Mound near Lunar South Pole formed by Unique Volcanic Process

A topographic view of the South Pole-Aitken Basin. Reds are high; blues are low. Mafic Mound, (the reddish area in the center) stands 800 meters above the surrounding surface. Credit: NASA/Goddard/MIT/Brown

Within a giant impact basin near the moon’s south pole, there sits a large mound of mysterious origin. Research by geologists suggests the mound was formed by unique volcanic processes set in motion by the impact that formed the basin. The formation, known as Mafic Mound, stands about 800m tall and 75 km across, smack in the middle of a giant impact crater known as the South Pole-Aitken Basin. This new study suggests that the mound is the result of a unique kind of volcanic activity set in motion by the colossal impact that formed the basin.

“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,” said Daniel Moriarty, PhD

Mafic Mound (mafic is a term for rocks rich in minerals such as pyroxene and olivine) was first discovered in the 1990s by Carle Pieters, a planetary geologist at Brown and Moriarty’s adviser. 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, whereas the surrounding rock is low-calcium.

Moriarty and Pieters 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 suggested Mafic Mound was created by 1 of 2 unique volcanic processes set in motion by the giant South Pole-Aitken impact >> created a cauldron of melted rock as much as 50 km deep. 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.

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.A sample return mission to the South Pole Aitken Basin would be a great way to try to verify the results.

“It’s the largest confirmed impact structure in the solar system and has shaped many aspects of the evolution of the Moon,” Moriarty said. https://news.brown.edu/articles/2015/10/mound