Impact melt in small lunar highland craters

Abstract

Impact melt deposits have been identified in small, simple impact craters within the lunar highlands. Such deposits are rare, but have been observed in craters as small as 170 m diameter. The melt occurs as well‐defined pools on the crater floor, as well as veneers on the inner crater wall and stringers of material extending over the rim and away from the crater. Model calculations indicate that the amount of melt formed in craters 100–2000 m diameter would amount to a few to ∼106 m3, representing <1% of the crater volume. Thus, significant, visible impact melt deposits would not be expected in such small craters as most of the melt material that was formed would be ejected. Variations in the properties of the projectile or the target cannot account for the amount of observed melt; the amount of melt produced is largely insensitive to such variations. Rather, we suggest that these small melt‐containing craters represent near‐vertical impacts in which the axes of melting and melt motion are essentially straight down, toward the base of the transient cavity. For a given event energy under vertical impact conditions, the volume of melt produced would be greater than in an oblique impact and the momentum of the material would be directed vertically downward with minimal lateral momentum such that most of the melt is retained within the crater interior. Since vertical impacts are relatively rare, such small craters with visible, interior melt deposits are rare. While we focus here on the highlands, such craters also occur on the maria.