High-Resolution Simulations of Giant Impacts: Efficient Spherical Initial Conditions and Next-Generation Performance with SWIFT.

We perform simulations of giant impacts onto the young Uranus using smoothed particle hydrodynamics (SPH) with over 100 million particles. This 100--1000$times$ improvement in resolution passes a key threshold to reveal previously unseen details in the post-impact evolution of the planetu0027s atmosphere and ejected debris. 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 present 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 SWIFT and the particle placement code are publicly released.