Lunar soil

The nuclei of iron are especially stable, giving it a comparatively high cosmic abundance (Chap. 1, p. 11), and it is thought to be the main constituent of the earth s core (which has a radius of approximately 3500 km, i.e. 2150 miles) as well as being the major component of siderite meteorites. About 0.5% of the lunar soil is now known to be metallic iron and, since on average this soil is 10 m deep, there must be 10 tonnes of iron on the moon s surface. In the earth s crustal rocks (6.2%, i.e. 62000ppm) it is the fourth most abundant element (after oxygen, silicon and aluminium) and the second most abundant metal. It is also widely distributed. 

Immature soil samples have S Te values that are indistinguishable from lunar rocks, whereas submature and mature soils have 5 Fe values that are greater than those of lunar rocks, and S Te values are positively correlated with Ig/FeO values (Fig. 12). Lunar regolith samples in general tend to have heavy isotopic compositions as compared to lunar rock samples, as demonstrated by isotopic analyses of O, Si, S, Mg, K, Ca, and Cd (Epstein and Taylor 1971 Clayton et al. 1974 Russell et al. 1977 Esat and Taylor 1992 Humayun and Clayton 1995 Sands et al. 2001 Thode 1976). The origin of isotopic compositions that are enriched in the heavy isotopes has been presumed to reflect sputtering by solar wind and vaporization, where preferential loss of the lighter isotope to space occurs. In contrast to previous isotopic studies, the Fe isotope compositions measured in the Lunar Soil Characterization Consortium samples can be related to a specific phase based on the positive correlation in Ig/FeO and 5 Fe values (Fig. 12). 

Figure 12. Plot of I/FeO versus 5 Fe values of lunar regolith samples from the Lunar Soil Characterization Consortium. The sub-scripted numbers after the sample numbers are the I,/FeO values measured for the <250 pm sized fraction. All analyses are for bulk samples of the different sized fractions error bars are 2a as calculated from 2 or more complete Fe isotope analyses. Modified from Wiesli et al. (2003a).

Bematowicz TJ, Nichols RH, Hohenberg CM (1994) Origin of amorphous rims on lunar soil grains. Lunar Planet Sci Conf XXV, 105-106... 

Esat TM, Taylor SR (1992) Magnesium isotope fractionation in lunar soils. Geochim Cosmochim Acta 56 1025-1031... 

Morris RV (1976) Surface exposure indices of lunar soils a comparative FMR study. Proc Lunar Planet Sci Conf7 315-335... 

Sands DG, Rosman KJR, Laeter de JR (2001) A preliminary study of cadmium mass fractionation in lunar soils. Earth Planet Sci Lett 186 103-111... 

Thode HG, Rees CE (1976) Sulphur isotopes in grain size fractions of lunar soils, Proc Lunar Sci Conf 7 459-468... 

Lunar soil samples being collected by an Apollo astronaut. Figure courtesy of NASA. 

Noble gas abundances in lunar soils and chondrites, (a) Elemental abundance patterns for trapped solar wind in lunar soils, normalized to solar system abundances, (b) Elemental abundance patterns for planetary trapped noble gases, normalized to solar system abundances. This diagram is intended to illustrate patterns only vertical positions are arbitrary. Modified from Ozima and Podosek (2002). 

SEM electron backscatter image of lunar soil particles, showing minute beads of metallic iron (white) in silicate glass coatings. The iron beads modify the spectra, accounting for the phenomenon of space weathering. 

Mare basalts include lavas that erupted from fissures and pyroclastic deposits that produced glass beads. Six of the nine missions to the Moon that returned samples included basalts. The mare basalts from different sites have distinctive compositions and are classified based on their Ti02 contents, and to a lesser extent on their potassium contents (Fig. 13.3). A further subdivision is sometimes made, based on A1203 contents. Mare basalts are compositionally more diverse than their terrestrial counterparts. Volcanic glass beads, formed by fire fountains of hot lava erupting into the lunar vacuum, were found at several Apollo sites and eventually were shown to be a constituent of virtually every lunar soil. The glasses are ultramafic in composition and formed at very high temperatures. 

The same ferroan anorthosite-mare basalt-KREEP components also define the compositions of lunar soils. The soils from each site contain different proportions of these end members. For example, Apollo 12 soils are mixtures of mare basalt and KREEP, whereas Apollo 15 soils contain all three components. 

Ion implantation is another common noble gas trapping mechanism in nature. For example lunar soils are implanted by solar emission particles that essentially consist of lighter noble gases. A few laboratory experiments have been carried out with the hope that experiments may be relevant to incorporation of noble gases in early solar... 

He, Ne, and Kr Ilmenite from lunar soil 71501 by the CSSE method (Benkert et ah, 1993 Wieler and Baur, 1994 see also Section 2.8). The soil trapped its solar gases in the last... 

Figure 7.1 Elemental abundance of noble gases relative to cosmic abundance (Anders Grevesse, 1989). Data for Earth (atmosphere), SW (solar wind implanted on A1 foils on the moon), Lunar (solar wind implanted on lunar soils), Q (chondrites), and Mars are from Table 3.2.

Figure 7.1 shows a noble gas elemental abundance relative to 36Ar for the Earth atmosphere, Q, SW, and lunar soils [cf. Table 3.2,3.3(a), and 3.3(b)]. We also included the supposed Martian atmospheric noble gas (e.g., Pepin, 1991). The abundances are normalized to the solar (cosmic) abundance. 

Black, D. C. (1972) On the origin of trapped helium, neon, and argon siotopic variations in meteorites - I. Gas-rich meteorites, lunar soil and breccia. Geochim. Cosmochim. Acta, 36, 347-75. 

Futagami, T., Ozima, M., Nagai, S., Aoki, Y. (1993) Experiments on thermal release of implanted noble gases from minerals and their implantations for noble gases in lunar soil grains. Geochim. Cosmochim. Acta, 57, 3177—94. 

Wieler, R., Baur, H. (1995) Fractionation of Xe, Kr, and Ar in the solar corpuscular radiation deduced by closed system etching of lunar soils. Astmphys. J., 453, 987-97. 

Wieler, R., Baur, H., Signer, R (1986) Noble gases from solar energetic particles revealed by closed system stepwise etching of lunar soil minerals. Geochim. Cosmochim. Acta, 50, 1997-2017. 

Charette, M. P., McCord, T. B., Pieters, C. Adams, J. B. (1974) Application of remote spectral reflectance measurements to lunar geology classification and determination of titanium content of lunar soils. J. Geophys. Res., 79,1605-13. 

Farr, T. G., Bates, B. A., Ralph, R. L. Adams, J. B. (1980) Effects of overlapping optical absorption bands of pyroxene and glass on the reflectance spectra of lunar soils. Proc. 11th Lunar Planet. Sci. Conf, Geochim. Cosmochim. Acta, Suppl. 11 (Pergamon Press, New York), pp. 19-29. 

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