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EI2GYB > ASTRO 13.10.25 21:26l 44 Lines 5601 Bytes #28 (0) @ WW
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Subj: The Moon's Biggest Crater Tells a New Story
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The Moon's Biggest Crater Tells a New Story
The gravitational interaction between the Earth and Moon has led to one hemisphere of the Moon being locked facing away from Earth. Don't be misled though, the Moon does rotate, it just takes as long to rotate once on its axis as it takes to complete an orbit of Earth. This is known as synchronous rotation and on the far side there is a massive crater called the South Pole-Aitken basin. Spanning over 1,930 km from north to south and 1,600 km east to west. This ancient impact crater formed roughly 4.3 billion years ago when a giant asteroid delivered a glancing blow to the young Moon. A new study from the University of Arizona reveals that this colossal crater holds secrets about the Moon's formation and early evolution.
Jeffrey Andrews-Hanna and his colleagues made the discovery by carefully analysing the shape of the South Pole-Aitken basin. Giant impact basins across the Solar System share a characteristic teardrop shape, narrowing in the down range direction of the impact. Previous assumptions held that the asteroid struck from the south, but the new analysis shows the basin actually narrows toward the south, meaning the impact came from the north instead. This seemingly subtle detail has profound implications for what Artemis astronauts will find when they land near the site.
Impact craters don't distribute their excavated material evenly. The down range end of a basin usually gets buried under a thick blanket of ejecta, material blasted from deep within the Moon's interior during the collision. The up range end receives far less of this debris. Since Artemis missions are targeting the southern rim of the basin, the corrected impact direction means astronauts will be landing in exactly the right spot to study material from the Moon's deep interior, essentially getting a geological core sample without needing to drill.
What makes this discovery particularly exciting is what that excavated material contains. Early in its history, the Moon was covered by a global magma ocean. As this molten layer cooled and crystallised over millions of years, heavy minerals sank to form the mantle while lighter minerals floated to create the crust. However, certain elements resisted incorporation into solid rock and instead became concentrated in the final dregs of liquid magma. These leftover elements; potassium, rare earth elements, and phosphorus, collectively known as KREEP, refused to solidify until the very end.
The mystery has always been why KREEP ended up concentrated almost entirely on the Moon's near side, the hemisphere facing Earth. This radioactive material generated heat that drove intense volcanism, creating the dark basaltic plains that form the familiar "face" we see from Earth. Meanwhile, the far side remained heavily cratered and largely volcanic-free. The new study offers an explanation that the Moon's crust must be significantly thicker on its far side, an asymmetry that scientists still don't fully understand. The team proposes that, as the far side's crust thickened, it squeezed the remaining magma ocean beneath it toward the thinner near side.
The South Pole-Aitken impact provides crucial evidence that supports this model. The western flank of the basin shows high concentrations of radioactive thorium, a signature element in KREEP rich material, while the eastern side does not. This asymmetry suggests the impact sliced through the lunar crust right at the boundary where a thin, patchy layer of KREEP enriched magma still existed beneath parts of the far side. The collision essentially opened a window into this transitional zone between the near side's KREEP rich region and the far side's more typical crust.
When Artemis astronauts collect samples from this radioactive region and return them to Earth, scientists will have the opportunity to test these models with unprecedented detail. Those rocks may finally explain how our Moon evolved from a molten sphere into the geologically diverse world we see today, with its dramatically different hemispheres telling two very different stories of the same past.
Source : Moon's biggest impact crater made a radioactive splash
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