What caused this weird mound on the moon?

"It's the largest confirmed impact structure in the solar system and has shaped many aspects of the evolution of the moon," says Daniel Moriarty. (Credit: NASA)

A huge mound near the moon’s south pole appears to be a volcanic structure unlike any other found on the lunar surface.

Mafic Mound stands about 800 meters (2,625 feet) tall and is 75 kilometers (47 miles) across. It’s smack in the middle of a giant impact crater known as the South Pole-Aitken Basin. Now researchers say the impact that formed the basin may have touched off volcanic processes.

“If the scenarios that we lay out for its formation are correct, it could represent a totally new volcanic process that’s never been seen before,” says Daniel Moriarty, a PhD student at Brown University.

The findings are published in Geophysical Physical Letters.

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Mafic Mound (mafic is a term for rocks rich in minerals such as pyroxene and olivine) was first discovered in the 1990s. What makes it curious, other than its substantial size, is the fact that it has a different mineralogical composition than the surrounding rock. The mound is rich in high-calcium pyroxene. The surrounding rock is low-calcium.

“This unusual structure at the very center of the basin begs the question: ‘What is this thing, and might it be related to the basin formation process?'” Moriarty says.

topographic view of the South Pole-Aitken Basin
A topographic view of the South Pole-Aitken Basin. Reds are high; blues are low. Mafic Mound is the reddish area in the center. (Credit: NASA/Goddard/MIT/Brown)

To investigate that, researchers looked at a rich suite of data from multiple lunar exploration missions. They used detailed mineralogical data from the Moon Mineralogy Mapper, which flew aboard India’s Chandrayaan-1 spacecraft. NASA’s Lunar Orbiter Laser Altimeter provided precise topographic data, and data from the GRAIL mission characterized gravitational anomalies in the region.

Those combined datasets suggest that Mafic Mound was created by one of two unique volcanic processes set in motion by the giant South Pole-Aitken impact. An impact of that size would have created a cauldron of melted rock as much as 50 kilometers deep, some researchers think.

As that sheet of impact melt cooled and crystalized, it would have shrunk. As it did, still-molten material in the middle of the melt sheet may have been squeezed out the top like toothpaste from a tube. Eventually, that erupted material may have formed the mound.

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Such a process could explain the mound’s strange mineralogy. Models of how the South Pole-Aitken melt sheet may have crystalized suggest that the erupting material should be rich in high-calcium pyroxene, which is consistent with the observed mineralogy of the mound.

Another scenario that fits the data involves possible melting of the moon’s mantle shortly after the South Pole-Aitken impact. The impact would have blasted tons of rock out of the basin, creating a low-gravity region. The lower gravity condition could have enabled the center of the basin to rebound upward. Such upward movement would have caused partial melting of mantle material, which could have erupted to form the mound.

These scenarios make for a strong fit to those very detailed datasets, Moriarty says. And if either is true, it would represent a unique process on lunar surface. A sample return mission to the South Pole Aitken Basin would be a great way to try to verify the results.

The basin has long been an interesting mission target for lunar scientists.

“It’s the largest confirmed impact structure in the solar system and has shaped many aspects of the evolution of the moon,” Moriarty says. “So a big topic in lunar science is studying this basin and the effects it had on the geology of the moon through time.”

A sample return mission to the basin could bring back bits of lunar mantle, the composition of which is still not fully understood. A returned sample could also put a firm date on when the impact occurred, which could be used as a standard to date other features on the surface.

Source: Brown University