THE NATURE AND ORIGIN OF RIMS ON LUNAR SOIL GRAINS

Space weathering processes that operate in the lunar regolith modify t he surfaces of lunar soil gains. Transmission electron microscope anal ysis of the lunar soil grains from the fine size fraction of several l unar soils show that most grains are surrounded by thin (60-200 nm thi ck) rims. The microstructure and chemical compositions of the rims can be used to classify rims into four broad categories: amorphous, inclu sion-rich, multiple, and vesicular. Amorphous rims are noncrystalline, generally lack crystalline inclusions, show evidence for preferential sputtering of cations, and are produced largely by solar-wind irradia tion damage. Inclusion-rich rims contain abundant nanometer-sized grai ns of Fe metal as randomly dispersed inclusions or as distinct layers embedded in an amorphous silica-rich matrix. Inclusion-rich rims are c ompositionally distinct from their host grains and typically contain a ccumulations of elements that are not indigenous to the host. Inclusio n-rich rims are formed largely by the deposition of impact-generated v apors with a contribution from the deposition of sputtered ions. A con tinuum in the chemical and microstructural properties exists between t ypical amorphous rims and typical inclusion-rich rims. Multiple-rims c onsist of a distinct radiation-damaged layer up to 50 nm thick, that i s overlain by vapor-deposited material of comparable thickness. Vesicu lar rims are compositionally similar to their hosts and are characteri zed by an abundance of small (<50 nm in diameter) vesicles concentrate d in the outer 100 nm of the rims. The formation of vesicular rims is apparently due to the evolution of solar-wind implanted gases in respo nse to a pulse-heating event. The formation of rims on lunar soils is complex and involves several processes whose effects may be superimpos ed. From this study, it is shown that one process does not dominate an d that the relative importance of vapor-deposition is comparable to ra diation-damage in the formation of rims on lunar silicate grains. The presence of rims on lunar soil grains, particularly those with nanomet er-sized Fe metal inclusions, may have a major influence on the optica l and magnetic properties of lunar soils. Copyright (C) 1997 Elsevier Science Ltd