Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30s.479a&link_type=abstract
Meteoritics, vol. 30, no. 5, page 479
Computer Science
1
Lunar Soil, Maturity, Reflectance, Spectral, Regolith, Weathering, Space
Scientific paper
Space weathering in the upper 1 mm of lunar soil includes the combined effects of micrometeorite impacts and solar wind interactions. Impacts melt small volumes of soil containing solar wind hydrogen and carbon. The melt quenches rapidly to agglutinitic glass. Space weathering reduces a soil's overall reflectance and spectral contrast, and creates a red-sloped continuum [1]. Agglutinitic glass formation in a strongly reducing environment causes reduction of Fe^2+ in the glass to nanophase iron metal (np-Fe^O). The relative concentration of np-Fe^O (Is) normalized to total iron oxide (I(sub)s/FeO) is a maturity index for lunar soils [2]. We reported changes in optical and magnetic properties of terrestrial minerals and glasses under reducing conditions [3,4]. We have now completed similar experiments using lunar soils. Optical and magnetic changes from these experiments resemble changes characteristic of space weathering. Experiments: Seventeen samples of lunar soil were reduced in hydrogen for 3 hrs at 1050 degrees C. Samples were analyzed by VIS/NIR reflectance spectroscopy and SEM. Concentrations of np-FeO were measured using ferromagnetic resonance (FMR). Fe^O and metal abundances were determined by Mossbauer spectroscopy. Optical Effects: Absolute reflectance at 0.56 ?lm is used to compare the overall brightness of a suite of pristine lunar soils [1]. Values range from 0.38 (immature) to 0.11 (mature). All of our reduced samples are darker, with reflectances of 0.13 to 0.05. The 1 micrometer absorption band depths for pristine lunar soils range from 15-2%, and decrease with increasing maturity [1]. In most of our reduced soils the band depth is 2% or less, and in several this band is undetectable. The continuum slope bounding the 1 llm absorption band increases from 0.25 to 1.00 with increasing maturity [1]. Slopes of spectra for reduced soils are generally lower than 0.30, and some approach zero. Magnetic Effects: Our pristine samples display FMR linewidths in the range 560-790 G, consistent with all lunar soils [2]. Reduction at 1050 degrees C increases linewidths to 821-1360 G. Our previous reduction experiments produced linewidths in this range for samples of orthopyroxene, plagioclase and glass, the major components of lunar soil [4]. Published I(sub)s/FeO values for our unreacted soils range from 3 (immature) to 100 (mature [2]). Values for our reduced soils change only slightly, ranging from 8 to 96. In most cases, the "maturity" of H2-reduced soils is less than that for pristine samples. Discussion: Optical and magnetic effects in both mature and experimentally-reduced lunar soil are attributable to iron blebs formed by reduction of Fe^2+ [5]. Metal particles absorb strongly in VIS/NIR wavelengths, accounting for the darkening and loss of spectral contrast in mature soil. Lower albedos and red slopes of reduced soils compared to mature (pristine) samples result from the higher total Fe^O content of the former. Our H2-reduced soils have 2-12% metal, while pristine soils have at most ~1% Fe^O [6]. Despite this increase in metal, I(sub)s/FeO in H2-reduced soils is similar to that for pristine soils. I(sub)s measures only the nanophase component of iron. Most metal particles produced in our experiments are substantially larger than metal formed naturally during maturation (~4-33 nm [4]). These larger blebs are associated with long annealing times in our experiments, compared to rapid quench times for agglutinitic glass. The decreased albedo, decreased spectral contrast, and red slope characteristic of natural maturation are associated with formation of metallic iron particles on the lunar surface. Our experiments should be considered extreme cases of this localized reduction which characterizes space weathering. References: [1] Fischer E. M. and Pieters C. M. (1994) Icarus, 111, 475-488. [2] Morris R. V. (1978) Proc. LPSC 9th, 2287-2297. [3] Allen C. C. et al.(1993) Icarus, 104, 291-300. [4] Morris R. V. and Allen C. C.(1994) LPS XXV, 937-938. [5] Allen C. C. et al. (1994) JGR, 99, 23173-23185. [6] Morris R. V. (1980) Proc. LPSC 11th, 1697-1712.
Allen Christine
McKay David S.
Morris Richard V.
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