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<\/a><\/p>\nThe South Pole-Aitken impact basin on the far side of the Moon formed in a southward impact. Image credit: Jeff Andrews-Hanna \/ University of Arizona \/ NASA \/ NAOJ.<\/p>\n
\u201cThe down-range end of the basin should be covered by a thick layer of material excavated from the lunar interior by the impact, while the up-range end should not,\u201d said Dr. Jeffrey Andrews-Hanna, a planetary scientist at the University of Arizona.<\/p>\n
\u201cThis means that the Artemis missions will be landing on the down-range rim of the basin \u2014 the best place to study the largest and oldest impact basin on the Moon, where most of the ejecta, material from deep within the Moon\u2019s interior, should be piled up.\u201d<\/p>\n
It has long been thought that the early Moon was melted by the energy released during its formation, creating a magma ocean covering the entire Moon.<\/p>\n
As that magma ocean crystallized, heavy minerals sunk to make the lunar mantle, while light minerals floated to make the crust.<\/p>\n
However, some elements were excluded from the solid mantle and crust and instead became concentrated in the final liquids of the magma ocean.<\/p>\n
Those \u2018leftover\u2019 elements included potassium, rare earth elements and phosphorus, collectively referred to as KREEP.<\/p>\n
According to Dr. Andrews-Hanna and his colleagues, these elements were found to be particularly abundant on the Moon\u2019s near side.<\/p>\n
\u201cIf you\u2019ve ever left a can of soda in the freezer, you may have noticed that as the water becomes solid, the high fructose corn syrup resists freezing until the very end and instead becomes concentrated in the last bits of liquid,\u201d Dr. Andrews-Hanna said.<\/p>\n
\u201cWe think something similar happened on the Moon with KREEP.\u201d<\/p>\n
\u201cAs it cooled over many millions of years, the magma ocean gradually solidified into crust and mantle.\u201d<\/p>\n
\u201cAnd eventually you get to this point where you just have that tiny bit of liquid left sandwiched between the mantle and the crust, and that\u2019s this KREEP-rich material.\u201d<\/p>\n
\u201cAll of the KREEP-rich material and heat-producing elements somehow became concentrated on the Moon\u2019s near side, causing it to heat up and leading to intense volcanism that formed the dark volcanic plains that make for the familiar sight of the face of the Moon from Earth.\u201d<\/p>\n
\u201cHowever, the reason why the KREEP-rich material ended up on the nearside, and how that material evolved over time, has been a mystery.\u201d<\/p>\n
\u201cThe Moon\u2019s crust is much thicker on its far side than on its near side facing the Earth, an asymmetry that has scientists puzzled to this day.\u201d<\/p>\n
\u201cThis asymmetry has affected all aspects of the Moon\u2019s evolution, including the latest stages of the magma ocean.\u201d<\/p>\n
\u201cOur theory is that as the crust thickened on the far side, the magma ocean below was squeezed out to the sides, like toothpaste being squeezed out of a tube, until most of it ended up on the near side.\u201d<\/p>\n
The new study of the South Pole-Aitken basin revealed a striking and unexpected asymmetry around the basin that supports exactly that scenario: the ejecta blanket on its western side is rich in radioactive thorium, but not in its eastern flank.<\/p>\n
This suggests that the gash left by the impact created a window through the Moon\u2019s skin right at the boundary separating the crust underlain by the last remnants of the KREEP-enriched magma ocean from the \u2018regular\u2019 crust.<\/p>\n
\u201cOur study shows that the distribution and composition of these materials match the predictions that we get by modeling the latest stages of the evolution of the magma ocean,\u201d Dr. Andrews-Hanna said.<\/p>\n
\u201cThe last dregs of the lunar magma ocean ended up on the near side, where we see the highest concentrations of radioactive elements.\u201d<\/p>\n
\u201cBut at some earlier time, a thin and patchy layer of the magma ocean would have existed below parts of the far side, explaining the radioactive ejecta on one side of the South Pole-Aitken basin.\u201d<\/p>\n