A very early origin of isotopically distinct nitrogen in inner Solar System protoplanets

Abstract

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.

Determination of the initial hydrogen isotopic composition of the solar system

“…3) during the formation of igneous CAIs, in the first 200,000 years of the solar system, while the presolar cloud envelope was still collapsing 22 . The relationship between N isotopes and heavy metal nucleosynthetic anomalies in iron meteorites 23 further testifies that the planetary N reservoir built up during early infall of interstellar material.…”

Rates of protoplanetary accretion and differentiation set nitrogen budget of rocky planets

“…N budget of larger planets with protracted growth history can be satisfied if they accreted planetary embryos that grew via instantaneous accretion. Because most of the N in those planetary embryos resides in their metallic portions, the cores were the predominant delivery reservoirs for N and other siderophile volatiles like C. Establishing the N budget of the BSE chiefly via the cores of differentiated planetary embryos from inner and outer Solar System reservoirs 9 obviates the need of late accretion of chondritic materials as the mode of N delivery to Earth. Also, a siderophile character of N and C suggests that their accretional pathways in the inner Solar System planets maybe decoupled from that of water which likely accreted from chondritic materials 1, 45 .…”

“…However, enstatite chondrites that source the inner Solar System reservoir 7 contain several hundreds of ppm of N and C in refractory phases 8 . The presence of isotopically distinct N in NC (non-carbonaceous chondrite affinity) iron meteorites also suggests that planetesimals and planetary embryos in the inner Solar System did not accrete volatile-poor material 9 . Consequently, post-accretion processes should have played an important role in establishing the N-and C-depleted character of the bulk silicate reservoirs of rocky bodies.…”

The effect of a strong pressure bump in the Sun's natal disk: Terrestrial planet formation via planetesimal accretion rather than pebble accretion

“…A distinct difference in stable isotopic composition between these two classes of meteorites has been documented for a wide range of non-volatile elements such as siderophile Mo, W, Ni, Ru and lithophile Cr, Ti (Warren 2011;Kruijer et al 2017;Kruijer et al 2020). Stable isotopic compositional dichotomy between NC and CC meteorites has recently been extended to volatile element nitrogen (N; Grewal et al 2021). Combined Cr-Ti-O isotope data also suggests no transport of chondrules -a particular type of pebbles (see Connolly & Jones 2016, for a review on chondrules formation models) -between the NC and CC reservoirs (Schneider et al 2020).…”