Abundances of ordinary chondrites in thermally evolving planetesimals

TL;DR

This study models the thermal evolution of planetesimals to explain the observed distribution of petrologic types of ordinary chondrites, revealing formation timing and size constraints that match fall statistics.

Contribution

It introduces a numerical thermal evolution model that links planetesimal formation timing and size to the observed chondrite petrologic type distribution.

Findings

Planetesimals forming within 2.0 Myr after CAIs can contain all chondrite types.

Larger planetesimals (>60 km) likely formed around 2.0 Myr after CAIs.

Smaller planetesimals (<50 km) probably formed within 1.5 Myr after CAIs.

Abstract

Chondrites are one of the most primitive objects in the solar system, and keep the record of the degree of thermal metamorphism experienced in their parent bodies. This thermal history can be classified by the petrologic type. We investigate the thermal evolution of planetesimals to account for the current abundances (known as the fall statistics) of petrologic types 3 - 6 ordinary chondrites. We carry out a number of numerical calculations in which formation times and sizes of planetesimals are taken as parameters. We find that planetesimals that form within 2.0 Myr after the formation of Ca-Al-rich inclusions (CAIs) can contain all petrologic types of ordinary chondrites. Our results also indicate that plausible scenarios of planetesimal formation, which are consistent with the fall statistics, are that planetesimals with radii larger than 60 km start to form around 2.0 Myr after CAIs and/or that ones with radii less than 50 km should be formed within 1.5 Myr after CAIs. Thus, thermal modelling of planetesimals is important for revealing the occurrence and amount of metamorphosed chondrites, and for providing invaluable insights into planetesimal formation.