The effect of pre-impact spin on the Moon-forming collision

Abstract

We simulate the hypothesised collision between the proto-Earth and a Mars-sized impactor that created the Moon. Amongst the resulting debris disk in some impacts, we find a self-gravitating clump of material. It is roughly the mass of the Moon, contains $\sim 1\%$ iron like the Moon, and has its internal composition resolved for the first time. The clump contains mainly impactor material near its core but becomes increasingly enriched in proto-Earth material near its surface. A graduated composition has recently been measured in the oxygen isotope ratios of Apollo samples, suggesting incomplete mixing between proto-Earth and impactor material that formed the Moon. However, the formation of the Moon-sized clump depends sensitively on the spin of the impactor. To explore this, we develop a fast method to construct models of multi-layered, rotating bodies and their conversion into initial conditions for smoothed particle hydrodynamical (SPH) simulations. We use our publicly available code to calculate density and pressure profiles in hydrostatic equilibrium, then generate configurations of over a billion particles with SPH densities within $1\%$ of the desired values. This algorithm runs in a few minutes on a desktop computer, for $10^7$ particles, and allows direct control over the properties of the spinning body. In comparison, relaxation or spin-up techniques that take hours on a supercomputer before the structure of the rotating body is even known. Collisions that differ only in the impactor's initial spin reveal a wide variety of outcomes: a merger, a grazing hit-and-run, or the creation of an orbiting proto-Moon.

Submitted to MNRAS. 11 pages, 9 figures

References in corpus (5)

• The structure of terrestrial bodies: Impact heating, corotation limits and synestias
• The early instability scenario: terrestrial planet formation during the giant planet instability, and the effect of collisional fragmentation
• Models of Saturn's Interior Constructed with Accelerated Concentric Maclaurin Spheroid Method
• Numerical aspects of Giant Impact simulations
• Observational constraint on the radius and oblateness of the lunar core-mantle boundary

Cited by in corpus (13)

• SWIFT: A modern highly-parallel gravity and smoothed particle hydrodynamics solver for astrophysical and cosmological applications
• Reduced atmospheres of post-impact worlds: The early Earth
• A super-massive Neptune-sized planet
• Immediate origin of the Moon as a post-impact satellite
• A recent impact origin of Saturn's rings and mid-sized moons
• Origin of Mars's moons by disruptive partial capture of an asteroid
• REMIX SPH -- improving mixing in smoothed particle hydrodynamics simulations using a generalised, material-independent approach
• A Systematic Survey of Moon-Forming Giant Impacts. II. Rotating bodies
• Exploring the catastrophic regime: thermodynamics and disintegration in head-on planetary collisions
• Dealing with density discontinuities in planetary SPH simulations
• No dilute core produced in simulations of giant impacts on to Jupiter
• A synchronous moon as a possible cause of Mars' initial triaxiality
• Investigating tidal stripping of a pre-existing moon as the origin of Saturn's young icy rings