Structure and evolution of terrestrial bodies as recorded by impact basins

Impact basins are the largest impact structures. Their formation is exclusive to the first ~800 Myrs since the formation of the Solar System, when the largest impactors left over from planetary accretion were still roaming [e.g., 1]. Impact basins were formed by planetary-scale impacts that remobilised considerable portions of the target’s lithosphere, significantly affecting the structure of the crust and mantle and its subsequent evolution. Through their study, it is possible to construct a better understanding of the thermal and geological evolution of the target at the time of impact, providing important insights into the early phases of planetary evolution in the Solar System [e.g., 2, 3, 4].

The properties of impact basins are controlled by the target’s lithospheric thickness and thermal properties at the time of impact. The formation of peak and multi-ring structures is predominantly governed by the thickness of the lithosphere. The formation of multi-rings is favoured by a thinner lithosphere, allowing the formation of more pronounced displacement along faults in the outskirts of the basins. The size of impact basins and the volume of impact melt produced by the impact are highly sensitive to the thermal conditions in the crust and upper mantle at the time of impact. Numerical simulations suggest that higher temperatures favour the formation of larger basins flooded by higher amounts of melt for a given set of impact conditions.

On the Moon, the thermal gradient played a dominant role in determining the final size of an impact basin. For a given set of impact conditions, impact basins formed in the Procellarum KREEP Terrane were larger than impact structures at other locations due to the unique thermal conditions present at the base of the crust [2]. Impact basins forming while the lunar magma ocean was still cooling resulted in a different morphology, with impact basins prone to easier erasure over time [3]. On Mars, the interplay between crustal thickness and thermal gradients in the Northern lowlands and Southern highlands during early geological epochs significantly influenced the final structure of impact basins [4]. For a given set of impact conditions, the thin crust typical for the lowlands favoured the formation of larger basins and increased the chances of multi-ring development, while the thicker crust typical for the highlands favoured the formation of smaller basins with less pronounced rings. The likelihood of the preservation of such basins is controlled by lithospheric temperature, with higher temperatures likely causing the formation or larger amounts of melt and preventing preservation of multi-rings at the surface. Mercury presents a scenario similar to the Moon, where impact basins formed in a thin crust and mantle under comparatively higher gravity conditions. Numerical simulations suggest that even the largest basins formed without significant interaction with the core despite its relatively larger size in relation to the planet’s volume in comparison to other rocky worlds. Similarly to Mars, higher lithospheric temperatures prevent the preservation of features associated to larger basins such as Caloris, resulting in flat impact structures almost entirely flooded by impact melt [e.g., 5]. Venus poses a more complex challenge. Regional variation in lithospheric thickness and apparent scarcity of impact basins complicate our understanding of its impact history but highlights the influence of unique planetary geophysical properties in the cratering record. It is likely that the high temperatures, especially in the upper lithosphere, and the subsequent intense crustal recycling erased most traces of older impact structures. Nevertheless, it is likely that records of such impacts can still be seen in the lithosphere, accessible through a combination of geodynamical and impact modelling.

This work systematically reviews the formation of impact basins on the Earth and Moon, Mars, Venus, and Mercury. We interrogate the existing cratering records of impact basins on these bodies to better understand the planet-defining impactor population. We compare insights from numerical impact simulations, with the current knowledge of global thermal evolution and crustal thickness models of the early geological epochs of the Moon and terrestrial planets, including numerical limitations and unknowns. By examining these properties, we can gain insights into the thermal and structural evolution of these bodies and enhance our understanding of the large impactor population in the early Solar System, shedding light on the frequency and scale of these planet-altering events.

[1] Marchi et al (2009) AJ 137 4936. [2] Miljkovic et al (2013) Science342,724-726. [3] Miljkovic et al (2021) Nat Commun 12, 5433 [4] Branco et al (2024) JGR: Planets, 129, e2023JE008217. [5] Gosselin et al. 2023. JGR: Planets 128, e2023JE007920.