The return of lunar soils more than 45 years ago changed forever our understanding of the characteristics of airless bodies and how we use optical tools to evaluate their composition remotely. When Apollo soils were evaluated in earth-based laboratories, it was quickly found that their optical properties do not look like rocks from the same site (1). Soils are darker (especially at visible wavelengths) and diagnostic absorptions present are considerably weaker. Since then we have learned much about processes that gradually alter materials exposed to the space environment., ie. space weathering (2, 3). These include several impact processes and products, irradiation by energetic particles and electromagnetic radiation, and repeated thermal cycles, stirring, and mixing with time (4). In addition, there are other variables such as rock abundance, size, and destruction sequence (e.g. 5) that affect the local soil formation rate and the accumulation of weathering products detected optically. It has become clear that space weathering affects all airless bodies, and that the relative importance of specific processes varies with composition of the surface and for different locations in the solar system (e.g. 6,7,8). The time scale for space weathering is also debated. It has been suggested that it might be as rapid as 10^6 yrs (9).
The lunar samples provide ‘ground truth’ data for the Moon that constrain the time scale of space weathering at 1 AU. From detailed sample analyses we have measurements of the age when specific lunar craters occurred and exposed fresh local material, i.e., the ‘exposure age’. The optical properties of these craters can be evaluated with modern remote sensing instruments to determine the degree to which their optical properties are ‘fresh’ or ‘mature’ relative to their surroundings. Key craters include: Copernicus (~800 My), Tycho (~100 My), North and South Ray Craters (50 & 2 My respectively).
Results. To a first order, lunar data indicate that it takes less than a billion years for a freshly exposed flat lunar surface to develop a soil with a level of maturity comparable to background soils. Flat surfaces associated with Copernicus are found to exhibit the properties of well-developed ‘mature’ soils. Surfaces measured spectrally at the best current resolution (140 m) in and around North and South Ray craters indicate these two young craters near Apollo 16 are definitely optically ‘fresh.’ The extensive ray system of Tycho indicates it is ‘immature', but it is so extensively affected by impact melt products, it is not possible to directly assess.
References. (1) J. B. Adams & McCord, (1971) Science 171, 567. (2) C. Pieters et al., (2000) Meteoritics & Planet. Sci. 35, 1101-1107. (3) B. Hapke (2001) JGR 106, E5, 10039. (4) D. Domingue et al. (2014) Space Sci Rev, 181:121–214 . (5) A. T. Basilevsky et al. (2013) PSS 89, 118-126 (6) J. Trombka et al. (2000) Science 289, 2101. (7) T. Noguchi et al. (2011) Science 333, 1121. (8) C. Pieters et al. (2012) Nature, 491, 79-82. (9) Vernazza et al. (2009) Nature, 458. 993-995. (9).