Planetary giant impacts: convergence of high-resolution simulations using efficient spherical initial conditions and swift

Abstract

We perform simulations of giant impacts onto the young Uranus using smoothed particle hydrodynamics (SPH) with over 100 million particles. This 100-1000× improvement in particle number reveals that simulations with below 10 7 particles fail to converge on even bulk properties like the post-impact rotation period, or on the detailed erosion of the atmosphere. Higher resolutions appear to determine these largescale results reliably, but even 10 8 particles may not be sufficient to study the detailed composition of the debris -finding that almost an order of magnitude more rock is ejected beyond the Roche radius than with 10 5 particles. We present two software developments that enable this increase in the feasible number of particles. First, we present an algorithm to place any number of particles in a spherical shell such that they all have an SPH density within 1% of the desired value. Particles in model planets built from these nested shells have a root-mean-squared velocity below 1% of the escape speed, which avoids the need for long precursor simulations to produce relaxed initial conditions. Second, we develop the hydrodynamics code SWIFT for planetary simulations. SWIFT uses task-based parallelism and other modern algorithmic approaches to take full advantage of contemporary supercomputer architectures. Both the particle placement code and SWIFT are publicly released.1 The SEAGen code is publicly available at github.com/jkeger/seagen and the python module seagen can be installed directly with pip.

Tilting Ice Giants with a Spin–Orbit Resonance

“…Instead, we observe a wide range of tilts with Uranus' as the extreme case at 98°. The leading hypothesis for Uranus' large obliquity is multiple giant impacts (Benz et al 1989;Korycansky et al 1990;Slattery et al 1992;Parisi & Brunini 1997;Morbidelli et al 2012;Izidoro et al 2015;Kegerreis et al 2018Kegerreis et al , 2019, which are expected during the early stages of planetary formation (e.g. formation of Earth's Moon (Canup & Asphaug 2001)); this model, however, has significant drawbacks, mainly that the impactors need to be near-Earth sized.…”

Atmospheric Erosion by Giant Impacts onto Terrestrial Planets

“…In this section we describe the initial conditions for the model planets, the range of impact scenarios, and the previous models to which we compare our results. The SPH simulations are run using the hydrodynamics and gravity code SWIFT 4 (Schaller et al 2016;Kegerreis et al 2019).…”

“…The radii of the outer edge of the core and mantle are 0.49 and 0.96R ⊕ for the target and 0.29 and 0.57R ⊕ for the impactor. We use the simple Tillotson (1962) iron and granite equations of state (EOSs; Melosh 2007, Table AII.3) to model these materials (Kegerreis et al 2019). 5 For the atmospheres, we use the Hubbard & MacFarlane (1980) hydrogen-helium EOS, as described in Kegerreis et al (2018).…”

“…Particles are then placed to precisely match these profiles using the stretched equal-area (SEA) method 6 described in Kegerreis et al (2019). This results in a relaxed arrangement of particles that have SPH densities within 1% of the desired profile values, mitigating the need for extra computation that is otherwise required to produce initial conditions that are settled and ready for a simulation.…”

“…Giant impacts are most commonly studied using smoothed particle hydrodynamics (SPH) simulations, where planets are modeled with particles that evolve under gravity and material pressure. It was recently shown that at least 10 7 (equal-mass) SPH particles can be required to converge on even the largescale results from simulations of giant impacts, and that the resolution requirements for reliable results depend strongly on the specific scenario and question (Hosono et al 2017;Kegerreis et al 2019).…”