Investigating the feasibility of an impact-induced Martian Dichotomy

Ballantyne, Harry A.; Jutzi, Martin; Golabek, Gregor J.; Mishra, Lokesh; Cheng, Kar Wai; Rozel, Antoine B.; Tackley, Paul J. (2023). Investigating the feasibility of an impact-induced Martian Dichotomy. Icarus, 392, p. 115395. Elsevier 10.1016/j.icarus.2022.115395

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A giant impact is commonly thought to explain the dramatic contrast in elevation and crustal thickness between
the two hemispheres of Mars known as the ‘‘Martian Dichotomy’’. Initially, this scenario referred to an impact
in the northern hemisphere that would lead to a huge impact basin (dubbed the ‘‘Borealis Basin’’), while
more recent work has instead suggested a hybrid origin that produces the Dichotomy through impact-induced
crust-production. The majority of these studies have relied upon impact scaling-laws inaccurate at such large-
scales, however, and those that have included realistic impact models have utilised over-simplified geophysical
models and neglected any material strength. Here we use a large suite of strength-including smoothed-particle
hydrodynamics (SPH) impact simulations coupled with a more sophisticated geophysical scheme of crust
production and primordial crust to simultaneously investigate the feasibility of a giant impact on either
hemisphere of Mars to have produced its dichotomous crust distribution, and utilise spherical harmonic analysis
to identify the best-fitting cases. We find that the canonical Borealis-forming impact is not possible without
both excessive crust production and strong antipodal effects not seen on Mars’ southern hemisphere today.
Our results instead favour an impact and subsequent localised magma ocean in the southern hemisphere that
results in a thicker crust than the north upon crystallisation. Specifically, our best-fitting cases suggest that
the projectile responsible for the Dichotomy-forming event was of radius 500–750 km, and collided with Mars
at an impact angle of 15–30◦ with a velocity of 1.2–1.4 times mutual escape speed (∼6–7 km/s).

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