Fe, Ti, and Is/FeO Maps for the Lunar Nearside: New Estimations by Optical Data

A method to separate contributions of the Fe and Ti abundance and maturity degree to spectral properties of the lunar surface is developed. A map of maturity degree for the Moon nearside is presented. Introduction. Laboratory studies of lunar samples showed that regolith of the same chemical composition but different maturity degree, characterizing by the parameter Is/FeO, has different spectral features. A technique to separate influence of the Fe and Ti abundance and maturity degree on spectral properties of the lunar surface has recently been proposed [1,2]. In the case of Fe, it is based on a choice of such a system of polar coordinates on the plane of albedo A(0.75 μm) and color-index C(0.95/0.75 μm) that the polar angle and radial coordinate are related to iron content and maturity degree, respectively. To make the choice, data for lunar samples are used. A similar approach is proposed to estimate Ti content and maturity degree [2]. Unfortunately, the technique faces to some problems. For instance, to derive the maturity degree, one can use both diagrams, A(0.75 μm) C(0.95/0.75 μm) and A(0.75 μm) C(0.75/0.42 μm). However, these two estimations of maturity give quite different results. We try here to improve this method. New approach. Usually one correlates directly chemical composition with optical properties of regolith surface, whereas, it should be done with optical properties of the material forming the surface. To derive information on composition and maturity from optical properties of the Moon nearside we used (1) telescope photometric images of the lunar nearside at 0.42, 0.65, 0.75, and 0.95 μm [3] and (2) a geometrical-optics model for albedo spectral dependence of regolith-like surfaces [4]. This model allows us to estimate the imaginary part of the refractive index k of regolithforming material using albedo measurements. To avoid the maturity ambiguities, we use in our analysis the 3Ddiagram: κ (0.42 μm) Cκ (0.42/0.65 μm) Dκ (0,95 μm), where κ(0.42 μm) is the absorption coefficient of regolith material, Cκ (0.42/0.65 μm) is the ratio of κ in red and blue light, and Dκ (0,95 μm) is the depth of absorption band near 1 μm, obtained from the wavelength dependence of κ. These parameters are derived from the model. The main idea of our approach is find such a coordinate system in the space of parameters κ (0.65 μm), Cκ(0.42/0.65 μm), and Dκ (0.95 μm) that new coordinates correlate with Fe and Ti content better than others. To calibrate the maps we use data of the Apollo, Luna, and Surveyor missions (see details in [3]). Results and discussion. The frequency histograms and maps of Fe and Ti content are presented in Figs. 1-4. Dark color in Fig. 3 and 4 corresponds to low content of Fe and Ti. There is resemblance of these maps and the distributions presented in [1,2]. The frequency histogram of Fe content is bimodal (Fig. 1). The maximum of the mode for low iron content lies near 6%. This is in a good agreement with the results for the lunar nearside obtained by Clementine data [1]. However, the location of the second mode is shifted toward greater values (17%) in comparison with 10% obtained in [1]. The frequency histogram for titanium has only one maximum and is very asymmetrical (Fig. 2). The maximum of distribution lies near 0,7%. The distribution of maturity degree, which is presented in Fig. 5, was calibrated in Is/FeO units by the laboratory data for lunar samples [5]. Clearly seen on the Is/FeO map are young craters and their ejecta (immature soils), e.g., Tycho crater and its ray system. Crater Copernicus shows up, too, but its ray system has similar maturity as its mare neighborhood. This crater is older than Tycho and, probably, its ejecta material is quite mixed with background material. The mare/highland boundary is practically not seen on the lunar nearside in parameter Is/FeO. That is in a good agreement with results of laboratory studies of lunar samples [6] the correlation between the parameters Is/FeO and iron content (and, consequently, albedo) does not exist for mare samples as well as for highland ones. It means that lunar images in the parameter Is/FeO should not feel the mare/highland boundary. Regions with high maturity degree were defined as Is/FeO≥70, whereas regions with Is/FeO≤50 (young craters and their ray systems) were defined as immature. Clearly seen on the map are zones of high maturity degree. This zone contains the south-west border of Mare Serenitatis, the west part of Mare Traquilitatis and Mare Nectaris. The high maturity zones correlate with some morphological features of the lunar surface. This work was supported by the CRDF, grant #UG2295. References: 1. Lucey P. et al. Science. 1995. 268. P. 1150. 2. Blewett D.T. et al. J. Geophys. Res. 102, No. E7, P. 16,319. 3. Shkuratov Yu.G. et al. Cosmic Sci. and Technol. 1997. P. 70. 4. Starukhina L.V., Shkuratov Yu.G. Astron.Vestnik. 1996. 30. P. 299. 5. Fisher E.M. PhD Thesis, Brown Univ. RI. 1995. 6. Morris R. Proc. Lunar Sci. Conf. 9th. 1978. LPI Houston. P. 2287. 0 5 10 15 20 25 FeO ,% 1 3