¹M.Ferrais et al. (>10)
Astronomy & Astrophysics 662, A71 Open Access Link to Article [DOI https://doi.org/10.1051/0004-6361/202243200]
¹Aix Marseille Université, CNRS, CNES, Laboratoire d’Astrophysique de Marseille, Marseille, France
Reproduced with permission (C)ESO

Context. Asteroid (22) Kalliope is the second largest M-type asteroid in the main belt and is orbited by a satellite, Linus. Whereas the mass of Kalliope is already well constrained thanks to the presence of a moon, its volume is still poorly known, leading to uncertainties on its bulk density and internal structure.

Aims. We aim to refine the shape of (22) Kalliope and thus its diameter and bulk density, as well as the orbit of its moon to better constrain its mass, hence density and internal structure.

Methods. We acquired disk-resolved observations of (22) Kalliope using the VLT/SPHERE/ZIMPOL instrument to reconstruct its three-dimensional (3D) shape using three different modeling techniques. These images were also used together with new speckle observations at the C2PU/PISCO instrument as well as archival images from other large ground-based telescopes to refine the orbit of Linus.

Results. The volume of (22) Kalliope given by the shape models, corresponding to D = 150 ± 5 km, and the mass constrained by its satellite’s orbit yield a density of ρ = 4.40 ± 0.46 g cm−3. This high density potentially makes (22) Kalliope the densest known small body in the Solar System. A macroporosity in the 10–25% range (as expected for this mass and size), implies a grain density in the 4.8–5.9 g cm−3 range. Kalliope’s high bulk density, along with its silicate-rich surface implied by its low radar albedo, implies a differentiated interior with metal contributing to most of the mass of the body.

Conclusions. Kalliope’s high metal content (40–60%) along with its metal-poor mantle makes it the smallest known Mercury-like body. A large impact at the origin of the formation of the moon Linus is likely the cause of its high metal content and density.

¹M. Mahlke,¹B. Carry,²P.-A. Mattei
Astronomy & Astrophysics 665, A26 Open Access Link to Article [DOI https://doi.org/10.1051/0004-6361/202243587]
¹Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, 06304 Nice Cedex 4, France
²Université Côte d’Azur, Inria, Maasai project-team, Laboratoire J.A. Dieudonné, UMR CNRS 7351, 06902 Sophia-Antipolis, France
Reproduced with permission (C)ESO

Context. The classification of the minor bodies of the Solar System based on observables has been continuously developed and iterated over the past 40 yr. While prior iterations followed either the availability of large observational campaigns or new instrumental capabilities opening new observational dimensions, we see the opportunity to improve primarily upon the established methodology.

Aims. We developed an iteration of the asteroid taxonomy which allows the classification of partial and complete observations (i.e. visible, near-infrared, and visible-near-infrared spectrometry) and which reintroduces the visual albedo into the classification observables. The resulting class assignments are given probabilistically, enabling the uncertainty of a classification to be quantified.

Methods. We built the taxonomy based on 2983 observations of 2125 individual asteroids, representing an almost tenfold increase of sample size compared with the previous taxonomy. The asteroid classes are identified in a lower-dimensional representation of the observations using a mixture of common factor analysers model.

Results. We identify 17 classes split into the three complexes C, M, and S, including the new Z-class for extremely-red objects in the main belt. The visual albedo information resolves the spectral degeneracy of the X-complex and establishes the P-class as part of the C-complex. We present a classification tool which computes probabilistic class assignments within this taxonomic scheme from asteroid observations, intrinsically accounting for degeneracies between classes based on the observed wavelength region. The taxonomic classifications of 6038 observations of 4526 individual asteroids are published.

Conclusions. The ability to classify partial observations and the reintroduction of the visual albedo into the classification provide a taxonomy which is well suited for the current and future datasets of asteroid observations, in particular provided by the Gaia, MITHNEOS, NEO Surveyor, and SPHEREx surveys.

¹Cody Schultz,^2,3Brendan A. Anzures,¹Ralph E. Milliken,¹Taki Hiroi,¹Kevin Robertson
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13946]
¹Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, Rhode Island, 02912 USA
²Lunar and Planetary Institute, USRA, Houston, Texas, 77058 USA
³NASA Johnson Space Center, ARES, Houston, Texas, 77058 USA
Published by arrangement with John Wiley & Sons

H2O and OH are readily detected in hydrated minerals in CM chondrites via reflectance spectroscopy due to their characteristic vibration absorptions at infrared wavelengths. Previous spectroscopic work on bulk powdered CM chondrites has shown that spectral parameters, like the wavelength position of the “3 μm absorption feature,” vary systematically with the extent to which the samples have been aqueously altered. However, it is yet unclear how these spectral features may vary across an intact meteorite chip when measured at spatial scales smaller than that of the individual components of the meteorite. Here, we explore the spatial variability of this spectral feature and others on intact CM2 chips which, unlike powders, retain their petrologic and textural characteristics. We also model the modal mineralogy of the bulk meteorite powders and correlate this with key spectral features, demonstrating that microscope Fourier transform infrared spectroscopic mapping provides a powerful, rapid, and non-destructive technique for assessing compositional diversity and variations in water–rock interactions in chondritic planetary materials. In all CM2 chondrites studied here, we find that variations in the position, shape, and strength of the 3 μm absorption feature reveal a single chondrite can exhibit as much spectral variation as the entire suite of CM2 chondrites. The observed variations in the position and shape of the 3 μm feature within individual CM2 chondrite chips suggest a range of alteration products (e.g., Mg-rich to Fe-rich phyllosilicates) are present and record sub-mm scale variations in the amount and/or chemistry of the altering fluids. The samples having experienced the most progressive aqueous alteration show the least amount of variability in features like the 3 μm absorption band minimum position, whereas the least altered samples exhibit the most variability. We also find that the bulk spectral signatures in the least altered samples appear to be biased toward the spectral signatures of clasts versus matrix. By extension, asteroid reflectance spectra exhibiting 3 μm absorption features consistent with those measured here may be interpreted in a similar framework in which the spectrum of what may appear to be the least altered asteroids represents an average that belies the true diversity of mineralogy and chemistry of the body.