Explanation for the Two-Faced Moon: Colossal Ancient Impact Linked to Differences Between the Moon’s Near and Far Sides

scitechdaily.com ^ | APRIL 10, 2022 | BROWN UNIVERSITY

[BenLurkin]

A new study published in the journal Science Advances shows that the impact that formed the Moon’s giant South Pole–Aitken (SPA) basin would have created a massive plume of heat that propagated through the lunar interior. That plume would have carried certain materials — a suite of rare-Earth and heat-producing elements — to the Moon’s nearside. That concentration of elements would have contributed to the volcanism that created the nearside volcanic plains.

The Moon’s nearside (left) is dominated by vast volcanic deposits, while the far side (right) has far fewer). Why the two sides are so different is an enduring lunar mystery. Credit: Brown University

A new study reveals that an ancient collision on the Moon’s south pole changed patterns of convection in the lunar mantle, concentrating a suite of heat-producing elements on the nearside. Those elements played a role in creating the vast lunar mare visible from Earth. Credit: Matt Jones

The nearside is home to a compositional anomaly known as the Procellarum KREEP terrane (PKT) — a concentration of potassium (K), rare earth elements (REE), phosphorus (P), along with heat-producing elements like thorium. KREEP seems to be concentrated in and around Oceanus Procellarum, the largest of the nearside volcanic plains, but is sparse elsewhere on the Moon.

KREEP material would have ridden the wave of heat emanating from the SPA impact zone like a surfer. As the heat plume spread beneath the Moon’s crust, that material was eventually delivered en masse to the nearside. The team ran simulations for a number of different impact scenarios, from dead-on hit to a glancing blow. While each produced differing heat patterns and mobilized KREEP to varying degrees, all created KREEP concentrations on the nearside, consistent with the PKT anomaly.

[Verginius Rufus]

So that’s what KREEP is. Sam Ervin’s Senate committee kept talking about it in 1973.

[Svartalfiar]

Nope, the Earth doesn't provide significant shielding for the Moon. It's 8,000 miles in diameter, certainly bigger than the Moon, but it's 300,000 miles distant, meaning it provides a pretty much negligible barrier to space rocks. Visualize 8 vs. 300 -- it's like an 8-inch pie plate at a distance of 25 feet.

But the Earth's gravitational field is much bigger. So for any rock coming at the moon from the Earth-side, there's a small circle around Earth that the rock has to hit to be swung into the Moon, otherwise it misses. Needs an exact velocity on the proper vector, whereas rocks hitting the other side only need to be on the right vector, velocity doesn't matter.

No idea how much that actually affects the odds, but it seems like it would narrow them somewhat.

[dayglored]

Yep, ~240,000 not ~300,000. I was misremembering.