Copernicus crater
April 3, 2013

Moon Mapper Data Shows Preserved Mineralogy Exists In Lunar Crater

April Flowers for - Your Universe Online

Large impacts on the Moon produce unimaginable amounts of energy; however, they may not wipe the mineralogical slate clean.

New research, led by Brown University geoscientists, has discovered a rock body with a distinct mineralogy snaking 18 miles across the floor of Copernicus crater — a 60-mile-wide feature on the Moon's near side. Mineralogical signatures of rock present before the impact that created the crater appear to make up the sinuous feature.

The deposit is part of a sheet of impact melt, which are the cooled remains of rocks melted during an impact. Prior to this discovery, geologists have assumed that melt deposits would retain little pre-impact mineralogical diversity.

Giant cauldrons of impact melt are produced by large impacts. These eventually cool and reform into solid rock. Geologists assumed that the energy from the impact would stir the molten cauldron thoroughly during the liquid phase. This would mix all the rock types together into an indistinguishable mass. The theory was that identifying any pre-impact variation in the minerals would be like putting a four-course meal into a blender and then attempting to pick out the potatoes.

The newly discovered feature in Copernicus, however, suggests that pre-existing mineralogy isn't always eradicated by the impact process.

“The takeaway here is that impact melt deposits aren´t bland,” said Deepak Dhingra, a Brown graduate student who led the research that was published in a recent issue of Geophysical Research Letters. “The implication is that we don´t understand the impact cratering process quite as well as we thought.”

Even though Copernicus is one of the best-studied features on the Moon, this deposit went unnoticed for decades. The Moon Mineralogy Mapper's (M3) imaging in 83 wavelengths of light — in the visible and near-infrared spectrum — made the deposit stand out however.

The M3, a state-of-the-art imaging spectrometer, is one of two scientific instruments contributed by NASA to the first Indian mission to the moon, Chandrayaan-1, which orbited the Moon for 10 months during 2008-09. The M3 instrument mapped nearly the entire lunar surface and has created the first mineralogical map of the lunar surface at high spatial and spectral resolution. M3 is part of NASA's Discovery Mission of Opportunity.

Different minerals reflect light in different wavelengths at variable intensities, making it possible to identify the minerals by observing the wavelength variations. The new feature appeared as an area that reflects less light at wavelengths around 900 and 2,000 nanometers in the M2 imaging. This indicates minerals rich in magnesium pyroxenes. The remaining crater floor showed a dominant dip beyond 950 nm and 2400 nm, indicating iron and calcium pyroxene rich minerals. “That means there are at least two different mineral compositions within the impact melt, something previously not known for impact melt on the Moon,” Dhingra said.

How or why this feature formed this way is unclear. The research team says that is an area for future study. The fact that the impact melt isn't always homogenous, however, changes the way geologists look at lunar impact craters.

“These features have preserved signatures of the original target material, providing ℠pointers´ that lead back to the source region inside the crater,” said James W. Head III, the Scherck Distinguished Professor of Geological Sciences. “Deepak´s findings have provided new insight into the fundamentals of how the cratering process works. These results will now permit a more rigorous reconstruction of the cratering process to be undertaken.”

Image 2 (below): Pre-existing mineral deposits on the Moon (sinuous melt, above) have survived impacts powerful enough to melt rock. Not detectable in the crater image (inset), deposits are visible only in light at certain wavelengths. Credit: NASA and Deepak Dhingra