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.
A monochrome mosaic of the Moon from NASA’s Lunar Reconnaissance Orbiter, centered in the middle of the South Pole-Aitken Basin. Image credit: NASA / Goddard / Arizona State University.
Mafic Mound was first discovered in the 1990s by team member Dr Carle Pieters.
The mound is roughly 2,625 feet (800 m) tall and 46.6 miles (75 km) across. It lies near the center of the vast South Pole-Aitken Basin.
The structure has a different mineralogical composition than the surrounding rock. It is rich in high-calcium pyroxene, whereas the surrounding rock is low-calcium.
A new study published in the Geophysical Research Letters 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 lead author Daniel Moriarty, a PhD student at Brown University.
Dr Pieters and Moriarty looked at datasets from Chandrayaan-1’s Moon Mineralogy Mapper, and NASA’s Lunar Orbiter Laser Altimeter and the agency’s GRAIL mission. Those datasets suggested that Mafic Mound was created by one of two volcanic processes set in motion by the giant South Pole-Aitken impact. Some researchers suggest that an impact of that size would have created a cauldron of melted rock as much as 31 miles (50 km) deep.
As that sheet of impact melt cooled and crystallized, 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.
Such a process could explain the strange mineralogy of Mafic Mound.
Volcanic processes touched off by a huge impact appear to have created Mafic Mound, an enigmatic feature near the center of the South Pole-Aitken Basin (the largest and oldest recognized impact basin on the Moon; it’s diameter is roughly 1,550 miles). Image credit: NASA / Goddard / Arizona State University.
Models of how the South Pole-Aitken melt sheet may have crystallized suggest that the erupting material should be rich in high-calcium pyroxene, which is consistent with the observed mineralogy of the structure.
Another scenario that fits the data involves possible melting of the lunar 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 said.
And if either is true, it would represent a unique process on the Moon’s 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,” Moriarty said.
_____
Daniel P. Moriarty III & Carle M. Pieters. The nature and origin of Mafic Mound in the South Pole-Aitken Basin. Geophysical Research Letters, published online October 15, 2015; doi: 10.1002/2015GL065718
