An experimental approach to understanding the optical effects of space weathering

The creation and accumulation of nanophase iron (npFe0) is a principal mechanism by which spectra of materials exposed to the space environment incur systematic changes referred to as “space weathering.” Since there is no reason to assume that cumulative space weathering products throughout the Solar System will be the same as those found in lunar soils, these products are likely to be very dependent on the specific environmental conditions under which they were produced. We have prepared a suite of analog soils to explore the optical effects of npFe0. By varying the size and concentration of npFe0 in the analogs we found significant systematic changes in the Vis/NIR spectral properties of the materials. Smaller npFe0 (<10 nm in diameter) dramatically reddens spectra in the visible wavelengths while leaving the infrared region largely unaffected. Larger npFe0 (>40 nm in diameter) lowers the albedo across the Vis/NIR range with little change in the overall shape of the continuum. Intermediate npFe0 sizes impact the spectra in a distinct pattern that changes with concentration. The products of these controlled experiments have implications for space-weathered material throughout the inner Solar System. Our results indicate that the lunar soil continuum is best modeled by npFe0 particles with bulk properties in the ∼15–25 nm size range. Larger npFe0 grains result in spectra that are similar in shape to the Mercury continuum. The continuum of S-type asteroid spectra appear to be best represented by low abundances of npFe0. The size of asteroidal npFe0 is similar to that of lunar soils, but slightly smaller on average (∼10–15 nm).