Imaging of lunar surface maturity

The physical evolution of the lunar surface with exposure to the space environment is termed maturation, and maturity is the degree to which a particular lunar soil possesses quantitative characteristics consistent with that exposure. Several quantitative measures or indices of maturity have been proposed and employed, including the abundance of solar wind gas, abundance of various types of agglutinates, various measures of grain size, and I-s/FeO. Among the changes attendant with space exposure are striking changes in the optical characteristics of soils. Mature lunar soils are dark red and exhibit reduced spectral contrast relative to immature soils. This paper presents an optical maturity index that quantifies the spectral effects of maturation. We show that this optical maturity index correlates with other maturity indices about as well as the accepted maturity indices correlate among themselves and is only weakly coupled to composition. The modest correlations among maturity indices suggest important controls on individual maturity indices other than age, uncorrelated variations in the rates of accumulation of individual indicators, or variations in depths over which indicators are emplaced. In addition to mixing, these effects conspire to reduce the equivalence of maturity and duration of surface exposure. Optical maturity illustrates some of these effects, showing that ejecta of large and small craters mature at different rates and that the interiors and ejecta of large craters exhibit systematically different optical maturities. The same or analogous effects are likely to influence other maturity indices.