Giant Impact Hypothesis to Explain the Martian Hemispheric Dichotomy

Equirectangular projection of post-impact crustal thickness for the best-fitting case of the 750-km-radius impactor and the best-fitting case overall. Credit: Ballantyne et al., 2023.

The northern hemisphere of Mars is dominated by a large plain called the Vastitas Borealis (Borealis Basin). As a result, the northern hemisphere has an average altitude of about five kilometers less than the southern hemisphere. This difference is called the martian hemispheric dichotomy. One prevalent hypothesis to explain the hemispheric dichotomy is that a celestial body with a radius of about 1000 kilometers (about half the size of the Moon) impacted Mars. Initially, this hypothesis stated that the Borealis Basin was the resulting crater. More recent hypotheses suggest that an impact in the southern hemisphere could have produced the hemispheric dichotomy by inducing mantle upwelling and melting, generating huge volumes of new crustal material there.

To determine the most likely giant impact scenario that could result in the formation of the martian hemispheric dichotomy, Harry Ballantyne from the University of Bern and colleagues simulated the impact and resulting mantle dynamics with a smoothed-particle hydrodynamics (SPH) model coupled to a crust production model. SPH simulations calculate the properties of a medium, such as the crust or the mantle, by using a collection of particles, which move according to fluid mechanics. The crust production model is based on the melt fraction, which is determined by the pressure and temperature of the simulation. Crucially, the model of Ballantyne and colleagues incorporates material strength, which has historically been neglected but was found to have a significant effect. Among a variety of impact angles, velocities, radii, and other parameters totaling 14,700 cases, Ballantyne and colleagues find that the most likely impactor had a radius of 750 km, a velocity of 7 km/s, and an angle of 15° away from a head-on impact, that the initial crustal thickness was 25 km, and that the impactor hit the southern hemisphere. Further investigation of giant impact models like this could improve understanding of the interior of Mars and the origin of the hemispheric dichotomy. READ MORE