Lunar Secondary Craters and Estimated Ejecta Block Sizes Reveal a Scale‐Dependent Fragmentation Trend

Abstract

Planetary impact events eject large volumes of surface material. Crater excavation processes are difficult to study, and in particular the details of individual ejecta fragments are not well understood. A related, enduring issue in planetary mapping is whether a given crater resulted from a primary impact (asteroid or comet) or instead is a secondary crater created by an ejecta fragment. With mapping and statistical analyses of six lunar secondary crater fields (including Orientale, Copernicus, and Kepler) we provide three new constraints on these issues: (1) estimation of the maximum secondary crater size as a function of distance from a primary crater on the Moon, (2) estimation of the size and velocity of ejecta fragments that formed these secondaries, and (3) estimation of the fragment size ejected at escape velocity. Through this analysis, we confirmed and extended a suspected scale‐dependent trend in ejecta size‐velocity distributions. Maximum ejecta fragment sizes fall off much more steeply with increasing ejection velocity for larger primary impacts (compared to smaller primary impacts). Specifically, we characterize the maximum ejecta sizes for a given ejection velocity with a power law and find that the velocity exponent varies between approximately −0.3 and −3 for the range of primary craters investigated here (0.83–660 km in diameter). Data for the Jovian moons Europa and Ganymede confirm similar trends for icy surfaces. This result is not predicted by analytical theories of formation of Grady‐Kipp fragments or spalls during impacts and suggests that further modeling investigations are warranted to explain this scale‐dependent effect.

Impactor material records the ancient lunar magnetic field in antipodal anomalies

“…Recent simulations of impact fragmentation suggest that ejecta capable of reaching the antipode produced by a 100-km-diameter impactor will consist of 50–1000 m scale fragments 24 . These estimates are consistent with extrapolation from observations of secondary craters of Orientale 25 . Even ejecta fragments as small as 0.5 m have a cooling timescale, = 70 h, which is longer than the flight time of most antipodal ejecta (>93%).…”

Low-speed Impacts into Ice–Dust Granular Mixtures

“…Secondary craters have been shown to be a key component of crater populations on various surfaces, ranging from the Moon (Dundas & McEwen 2007) to Mars (McEwen et al 2005) to icy moons such as Europa and Ganymede (Bierhaus et al 2005;Zahnle et al 2008;Singer et al 2013). The study of such craters is not only important for accurately dating these surfaces (Bierhaus et al 2018), but also for determining the ejecta size distributions from primary impacts (Singer et al 2013(Singer et al , 2020, thus verifying the validity of the cratering models (Melosh 1984) and scaling laws (Housen & Holsapple 2011) at planetary scales.…”

The appearance of a 'fresh' surface on 596 Scheila as a consequence of the 2010 impact event

“…The reddening of the spectrum after the impact suggests that finer particles may be present compared to before the impact (e.g., Johnson & Fanale 1973;Vernazza et al 2016). Fine particles ejected from craters by the excavation flow can have a wide range of ejection velocities (Singer et al 2020). However, finer particles have faster ejection velocities on average than larger particles, meaning that large particles should have preferentially been re-accreted at the surface of 596 after the impact.…”