SPACE WEATHERING OF LUNAR ROCKS AND
REGOLITH GRAINS. L. P. Keller. Robert M. Walker Laboratory for Space Science, Code KR, ARES, NASA/JSC, Houston,TX 77058. E-mail: Lindsay.P.Keller@nasa.gov.
Introduction: The exposed surfaces of lunar soil grains andlunar rocks become modified and coated over time with a thinrind of material (patina) through complex interactions with thespace environment. These interactions encompass many processes including micrometeorite impacts, vapor and melt deposition,and solar wind implantation/sputtering effects that collectivelyare referred to as "space weathering". Studies of space weathering effects in lunar soils and rocks provide important clues to understanding the origin and evolution of the lunar regolith as wellas aiding in the interpretation of global chemical and mineralogical datasets obtained by remote-sensing missions. The interpretation of reflectance spectra obtained by these missions is complicated because the patina coatings obscure the underlying rockmineralogy and compositions.
Much of our understanding of these processes and products
comes from decades of work on remote-sensing observations of
the Moon, the analysis of lunar samples, and laboratory experiments. Space weathering effects collectively result in a reddened
continuum slope, lowered albedo, and attenuated absorption features in reflectance spectra of lunar soils as compared to finely
comminuted rocks from the same Apollo sites. Space weathering
effects are largely surface-correlated [1], concentrated in the finesize fractions, and occur as amorphous rims on individual soilgrains. Rims on lunar soil grains are highly complex and span therange between erosional surfaces modified by solar wind irradiation to depositional surfaces modified by the condensation of
sputtered ions and impact-generated vapors [2]. The optical effects of space weathering effects are directly linked to the production of nanophase Fe metal in lunar materials [3]. The size of
distribution of nanophase inclusions in the rims directly affectoptical properties given that large Feo grains (~10 nm and larger)
darken the sample (lower albedo) while the tiny Feo grains(<5nm) are the primary agent in spectral “reddening” [4,5].
More recent work has focused on the nature and abundanceof OH/H2O in the lunar regolith using orbital data [e.g. 6] andsamples analyses [7]. Advances in sample preparation techniques have made possible detailed analyses of patina-coatedrock surfaces [8]. Major advances are occurring in quantifying
the rates and efficiency of space weathering processes throughlaboratory experimentation [e.g. 9-11].
References: [1] Pieters C. M. 1993. JGR 98, 20,817-20,824.
[2] Keller L. P. and McKay D. S. 1997. GCA 61, 2331-2341.
[3] Taylor L. A. et al. 2001. JGR-E, 106, 27,985-27,999. [4] Keller L. P. et al. 1998. 29th LPSC, #1762. [5] Noble S. K. et al.
2007. Icarus 192, 629-642. [6] Pieters C. M. et al. 2009. Science,326:569-572. [7] Liu Y. et al. 2012. Nature Geo. 5:779-782. [8]Noble S. K. et al. 2013. 44th LPSC, #1298. [9] Sasaki S. et al.
2001. Nature, 410, 555-557. [10] Christoffersen R. et al. 2010.
41st LPSC, #1532. [11] Zhang S. and Keller L. P. 2012. MAPS,47, #5267.