Optical Effects of Space Weathering: The Role of the Finest Fraction

The optical properties of lunar softs are different than those of rocks from which they are derived. As a consequence of lunar space weathering, soils are darker and exhibit a distinctive red-sloped continuum and weaker mineral absorption bands. The accumulation of dark glass-welded aggregates (agglutinates) has been thought to account for these optical effects of space weathering on lunar soils. Spectroscopic analyses of agglutinate separates and size fractions for a suite of lunar softs presented here indicate that the agglutinate paradigm is insufficient to fully account for lunar optical alteration. It is the finest fraction of lunar soils (<25 gm which constitute -25 wt %) that dominates the optical properties of the bulk soil. Unlike size fractions of most silicates for which the finest fraction is the brightest, the lunar soil size fractions all have comparable albedos in the short-wavelength visible. In the near infrared, however, it is the finest fraction that exhibits the steep red continuum and weak absorption bands. The properties of the finest fraction cannot be duplicated by preparing a fine fraction by grinding larger agglutinate-rich soil particles. These results suggest space weathering on airless bodies is dominated by surface correlated processes (perhaps associated with the development of fine-grained Fe ø on or near the surface of grains), and the accumulation of the larger agglutinates is not necessarily required to account for lunar optical alteration.