Chondrules from high-velocity collisions: thermal histories and the agglomeration problem

TL;DR

This paper investigates the formation of chondrules through high-velocity collisions in the early solar system, analyzing their thermal histories and the challenges of their aggregation into meteorite parent bodies.

Contribution

It provides models for chondrule formation via super-km/s collisions and discusses the implications for their thermal histories and accretion processes.

Findings

CB chondrules' thermal histories match collisions of 10-100 km bodies.

Non-CB chondrules' cooling rates suggest larger colliders (>500 km).

Reaccretion of chondrules onto planetesimals is inefficient due to orbital dynamics.

Abstract

We assess whether chondrules, once-molten mm-sized spheres filling the oldest meteorites, could have formed from super-km/s collisions between planetesimals in the solar nebula. High-velocity collisions release hot and dense clouds of silicate vapor which entrain and heat chondrule precursors. Thermal histories of CB chondrules are reproduced for colliding bodies $\sim$10--100 km in radius. The slower cooling rates of non-CB, porphyritic chondrules point to colliders with radii $\gtrsim$ 500 km. How chondrules, collisionally dispersed into the nebula, agglomerated into meteorite parent bodies remains a mystery. The same orbital eccentricities and inclinations that enable energetic collisions prevent planetesimals from re-accreting chondrules efficiently and without damage; thus the sedimentary laminations of the CB/CH chondrite Isheyevo are hard to explain by direct fallback of collisional ejecta. At the same time, planetesimal surfaces may be littered with the shattered remains of chondrules. The micron-sized igneous particles recovered from comet 81P/Wild-2 may have originated from in-situ collisions and subsequent accretion in the proto-Kuiper belt, obviating the need to transport igneous solids across the nebula. Asteroid sample returns from Hayabusa2 and OSIRIS-REx may similarly contain chondrule fragments.