Accretion and differentiation of the terrestrial planets with implications for the compositions of early-formed Solar System bodies and accretion of water

Abstract

a b s t r a c tIn order to test accretion simulations as well as planetary differentiation scenarios, we have integrated a multistage core-mantle differentiation model with N-body accretion simulations. Impacts between embryos and planetesimals are considered to result in magma ocean formation and episodes of core formation. The core formation model combines rigorous chemical mass balance with metal-silicate element partitioning data and requires that the bulk compositions of all starting embryos and planetesimals are defined as a function of their heliocentric distances of origin. To do this, we assume that non-volatile elements are present in Solar System (CI) relative abundances in all bodies and that oxygen and H 2 O contents are the main compositional variables. The primary constraint on the combined model is the composition of the Earth's primitive mantle. In addition, we aim to reproduce the composition of the martian mantle and the mass fractions of the metallic cores of Earth and Mars. The model is refined by least squares minimization with up to five fitting parameters that consist of the metal-silicate equilibration pressure and 1-4 parameters that define the starting compositions of primitive bodies. This integrated model has been applied to six Grand Tack N-body accretion simulations. Investigations of a broad parameter space indicate that: (1) accretion of Earth was heterogeneous, (2) metal-silicate equilibration pressures increase as accretion progresses and are, on average, 60-70% of core-mantle boundary pressures at the time of each impact, and (3) a large fraction (70-100%) of the metal of impactor cores equilibrates with a small fraction of the silicate mantles of proto-planets during each core formation event. Results are highly sensitive to the compositional model for the primitive starting bodies and several accretion/core-formation models can thus be excluded. Acceptable fits to the Earth's mantle composition are obtained only when bodies that originated close to the Sun, at <0.9-1.2 AU, are highly reduced and those from beyond this distance are increasingly oxidized. Reasonable concentrations of H 2 O in Earth's mantle are obtained when bodies originating from beyond 6-7 AU contain 20 wt% water ice (icy bodies that originated between the snow line and this distance did not contribute to Earth's accretion because they were swept up by Jupiter and Saturn). In the six models examined, water is added to the Earth mainly after 60-80% of its final mass has accreted. The compositional evolution of the mantles of Venus and Mars are also constrained by the model. The FeO content of the martian mantle depends critically on the heliocentric distance at which the Mars-forming embryo originated. Finally, the Earth's core is predicted to contain 8-9 wt% silicon, 2-4 wt% oxygen and 10-60 ppm hydrogen, whereas the martian core is predicted to contain low concentrations (<1 wt%) of Si and O.

Tracing the ingredients for a habitable earth from interstellar space through planet formation

“…Thus, substantial loss of N to space seems to be required. This may be most consistent with scenarios that include (i) late delivery of volatiles, chiefly from comparatively oxidized and metal-poor bodies (48), thereby adding volatile-rich material to the mantle without loss of metal to the core; (ii) multiple magma oceans punctuated by large atmospheric loss events (11); and (iii) atmospheric ablation from many smaller impacts following magma ocean solidification (32). The high C/N ratio of the BSE therefore appears to be a sensitive indicator of the balance of volatile accretion and loss during the final stages of the Earth's assembly.…”

Nitrogen isotope fractionation during terrestrial core-mantle separation

“…The high end of these C N BE values may be higher than the nitrogen concentration in any known chondrites (Krot et al, 2014), which thus indicates that a hybrid Rayleigh-equilibrium model may be more realistic for the segregation of nitrogen in Earth's core if D N metal/silicate is 15-20. Such a hybrid model (Rubie et al, 2015) assumes that during the early stages of accretion, the impacting objects are relatively small and their metal phase sequentially extracts nitrogen from the magma ocean according to a Rayleigh model; however, towards the end of accretion, large, differentiated planetesimals collide with the growing Earth. The cores of these large objects are in bulk equilibrium with their mantles, but due to their large size, they do not exchange nitrogen with the silicate magma ocean anymore.…”

“…The C N BE required to achieve the C N mantle of 0.8 ppm by Marty (2012) would be about 40 ppm. According to Rubie et al (2015), 80-100 % of the core-forming metal may have equilibrated with the silicate magma ocean according to the Rayleigh model. However, it should be noted that the resulting d 15 N mantle of -17 ‰ in the hybrid model above is still significantly lower than the present-day mantle d 15 N of −5 ‰, which indicates a much reduced efficiency of the hybrid Rayleigh-equilibrium model in increasing d 15 N mantle compared to the pure Rayleigh model.…”

In search of late-stage planetary building blocks

“…Late stages of major terrestrial planetary accretion may have included the participation of materials that formed in different portions of the protoplanetary disk, and included water-and organic-rich materials (Weidenschilling et al, 1997;Chambers, 2001;. Thus, although limited in mass, late stage planetary growth may have had a disproportionate effect on the volatile contents of the rocky planets (e.g., Kerridge, 1985;Balsiger et al;Eberhardt et al, 1995;Albarede et al, 2013;O'Brien et al, 2014;Rubie et al, 2015a). Further, late stage additions may have carried genetically distinct elemental and isotopic fingerprints.…”