Ancient Impact Crater Holds Clues About Moon's Mantle

Our moon may hold more secrets than we first thought. Researchers have discovered some mineralogical surprises in the moon's largest impact crater, revealing a bit more about its mantle.

The new data comes from the South Pole Aitken (SPA) basin. At 2,500 kilometers across, the SPA is the largest impact basin on the moon and perhaps even the largest in the solar system. Impacts of this size can turn tons of solid rock into molten slush. While most researchers assumed that this melting process would obliterate any distinct signatures of pre-existing mineralogical diversity through extensive mixing, this new research reveals that this may not be the case.

The scientists used data from the Moon Mineralogy Mapper that flew aboard India's Candrayaan-1 lunar orbiter. This data showed a diverse mineralogy in the subsurface of the giant SPA. More specifically, the researchers looked at smaller craters within the larger SPA basin made by impacts that happened millions of years after the impact that formed the basin itself. These impacts uncovered material from deep within the basin; in addition, they offer clues about what lies beneath the surface.

More specifically, the researchers examined central peaks of four craters within the basin. Central peaks form when material under the impact zone rebounds, forming an upraised rock formation in the middle of the crater. The scientists examined the light reflected from each of the four central peaks. The spectra of reflected light gave scientists clues about the makeup of the rocks and showed substantial differences from peak to peak. This varying mineralogy shows that the SPA subsurface may be far more diverse than previously thought.

"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," said Dan Moriarty, one of the researchers, in a news release. "We looked in a little more detail and found significant compositional differences between these  central peaks. 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."

Now, the researchers plan to look at exactly where this diversity came from. It's possible that the distinct minerals formed as the molten rock from the SPA impact cooled, but it's also possible that the diversity reflects minerals that were there before the impact. Scientists are currently looking a bit closer at the data, hoping to find out a little bit more about the moon's mantle.

The findings are published in the Journal of Geophysical Research: Planets.