Do large impacts always erase surface mineralogy?

Our present all-encompassing view of the nearside landmark lunar crater Copernicus seems only a little different than the best photography from Earth. Only 20 degrees west and less than 10 degrees north of the 'center' of the Moon's tidally-locked hemisphere, the round rim appears only slightly oblong from angle seen from our backyards. Its general brightness, both inside and out, wash out much of the detail seen in this LROC Wide Angle Camera (WAC) monochrome (649 nm) montage made up of observations in six orbital passes in January 2010. Long recognized differences can easily be confirmed between the northwest quadrant and the remaining three-quarters of the crater floor may be more extraordinary than previously believed possible [NASA/GSFC/Arizona State University].

EDITORIAL NOTE: One of a handful of features on the Moon's nearside detectible to the naked eye, it's amazing what there is still to be learned about the majestic crater Copernicus. Not as young and bright as Tycho, with rays streaming over an entire hemisphere, it's rays are impressive enough and the larger Copernicus is distinctive enough be the namesake for an entire lunar Age, the "Copernican," the Moon's most modern period, encompassing features less than about 1.1 billion years old.

This has made the Copernican family of excavations very valuable to planetary scientists who utilize Earth's Moon as the Rosetta Stone of the Solar System (and, increasingly, our Earth - the planet with which it has shared precisely the same space in the universe for approximately 4.575 billion years.

Most likely mapped first by Galileo, the 93 kilometer impact crater has since his time been drawn and redrawn with with increasing precision and appreciation. Since the 19th century, and definitely since the latter half of the 20th century, Copernicus may be the lunar crater most individually photographed from Earth. A highly oblique orbital image captured from Lunar Orbiter 2, November 24, 1966, is one of only a handful of images popularly celebrated as a "Photograph of the Century." 

Detail (highly resampled) from Lunar Orbiter 2-162, oblique view from 26 km over the lunar surface south of Copernicus crater, November 24, 1966 [Moonviews].

That delicate telemetry was recovered and reprocessed by the phenomenal Lunar Orbiter Image Recovery Project (LOIRP) in 2009. Without question, we have learned a great deal about the Moon since 1957, but, until recently, not very much more about Copernicus crater than might have been inferred using a decent telescope here on Earth. It's complexity and subtle albedo has defied definitive understanding.

We have known for some, for example, it has at least two (or three) central peaks, and mulled over tantalizing indications of a twin set of widespread rays - hinting at a near simultaneous double impact. High-resolution analysis of its wide interior achieved greatest progress in study of the continued arrival of unambiguously remote sensing from India's Chandrayaan-1 and the U.S. Lunar Reconnaissance Orbiter (LRO) after beginning its on-going mission in close polar orbit in 2009. 

Hints that the northwest quadrant of its interior floor is very distinct from the remaining three-fourths slowly have finally come into focus, hopefully to stay.

The demarcation between the character of the western and eastern north floor of Copernicus has just become more obvious as images from LRO continued to improve. Those differences are particularly striking in spectral analysis, by the Clementine orbiter in 1994, for example. LROC WAC observation M147109260CE (643 nm), spacecraft orbit 6813, December 16, 2010; angle of incidence 77.97° at 60 meters per pixel resolution, from 43.13 km [NASA/GSFC/Arizona State University].

Now the distinguished lunar and planetary scientist Carle Pieters and a team at Brown University have added another set of clues to sharp lines from remote sensing of the northwest quarter of the floor of Copernicus that might have everyone refining or completely revising set theories about what happens in those fantastic and brief hours immediately following a highly energetic crater-forming impact.

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 [NASA/Deepak Dhingra].

Brown University — April 2 — Despite the unimaginable energy produced during large impacts on the Moon, those impacts may not wipe the mineralogical slate clean, according to new research led by Brown University geoscientists.

The researchers have discovered a rock body with a distinct mineralogy snaking for (28.9 km) across the floor of Copernicus crater, a 60-mile-wide hole on the Moon’s near side. The sinuous feature appears to bear the mineralogical signature of rocks that were present before the impact that made the crater.

The deposit is interesting because it is part of a sheet of impact melt, the cooled remains of rocks melted during an impact. Geologists had long assumed that melt deposits would retain little pre-impact mineralogical diversity.

Large impacts produce giant cauldrons of impact melt that eventually cool and reform into solid rock. The assumption was that the impact energy would stir that cauldron thoroughly during the liquid phase, mixing all the rock types together into an indistinguishable mass. Identifying any pre-impact mineral variation would be a bit like dumping four-course meal into a blender and then trying to pick out the potatoes.

But this distinct feature found at Copernicus suggests that pre-existing mineralogy isn’t always blended away 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. “The implication is that we don’t understand the impact cratering process quite as well as we thought.”

Close up view of the feature marked with light green, designated "Surrounding Melt (Fe-Ca rich Pyroxene)" in the study illustration immediately above. LROC Narrow Angle Camera (NAC) observation M175408129R, spacecraft orbit 10984, November 8, 2011; resolution 41 cm per pixel from 26.06 kilometers [NASA/GSFC/Arizona State University].

The findings are published in online early view in the journal Geophysical Research Letters .

Copernicus is one of the best-studied craters on the Moon, yet this deposit went unnoticed for decades. It was imaging in 83 wavelengths of light in the visible and near-infrared region by the Moon Mineralogy Mapper — M3 — that made the deposit stand out like a sore thumb.

M3 orbited the Moon for 10 months during 2008-09 aboard India’s Chandrayaan-1 spacecraft and mapped nearly the whole lunar surface. Different minerals reflect light in different wavelengths at variable intensities. So by looking at the variation at those wavelengths, it’s possible to identify minerals.

In the M3 imaging of Copernicus, the new feature appeared as an area that reflects less light at wavelengths around 900 and 2,000 nanometers, an indicator of minerals rich in magnesium pyroxenes. In the rest of the crater floor, there was a dominant dip beyond 950 nm and 2400 nm, indicating minerals rich in iron and calcium pyroxenes. “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.

It is not clear exactly how or why this feature formed the way it did, the researchers say. That’s an area for future study. But the fact that impact melt isn’t always homogenous 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 and one of the authors of the study. “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.”

Carle Pieters, a professor of geological sciences at Brown and the principal investigator of the M3 experiment, was one of the co-authors on the paper, with Peter Isaacson of the University of Hawaii.