Impact: Research at Brown

Understanding the Moon’s Two Faces

New research led by Brown scholars offers a credible explanation for one of the most enduring mysteries about the Moon: why its two sides are so different.

New research led by Brown scholars offers a credible explanation for one of the most enduring mysteries about the Moon: why its two sides are so different.

The face that the Moon shows to Earth looks far different from the one it hides on its far side. The nearside is dominated by the lunar mare—the vast, dark-colored remnants of ancient lava flows. The crater-pocked far side, on the other hand, is virtually devoid of large-scale mare features. Why the two sides are so different is one of the Moon’s most enduring mysteries.

Now researchers have an explanation. A new study 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 throughout the lunar interior. That plume would have carried certain materials to the Moon’s nearside, and that concentration of elements would have contributed to the volcanism that created the nearside volcanic plains.

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

The study was a collaboration between Jones and Alexander Evans, assistant professor of earth, environmental, and planetary sciences at Brown, with researchers from Purdue University, Stanford University, University of Arizona, and NASA’s Jet Propulsion Laboratory.

“ 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. ”

Matt Jones PhD candidate at Brown and the study’s lead author

The nearside is home to a compositional anomaly known as the Procellarum KREEP Terrane (PKT)—a concentration of potassium (K), rare earth elements (REE), and 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.

“How the PKT formed is arguably the most significant open question in lunar science,” Jones said. “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 really exciting.”