Moon's biggest impact crater made a radioactive splash

(Press-News.org) When astronauts land near the moon's south pole as part of NASA's Artemis program in a few years, they likely will find themselves in an unexpected treasure trove of clues that could help scientists better understand how Earth's only natural satellite came to be. That's according to a new study led by Jeffrey Andrews-Hanna, a planetary scientist at the University of Arizona.

Published on Oct. 8 in the journal Nature, the paper also provides a snapshot of the moon's tumultuous past that could help explain longstanding puzzles such as why the moon's crater-riddled far side is so dramatically different from its smooth near side, which provided the backdrop for the Apollo moon landings in the 1960s and 1970s.

Roughly 4.3 billion years ago, when the solar system was still in its infancy, a giant asteroid slammed into the far side of the moon, blasting an enormous crater referred to as the South Pole-Aitken basin, or SPA. This impact feature is the largest crater on the moon, spanning more than 1,200 miles north to south, and 1,000 miles east to west. The oblong shape of the basin is the result of a glancing blow rather than a head-on impact.

By comparing the shape of SPA to other giant impact basins across the solar system, Andrews-Hanna and his team found that these features get narrower in the down-range direction, with a shape resembling a teardrop or an avocado. Upending conventional wisdom that SPA was formed by an asteroid coming in from a southern direction, the new analysis reveals that SPA's shape narrows toward the south, indicating an impact coming from the north instead. The 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, Andrews-Hanna explained. 

"This means that the Artemis missions will be landing on the down-range rim of the basin – 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's interior, should be piled up," he said.

In the paper, the group presents additional evidence supporting a southward impact from analyses of the topography, the thickness of the crust and the surface composition. In addition, the results offer new clues about on the interior structure of the moon and its evolution through time, according to the authors.

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. As that magma ocean crystallized, heavy minerals sunk to make the lunar mantle, while light minerals floated to make the crust. However, some elements were excluded from the solid mantle and crust and instead became concentrated in the final liquids of the magma ocean. Those "leftover" elements included potassium, rare earth elements and phosphorus, collectively referred to as "KREEP " – the acronym's first letter reflecting potassium's symbol in the periodic table of elements, "K." According to Andrews-Hanna these elements were found to be particularly abundant on the moon's near side.

"If you've 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," he said. "We think something similar happened on the moon with KREEP."

As it cooled over many millions of years, the magma ocean gradually solidified into crust and mantle. "And 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's this KREEP-rich material," he said.

"All of the KREEP-rich material and heat-producing elements somehow became concentrated on the moon's 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, according to Andrews-Hanna.  However, the reason why the KREEP-rich material ended up on the nearside, and how that material evolved over time, has been a mystery. 

The moon's 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. This asymmetry has affected all aspects of the moon's evolution, including the latest stages of the magma ocean, Andrews-Hanna said.

"Our 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," he said.

The new study of the SPA impact crater 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. This suggests that the gash left by the impact created a window through the moon's skin right at the boundary separating the crust underlain by the last remnants of the KREEP-enriched magma ocean from the "regular" crust.

"Our 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," Andrews-Hanna said. "The last dregs of the lunar magma ocean ended up on the near side, where we see the highest concentrations of radioactive elements. But at some earlier time, a thin and patchy layer of magma ocean would have existed below parts of the far side, explaining the radioactive ejecta on one side of the SPA impact basin."

  Many mysteries surrounding the moon's earliest history still remain, and once astronauts bring samples back to Earth, researchers hope to find more pieces to the puzzle. Remote sensing data collected by orbiting spacecraft like those used for this study provide researchers with a basic idea of the composition of the moon's surface, according to Andrews-Hanna. Thorium, an important element in KREEP-rich material, is easy to spot, but getting a more detailed analysis of the composition is a heavier lift.  

“Those samples will be analyzed by scientists around the world, including here at the University of Arizona, where we have state -of-the-art facilities that are specially designed for those types of analyses," he said.

"With Artemis, we'll have samples to study here on Earth, and we will know exactly what they are," he said. "Our study shows that these samples may reveal even more about the early evolution of the moon than had been thought."

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