High pressure-temperature experiments suggest late delivery of life-essential elements to Earth by a large differentiated body.

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Understanding the origin of life-essential volatiles like nitrogen (N) in the SolarSystem and beyond is critical to evaluate the potential habitability of rocky planets [1][2][3][4][5] .Whether the inner Solar System planets accreted these volatiles from their inception or had an exogenous delivery from the outer Solar System is, however, not well understood. Using previously published data of nucleosynthetic anomalies of Ni, Mo, W and Ru in iron meteorites along with their 15 N/ 14 N ratios, here we show that the earliest formed protoplanets in the inner and outer protoplanetary disk accreted isotopically distinct N.While the Sun and Jupiter captured N from nebular gas 6 , concomitantly growing protoplanets in the inner and outer disk possibly sourced their N from organics and/or dust with each reservoir having a different N isotopic composition. A distinct N isotopic signature of the inner Solar System protoplanets coupled with their rapid accretion 7,8 suggests that non-nebular, isotopically processed N was ubiquitous in their growth zone at ~0-0.3 Myr after the formation of CAIs. Because 15 N/ 14 N ratio of the bulk silicate Earth falls between that of inner and outer Solar System reservoirs, we infer that N in the present-day rocky planets represents a mixture of both inner and outer Solar System material.

The effect of protoplanetary differentiation on the fate of life-essential volatiles like nitrogen and carbon and its subsequent effect on the dynamics of planetary growth is unknown. Because the dissolution of nitrogen in magma oceans depends on its partial pressure and oxygen fugacity, it is an ideal proxy to track volatile re-distribution in protoplanets as a function of their sizes and growth zones. Using high pressuretemperature experiments in graphite-undersaturated conditions, here we show that the siderophile (iron-loving) character of nitrogen is an order of magnitude higher than previous estimates across a wide range of oxygen fugacity. The experimental data combined with metal-silicate-atmosphere fractionation models suggest that asteroid-sized protoplanets, and planetary embryos that grew from them, were nitrogen-depleted.However, protoplanets that grew to planetary embryo-size before undergoing differentiation had nitrogen-rich cores and nitrogen-poor silicate reservoirs. Bulk silicate reservoirs of large Earth-like planets attained nitrogen from the cores of latter type of

Terrestrial reservoirs BSE 0.011 AE 0.002 0.49 AE 0.14 40.00 AE 8.00 1.13 AE 0.20 Mantle 0.008 AE 0.002 0.36 AE 0.10 72.73 AE 37.72 2.00 AE 0.70 Crust-ocean-atmosphere 0.002 AE 0.000 8.89 AE 2.22 12.5 AE 3.22 0.51 AE 0.04 Core 0.50 (?) 0.32 (?) 85.00 (?) 8.40 (?) Carbonaceous chondrites CO 0.63 AE 0.24 0.32 AE 0.12 21.74 AE 19.16 10.40 AE 0.00 CV 0.92 AE 0.43 0.42 AE 0.20 23.96 AE 24.16 10.20 AE 0.00 CM 1.89 AE 0.48 0.58 AE 0.15 21.01 AE 8.35 3.20 AE 0.00 CI 4.24 AE 0.77 0.72 AE 0.13 19.66 AE 11.69 4.80 AE 0.00 Enstatite chondrites EH 0.36 AE 0.13 0.06 AE 0.02 13.73 AE 7.26 11.00 AE 0.00 EL 0.48 AE 0.16 0.15 AE 0.05 24.43 AE 9.40 11.00 AE 0.00 Ordinary chondrites H 0.11 0.06 >104.00 2.50 AE 0.70 L 0.09 0.04 46.10 AE 1.00 3.00 AE 1.00 LL 0.09 0.05 51.22 AE 1.20 2.60 AE 0.50 The mantle, the crust-ocean-atmosphere, and the BSE averages and 1σ standard deviations are from Hirschmann and Dasgupta 7 for C/H, Bergin et al. 10 for C/N, and Hirschmann 11 for C/S. The carbonaceous chondrite data for C, H, and N are from Figure 2.3, E-chondrite C/N data are from Grady et al. 52 and Alexander et al. 53 OC C/N data are from Bergin et al. 10 and references therein. The C/S data for all chondrites are from Wasson and Kallemeyn, 37 and the C/H data for enstatite and OCs are from Hirschmann 11 and references therein. The core compositional estimates are using bulk Earth concentrations from McDonough, 83 and D alloy=silicate x values are from Figure 2.5 and assuming complete core-mantle equilibration. The data for C contents are from Bergin et al. 10 for terrestrial reservoirs, from Alexander et al. 48 for carbonaceous chondrites, from Grady et al. 52 for E-chondrites, and from Wasson and Kallemeyn 37 for OCs.

Carbon-enriched rocky exoplanets have been proposed around dwarf stars as well as around binary stars, white dwarfs and pulsars. However, the mineralogical make up of such planets is poorly constrained. We performed high-pressure high-temperature laboratory experiments (P = 1´2 GPa, T = 1523´1823 K) on chemical mixtures representative of C-enriched rocky exoplanets based on calculations of protoplanetary disk compositions. These P´T conditions correspond to the deep interiors of Pluto-to Mars-size planets and the upper mantles of larger planets.Our results show that these exoplanets, when fully-differentiated, comprise a metallic core, a silicate mantle and a graphite layer on top of the silicate mantle. Graphite is the dominant carbon-bearing phase at the conditions of our experiments with no traces of silicon carbide or carbonates. The silicate mineralogy comprises olivine, orthopyroxene, clinopyroxene and spinel, similar to the mineralogy of the mantles of carbon-poor planets such as the Earth, and largely unaffected by the amount of carbon. Metals are either two immiscible iron-rich alloys (S-rich and S-poor) or a single iron-rich alloy in the Fe-C-S system with immiscibility depending on the S/Fe ratio and core pressure.We show that for our C-enriched compositions the minimum carbon abundance needed for C-saturation is 0.05´0.7 wt% (molar C/O " 0.002´0.03). Fully differentiated rocky exoplanets with C/O ratios more than needed for C-saturation would contain graphite as an additional layer on top of the silicate mantle. For a thick enough graphite layer, diamonds would form at the bottom of this layer due to high pressures.We model the interior structure of Kepler-37b and show that a mere 10 wt% graphite layer would decrease its derived mass by 7%, suggesting future space missions that determine both radius and mass of rocky exoplanets with insignificant gaseous envelopes could provide quantitative limits on their carbon content. Future observations of rocky exoplanets with graphite-rich surfaces would show low albedos due to the low reflectance of graphite. The absence of life-bearing elements other than carbon on the surface likely makes them uninhabitable.