Biggest impact crater on Moon made radioactive splash
The formation of one of the biggest impact basins in the Solar System in the chaotic infancy of the Solar System splashed material from the lunar magma ocean onto the surface. Explorers around the South Pole of the Moon may stumble across this material.
About 4.3 billion years ago, soon after the formation of the Sun, a giant asteroid slammed into the far side of the Moon, resulting in one of the biggest impact craters in the Solar system, known as the South Pole-Aitken or SPA basin. This is the largest crater on the Moon, spanning nearly 2,000 km north to south and 1,600 km in the east-west direction. The asteroid struck a glancing blow instead of a head-on impact, resulting in the oblong crater. New analysis indicates that the shape of the SPA basin narrows in the south, indicating that the impactor struck from the north. It was previously believed that the impactor struck from the south.
The new research suggests that the down-range end of the basin should be covered by a thick layer of material ejected from the interior of the Moon, which contained a magma ocean, with very little such material up-range from the crater. Scientists have long known that the far side of the Moon is geochemically different from the near side, which has been reinforced by the research based on samples returned from the lunar far side for the first time by the Chinese Chang'e 6 mission. The new research indicates that this discrepancy may be because of material ejected from the interior of the Moon. The research also provides new insights into the interior of the Moon and its evolution over time.
A magma moon
In the infancy of the solar System, the Moon was a magma ocean. As it crystallised, the heavier minerals sunk to create the lunar mantle, with the lighter material floating on top to form the crust. Then, the Moon froze from the outside in. Some elements were excluded from the solid mantle crust, and became concentrated in the final liquids within the magma ocean. These included potassion, rare earth elements and phosphorous, that are together called 'KREEP'. This KREEP-rich material was sandwiched between the mantle and the crust. The research indicates that the impact ejected this material to the surface. All major spacefaring nations in the world are headed to the south pole of the Moon. Future explorers of the lunar surface may discover this material ejected billions of years ago from the interior of the Moon.