The origin of the late veneer of the terrestrial planets and of the lunar bombardment has been the subject of numerous studies in the field of cosmochemistry and in the field of planet formation and dynamical evolution of the early solar system. In the last years, we have studied [1,..,5] the dynamical and collisional evolution of the population of planetesimals originally in the terrestrial planet region and still “alive” at the time of the Moon-forming event. We have shown that this population of leftover planetesimals can explain the late veneer of Earth, Mars and Vesta as constrained by the amount of highly siderophile elements (HSE) in their mantles as well as the number of late impact basins on the Moon. The low concentration of HSE in the lunar mantle can be explained by a late sequestration of lunar mantle HSEs into the core at the time of the lunar mantle overturn. The origin of the late veneer carrier from the terrestrial planet region is consistent with the isotopic constraints on the source of the late veneer, indicating a non-carbonaceous source. This suggests that the carbonaceous projectiles that delivered part of the terrestrial volatile elements had already decayed by the time the late veneer started.

[5]Nesvorný, D., et al. 2023, Icarus, 399, 115545.

[4]Nesvorný, D., et al. 2022, ApJL, 941, L9.

[3]Zhu, M.-H. et al. 2021, Nature Astronomy, 5, 1286.

[2]Zhu, M.-H. et al. 2019, Nature Astronomy, 571, 226.

[1]Morbidelli, A., et al. 2018, Icarus, 305, 262.

Planetary formation models suggest that Earth experienced multiple high-energy impacts. Among those, the Moon-forming event is thought to be responsible for melting a large fraction of proto-Earth’s silicate mantle. Mixing of the impactor’s metallic core into Earth's silicate mantle controlled the chemical equilibration between metal and silicates, and hence the respective compositions of Earth's core and mantle. The extent of this mixing is, however, still debated. Previous studies explore mixing upon large impacts either with numerical modelling or with analog laboratory experiments. Numerical simulations are efficient in that they reproduce the shock physics of hypervelocity impacts. However, their spatial resolution is too limited to produce the turbulent features responsible for metal-silicate mixing in a magma ocean. Liquid impact experiments on the other hand are subsonic and hence neglect compressibility effects. However, they produce small-scale mixing and turbulence, which is crucial in estimating metal-silicate equilibration. Recent simulations and experiments disagree on the degree of mixing between the impactor and target materials. The origin of these differences is still unclear and requires further investigation. We present a scaling-law developed to extend the laboratory experiments results to hypervelocity cases and its further application the the metal-silicate mixing upon impact. We find that the Mach number (impact velocity to sound speed ratio) affects the metal-silicate mixing upon impacts, but that its effect depends on the other impact parameters such as the impactor size.

The influence of asteroid impacts during the late accretion phase on Earth’s present day composition is still not fully understood. One important question here is if the mixing of metal cores from differentiated impactors into an existing magma ocean could explain the relatively high concentrations of highly siderophile elements observed in Earth’s mantle. For this it is essential to know how much the impactor cores break apart during the impact process, since a more fragmented body will allow more mixing with the surrounding material.

We simulate the impacts of differentiated impactors into magma oceans using the grid-based Eulerian shock physics code iSALE. We developed and implemented a new method to improve the fragmentation behavior in such Eulerian codes and used it to study the fragmentation and dispersion of the metal core of the differentiated impactor. We vary the size and velocity of the impactor as well as target properties like the depth of the magma ocean and its viscosity.

We see significant fragmentation of the impactor core under most tested parameters. Higher impact velocity and greater magma ocean depth show an especially pronounced increase in core fragmentation.

Acknowledgments: We gratefully acknowledge the developers of iSALE-2D, including Gareth Collins, Kai Wünnemann, Dirk Elbeshausen, Tom Davison, Boris Ivanov and Jay Melosh. This work was funded by the Deutsche Forschungsgemeinschaft (SFB-TRR170, subproject C2 and C4).

The formation of a segregated metallic core is viewed as an inevitable consequence of the growth of larger protoplanets. However, the effect of this process on the distribution of siderophile elements is hugely variable depending on the physiochemical nature of the protoplanet and the pressure and temperature at which metal-silicate equilibration occurs. On Earth, study of this process can be complicated by the overprinting effect of late accretion, which delivered additional siderophile element mass to the Earth. Along with Precambrian-aged mantle-derived rocks, ocean island basalts (OIB) are now recognized as an important source of information about the early siderophile evolution of the deep Earth. We demonstrate that the combined W isotopic and highly siderophile element (HSE) characteristics of major global hotspots (Hawaiʻi, Iceland, Réunion) preserve geochemical signatures secondary to Hadean metal-silicate equilibration that have not been overprinted by late accretion. Further, some OIB preserve Ru/Ir ratios that are higher than expected, either for chondritic material delivered by late accretion or for the more highly processed primitive mantle. These elevated Ru/Ir signatures are not always easily explained by partial melting and/or magma differentiation processes and must in part reflect elevated Ru/Ir ratios in the deep mantle sources of OIB. Ruthenium has previously been investigated for its unique behavior among HSE during metal-silicate equilibration and heterogeneous, pre-late accretion Ru isotopic signatures have been recognized in some Archean-aged mantle-derived rocks. This implies that OIB may be an untapped source of information about the state of Earth’s interior during and after core formation.