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

TL;DR

The paper tackles the origin of petrologic diversity in LL chondrites by testing whether a single onion-shell parent body or multiple parent bodies better explain the LL3–LL6 distribution. It combines asteroid-family spectroscopy, LL-chondrite spectra, NEO data from the MITHNEOS survey, and thermal-evolution modeling with revised parent-body sizes to link LL chondrites to inner-belt sources. The results indicate two primary sources, NysaS for unequilibrated LL3 material and Flora for higher-grade LL5–7 material, supporting a multi-source LL chondrite origin and constraining accretion timing to a span of roughly $0.5-0.7$ Myr after CAI formation. This has significant implications for the delivery of unequilibrated material to Earth and the thermal histories of OC parent bodies in the early solar system.

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

Figures (10)

• Figure 1: Petrologic distribution of ordinary chondrites. The filled histogram shows the distribution of meteorite falls as a function of mineralogical class (H, L, LL), compiled from the Meteoritical Bulletin Database. The dashed black outline shows the corresponding distribution after applying the revised petrologic-type ratios reported by Eschrig:2022
• Figure 2: Reflectance spectra of NysaS family members. Spectra were obtained with the SpeX spectrograph on NASA’s IRTF on 28 March and 23 June 2025 (Table \ref{['tab:pb_sizes']}). All spectra are normalized to unity at 0.9 $\mu$m. The asteroid number is indicated in each panel.
• Figure 3: Spectral comparison between LL ordinary chondrites and the NysaS and Flora families. Near-infrared reflectance spectra of unequilibrated (petrological type 3; top) and highly equilibrated (petrological types 6 and 7; bottom) LL chondrites are compared with the average spectra of the NysaS (blue) and Flora (orange) asteroid families. All spectra are normalized near the 1 $\mu$m band minimum and dereddened using a space weathering exponential model (see Section \ref{['sec:spectra']}). The small discontinuity near 1.9 $\mu$m observed in the average LL3 spectrum is caused by the detector transition of the SHADOWS spectrometer Potin:2018 used to acquire the SSHADE dataset. The $\chi^2$ values reported in the legend are used as a relative distance metric to identify the best spectral matches. The comparisons show that unequilibrated LL3 meteorites are highly consistent with the NysaS family, whereas equilibrated LL6–7 meteorites preferentially match Flora, particularly in the shape of the 1 $\mu$m absorption band -- the most diagnostic feature of silicate composition and, in particular, of the olivine-to-pyroxene ratio. This dual correspondence supports a multiple-parent-body origin for LL chondrites rather than a single onion-shell structure.
• Figure 4: Fractions of LL chondrites spectrally associated with the NysaS and Flora families. Pie charts show, for each petrologic type, the fraction of meteorites assigned to each family based on the classifications in Table \ref{['tab:met']}, with numbers N indicating the total sample size for each subtype. The SSHADE dataset comprises two meteorites classified as LL7, which we included here in the LL6 group. LL3 meteorites are only associated with NysaS-like spectra, whereas the more equilibrated LL chondrites (LL4-7) are dominated by Flora.
• Figure 5: Mineralogy of LL chondrites, LL-chondrite-like NEOs and corresponding asteroid families. Histograms of the ol/(ol + opx) ratios derived for each spectrum from the Shkuratov model show that the NysaS family exhibits values close to those of unequilibrated, type-3 LL chondrites, whereas the Flora and Eunomia families are shifted toward values characteristic of more thermally equilibrated, higher-petrologic-type material. Approximately 3% offsets between LL3 and NysaS, and between LL6 and Flora, is likely due to limitations of the model, which is sensitive to differences in space-weathering states between meteorites and asteroids. In the meteorite panels, empty black histograms correspond to the combined RELAB+SSHADE dataset, while filled grey histograms show SSHADE Eschrig:2022 data only. For the NEO panels, the sample is restricted to objects with a mean ol/(ol+opx) ratio ${>} 0.72$, corresponding to the dashed vertical line, in order to minimize contamination from L-chondrite-like NEOs associated with the Massalia family Marsset:2024Nature.