# Planetary Giant Impacts: Convergence of High-Resolution Simulations using Efficient Spherical Initial Conditions and SWIFT

## TL;DR

This paper demonstrates that high-resolution SPH simulations with over 10^8 particles are crucial for accurately modeling giant impacts on Uranus, revealing convergence issues at lower resolutions and introducing new software tools for efficient, realistic initial conditions and simulations.

## Contribution

The authors develop a novel particle placement algorithm and an advanced hydrodynamics code, SWIFT, enabling high-resolution planetary impact simulations with improved accuracy and efficiency.

## Key findings

- Simulations with over 10^7 particles are needed for convergence of bulk properties.
- Higher resolutions significantly affect the amount and distribution of ejected debris.
- The new software tools are publicly available for the community.

## Abstract

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 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 large-scale 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.