Impact vaporization of planetesimal cores in the late stages of planet formation

Abstract

Di erentiated planetesimals delivered iron-rich material to the Earth and Moon in high-velocity collisions at the end stages of accretion. The physical process of accreting this late material has implications for the geochemical evolution of the Earth-Moon system and the timing of Earth's core formation 1-3 . However, the fraction of a planetesimal's iron core that is vaporized by an impact is not well constrained as a result of iron's poorly understood equation of state. Here we determine the entropy in the shock state of iron using a recently developed shock-and-release experimental technique implemented at the Sandia National Laboratory Z-Machine. We find that the shock pressure required to vaporize iron is 507 (+65, −85) GPa, which is lower than the previous theoretical estimate 4 (887 GPa) and readily achieved by the high velocity impacts at the end stages of accretion. We suggest that impact vaporization of planetesimal cores dispersed iron over the surface of the growing Earth and enhanced chemical equilibration with the mantle. In addition, the comparatively low abundance of highly siderophile elements in the lunar mantle and crust 5-8 can be explained by the retention of a smaller fraction of vaporized planetesimal iron on the Moon, as compared with Earth, due to the Moon's lower escape velocity.Estimates of the timing of the end of Earth's core formation range from ∼30 to >100 Myr after the start of the Solar System 2 and depend on approximations for the magnitude of metal-silicate chemical equilibration for impactors of different sizes and impact velocities. Complete equilibration via emulsification of iron in a mantle magma ocean may occur by mixing the core material to centimetre length scales 9 . However, numerical simulations of giant impacts generally find that the impactor's core penetrates through the mantle to Earth's core 10,11 , and calculations of Rayleigh-Taylor instabilities and turbulent mixing do not achieve emulsification of iron cores larger than about 10 km (ref. 12). Even when cores are emulsified by instabilities, they may only equilibrate with a fraction of Earth's mantle unless there is significant post-emulsification mixing 12 . Thus, studies of the physics of core formation have suggested limited chemical equilibration of impactor cores with Earth's mantle. In contrast, chemical 1 and W-isotopic evidence 2 suggest that more substantial equilibration must have occurred.Core formation removes highly siderophile elements (HSEs: Re, Os, Ir, Ru, Rh, Pt, Pd, Au) from the mantle, but Earth's mantle contains orders of magnitude higher concentrations of HSEs than predicted by their low-pressure metal-silicate partitioning coefficients 6,13 . The concentrations and chondritic proportions of HSEs in the mantles of the Earth, Moon and other planets are used to infer the late accretion of chondritic planetesimals throughout the inner Solar System 3,13 . Perplexingly, the abundance of HSEs in the lunar mantle and crust has been estimated to be about one to two orders of magnitude smaller than...

Earth's moderately volatile element composition may not be chondritic: Evidence from In, Cd and Zn

“…A range of processes such as core formation, impact induced element loss during accretion via collisional erosion (O'Neill and Palme, 2008), vaporization (e.g., Kraus et al, 2015) and late addition might have influenced the depletion trend of volatile elements in the BSE. On the other hand, previous estimates of In abundances in the BSE were based on limited data, and different methods suggested different abundances (McDonough and Sun, 1995;Witt-Eickschen et al, 2009;Yi et al, 1995Yi et al, , 2000.…”

Degassing Processes and Chemistry

“…In fact, the arguments given here for conditions following somewhat smaller impactors (see Table 4.1) would also apply to the MFI, with suitable modifications to adjust for the magnitude of the event. Although MFI scenarios suggest that the core of the impactor is largely retained by Earth (Cameron 1997;Canup 2004), it would be surprising if some fraction of the metallic inventory were not vaporized during the impact (Kraus et al 2015). If as little as 10 % of the core were vaporized (and added to vaporized ferrous iron from the silicate shell) the amount of hydrogen produced by reaction with water would be at least as much as that produced in the smaller event of a 1000 km impactor (Table 4.1).…”

“…If the Moon's surface remained active for 50-100 MY following the MFI, or if lunar reaccretion from the post-MFI "cloud" were delayed for a similar interval (perhaps due to tidal disruption), any late veneer material added to the Moon would have been simply mixed in before the solidification of the lunar surface, including the lunar highland rocks. Alternatively, the vaporization of impactors on the moon might have resulted in vapor clouds that would simply escape from Moon's lower gravity (Kraus et al 2015). For Earth the surface would have stabilized rapidly, the presumed post-MFI magma ocean solidifying in about 20 million years or less (Sleep 2010).…”

The late Earth's accretion: Processes and materials

“…In case of large (up to ≈ 10 3 km) impactors [Raymond et al, 2013] melting and evaporation of impactor, crustal and mantle materials are the principal processes [Kraus et al, 2015]. To prevent fractionation of NM, special additional features of these processes are considered.…”