A theoretical model of lunar optical maturation: Effects of submicroscopicreduced iron and particle size variations

Diagrams, color index C(0.95/0.75 mum) versus albedo A(0.75 mum), are used to estimate composition and maturity degree of the lunar regolith. We simul ate maturation curves on the diagrams with the model (Shkuratov, Yu. G., L. V. Starukhina, H. Hoffmann, and G. Arnold 1999. Icarus 137, 235-246). In s imulation, decrease of characteristic particle size as well as increase of concentration of reduced iron were taken into account. For rather small cha nges of maturation parameters, almost linear A(0.75 mum)-(0.95/0.75 gm) mat uration trends are found which is in accordance with the method of determin ation of iron content and maturation degree proposed by Lucey, P. G., D. Bl ewett, and J. Johnson (1995. Science 268, 1150-1154). With increased change s in maturation parameters, the maturation trends are no longer linear and the branches with the opposite direction of optical maturation are obtained . This is in accordance with spectral measurements of particle size fractio ns of lunar soils. The maturation trends on A(0.75 mum)-C(0.95/0.75 mum) pl ots have in general no common origin. In most cases there is an area, where the trends can be focused, but the position of the area differs for differ ent sets of data. Thus, Lucey's assumption about the common origin of matur ation trends is valid only as an approximation. Elongated clusters on A(0.7 5 mum)-(0.95/0.75 Am) diagram for the lunar nearside (Shkuratov, Yu. G., V. G. Kaydash, and N. V. Opanasenko 1999. Icarus 137, 222-234) can be describ ed by maturation trajectories corresponding to different Fe2+ abundance. Th e upper edge of the diagram, i.e., the greatest values of C(0.95/0.75 mum), can be determined by the processes of particle size decrease in maturation . (C) 2001 Academic Press.