Phosphorus-nitrogen systematics of first-generation planetesimals constrain life-essential element delivery to Earth.

Abstract

Habitability of rocky planets relies on the budgets of life-essential elements (LEEs) in their building blocks. The provenance and geochemistry of the planetesimals that supplied the LEEs to Earth remain debated. Traditional models argue LEE delivery via outer Solar System chondrites, but their 2 to 4 million-year (Myr) accretion ages preclude them as the first feedstock. To investigate the initial LEE distribution, we reconstructed the phosphorus-nitrogen (P-N) budget of the iron meteorite parent bodies (IMPBs), which accreted <1 Myr of Solar System formation. High-pressure-temperature experiments of P-N partitioning between solid and liquid alloys combined with geochemical models reveal higher P/N ratios in outer Solar System IMPBs than in inner ones-a trend reversed in chondrites. This evolution reflects early refractory schreibersite delivery to the outer disk, later curtailed by Jupiter's growth. Further modeling in combination with previous elemental and isotopic data on volatile LEEs suggests that both early and later inner Solar System planetesimals are chief contributors to Earth's LEE inventory.