Margarita Marinova, NASA Ames Research Center
The Solar System started out as a cloud of gas, coalescing into kilometer-sized objects, and in the final stages planetesimals – objects a few thousand kilometers across –accreted into the planets we observe today. These last, violent stages of planet formation left behind planetary-sized scars: impact craters thousands and tens of thousands of kilometers across. Such craters are observed throughout the Solar System: Caloris Basin on Mercury, South Pole-Aitken Basin on the Moon, Hellas Basin on Mars, Herschel Crater on Saturn’s moon Mimas, and others. The geophysical consequences of these impacts determine the features we see today. And understanding these impact processes provides insight into their thermal and geochemical consequences for the young planet.
We use a three-dimensional Smoothed Particle Hydrodynamics (SPH) model to simulate the impacts and capture the surface curvature of the target, the radial gravity, and the large relative size of the impactor to the planet. We explore impacts into Mars, with energies over two orders of magnitude (1.98x1027 to 5.89x1029 J), impact velocities of 6 to 50 km/s, impact angles from 0° (head-on) to the highly-oblique 75°, and a range of single-composition and differentiated impactors. In all cases the impacts penetrate through and remove the surrounding crust, forming a crater cavity. The resulting crater cavities are about 2,000 to 13,000 km across, equivalent to 9% to more than half (61%) of Mars’ circumference.
For these planetary-scale impacts, unlike for smaller impacts, no rim is present around the crater and the crater becomes elliptical at intermediate impact angles. Much of the impact energy is deposited in the mantle of the impacted planet; however, also a significant amount of material is ejected from the planet or put into orbit. The significant momentum of the impact can spin up the planet’s rotation rate to a period of less than a day.
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