A crater covering nearly a quarter of the lunar surface has revealed new information about how Earth’s natural satellite buddy came to be – and the findings have huge implications, 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 of the development of the lunar mantle and crust by using radioactive thorium to determine the sequence of events. track down.
“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 moon’s formation and evolution.”
On a moon absolutely covered with impact scars, the South Pole-Aitken basin really stands out. At 2,500 kilometers (1,550 miles) in diameter 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 the time, the moon was still quite warm and malleable, and the impact would have “splashed” a significant amount of material from beneath the surface.
Since the basin is on the other side of the moon, it was not as easy to study as the side of the moon facing us. Researchers have now performed a new simulation of the South Pole-Aitken impact splash pattern and found that where the ejecta should have dropped corresponds to thorium deposits on the lunar surface.
One of the strange things about the moon is that the near side and the other side are very different from each other. The nearest side – always facing the Earth – is covered with dark spots. These are the lunar maria, wide expanses of dark basalt from ancient volcanic activity in the moon.
The other side, on the other hand, is much paler, with fewer basalt patches and many more craters. The crust on the other side is also thicker and has a different composition than the other side.
Most of the thorium we 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 point, about 4.5 to 4.4 billion years ago, the moon is believed to have been covered with molten rock that gradually cooled and solidified.
During this process, thicker minerals sink to the bottom of the molten layer to form the mantle, and lighter elements float to the top to form the crust. Because thorium is not easily incorporated into mineral structures, it would have remained in the molten layer between these two layers and would not sink to the core until 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 heap of thorium from this layer, splashing it on the near side of the lunar surface.
This means that the impact would have occurred before the thorium layer subsided. It also suggests that the thorium layer must be scattered globally at that point, rather than concentrated on the near moon.
The South Pole-Aitken impact also melted rock from greater depths than the ejecta. This is very different in composition from the material sprayed over the surface, with very little thorium. This, in turn, suggests that the upper mantle at the time of impact had two different compositionally different layers exposed in different ways.
The impact splash material has since been covered by craters and weathering and volcanic activity for more than 4 billion years, but the team has managed to locate several pristine thorium deposits in recent impact craters. These will be important locations to visit in future lunar missions.
“Formation of the South Pole-Aitken Basin is one of the oldest and most important events in the history of the Moon. It not only affected the thermal and chemical evolution of the lunar mantle, but it also preserved heterogeneous mantle materials on the lunar surface in the Moon. shape of ejecta and impact melt, ”the researchers wrote in their paper.
“As we enter a new era of international and commercial lunar exploration, these lunar surface mantle materials should be regarded as one of the top priority goals for the advancement of planetary science.”
The research is published in JGR planets