Hydrocode modeling of oblique impacts into terrestrial planets

Jordan D Kendall, Purdue University

Abstract

The abundance of moderately siderophile elements (“iron-loving”; e.g., Co, Ni) in the Earth’s mantle is 10 to 100 times larger than predicted by chemical equilibrium between silicate melt and iron at low pressure, but it does match expectation for equilibrium at high pressure and temperature. Recent studies of differentiated planetesimal impacts assume that planetesimal cores survive the impact intact as concentrated masses that passively settle from a zero initial velocity and undergo turbulent entrainment in a global magma ocean; under these conditions, cores greater than 10 km in diameter do not fully mix without a sufficiently deep magma ocean. I have performed hydrocode simulations that revise this assumption and yield a clearer picture of the impact process for differentiated planetesimals possessing iron cores with radius = 100 km that impact into magma oceans. The impact process strips away the silicate mantle of the planetesimal and then stretches the iron core, dispersing the liquid iron into a much larger volume of the underlying liquid silicate mantle. Lagrangian tracer particles track the initially intact iron core as the impact stretches and disperses the core. The final displacement distance of initially closest tracer pairs gives a metric of core stretching. The statistics of stretching imply mixing that separates the iron core into sheets, ligaments, and smaller fragments, on a scale of 10 km or less. The impact dispersed core fragments undergo further mixing through turbulent entrainment as the molten iron fragments sink through the magma ocean and settle deeper into the planet. My results thus support the idea that iron in the cores of even large differentiated planetesimals can chemically equilibrate deep in a terrestrial magma ocean. The largest known impact on the Moon formed the South Pole-Aitken (SP-A) basin and excavated material as deep as the mantle. Here I suggest that large impacts eject enough material to cover the farside of the Moon. During the impact process, ejecta leave the crater and travel well beyond the transient crater. Ejecta blankets depend on impactor size and angle. I use iSALE, an impact hydrocode, to determine the ejecta distribution, volume, and thickness. I calculate the trajectory of ejecta that leave the crater and return to the lunar surface. In these simulations, an ejecta blanket forms, with a thickness of kilometers, over the lunar farside. The ejecta blanket thicknesses are comparable to the difference between nearside and farside crustal thickness. Previous studies suggest other possible mechanisms for the lunar farside-nearside dichotomy. However, the impact that formed SP-A basin was large enough to eject material onto the farside. I also suggest a differentiated impactor’s core would disperse downrange of the impact point underneath the basin. Doublet craters form within crater rays on terrestrial bodies. The near simultaneous impact of two projectiles results in overlapping craters. This process results in modified crater morphologies and ejecta morphologies. I modeled the impact of two identical projectiles and vary the angle, timing, and initial separation distance. In this work, I identified projectiles with a separation distance of four times their initial diameter will form distinct craters, but the ejecta from the uprange crater will overfill the downrange crater and result in a smaller crater depth. This result implies the direction of the impactor may be inferred from the crater depths. Also, I found impacts that form closer together result in elliptical or dumbbell craters depending upon the impact parameters. The ejecta curtains interact in each simulation and result in structures similar to the V-shaped ridges or “herringbone” patterns traversing clusters of secondary craters in observations. The ejecta that lands within the ridges comes from a depth that is 100 to 125 m for a 500 m impactor traveling at 1 km/s. This is less deep than the maximum excavation depth of 125 to 150 m, depending upon the impact angle. This work represents a first step towards a more comprehensive method for not only determining how doublet craters form and how aberrant craters form, such as Messier A on the Moon, but also determining how the regolith changes and the ejecta blanket forms for such impacts.

Degree

Ph.D.

Advisors

Melosh, Purdue University.

Subject Area

Geology|Geophysics|Planetology

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