Comparison of Reflectance Spectra of Apollo Regolith Samples to Crushed Lunar Meteorites

Apollo regolith samples provide a spectroscopic baseline against which regoilith simulants can be judged. The lunar surface undergoes space weathering, which involves a number of processes. These include the production of nanophase iron in the outermost surfaces of mineral grains. This process is unique to airless bodies and does not occur on the Earth. As a result, terrestrial analogues of lunar surface rocks may not be good spectral analogues. The result of nanophase iron production is to darken and redden the lunar surface. As a result, lunar regolith spectra are uniformly red-sloped over the 500 to 2500 nm range. 

With increasing maturity, highland and mare spectra become darker and more red-sloped. To determine whether lunar meteorite powders can serve as spectral analogues, we have measured the reflectance spectra of a suite of powdered (<1000 micron grain size) lunar meteorites, which are largely brecciated with obvious highland and mare clasts. 

The 350-2500 nm reflectance of a suite of Apollo samples, sieved to <1000 microns, were measured at the University of Winnipeg's Centre for Terrestrial and Planetary Exploration (C-TAPE). It was found that with increasing maturity, both highland and mare regolith samples become darker and more red-sloped, as expected. Visible region reflectance (at 550 nm) ranges from 14-32% for highland samples, and 5-12% for mare samples. Spectral slopes, as measured by the 2500/500 nm reflectance ratio, ranges from 1.5 to 2.9 for highland regolith, and 2.2 to 4.0 for mare samples.

Applying the same metrics to our suite of lunar meteorite powders, we find that 550 nm reflectance is wide-ranging, from 6 to 54%; nearly as low as mature mare and brighter than immature highlands. However, spectral slope (2500/500 nm reflectance ratio) is more limited, ranging from ~1 (flat overall spectral slope) to 1.7, just within the range of immature highland regolith. Consequently, lunar meteorites are not good spectral analogues for the Moon with the possible exception of immature highlands, as exemplified by the Apollo 61221 regolith sample.

Our results indicate that lunar meteorites, while compositionally identical to the Moon, require space weathering, likely production of nanophae iron, to serve as spectral analogues for lunar regolith.