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Impact Melting of Regolith Particles by Micrometeorites as a Mechanism of Soil Maturation

Introduction: The mechanisms of maturation of the regolith on the surfaces of atmosphereless celestial bodies were discussed from the very beginning of the studies of lunar soils. The most puzzling feature of a mature soil is presence of 1000 Å-thick amorphous rims containing nanometer size grains of reduced iron (npFe 0) on the surfaces of regolith particles [1]. One group of researchers (e.g., [2]) suggests that the rims with npFe 0 are due to solar wind (SW), whereas the alternative hypothesis was condensation of impact vapor in meteoritic bombardment [3]. Here a new approach to rim formation is presented, with an emphasis on the importance of impact melting of regolith particles , especially, melting of their surfaces in the impacts of submicron scale. The contributions of solar wind, impact evaporation, and impact melting to rim formation are compared. Experimental basis: There are experiments indicating that heating of Fe-bearing silicate materials to melting or submelting temperatures is enough for formation of npFe 0 , provided that oxygen pressure is low enough. Subsolidus reduction of Fe 2+ ions in olivine Fo89 and pyroxene En88 was observed in [4]. An indication to Fe reduction in silicate melt may be found in laser heating experiments [5]. On the irradiated oli-vine particles, amorphous rims with npFe 0 were observed , whereas in the irradiated pyroxene powder neither no amorphous rims with npFe 0 were detected. Yet a small number of large (~100µm in size) dark amorphous enstatite particles that contained npFe