A new explanation for the two-faced Moon

Differences between the Moon’s two sides are linked to colossal ancient impact.


Moon’s far side looks far different than Moon’s near side. The Moon’s near side consists of the lunar mare — the vast, dark-colored remnants of ancient lava flows, whereas the far side of the Moon is virtually devoid of large-scale mare features. 

Why are these two sides of the Moon so different?

A new study offered a new explanation for this. It may relate to a giant impact billions of years ago near the Moon’s south pole.

The study suggests that the impact that formed the Moon’s giant South Pole–Aitken (SPA) basin must have generated a massive heat plume. That plume- consisting of a suite of rare-Earth and heat-producing elements- would have disseminated through the lunar interior.

Scientists think that the concentration of elements within the heat may probably lead to volcanism that generated nearside volcanic plains.

Matt Jones, a Ph.D. candidate at Brown University and the study’s lead author, said, “We know that big impacts like the one that formed SPA would create a lot of heat. The question is how that heat affects the Moon’s interior dynamics. We show that under any plausible conditions at the time that SPA formed, it ends up concentrating these heat-producing elements on the nearside. We expect this contributed to the mantle melting that produced the lava flows we see on the surface.”

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

Moon's nearside
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.

According to scientists, there might be a connection between the PKT and the nearside lava flows. However, it remains unclear why that suite of elements was concentrated on the nearside.

Using computer simulations of how heat generated by a giant impact would alter convection patterns in the Moon’s interior and how that might redistribute KREEP material in the lunar mantle, scientists show that widespread mantle heating from the SPA impact can catalyze the formation of the long-lived nearside-farside lunar asymmetry in incompatible elements and surface volcanic deposits.

The model suggests that the KREEP material would have ridden the plume of heat emanating from the SPA impact zone like a surfer. As the heat plume spread beneath the Moon’sMoon’s crust that material was eventually carried to a nearby region.

Scientists ran simulations for several possible impact scenarios. Each produced different heat patterns and mobilized KREEP to vary degrees, all created KREEP concentrations on the nearside, consistent with the PKT anomaly.

Scientists noted“The work provides a credible explanation for one of the Moon’s most enduring mysteries.”

“How the PKT formed is arguably the most important open question in lunar science. And the South Pole–Aitken impact is one of the most significant events in lunar history. This work brings those two things together, and I think our results are fascinating.”

Journal Reference:

  1. Marr J. Jones et al. A South Pole–Aitken impact origin of the lunar compositional asymmetry. DOI: 10.1126/sciadv.abm8475
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