Moon's Largest Crater May Serve as a Window into Lunar Interior

An analysis of the Moon's largest impact crater reveals a surprising diversity of minerals, suggesting the site may present a rare window into the Moon's inner contents as well as the satellite's past. 

At 2,500 km across, the South Pole Aitken (SPA) basin may be the largest impact basin in the solar system, and according to a review of data gathered by India's Chandrayaan-1 lunar orbiter, is home to a far more diverse mineralogy than once thought.

Chandrayaan-1, launched in 2008, marked India's first unmanned lunar probe. Though the mission was cut short due to technical failures, the probe was able to peer inside the SPA using its Moon Mineralogy Mapper, gathering data on the overall crater, believed to date back some 4 billion years, as well younger ones located inside it.

Scientists from the University of Hawaii and Brown University used the instrument to study the spectra of light reflected off of four central peaks formed when the material beneath the impact zone rebounded.

The results were surprisingly varied, with some peaks showing signs of higher levels of magnesium than others, suggesting the ground beneath the SPA is likely more diverse than prior estimates gave it credit for. 

"Previous studies have suggested that all the central peaks look very similar, and that was taken as evidence that everything's the same across the basin," explained Dan Moriarty, a graduate student from Brown who led the study.

The results, published in the Journal of Geophysical Research: Planets, argue otherwise.

"We looked in a little more detail and found significant compositional differences between these central peaks," Moriarty said. "The Moon Mineralogy Mapper has very high spatial and spectral resolution. We haven't really been able to look at the Moon in this kind of detail before."

It's possible, the researchers note, that the distinct minerals formed as the molten rock from the SPA impact cooled, versus pre-existing mineral differences. Recent research performed at Brown as well as other places suggest that such mineral formation during impact melt is possible.

Currently, Moriarty is working on a much larger survey of SPA craters in hopes of teasing out the true cause of the mineralogical variety.

"If you do the impact scaling from models, [the SPA impact] should have excavated into the mantle," Moriarty said. "We think the upper mantle is rich in a mineral called olivine, but we don't see much olivine in the basin. That's one of the big mysteries about the South Pole Aitken basin. So one of the things we're trying to figure out is how deep did the impact really excavate. If it melted and excavated any material from the mantle, why aren't we seeing it?"

Determining the answers to these questions, the researcher explains, is a key part in piecing together the history of the Moon's formation.