Formation of glass spheres on the lunar surface

Abstract

The lunar fines contain an appreciable proportion of spherical glass particles. They are formed by local melting and splash due to meteorite impact. The observed particle size of the spheres is discussed in relation to the physical processes controlling their formation. Detailed studies of the structure of some larger spheres have been made. Three aspects are reported and discussed, the secondary cratering on the surface, the porosity of the spheres and their chemical homogeneity.

Lunar Impact Glasses: Probing the Moon's Surface and Constraining its Impact History

“…Investigations of the formation mechanisms of lunar impact glasses, including shapes, were conducted shortly after the first samples were brought to Earth (e.g., Chernyak & Nussinov, 1976;Scarlett et al, 1977), and multiple laboratory impact experiments using various kinds of projectiles and targets have produced glassy material (e.g., Hörz & Cintala, 1997;Hörz & Schaal, 1979). However, to date, "there are no impact experiments … that directly address the origin and compositional variety of these [lunar impact] glasses" (Hörz & Cintala, 1997).…”

“…Homogeneous glasses, with no internal compositional variation, formed either via impact into a single-mineral target or during an impact that was big enough and hot enough to homogenize the target material (e.g., Dence, 1971;Grieve, 1978;Grieve et al, 1974Grieve et al, , 1977von Engelhardt, 1972). Heterogeneous glasses, by contrast, most likely formed via impacts into multiple-component targets, contain fragments of other material (e.g., Chao et al, 1970;Scarlett et al, 1977), cooled slowly to form crystals (e.g., Symes et al, 1988), or some combination thereof.…”

“…None of the glasses discussed are ropy or agglutinates. Unlike volcanic glasses that were produced in residual liquids at the end stages of crystallization of the lunar magma ocean (e.g., Delano, 1986;Taylor & Jakes, 1974), impact glasses are formed by melted impact ejecta during ballistic flight (e.g., Chao et al, 1970;Delano & Livi, 1981;Glass, 1971), at temperatures upwards of 1,000 °C (e.g., Scarlett et al, 1977;Symes et al, 1988). They have a variety of distinct characteristics indicative of impact formation (e.g., Chao et al, 1970;LSPET, 1969;McKay et al, 1970 and references therein), including 1. mineral or rock fragment inclusions, some of which may have been derived from the impacting meteorite; 2. a single composition related to the mineral which melted upon impact; 3. flow features and/or shapes (e.g., dumbbell, teardrop, oblate spheroid, i.e., "splash-form" in the terrestrial literature) representative of rapid fusion and quenching; and/or 4. a coating of impact-induced "flour" (Chao et al, 1970) that adhered to the outer surface when produced simultaneously with the glass in the impact.…”