Impact craters provide valuable insights into the formation and geological evolution of planetary bodies. However, the origin of the impactor often remains uncertain. Typically, two different origins are considered: a planetocentric origin, where impactors orbited a parent-planet, or a heliocentric origin, where impactors orbited the Sun. This distinction is crucial, because the velocity is different for planetocentric and heliocentric impactors, and using one model or the other can lead to significantly different outcomes. To address this issue, further research into this topic is required.

To help determine the origin of impactors, this report explores potential methods to distinguish heliocentric and planetocentric impactors based on crater characteristics on icy moons, using hydrocode impact simulations. While previous studies have examined cratering in ice, they primarily focused on crater diameter and depth. Here, additional characteristics are explored, including damage, temperature, and ice phases, to identify correlations between these crater properties, impact velocity, and impactor size. From these correlations, the probability that a crater was formed by a heliocentric or planetocentric impactor can be inferred.

This report focuses on impact simulations targeting a Ganymede-like surface, with velocities of 5 km/s and 15 km/s representing planetocentric and heliocentric impactors, respectively.
Simulation results reveal distinct differences in crater morphology for the depth-to-diameter (d/D) ratio between 5 km/s and 15 km/s. Thus, an impact velocity can be inferred for a given crater diameter and depth by determining its position relative to these d/D curves.
Fracturing was observed to be significantly more extensive in the 15 km/s simulations, although observations of actual craters often show limited fracturing due to freezing, post-impact relaxation, and tectonic movements. However, in larger craters, extensive fracturing can allow drainage of impact melt, forming central pits — a feature that could act as an indicator of high-velocity impacts, since fracturing is mostly prominent for the 15 km/s simulations.
Temperature results show high-temperature crystalline ice forming on the crater floor and walls, with a surrounding layer of warm material, both more prominent in the 15 km/s simulations. Comparisons with observational data reveal that a 1000 m, 15 km/s simulation aligns well with craters showing low amorphous ice mass fractions and large ice grain sizes near the crater, suggesting that such craters could similarly result from high-velocity impactors.

Overall, the combined findings provide a good preliminary indication of the impactor origin based on observable crater properties. Nonetheless, uncertainties remain and adjustments and improvements could be made to attain a more concrete result. Future research should expand simulations to other icy moons to compare the response of differing environments, incorporate alternative viscosity models, and explore a wider range of impactor sizes and velocities thus increasing the number of simulations. Finally, introducing 3D simulations could provide new perspectives regarding oblique impacts, ejecta patterns, and central peaks.

This is the final report in a project which aims to design a mega-constellation to survey the Earth for environmental, safety and security reasons: monitoring gases, humans, vehicles, and potential other aspects such as forest fires...