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

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 indicate that planetesimal accretion within the OC reservoir extended over 0.5-0.7 Myr.

💡 Research Summary

The paper tackles the long‑standing problem of the origin of LL ordinary chondrites (OCs), which display a strikingly bimodal distribution of petrologic types: most falls are either LL3 (unequilibrated) or LL6 (equilibrated). The classic “onion‑shell” model, in which a single parent body is thermally stratified by depth, cannot readily account for this distribution. To test the competing hypothesis that multiple parent bodies contributed, the authors combine near‑infrared (0.8–2.5 µm) spectroscopy, mineralogical modeling, dynamical analysis, and thermal evolution simulations.

Spectroscopic data were gathered for the bright S‑type component of the Nysa family (Nysa S) and for the Flora family. The dataset includes 14 spectra of 13 Nysa S members (plus seven new observations obtained with NASA‑IRTF SpeX) and 47 spectra of 44 Flora members, supplemented by literature measurements. Each spectrum was de‑reddened using the Brunetto exponential space‑weathering correction and normalized at the 1 µm band minimum to allow direct comparison of the silicate absorption features.

Laboratory spectra of 151 LL chondrites from RELAB and SSHADE were divided into unequilibrated LL3.x and highly equilibrated LL5‑6.x groups based on petrographic classifications. χ² fitting of the family‑average spectra to the meteorite spectra showed a near‑perfect match between Nysa S and LL3 material, while Flora’s average spectrum aligns closely with LL5‑6 material. When each individual meteorite spectrum is forced to be assigned to either family, all LL3 samples map to Nysa S, whereas the higher‑grade samples preferentially map to Flora, with only a few ambiguous cases.

To quantify compositional differences, the authors applied the Shkuratov radiative‑transfer model (as implemented by Vernazza et al.) using olivine, orthopyroxene, and chromite as end‑members, and incorporated space‑weathering via an exponential reddening term. The model retrieved higher olivine‑to‑orthopyroxene ratios for Nysa S (≈1.2) and lower ratios for Flora (≈0.9), consistent with the mineralogical trends expected for LL3 versus LL5‑6 material.

A dynamical link is established by analysing the MITHNEOS near‑Earth object (NEO) dataset. Among 325 S‑type NEOs identified as LL‑chondrite analogues, spectral clustering reveals two distinct subpopulations that correspond to the Nysa S and Flora families, confirming that both families currently supply LL‑like NEOs that can intersect Earth’s orbit.

Thermal‑history modeling, based on revised parent‑body sizes (≈30–40 km) and heating by ²⁶Al decay, indicates that the Nysa S and Flora parent bodies accreted 0.5–0.7 Myr apart. The earlier‑forming Nysa S experienced a rapid, relatively shallow heating episode that preserved unequilibrated material, whereas the later‑forming Flora underwent a longer, deeper heating phase that produced equilibrated LL5‑6 material. This timescale matches constraints from isotopic dating and supports a scenario of staggered planetesimal formation within the ordinary‑chondrite reservoir.

Overall, the study provides compelling, multi‑disciplinary evidence that LL chondrites originate from at least two distinct parent bodies: the Nysa S family supplies the unequilibrated LL3 component, and the Flora family supplies the equilibrated LL5‑6 component. This multiple‑parent‑body model better explains the observed petrologic bimodality than the single‑onion‑shell hypothesis and implies that planetesimal accretion and thermal processing in the inner main belt were ongoing over several hundred thousand years during the first million years of Solar System history.