Secondary craters and ejecta across the solar system: Populations and effects on impact‐crater–based chronologies

Abstract

We review the secondary‐crater research over the past decade, and provide new analyses and simulations that are the first to model an accumulation of a combined primary‐plus‐secondary crater population as discrete cratering events. We develop the secondary populations by using scaling laws to generate ejecta fragments, integrating the trajectories of individual ejecta fragments, noting the location and velocity at impact, and using scaling laws to estimate secondary‐crater diameters given the impact conditions. We also explore the relationship between the impactor size–frequency distribution (SFD) and the resulting secondary‐crater SFD. Our results from these analyses indicate that the “secondary effect” varies from surface to surface and that no single conclusion applies across the solar system nor at any given moment in time—rather, there is a spectrum of outcomes both spatially and temporally, dependent upon target parameters and the impacting population. Surface gravity and escape speed define the spatial distribution of secondaries. A shallow‐sloped impactor SFD will cause proportionally more secondaries than a steeper‐sloped SFD. Accounting for the driving factors that define the magnitude and spatial distribution of secondaries is essential to determine the relative population of secondary craters, and their effect on derived surface ages.

Lunar Cold Spots and Crater Production on the Moon

“…Studies of Apollo drill cores however find that vertical mixing of nanophase FeO, cosmic‐ray tracks, cosmogenic radionuclides, and other products of regolith maturation that accumulate at the surface (e.g., Blanford, ; Fruchter et al, ; Morris, ) require much higher overturn rates. Secondary craters, craters formed by fragments ejected by the original primary impact event, have been suggested to form a substantial fraction of the total population of craters (e.g., Bierhaus et al, ; McEwen et al, ; Shoemaker, ; Williams, ), and Costello et al () find that estimates of regolith overturn‐depth rates that include secondary cratering are in good agreement with the analysis of the Apollo cores. This model estimates that in 300 kyr, there is a 99% chance of the regolith being overturned one or more times to a depth of ~12 cm and a 99% chance of the regolith being overturned 100 or more times at a depth of 5 cm, which is broadly consistent with our estimates of the cold spot lifetimes of less than ~1 Myr (i.e., 5 cm to >10 cm overturned in a few hundred kiloyear).…”

Measuring impact crater depth throughout the solar system

“…; McEwen and Bierhaus ; Bierhaus et al. ), although recent work shows a wide range of values for Martian secondaries, up to >0.2 for impact velocities exceeding 1 km/sec (Watters et al. in press).…”

“…6). Prior studies report that secondary craters tend to have d/D % 0.1 (Shoemaker 1965;Pike and Wilhelms 1978;Schultz and Singer 1980;McEwen et al 2005;McEwen and Bierhaus 2006;Bierhaus et al 2018), although recent work shows a wide range of values for Martian secondaries, up to >0.2 for impact velocities exceeding 1 km/sec (Watters et al in press).…”

“…Low‐velocity, subsonic impacts are typical of secondary impact cratering (e.g., Roberts ; Moore ; Bierhaus et al. ) and also tend to exhibit shallower cavities. Such impacts are dominated by transfer of momentum (“pushing aside” of surface material) rather than the effects of shock compression and subsequent unloading of the target.…”

“…Impacts at low angles (generally ≲10°; e.g., Collins et al 2011) tend to deposit energy at a shallower depth and hence exhibit less cratering efficiency, producing shallower impact craters (e.g., Gault and Wedekind 1978;Gault and Sonett 1982). Low-velocity, subsonic impacts are typical of secondary impact cratering (e.g., Roberts 1964;Moore 1971;Bierhaus et al 2018) and also tend to exhibit shallower cavities. Such impacts are dominated by transfer of momentum ("pushing aside" of surface material) rather than the effects of shock compression and subsequent unloading of the target.…”