Collisional Stripping and Disruption of Super-Earths

The final stage of planet formation is dominated by collisions between planetary embryos. The dynamics of this stage determine the orbital configuration and the mass and composition of planets in the system. In the solar system, late giant impacts have been proposed for Mercury, Earth, Mars, and Pluto. In the case of Mercury, this giant impact may have significantly altered the bulk composition of the planet. Here we present the results of smoothed particle hydrodynamics simulations of high-velocity (up to ~5 v_esc) collisions between 1 and 10 M_Earth planets of initially terrestrial composition to investigate the end stages of formation of extrasolar super-Earths. As found in previous simulations of collisions between smaller bodies, when collision energies exceed simple merging, giant impacts are divided into two regimes: (1) disruption and (2) hit-and-run (a grazing inelastic collision and projectile escape). Disruption occurs when the impact parameter is near zero, when the projectile mass is small compared to the target, or at extremely high velocities. In the disruption regime, we derive the criteria for catastrophic disruption (when half the total colliding mass is lost), the transition energy between accretion and erosion, and a scaling law for the change in bulk composition (iron-to-silicate ratio) resulting from collisional stripping of a mantle.

💡 Research Summary

The paper investigates the final stage of terrestrial planet formation, when planetary embryos collide at high velocities, by performing a suite of three‑dimensional smoothed particle hydrodynamics (SPH) simulations of collisions between 1–10 M⊕ bodies that initially consist of a 30 wt% iron core and a 70 wt% silicate mantle. The authors explore a broad parameter space: impact velocities ranging from 1 to 5 times the mutual escape velocity (v_esc), impact parameters (b) from head‑on (b≈0) to grazing (b≈0.75 R_target), and projectile‑to‑target mass ratios (μ) from 0.1 to 1.0. Each simulation uses ≈10⁶ SPH particles and an ANEOS‑based equation of state to capture high‑pressure, high‑temperature behavior of both metal and silicate phases.

The results confirm the dichotomy previously identified for smaller bodies: collisions separate into (1) a disruption regime and (2) a hit‑and‑run regime. Disruption occurs when the impact is near‑central, the projectile is much smaller than the target, or the impact velocity is extreme. In this regime the authors derive a catastrophic disruption criterion: the specific impact energy Q*_RD at which 50 % of the total colliding mass is lost scales as Q*_RD ∝ (M_total)^0.75, consistent with earlier scaling laws for planetesimals. They also identify a transition energy Q_trans that marks the boundary between net accretion (Q < Q_trans) and net erosion (Q > Q_trans). The mass‑loss fraction follows a power‑law dependence on the normalized impact energy, Q/Q*_RD, with an exponent of ≈0.5, indicating that loss efficiency rises slowly with increasing energy.

In the hit‑and‑run regime, which dominates for impact parameters larger than ~0.5 R_target and for projectile‑to‑target mass ratios μ ≥ 0.3, the two bodies experience a brief inelastic contact and then separate. The target suffers little permanent deformation, while the projectile may lose a modest amount of surface material (typically <5 % of its mass for v ≤ 2 v_esc). The authors quantify the dependence of projectile mass loss on impact angle and velocity, showing a rapid decline in stripping efficiency for more oblique impacts.

A key focus of the study is compositional alteration through mantle stripping. In disruptive collisions, the silicate mantle is preferentially removed, leading to an increase in the iron‑to‑silicate mass ratio of the largest remnant. The authors present an empirical scaling law for the change in bulk Fe/Si ratio: Δ(Fe/Si) ≈ k · (E/E_crit)^β, where k ≈ 0.2 and β ≈ 0.7, and E_crit is the catastrophic disruption energy. For the most energetic, near‑central impacts (v ≈ 5 v_esc, b ≈ 0), the Fe/Si ratio of the remnant can double relative to the pre‑impact value, reproducing the high core fractions inferred for Mercury and several dense super‑Earths.

The paper discusses the astrophysical implications of these findings. The disruption scaling provides a straightforward prescription for incorporating realistic collision outcomes into N‑body planet formation models, allowing the simultaneous tracking of mass growth and compositional evolution. The authors argue that Mercury’s anomalously large core could be the product of a single high‑velocity, low‑impact‑parameter event, while the observed diversity of densities among exoplanet super‑Earths may reflect a spectrum of giant‑impact histories, ranging from gentle hit‑and‑run encounters to catastrophic mantle stripping.

In summary, the study delivers (i) quantitative criteria for catastrophic disruption and the accretion‑erosion transition for Earth‑mass to super‑Earth‑mass bodies, (ii) a robust scaling law for impact‑driven compositional change, and (iii) a practical framework for embedding these results into larger‑scale planet formation simulations. These contributions advance our understanding of how high‑energy collisions sculpt the mass, orbit, and interior structure of terrestrial planets both in our Solar System and in extrasolar planetary systems.