Geochemistry Group Seminar - Cosmochemical constraints on terrestrial planet formation - Isaac Onyett

We are pleased to announce a Geochemistry Group Seminar by Isaac Onyett on the topic of: Cosmochemical constraints on terrestrial planet formation.

Abstract:

Nucleosynthetic isotope variability among Solar System objects offers a powerful tool for tracing the genetic relationships between meteorite groups and the rocky planets—Mercury, Venus, Earth, and Mars. Previous studies have shown that no known primitive meteorite perfectly matches Earth’s isotopic composition, leaving the exact materials that formed Earth largely unknown. However, these conclusions assume that nucleosynthetic variability in the inner Solar System primarily reflects spatial heterogeneity rather than a temporal evolution in the composition of the protoplanetary disk. Additionally, earlier investigations relied on isotope tracers present only in trace amounts on Earth (e.g. Cr, Ti), which has led to ambiguities in interpreting data.

In this presentation, I use the mass-independent isotopic composition of silicon, a major refractory element, to show that the nucleosynthetic variability in the inner Solar System is largely driven by a rapid evolution in the isotopic makeup of protoplanetary disk solids during the early stages of mass accretion onto the proto-Sun. The silicon isotope ratios in meteorites exhibit a positive correlation with the accretion timescales of their parent bodies, suggesting a secular evolution of the silicon isotopic composition in the region where terrestrial planets formed.

When compared to another major lithophile tracer, ⁴³Ca, Solar System materials—including Earth and Mars—fall along a mixing line between ureilites and CI chondrites. This supports the idea that Earth accreted from a mixture of volatile-poor inner-disk planetesimals (~80%) and radially drifting, volatile-rich, CI-like pebbles from the outer Solar System (~20%).

The predictable delivery of volatiles, such as water, through pebble accretion—rather than through the stochastic nature of volatile-rich impact event—suggests that the formation of water-rich worlds like Earth may be more common in exoplanetary systems than previously thought.