Mafic Mound, a distinctive and enigmatic feature near the Moon’s South Pole, is a volcanic structure unlike any other found on the surface of the Earth’s only natural satellite, says a team of planetary geologists at Brown University. It stands about 2,600 feet tall and 47 miles across and lies in the middle of a giant impact crater known as the South Pole-Aitken 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, a doctoral student in Brown University’s Department of Earth, Environmental and Planetary Sciences and the study’s lead author, in a press release.
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. Along with its size, Mafic Mound has been found to have a different mineralogical composition than the surrounding rock. The mound is rich in high-calcium pyroxene, whereas 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 said.To investigate that, Moriarty and Pieters looked at 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.
Together, the data suggests 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 30 miles deep. As that sheet of impact melt cooled and crystalized, it would have shrunk and molten material from the melt sheet would have squeezed out of the top, possibly forming the mound. Such a process could explain the mound’s strange mineralogy. Another scenario relates to the 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.
Either scenario fits with the data sets. But to verify the results, Moriarty said the next step would be a sample return mission to the moon’s South Pole-Aitken Basin. Such a mission would also give scientists a chance to collect bits of lunar mantle, the composition of which is still not fully understood. It 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.
“It’s the largest confirmed impact structure in the solar system and has shaped many aspects of the evolution of the Moon,” Moriarty said. “So a big topic in lunar science is studying this basin and the effects it had on the geology of the Moon through time.”
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