Compositional characterisation of asteroid (84) Klio with JWST

TL;DR

Using JWST/NIRSpec, the authors analyze the 0.97–5.10 μm spectrum of asteroid (84) Klio to constrain surface composition and assess links to hydrated CM2 chondrites. They separate reflected and thermal flux via the NEATM model, deriving $D = 78.1 \,\pm\, 23.9$ km and $p_V = 0.05 \,\pm\, 0.03$, and isolating the reflectance for mineralogical analysis. The spectrum reveals a prominent phyllosilicate-related band at $2.776 \,\pm\, 0.001$ μm, a possible carbonate feature near $3.9$ μm, and an inconclusive 3.4 μm organics region, aligning Klio with CM2-like hydrated minerals rather than Ryugu or Bennu. The results imply a CM2-like aqueous alteration history for Klio and demonstrate JWST's capability to characterize primitive asteroids in the 3 μm region, with implications for the distribution of volatiles in the inner main belt.

Abstract

The analysis of the composition of primitive C$-$complex asteroids is essential to understand the distribution of volatiles in the Solar System since its formation. Primitive low-albedo families within the inner main asteroid belt are of particular interest because they are a significant source of carbonaceous near-Earth asteroids, such as Ryugu and Bennu. This study, part of the JWST SAMBA3 project (Spectral Analysis of Main Belt Asteroids in the 3 $μ$m region), report the first spectroscopic analysis of asteroid (84) Klio in the 3 $μ$m region, in order to better constrain its composition. We analysed the infrared (0.97$-$5.10 $μ$m) Spectrum of Klio measured by the NIRSpec instrument on board JWST. We used the NEATM thermal model to extract the reflectance spectrum of the asteroid. Several spectral features were then analysed in the 2.8, 3.4, and 3.9 $μ$m regions by different Gaussian fitting. The Spectrum of Klio shows an absorption band at 2.776 $\pm$ 0.001 $μ$m that we attributed to phyllosilicates. We compared the position and shape of the feature with that observed in primitive materials such as carbonaceous chondrites and returned samples from Ryugu and Bennu. The position and shape of the 2.8 $μ$m band, as well as the presence of a 0.7 $μ$m band in the visible, suggest that Klio's spectrum is similar to certain CM2 meteorites. We observed an absorption band around 3.9 $μ$m, with a depth of $0.020 \pm 0.001$ that could be attributed to carbonates. We could not clearly detect any absorption associated with organics at 3.4 $μ$m.

Figures (13)

• Figure 1: Proper inclination as a function of the proper semi-major axis of the low-albedo families in the inner main belt. For the Polana and Eulalia families, we used the definition from 2013Icar..225..283W, which was also used in 2016Icar..266...57D. For the other families, we applied the definition from 2015aste.book..297N. The yellow diamond marks the location of the asteroid (84) Klio.
• Figure 2: Visible and near-infrared spectra of Klio. The two spectra covering the 0.43 - 0.92 $\mu$m range are from the SMASS-II 2002Icar..158..106B and PRIMASS 2024pds..data..114P surveys, the latter after phase-reddening correction (see main text). The two spectra covering the 0.7 - 2.5 $\mu$m range were acquired by SpeX/IRTF, in 2003 REDDY_MBASpec and in 2024 (in purple). The grey areas represent the absorption by atmospheric H2O. The complete visible to near-infrared spectra are normalised to unity at 1.6 $\mu$m.
• Figure 3: Irradiance spectrum of Klio obtained with NIRSpec using the three grating-filter combinations (in different shades of blue) shown in Table \ref{['tab:properties']}. The irradiance spectrum of the P330E star is shown in red, scaled to fit Klio's spectrum in the 2.2–2.6 $\mu$m region (zoomed region in the centre of the panel). Note that above $\sim$3.3 $\mu$m, Klio's spectrum becomes significantly brighter than that of P330E, indicating that thermal emission starts to dominate over reflected light.
• Figure 4: Thermal emission in Klio's spectrum (blue) fitted with the NEATM model (red). The deviation below 3.5 $\mu$m is due to the presence of a broad absorption band around 2.8 - 3 $\mu$m in Klio's spectrum.
• Figure 5: Klio reflectance spectrum. Small normalisation factors have been applied for a better fitting between the gratings, in the common wavelength region: the reflectance for grism 140M is multiplied by 0.99 and that of 395M (after thermal subtraction) is multiplied by 1.04.