Summary of Marinova et al., 2008 – Michael Bramble – GEOL 2810
Marinova et al. (2008) use a three-dimensional shock physics model to simulate a giant impact forming the northern lowlands of Mars. Under a moderately wide distribution of parameters, a modeled crater results in the northern hemisphere that matches characteristics of the northern lowlands. These parameters include (3–6)x 1029 J for the impact energy, 6–10 km/s for the impact velocities, impact angles of 30–60 degrees. The morphology of the absence of crust in the northern hemisphere matches that of the model, and using these parameters the impact melt remains largely inside of the cavity. The temporal location of the lowlands formation is suggested to correlate with the period of the early solar system when 1,600–2,700 km diameter bodies were present in Mars-crossing orbits.
The northern lowlands can be fit with an ellipse 10,650 by 9,520 km. Scarps, massifs, and plateaus concentric around the martian dichotomy boundary are suggested as evidence of the lowlands being an impact basin, with some similarity to features observed at the South Pole–Aitken basin on the Moon, Caloris basin on Mercury, and Hellas basin on Mars.
The modeling uses the SPH code, and tests the parameter space of (0.1–5.9) x 1029 J impact energies, crater diameters of 4,000–12,000 km, impact velocities of 6-50 km/s, impact angles of 0, 15, 30, 45, 60, 75 degrees, and impactor diameters of 400 to 2,700 km. Their simulations show that there is a ‘sweet spot’ in parameter space where the characteristics of the lowlands appears in their model (Figure 1).
Some findings of their simulations suggest that impact velocity and impact angle fundamen-tally alter the boundary of crustal excavation, its ellipticity, and the amount and distribution of melt in comparison to smaller, half-space craters. The rarefaction wave is suggested to completely remove the surrounding crust, and this crustal excavation boundary is suggested to correspond with the northern lowlands basin. Also, while holding many of the parameters steady, it is found that increasing the impact energy results in an increase in elongation of the excavation at shallow angles. For all but the highest energies investigation (i.e. <10,000 km crater diameters), it is found that the majority of the melt remains in the cavity. About 50–70% of the melt remains inside the dichotomy boundary, 25–30% is found beyond the boundary, and the remainder reaches escape velocity.
Select versions of their simulations suggest that significant antipodal effects should be observed from impacts that are large enough to form the lowlands, yet no such prominent structures are seen on the southern highlands. This affects their modelling in that they only consider impacts that form antipodal features smaller than 10 degrees in diameter.