Untrackable distal ejecta on planetary surfaces

Heterogeneous shock impedances of planetary materials cause abrupt changes of ejection angles, forming non-radial ejecta. Interpretations for provenances of surface deposits and ages derived from crater counts are affected by such untrackable ejecta. Impact ejecta are important references to establish regional and global stratigraphy of planetary bodies. Canonical views advocate radial distributions of distal ejecta with respect to the source crater, and their trajectories are significantly deflected on fast-rotating bodies. The Hokusai crater on Mercury formed a peculiar ray that features a hyperbola shape, and the sharp swerve of orientation was interpreted as a sign of a faster planetary rotation in the near past. Here, we show that this ray was not caused by a hypothesized larger Coriolis force, but due to abruptly-steepened ejection angles. Heterogeneous shock impedances of pre-impact impactor and/or target, such as topographic undulations, affect local propagation paths of shock and rarefaction waves, causing sudden changes of ejection angles. Distal ejecta with non-radial distributions are an inherent product of planetary impacts, and their unobvious provenances could mislead stratigraphic interpretations and hamper age estimations based on spatial densities of impact craters.

Automated summary

The heterogeneous shock impedances of planetary materials cause abrupt changes in ejection angles, resulting in non-radial ejecta. These untrackable ejecta affect interpretations of surface deposits and ages derived from crater counts. Impact ejecta serve as important references for establishing regional and global stratigraphy of planetary bodies.