On the Nature of the Impactor That Formed the Shackleton Crater on the Moon

Abstract

The present paper attempts to assess the characteristics of the impactor that formed the Shackleton crater, located at the south pole of the Moon. The crater's morphometric parameters were analyzed based on the data of the Lunar Orbiter Laser Altimeter aboard the Lunar Reconnaissance Orbiter. Conclusions were drawn regarding the possible range of the impact angle and the parameters of the transient crater, such as depth and volume. The thickness of ejecta deposits on the transient crater rim and the volume of these deposits at a certain distance from the crater rim were assessed. These assessments enabled determining the type and characteristics of impactors (velocity, density, size, and impact angle) that could have formed the Shackleton crater. It was shown that the Shackleton crater could have been formed by an impact of a low-velocity (3 km/s) comets with diameter 4-4.5 km, chondrite or achondrite with a diameter of 2 km at a 45°-50°angle, whose velocity did not exceed 6 km/s, as well as stony-iron or iron-nickel impactors with a 1-2 km diameter for stony-iron asteroids and 1-1.5 km for iron-nickel asteroids. The impact velocity of stony-iron impactors, according to the authors' calculations, can reach 12 km/s. The impact velocities of iron-nickel asteroids range from 6 to 9 km/s. The impactor's substance mass that could have remained in the crater after it was formed was assessed.

“Isocrater” impacts: Conditions and mantle dynamical responses for different impactor types

“…Geochemical arguments, especially isotopic studies of water and nitrogen for the Earth, the Moon, and Mars, point to a strong dominance of asteroidal impactors but also confirm the existence of a contribution of cometary impactors on the order of a few percent (e.g., Barnes et al, 2016); these authors also invoke carbonaceous chondrites as a major asteroidal source for the water, which would indicate that many impactors were C-type asteroids (Carry, 2012). Near-surface geological aspects of this ambiguity have been addressed in the literature in some cases (e.g., Pugacheva et al, 2016, for the Shackleton crater on the Moon), but the implications for the deep interior evolution have not received much attention so far.…”