The Nysa family as the main source of unequilibrated LL ordinary chondrites

TL;DR

This study suggests that the diverse petrologic types of LL ordinary chondrites originate mainly from the Nysa family and Flora family asteroids, supporting a multiple parent bodies model over the onion-shell model.

Contribution

It provides spectral, mineralogical, and thermal history evidence favoring multiple parent bodies for LL chondrites, especially linking unequilibrated LL3 chondrites to the Nysa family.

Findings

LL3 chondrites originate exclusively from NysaS.

Spectral and mineralogical diversity aligns with contributions from NysaS and Flora families.

Results support multiple parent bodies rather than a single stratified parent body.

Abstract

Context. The origin of the petrologic diversity observed in ordinary chondrites (OCs), the most common meteorites on Earth, remains debated. Competing models invoke either depth-dependent sampling of a single thermally stratified ("onion-shell") parent body or contributions from multiple distinct parent bodies. Aims. We aim to determine which of the two models is preferred for LL chondrites. These are unique among OCs in exhibiting a bimodal petrologic distribution, with most meteorites being LL3 or LL6. Methods. We compare the spectral and mineralogical properties of LL chondrites and corresponding LL-chondrite-like near-Earth objects (NEOs) with their possible sources in the main asteroid belt. We also model the thermal histories of the proposed parent bodies, based on revised estimates of parent-body sizes. Results. The spectral and mineralogical diversity of LL chondrites is consistent with contributions from the bright, S-type component of the Nysa family (NysaS) and the Flora family, with NysaS supplying mainly low-petrologic-type material and Flora higher-grade material. Unequilibrated, LL3 chondrites appear to originate exclusively from NysaS. Similarly, LL-chondrite-like NEOs form two distinct subpopulations consistent with origins in these same families. Conclusions. Our results favour multiple parent bodies for LL chondrites. The petrologic differences between the NysaS and Flora parent bodies can be explained by differences in their sizes, without requiring different formation times.