Analogues of enigmatic L-types: The effect of space weathering on CV, CO, CK, and CL chondrites

L-type asteroids are rare asteroids located throughout the Main Belt. They are unique among the minor bodies as their spectral appearance in the visible-near-infrared (VisNIR) is thought to be dominated by calcium–aluminium-rich inclusions (CAIs): agglomerates of minerals found in chondritic meteorites that contain the first solids that condensed in the solar nebula [1, 2].  A primary mineral phase in CAI is the aluminium oxide spinel (MgAl₂O₄), which presents an absorption feature at 2μm when enriched in iron due to thermal metamorphism. This absorption feature is observed in L-types, suggesting an enrichment of refractory inclusions in these bodies [3]. Therefore, the parent bodies of L-types might have formed at an early time and within a region of the protoplanetary nebula with a high concentration of CAI. If this is the case, the significance of L-type asteroids as tracers of planetary compositional and dynamical evolution cannot be overstated. The key question is: How rich in refractory inclusions are L-type asteroids?

Sunshine et al. (2008) concluded that some L-types contain between 22%-39% of FeO-rich CAI [4], an order of magnitude larger than what has been observed in the meteorite collection (<3vol%, [5]). Devogèle et al. (2018) confirmed the enrichment of most (but not all) L-types using the same methodology on a larger sample of L-type asteroids [6]. On the other hand, Mahlke et al. (2023) identified potential meteoritic analogues for several L-type asteroids among CO and CV chondrites [7], the meteorites with the highest CAI abundances, suggesting that L-types are at least partially sampled in the meteorite collection.

The discrepancy between the abundance models of asteroid spectra and direct comparisons with meteorite analogues may be in part due to the unknown spectral response of L-type material to secondary factors such as space weathering and phase-angle variations. While Lantz et al. (2017) have shown that CV and CO chondrites redden with increasing surface age [8], the response of the crucial 2μm band is unknown, though this information is paramount for deriving the CAI abundance in asteroids.

To quantify the effects of space weathering and phase-angle variations on spectra of L-type material, we collected nine samples of possible meteoritic analogues: 6 CV (2 of each subtype: reduced, oxidized-Allende-like, oxidized-Bali-like), 1 CO, 1 CK, and 1 CL chondrite.¹ Using the SHADOWS spectro-goniometer at IPAG, Grenoble [10], we acquired VisNIR spectra of the samples under multiple observation geometries to simulate the phase-angle variations of asteroidal targets as function of surface composition. For all samples, spectra are available from freshly-ground powders (in part newly acquired, in part from [11]) and from pressed pellets, further describing spectral changes due to surface texture and packing.

A particular focus of our experiment lies on the weathering-response of CAIs. To this end, we identified a large (>1mm in diameter) CAI rich in iron-bearing spinel in a section of CV Allende via scanning-electron microscopy, analogous to the inclusion expected on the asteroidal surfaces. For all samples (chondrites and CAI), we have concluded a dense spectral characterisation over a large wavelength range (VisNIR as described above and mid-infrared spectra at Synchrotron SOLEIL, Saint-Aubin), which is to be followed-up by ion irradiation using the INGMAR experiment (IAS & ICJLab, Orsay) to simulate space weathering by the solar wind [12]. We aim to acquire VisNIR spectra during the irradiation process at different dosages of ion implementation, capturing different degrees of space weathering. After irradiation, we aim to repeat the spectral characterisation executed before to capture and quantify spectral changes relevant for the interpretation of asteroidal spectral, including the common VisNIR range but also the mid-infrared range, now more accessible to remote-sensing observations than previously thanks to NASA JWST (e.g. [13]).

The irradiation and post-irradiation characterisation are scheduled to be concluded by July 2024. We will present first results of the irradiation experiments and further on-going efforts to describe the response of the samples to irradiation and phase-angle variations, crucial for the interpretation of remote-sensing observations of asteroids. Ultimately, the results obtained in this study will guide us in further constraining the nature of L-type asteroids by determining the closest analogue material within the meteorite collection and by aiding the interpretation of new spectra of L-type asteroids acquired in an on-going  observational campaign with ESO's VLT/X-SHOOTER as part of this project.

¹ The Loongana (CL) group is a recent addition to the taxonomy of chondrites [9].

References:
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[4] Sunshine, J. M., et al. (2008), Science, 320, 514
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[6] Devogèle, M., et al. (2018), Icarus, 304, 31
[7] Mahlke, M., et al. (2023), A&A, 676, A94
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[13] De Kleer, K., et al. (2024), LPSC Abstract, 3040

Acknowledgements: This work was supported by the Programme National dePlanétologie (PNP) of CNRS-INSU co-funded by CNES.