Simple craters are ubiquitous on the Moon and are mainly formed by impactors of <1 km in diameter. Prevailing knowledge suggested that gravity-dominated simple craters follow uniform morphometric scaling, which is routinely employed to probe lunar subsurface structures and compositions. However, recent observations revealed that the morphometry of some pristine simple craters on the Moon deviate from the empirical scaling. The empirical morphometric scaling was largely established from impact experiments using spherical impactors, whereas small impactors to the Moon such as near-Earth objects (NEOs) are typically of irregular or ellipsoidal shapes. The effect of impactor oblateness on the formation of lunar simple craters remains unexplored. We aim to investigate the effect of impactor oblateness on crater formation and their implications for the Moon and NEOs. We used numerical modelling to study the effect of impactor oblateness (i.e., aspect ratios of 1:10 to 10:1) on crater formation. For impactors with identical kinetic energy, larger oblateness yields smaller and shallower craters, but critical morphometric parameters remain unaffected, suggesting that simple crater morphology is not diagnostic evidence of impactor shape. Impactor oblateness affects the sizes of shocked zones that experienced peak pressures lower than a given value, yielding two-orders-of-magnitude variations in impact melt volumes for different impactor oblateness. Oblate impactors generate rarefaction waves earlier than spherical impactors, accelerating shock energy dissipation beyond the melt zone and reducing the maximum excavation depth and cratering efficiency by up to 22% and 80%, respectively. Empirical scaling relationships between the maximum excavation depth and crater geometries remain robust, verifying the reliability of using lunar simple craters as probes for subsurface structures and ejecta provenances. Considering the generally non-spherical shapes of NEOs with diameters of tens to hundreds of meters, their size distribution estimated using lunar simple crater diameters should have a revised power-law slope of -2.7, which is ∼3.8% steeper than prior estimations.
Wang et al. (Mon,) studied this question.