^1,2,3Kateřina Chrbolková,⁴Patricie Halodová,^1,3Tomáš Kohout,²Josef Ďurech,⁵Kenichiro Mizohata,⁶Petr Malý,⁷Václav Dědič,⁸Antti Penttilä,⁶František Trojánek,⁴Rajesh Jarugula
Astronomy & Astrophysics 665, A14 Open Access Link to Article [DOI https://doi.org/10.1051/0004-6361/202243282]
¹Department of Geosciences and Geography, PO Box 64, 00014 University of Helsinki, Helsinki, Finland
²Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 18000 Prague, Czech Republic
³Czech Academy of Sciences, Institute of Geology, Rozvojová 269, 16500 Prague, Czech Republic
⁴Research Centre Řež, Hlavní 130, 250 68 Husinec–Řež, Czech Republic
⁵Department of Physics, PO Box 43, 00014 University of Helsinki, Helsinki, Finland
⁶Department of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 12116 Prague, Czech Republic
⁷Institute of Physics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 12116 Prague, Czech Republic
⁸Department of Physics, PO Box 64, 00014 University of Helsinki, Helsinki, Finland
Reproduced with permission (C)ESO

Context. Airless planetary bodies are studied mainly by remote sensing methods. Reflectance spectroscopy is often used to derive their compositions. One of the main complications for the interpretation of reflectance spectra is surface alteration by space weathering caused by irradiation by solar wind and micrometeoroid particles.

Aims. We aim to evaluate the damage to the samples from H+ and laser irradiation and relate it to the observed alteration in the spectra.

Methods. We used olivine (OL) and pyroxene (OPX) pellets irradiated by 5 keV H+ ions and individual femtosecond laser pulses and measured their visible (VIS) and near-infrared (NIR) spectra. We observed the pellets with scanning and transmission electron microscopy. We studied structural, mineralogical, and chemical modifications in the samples. Finally, we connected the material observations to changes in the reflectance spectra.

Results. In both minerals, H+ irradiation induces partially amorphous sub-surface layers containing small vesicles. In OL pellets, these vesicles are more tightly packed than in OPX ones. Any related spectral change is mainly in the VIS spectral slope. Changes due to laser irradiation are mostly dependent on the material’s melting temperature. Of all the samples, only the laser-irradiated OL contains nanophase Fe particles, which induce detectable spectral slope change throughout the measured spectral range. Our results suggest that spectral changes at VIS-NIR wavelengths are mainly dependent on the thickness of (partially) amorphous sub-surface layers. Furthermore, amorphisation smooths micro-roughness, increasing the contribution of volume scattering and absorption over surface scattering.

Conclusions. Soon after exposure to the space environment, the appearance of partially amorphous sub-surface layers results in rapid changes in the VIS spectral slope. In later stages (onset of micrometeoroid bombardment), we expect an emergence of nanoparticles to also mildly affect the NIR spectral slope. An increase in the dimensions of amorphous layers and vesicles in the more space-weathered material will only cause band-depth variation and darkening.

¹C.Liu et al. (>10)
Astronomy & Astrophysics 658, A67 Link to Article [DOI https://doi.org/10.1051/0004-6361/202141398]
¹Shandong Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai 264209, PR China
Reproduced with permission (C)ESO

Context. Chang’e-4 (CE-4) provides unprecedented information about lunar materials exposed by the South Pole-Aitken (SPA) basin. Diverse results have been obtained from previous interpretations of CE-4 visible and near-infrared (VNIR) spectra. Some studies suggest that materials at the CE-4 landing site are dominated by olivine and orthopyroxene, but others argue that only a small amount of olivine should be exposed at the CE-4 landing site.

Aims. Laboratory spectroscopy studies using the Engineering Model of CE-4 Visible and Near-infrared Imaging Spectrometer (VNIS) are critical in constraining the accurate mineral proportions and composition of soils and boulders at the CE-4 landing site.

Methods. VNIR spectra of nine lunar analogs – prepared by mixing orthopyroxene (OPX), clinopyroxene (CPX), olivine (OL), and plagioclase – were acquired using the CE-4 VNIS Engineering Model. The spectral indices model and modified Gaussian model were developed to estimate CPX/(CPX+OPX) and OL/(OL+CPX+OPX) and are applicable to the in situ spectra acquired by the Yutu-2 VNIS spectrometer.

Results. The lunar rocks and regolith at the CE-4 landing site excavated by the Finsen impact are CPX-rich with limited OL (CPX:OPX:OL = 56:29:17). The mineral chemistries of the four lunar rocks show Mid-Ca, Fe pyroxene, and Mid-Mg OL (Fo60−79), providing critical constraints for mineral compositions in the SPA compositional anomaly. These rocks exhibit high M1 intensity ratios, indicating that they were crystallized at a high temperature (980–1300 °C) and a rapid-cooling magmatic system produced by impact melt differentiation or volcanic resurfacing events.

^1,10P. Zhang (张鹏飞) et al. (>10)
Astronomy & Astrophysics 659, A78 Link to Article [DOI https://doi.org/10.1051/0004-6361/202142590]
¹State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, PR China<
¹⁰CNSA Macau Center for Space Exploration and Science, Macau, PR China
Reproduced with permission (C)ESO

Context. Space weathering (SW) is crucial to improve the understanding of the evolution of optical characteristics on airless bodies. The classical view based on research of the Moon suggests that SW decreases albedo (darkening) and steepens spectral slope (reddening) in visible to near-infrared (VIS-NIR) wavelengths, producing nanophase iron (npFe0). However, this conclusion is not perfectly applicable to asteroids.

Aims. In this study, we focus on investigating the space weathering spectral alteration effects (SWSAE) and the causes of spectral alteration on various types of asteroids after long-term continuous micrometeoroid bombardments.

Methods. We used a pulsed laser to irradiate eight meteorites at the same energy, namely, of 28 mJ, in ten shots, including ordinary chondrites (OCs), aubrite (Aub), enstatite chondrites (ECs), CO, CV, and CM carbonaceous chondrites. Then we measured and compared the virgin and irradiated VIS-NIR reflectance spectra of these meteorites. We further surveyed the causes of spectral alteration through a scanning electron microscope and transmission electron microscope.

Results. Three different SWSAE are shown: (1) darkening and reddening on OCs, Aub, CO, and CV chondrites; (2) brightening and reddening on ECs; (3) brightening and bluing on CM chondrite. After irradiation, npFe0 and nanophase iron-nickel particles were respectively found in CV and CO chondrites; thick amorphous layers without any nanophase particles were found in Aub; melting and sputtering of metal were observed in ECs; a great deal of vesicles or bubbles without any nanophase particles were found in CM chondrite.

Conclusions. The long-term SW via micrometeoroid bombardments can spectrally remodel asteroid surfaces in different ways: darken and redden anhydrous silicate asteroids (e.g., S-, E-, and K-types); brighten and redden metal-rich M-type objects. The SWSAE of volatiles-rich carbonaceous asteroids (e.g., Ch-, Cgh-, and D-types) is related to SW degree: darkening and bluing at low degree then brightening and continue bluing as the SW degree increases. The various spectral units on Ryugu, Bennu, and Phobos can be created by the heterogeneity of the degree of SW.

¹Gregory J.Retallack,¹Shane Jepson,¹Adrian Broz
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2023.115436]
¹Department of Earth Sciences, University of Oregon Eugene, Oregon 97403-1272, United States
Copyright Elsevier

Unlike the water planet Earth, or furnace planet Venus, Mars is a frigid soil planet, most like the Dry Valleys of Antarctica, which also has paleosols revealing a different past. This study examined rocks in early Amazonian (3000 Ma) sequences of western Candor Chasma, cemented by sulfates and iron oxides. Mars Reconnaissance Orbiter data were used to quantify elevations, and the gypsic bands proved to follow ancient dune surfaces, like petrogypsic horizons of soils. Hesperian-early Amazonian (3700-3000 Ma) gypsic paleosols are widespread on Mars, which also has Noachian (3800-4000 Ma) deeply weathered, kaolinitic paleosols. The Archean (3700-3000 Ma) Earth was similar with both gypsic and deeply weathered profiles. Archean fossil microbes and soils on Earth include acid sulfate and deeply weathered soils, but both life and soil diversified afterward on Earth. There is not yet a fossil record on Mars, but the red planet does have acid sulfate and deeply weathered paleosols of geological ages equivalent to Archean on Earth. Unlike Earth however, there is little evidence of later significant soil formation on Mars.

¹Libby D. Tunney,¹Patrick J. A. Hill,¹Christopher D. K. Herd,^1,2Robert W. Hilts
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13948]
¹Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3 Canada
²Department of Physical Sciences, MacEwan University, Edmonton, Alberta, T6J 4S2 Canada
Published by arrangement with John Wiley & Sons

Organic matter in astromaterials can provide important information for understanding the chemistry of our solar system and the prebiotic conditions of the early Earth. However, once astromaterials reach the Earth’s surface, they can be readily contaminated through contact with the Earth’s surface as well as during processing and curation. Here, we investigate how typical handling and curation materials interact with meteorite specimens by documenting hydrophobic organic compound contamination in the laboratory environment and on materials that might be used for their collection and storage. We use gas chromatography–mass spectrometry analysis of soluble organic compounds in dichloromethane extracts of these materials to gain insights into what materials and methods are best for the collection and curation of astromaterials. Our results have implications for how extraterrestrial samples—especially those containing significant intrinsic organic matter—are handled and curated to preserve them in their most pristine states. Following recommendations of other researchers in the area of returned sample curation, we advocate for a thorough investigation into the materials used in handling and curation of meteorites to create a contamination baseline to inform soluble organic analyses on astromaterials and enable the discrimination of terrestrial and extraterrestrial compounds.

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.

¹A. P. Jones,¹N. Ysard
Astronomy & Astrophysics 657, A128 Open Access Link to Article [DOI https://doi.org/10.1051/0004-6361/202141793]
¹Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
Reproduced with permission (C)ESO

Context. Nano-diamonds are an enticing and enigmatic dust component yet their origin is still unclear. They have been unequivocally detected in only a few astronomical objects, yet they are the most abundant of the pre-solar grains, both in terms of mass and number.

Aims. Our goal is to derive a viable set of nano-diamond optical constants and optical properties to enable their modelling in any type of astrophysical object where, primarily, the local (inter)stellar radiation field is well-determined.

Methods. The complex indices of refraction, m(n, k), of nano-diamonds, constrained by available laboratory measurements, were calculated as a function of size, surface hydrogenation, and internal (dis)order, using the THEMIS a-C(:H) methodology optEC(s)(a).

Results. To demonstrate the utility of the optical properties (the efficiency factors Qext, Qsca, and Qabs), calculated using the derived m(n, k) data, we show that nano-diamonds could be abundant in the interstellar medium (ISM) and yet remain undetectable there.

Conclusions. The derived optical constants provide a means to explore the existence and viability of nano-diamonds in a wide range of astronomical sources. Here we show that up to a few percent of the available carbon budget could be hidden in the form of nano-diamonds in the diffuse ISM, in abundances comparable to the pre-solar nano-diamond abundances in primitive meteorites.

¹J. Bourdelle de Micas,^1,2S. Fornasier,³C. Avdellidou,³M. Delbo,⁴G. van Belle,^5,6P. Ochner,⁴W. Grundy,⁴N. Moskovitz
Astronomy & Astrophysics 665, A83 Open Access Link to Article [DOI https://doi.org/10.1051/0004-6361/202244099]
¹LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
²Institut Universitaire de France (IUF), 1 rue Descartes, 75231 Paris Cedex 05, France
³Université Côte d’Azur, CNRS-Lagrange, Observatoire de la Côte d’Azur, CS 34229, 06304 Nice Cedex 4, France
⁴Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001, USA
⁵INAF Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁶Dipartimento di Fisica e Astronomia G. Galilei, Universitá di Padova, Vicolo dell’ Osservatorio 3, 35122 Padova, Italy
Reproduced with permission (C)ESO

Aims. We carried out a spectroscopic survey in order to investigate the composition of 64 asteroids of the inner main belt, which are leftovers of the original planetesimals of our Solar System (we call them inner main belt planetesimals or IMBPs). Following published methods, we identified IMBPs in the inverse size versus semimajor axis (α) space, after the removal of all asteroids belonging to collisional families.

Methods. We conducted several ground-based observational campaigns of these IMBPs in the visible range at the 1.82 m Asiago telescope, and in the near-infrared range at the Telescopio Nationale Galileo, the Lowell Discovery Telescope, and the NASA InfraRed Telescope Facility telescopes. As several of the identified planetesimals already have spectra published in the literature, we collected all the available data and focused the telescope time to investigate those never observed before, or to complete the 0.45–2.5 μm range spectrum for those for which there is only partial spectral coverage or data with poor signal-to-noise ratio. In this way, we obtained new spectra for 24 IMBPs. Combining new and literature observations, we present spectra for 60 IMBPs in both the visible and near-infrared range, and 4 IMBPs in the visible only. All spectra were classified following well-established taxonomies. We also characterized their spectral absorption bands – when present –, their spectral slopes, and their mineralogy. In addition, we performed curve matching between astronomical and laboratory spectra in order to identify the closest meteorite analog using the RELAB database.

Results. The majority of the IMBPs belong to the S-complex; the latter are best matched with ordinary chondrite meteorites, and their olivine/(olivine and pyroxene) abundance ratio is not correlated with the semi-major axis. This result does not support the hypothesis that this ratio increases with heliocentric distance. Furthermore, ~27% of the IMBPs belong to the C-complex, where Ch/Cgh types dominate, meaning that most of the carbonaceous-rich planetesimals were aqueously altered. These are best fitted by CM2 carbonaceous chondrite meteorites. Finally, the remaining IMBPs (~20%) belong to the X-complex, and have various mineralogies and meteorite matches, while a few are end-member classes, including L-, K-, V-, and D- or T-types.

Conclusions. Our spectroscopic investigation of IMBPs confirms that silicate-rich bodies dominated the inner main belt where temperature has permitted the condensation of silicate rocks. However, almost all the spectral types are found, with the notable exception of olivine-rich A-types and Q-type asteroids. Their absence, as well as the absence of the R- and O-types among planetesimals, might be due to the rarity of these types among large asteroids. However, the absence of Q-types among primordial planetesimals is expected, as they have undergone surface rejuvenating processes. Therefore, Q-types have relatively young and less weathered surfaces compared to other types. Our results support the hypothesis of compositional mixing in the early Solar System. In particular, the fact that most of the C-complex planetesimals are aqueous altered, and the presence of three D- or T-type asteroids among them indicate that these bodies migrated from beyond 3 au to their current position.