Size-frequency distribution of different secondary crater populations: 1. Equilibrium caused by secondary impacts

Abstract

Accumulation of impact craters is the major reason causing equilibrium of crater populations on airless planetary surfaces. Besides primary craters, the effect of widespread secondaries on the equilibrium of local crater populations is little studied. Here the different secondary crater populations formed by the Hokusai crater on Mercury are systematically studied, and they are compared with those on the Moon to investigate their contribution to the evolution of local crater populations. Self‐secondaries cause equilibrium on continuous ejecta deposits in a short time, and the equilibrium crater population has a differential size‐frequency distribution (SFD) slope of about −3. Background secondaries are abundant on Mercury, and equilibrium caused by a combination of primaries and potential background secondaries follows the same pattern on the Moon and Mercury. The spatial dispersion of fragments that form both near‐field and distant secondaries is the major factor affecting the degree of mutual destruction and thus the final crater SFD. Some clustered distant secondaries on Mercury are likely formed by individual fragments considering their large spatial dispersion and identical morphology with same‐sized primaries, and the SFD rollovers of these secondaries possibly reflect the inherent SFD rollovers of the impact fragments. Near‐field secondaries and many other distant secondaries have morphology and spatial distribution that are consistent with being formed by clustered fragments, and mutual destruction of secondaries may be the major reason causing the observed SFD rollovers. Heterogeneous secondary impacts are a potential explanation for both different crater densities within the equilibrium diameter range and different regolith thicknesses on coeval surfaces.

On the importance of self-secondaries

“…These secondaries are termed as background secondaries or field secondaries (e.g., McEwen and Bierhaus 2006). The high efficiency in the production of distant secondaries have been noticed on Mercury (Strom et al 2011;Xiao 2016), the Moon (e.g., Hirata and Nakamura 2006;Dundas and McEwen 2007;Robinson et al 2015), Mars (e.g., McEwen et al 2005;Robbins and Hynek 2014), and Europa (Bierhaus et al 2005). However, whether or not background secondaries dominate crater populations within their diameter ranges (e.g., < ~1 km on the Moon and Mercury; < 8 km on Mercury) has been a debate for more than half a century (e.g., Shoemaker 1962;Neukum and Ivanov 1994;McEwen and Bierhaus 2006;Hartmann 2007;Werner et al 2009;Strom et al 2011;Xiao and Strom 2012;Williams et al 2014;Xie et al 2017).…”

“…The resolution of this debate is critically important to both the crater chronology and the application of crater statistics, because: (1) crater counts at the Apollo and Luna landing sites are dominated by sub-kilometer or kilometer diameter crater populations, so that possible contamination by background secondaries might have caused larger crater densities than those of the primary crater populations; (2) geological study for small planetary surface units are recently being enabled with more and more high-resolution images obtained, but only small craters are visible on small geological units, thus the effect of potential background secondaries cannot be ignored. Although new observations on Mercury suggest that background secondaries do abundantly exist outside of impact rays (Xiao 2016), solid observations for populations of background secondaries and their SFD are still lacking, since in theory, background secondaries are not distinguishable from primaries in terms of morphology and spatial distribution. Therefore, there have been three different schools in this debate: (1) Background secondaries dominate small crater populations on Mercury (Strom et al 2011), the Moon (e.g., Shoemaker 1962;Dundas and McEwen 2007;Xiao and Strom 2012), Mars (McEwen et al 2005;Robbins and Hynek 2014), and Europa (Bierhaus et al 2005); (2) Background secondaries are not abundant on planetary surfaces so they have negligible effect on crater statistics (e.g., Neukum and Ivanov 1994;Ivanov 2006;Werner et al 2009;Xie et al 2017); (3) Background secondaries do exist in populations of small impact craters, but the current crater chronology have integrated both primaries and background secondaries, so the effect of background secondaries has already been bracketed (Hartmann 2007;Hartmann and Daubar 2017).…”

“…2c). While most of these secondaries can be confidently recognized and excluded when dating planetary surfaces using crater statistics, some of them have identical plane morphology with similar-sized primaries (Xiao et al 2014), and they may no longer be differentiable from primaries once been degraded (Xiao 2016).…”

“…The base mosaic is an enhanced color mosaic of Mercury (3.74 km/pixel; equirectangular projection). The red, green, and blue channels are the second principal component, the first principal component, and the ratio of the 430 nm/1000 nm filters, respectively, of the MDIS 8-band global mosaics (Xiao 2016). b Approximate outer boundaries of the continuous ejecta deposits and continuous secondaries facies of the Hokusai crater (yellow lines).…”

“…Therefore, there have been three different schools in this debate: (1) Background secondaries dominate small crater populations on Mercury (Strom et al 2011), the Moon (e.g., Shoemaker 1962;Dundas and McEwen 2007;Xiao and Strom 2012), Mars (McEwen et al 2005;Robbins and Hynek 2014), and Europa (Bierhaus et al 2005); (2) Background secondaries are not abundant on planetary surfaces so they have negligible effect on crater statistics (e.g., Neukum and Ivanov 1994;Ivanov 2006;Werner et al 2009;Xie et al 2017); (3) Background secondaries do exist in populations of small impact craters, but the current crater chronology have integrated both primaries and background secondaries, so the effect of background secondaries has already been bracketed (Hartmann 2007;Hartmann and Daubar 2017). A full understanding of the spatial dispersion and SFD of impact fragments during the cratering process is required to investigate the heterogeneity of background secondaries in terms of diameter ranges and spatial locations, which may settle down the debate (Xiao 2016).…”