Ring formation in impact basins on Mars

Impact basins, the largest type of impact craters, form during the early stages of planetary evolution in the first several hundred million years following planetary accretion. Their size and morphology are mainly controlled by impact speed, impactor mass and geological properties of the target at the time of impact. Significant progress has been made in comprehending impact basins in recent years, with most efforts aimed at the Moon, where basins are best preserved. However, our comprehension of Mars basins remains comparatively limited. To fill this gap, we used numerical simulations to study the formation of impact basins on Mars at different geological epochs and regions.

We simulated the formation of impact basins using iSALE-2D, a multi-material, multi-rheology shock physics code that can simulate impacts in geological materials [www.isale-code.de]. We considered vertical impacts of dunite impactors travelling at 10 km/s on a target composed of a basaltic crust and a dunitic mantle with a surface temperature of 213 K. Projectile sizes varied from 20 to 400 km in diameter. The target was considered flat for simulations involving impactors smaller than 200 km; larger impacts were evaluated using both flat and spherical target geometry. We adopted three temperature profiles representing the planet's thermal state at 4.4 Ga (soon after the crust formed, when it was relatively hot), 4.0 Ga (an intermediate state) and 3.5 Ga (when the crust was comparatively colder). Two crustal thicknesses were considered, 47 and 91 km, corresponding to the average values of the martian lowlands and highlands.

Our simulations were presented as cross-sections of half craters showing two distinct layers, crust and mantle. We enabled material tracing to follow the material movement. We measured the spacing of rings indicated by tracer displacements and compared their position to the overall morphology of the simulated basins. We observed the formation of a listric normal fault at the main rim, crosscutting the crust and mantle to depths comparable to the maximum excavation depth. This structure, referred to as "main fault", was observed across all basin sizes considered in this study, and thus was considered as a suitable marker for the final basin size. In sufficiently large basins, we observed an outer ring defined by a planar normal fault crosscutting crust and mantle to comparatively larger depths. This structure, referred to as "outer ring fault", was associated with a topographical high and a zone of localised crustal thinning, forming crustal blocks similar to the ones observed in regions of extensional tectonics.

The spacing of rings in our simulated basins is similar to the reported spacing of rings in lunar basins. On average, the inner and outer rings are positioned at  0.46 and 1.41 times the final basin radius respectively (Figure 1). This spacing is controlled by age and crustal thickness. Rings of basins formed in older basins or hotter crust tend to be positioned closer to the impact point than those formed in younger basins or colder crust. Crustal thickness excerpt a similar effect, especially on the position of outer rings. Outer rings of basins formed in a thin crust are ~14% closer to the impact point than outer rings of basins formed in a thick crust.

Our results have implications for the formation and evolution of impact basins on Mars, helping to deepen our understanding of basin’s inner structure and multi-ring basin formation. Moreover, our results can be used to constrain the thermal state and crustal structure on Mars at the time of basin formation by comparing our findings with the observed morphology of Martian basins. This provides important insights to the bombardment history of Mars and to our knowledge of large impacts in the early Solar System.

Figure 1 - Relative ring spacing of the inner (r_inn) and outer (r_out) rings measured from 60 numerical simulations of impact basins on Mars created by projectiles from 20 to 200 km in diameter. The plots present, from top to bottom, the complete dataset, and simulations for 3.5, 4.0 and 4.4 Ga. Inner ring ratios for a thin and thick crust are presented in orange and blue respectively; outer ring ratios for a thin and thick crust are presented in red and green respectively. In the rare cases more than one outer ring was present, only the inner most radii were recorded.