Lunar regolith

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).

Clayton DD (1999) Radiogenic iron. Meteor Planet Sci 34 A145-A160 Clayton RN (1993) Oxygen isotopes in meteorites. Ann Rev Earth Planet Sci 21 115-149 Clayton RN, Mayeda TK, Hurd JM (1974) Loss of oxygen, silicon, sulfur, and potassiimi from flie limar regolith. Proc Lunar Sci Conf 5 1801-1809... 

Wiesli RA, Beard BL, Taylor LA, Johnson CM (2003a) Space weathering processes on airless bodies Fe isotope fractionation in the lunar regolith. Earth Planet Sci Lett 216 457-465... 

Bugbee, B. G., Salisbury, F. B. (1989). Controlled environment crop production Hydroponic vs. lunar regolith. In D. W. Ming,D. L. Henninger (Eds.), Lunar Base Agriculture Soils for Plant Growth (pp. 107-129). Amer. Soc. Agronomy, Madison, WI, USA. 

We also have over 120 lunar meteorites in our collections. Because the Moon has no atmosphere, the irradiation history of these meteorites can include an extended period in the lunar regolith. The transit times from the Moon to the Earth range from a few x 104 years to nearly 10 Myr. Detailed analysis of exposure ages and terrestrial ages indicate that at least three impact events in the lunar highlands and five events in the lunar mare ejected the meteorites that have been recovered to date. 

Lunar meteorites (see review by Korotev et al., 2003) are mostly brecciated samples of highlands crust (FAN) and regolith, although a few mare basalts are included in this collection. It is likely that the source craters for the meteorites are randomly distributed and thus include materials from the lunar farside. As we will see, these meteorites provide a better estimate of the crustal composition than do the geographically biased samples returned by spacecraft. 

The Apollo lunar samples have provided the first opportunity for the analysis of material collected from an extraterrestrial body under carefully controlled conditions. No definitive evidence about the concentration and nature of lunar carbon was provided by the a back-scattering experiments carried out by the Sureveyor unmanned landers. However, these remarkably successful experiments did indicate a magmatic origin for the samples analysed1"3. This was confirmed by the Apollo missions, three distinct samples types being collected from the lunar regolith ... 

The enrichment in the fines and regolith breccias is thought to result from exposure on the lunar surface, resulting in either preferential removal of 12C or addition of 13C. A number of mechanisms have been proposed to explain these observations for the fines and must also be applicable to the regolith breccias. 

It has been suggested24 that the CH4 released by deuterated acid dissolution could arise from reaction of lunar carbide on grain surfaces with adsorbed terrestrial H2O. This has been shown to be unlikely exposure of the fines to D20 at 200 °C 5), or at ambient for periods up to several weeks47, failed to release CD4. Also, dissolution of freshly exposed chips of an Apollo 11 regolith breccia released CH4 and CD4in concentrations similar to those released from Apollo 11 fines47. ... 

The available data show that the carbon chemistry of the lunar regolith is highly dependent on the processes occuring at the lunar surface, including solar... 

The carbon chemistry of the regolith has been established as a significant indicator of exposure and reworking. In conjunction with other parameters indicative of exposure of the fines and breccias on the lunar surface, it should contribute to an understanding of the complex history of the regolith. 

The values for the Al/Si ratio of the regolith in mare areas, as determined from moon samples as well as via the x-ray fluorescence technique, also differ from the values found in the rock samples of mare basalts (Tables 2 and 3), the latter being somewhat lower. These differences can be explained by the admixture of anorthositic material from the lunar highlands, as found at the... 

For all four mare sites the meteoritic components amount to 1.55—1.78%, evidencing a nearly constant admixture in the lunar regolith. Only slightly lower values are obtained when the highly volatile elements are used as indicators. Hence, the assumption that C 1 chondritic material is the meteoritic component is substantially correct, although some admixture of fractionated matter (less primitive meteorite classes) is seen, in the Apollo 11 and 12 soil samples, especially. 

The evidence from seismic experiments is fully in agreement with the model outlined above. In the Fra Mauro region of Oceanus Procellarum the crust is about 65 km thick and overlies a solid mantle 126 The crust, in turn, is divided into two parts covered with a thin regolith. For comparison with the returned lunar samples, the upper layer has been inferred to be mare basalt and the lower one anorthositic gabbros. 

Figure 31 Lunar meteorite North West Africa (NWA) 773 consists of two distinct lithologies cumulate olivine norite and regolith breccia. The cumulate portion is composed of olivine, pigeonite, augite, feldspar, and opaques (troilite, chromite, Fe-metal). The breccia portion contains fragments of cumulate portion as well as silica glass, hedenbergitic pyroxene, volcanic rocks, and unusual lithic clasts with fayalite + Ba-rich K-feldspar + silica + plagioclase (photograph courtesy of M. Killgore).

Warren P. H., Taylor J. G., and Keil K. (1983) Regolith breccia Allan HiUs A81005 evidence of lunar origin, and petrography of pristine and nonpristine clasts. Geophys. Res. Lett 10, 779-782. 

Kerridge J. F. (1993) Long-term compositional variation in solar corpuscular radiation evidence from nitrogen isotopes in the lunar regolith. Rev. Geophys. 31, 423-437. 

Many lunar meteorites are regolith breccias, meaning that over time at least some of their constituent grains resided not only below the surface waiting for launch as just described, but also at the very surface, where the grains could collect solar wind ions, and presumably at other depths as well. Such grains may have extremely... 

Numerous authors (e.g., Warren, 1994 Wieler, 2002 Nyquist et al., 2001) have contrasted the exposure histories and other properties of lunar and martian meteorites. On average, we would expect key systematic differences to relate to their respective distances from the Earth (or more precisely how easily their ejecta could attain Earth-crossing orbits), the respective depths of their gravitational wells, the mechanical properties of their regoliths, and the relative fluxes of impacting bodies. 

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