Silicon isotope constraints on terrestrial planet accretion

Area of Science:

• Planetary Science
• Cosmochemistry
• Isotope Geochemistry

Background:

• Understanding terrestrial planet formation requires knowledge of precursor materials.
• Nucleosynthetic variations in rocky bodies trace planetary building blocks.
• Silicon isotopes (μ³⁰Si) are key tracers of refractory elements.

Purpose of the Study:

• To determine the nucleosynthetic composition of silicon (μ³⁰Si) in meteorites.
• To identify the precursor materials of terrestrial planets.
• To reconstruct the accretion history of the inner Solar System.

Main Methods:

• Analysis of silicon isotope composition (μ³⁰Si) in primitive and differentiated meteorites.
• Comparison of μ³⁰Si values between inner and outer Solar System bodies.
• Correlation of μ³⁰Si values with accretion ages and other isotopic tracers (Mo, Zr, Ni).

Main Results:

• Inner Solar System bodies (including Mars) show μ³⁰Si deficits, while chondrites exhibit μ³⁰Si excesses.
• Chondritic bodies are not primary building blocks; differentiated asteroid material is a major constituent.
• Earth's precursors incorporated ~26% μ³⁰Si-rich outer Solar System material, while Mars formed earlier, avoiding this enrichment.
• μ³⁰Si compositions of Mars and proto-Earth are consistent with rapid formation (<3 million years) via collisional growth and pebble accretion.
• Earth's isotopic composition aligns with pebble accretion models, considering volatility and the Moon-forming impact.

Conclusions:

• Terrestrial planet formation involved the mixing of distinct inner and outer Solar System materials.
• Early-formed differentiated asteroids, not chondrites, represent a significant component of terrestrial planets.
• Rapid accretion models explain the isotopic signatures of Mars and Earth, highlighting the role of pebble accretion.