion years of cratering and weathering and volcanic activity, but the team has managed to locate several intact thorium deposits in recent craters. These will be important places 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 materials on the lunar surface in the form of ejections and impact cannons,” they wrote. researchers in their work.
“As we enter a new era of international and commercial lunar exploration, these mantle materials on the lunar surface must be considered among the priority goals for the advancement of planetary science.”
The research was published in JGR Planets.