The self‐secondary crater population of the Hokusai crater on Mercury

Abstract

Whether or not self‐secondaries dominate small crater populations on continuous ejecta deposits and floors of fresh impact craters has long been a controversy. This issue potentially affects the age determination technique using crater statistics. Here the self‐secondary crater population on the continuous ejecta deposits of the Hokusai crater on Mercury is unambiguously recognized. Superposition relationships show that this population was emplaced after both the ballistic sedimentation of excavation flows and the subsequent veneering of impact melt, but it predated the settlement and solidification of melt pools on the crater floor. Fragments that formed self‐secondaries were launched via impact spallation with large angles. Complex craters on the Moon, Mercury, and Mars probably all have formed self‐secondaries populations. Dating young craters using crater statistics on their continuous ejecta deposits can be misleading. Impact melt pools are less affected by self‐secondaries. Overprint by subsequent crater populations with time reduces the predominance of self‐secondaries.

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

“…A recent study on the self‐secondary crater population of Hokusai indicates that forming self‐secondaries is inherent to all complex craters on terrestrial bodies [ Xiao et al ., ]. Theoretically, impact fragments that form self‐secondaries are near‐vertically launched during early spallation [ Shoemaker et al ., ; Xiao et al ., ], so that they are relatively scattered in the spatial distribution compared with typical chains and clusters of secondaries (e.g., Figure d and Table S2 in the supporting information). Self‐secondaries that are formed on top of relatively porous ejecta deposits have similar morphology with same‐sized primaries, and those on impact melt deposits have irregular‐shaped rims and shallower depths, which may be due to a combined effect of the relatively small impact velocities and the then still‐molten impact melt [ Plescia , ].…”

“…Considering all the possibilities that might have caused the observed change in the crater SFD of the normal ejecta deposits, the observed −3 segment is most likely caused by equilibrium of self‐secondaries. The ballistic trajectory of fragments that form self‐secondaries has a maximum duration when their ejection velocities are comparable to the escape velocity, e.g., ~48 min on the Moon and ~42 min on Mercury [ Xiao et al ., ]. Although individual fragments have different flight times as indicated by their occurrences on both the normal ejecta and melt pools, equilibrium caused by self‐secondaries is a transient process compared to geological time scale.…”

“…Many of Hokusai's near‐field secondaries appear to be more circular in shape and more isolated in spatial distribution than their typical lunar counterparts [ Xiao et al ., ]. This characteristic is most obvious at the uprange (i.e., northeast) [ Xiao et al ., ], e.g., Figure b versus Figure c. Using MDIS WAC images obtained with large incidence angles (60°–75°), craters at both the uprange and downrange of the near‐field secondaries facies are separately collected to investigate the effect of the near‐field secondaries on the background crater populations (Figure a).…”

The Surface Roughness of Large Craters on Mercury

“…The map of the surface roughness of Hokusai has a smaller region of enhanced surface roughness values compared to other craters over 50 km in diameter (e.g., Abedin). Previous studies of Hokusai have noted extensive melt and unusual ejecta (rampart like structures; Barnouin et al, ; Xiao & Komatsu, ; Xiao et al, ). Arecibo radar data noted a region of elevated roughness values around Hokusai (Harmon et al, ) likely due to Hokusai having extensive melt, which is rough in radar wavelength‐scale (S band) surface roughness (Neish et al, ) due to the centimeter‐scale structure and smooth at L = 1‐km surface roughness since melt will infill rougher topography.…”

“…The lower density of secondary craters could result in lower surface roughness values exterior to the crater rim. From cross‐cutting relationships of distant secondaries and crater rays, Hokusai is believed to be the youngest complex crater on the surface (Xiao et al, ), but other young complex craters on the surface do not display the same large amount of melt. Additionally, the MLA coverage around Hokusai is not as dense as the coverage around Abedin, but the radial surface roughness plot had sufficient Δ h present to calculate RMS deviation (see Figure ).…”