¹L. Krämer Ruggiu,²P.Beck,¹J.Gattacceca,²J.Eschrig
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114393]
¹Aix Marseille Univ, CNRS, IRD, INRAE, CEREGE, Aix-en-Provence, France
²Univ. Grenoble Alpes, CNRS, IPAG, Grenoble, France
Copyrigh Elsevier

The composition of asteroids gives crucial insights into the formation and evolution of the Solar System. Although spectral surveys and spacecraft missions provide information on small bodies, many important analyses can only be performed in terrestrial laboratories. Meteorites represent our main source of samples of extraterrestrial material. Determining the source asteroids of these meteorites is crucial to interpret their analyses in the broader context of the inner Solar System. For now, the total number of parent bodies represented in our meteorites collection is estimated to about 150 parent bodies, of which 50 parent bodies represented by the poorly studied ungrouped chondrites. Linking ungrouped meteorites to their parent bodies is thus crucial to significantly increase our knowledge of asteroids. To this end, the petrography of 25 ungrouped chondrites and rare meteorite groups was studied, allowing grouping into 6 petrographic groups based on texture, mineralogy, and aqueous and thermal parent body processing. Then, we acquired visible-near-infrared (VIS-NIR) reflectance spectroscopy data of those 25 meteorites, in order to compare them to ground-based telescopic observations of asteroids. The reflectance spectra of meteorites were obtained on powdered samples, as usually done for such studies, but also on raw samples and polished sections. With asteroids surfaces being more complex than fine-grained regolith (e.g., asteroid (101955) Bennu), in particular near-Earth asteroids, the use of raw samples is a necessary addition for investigating parent bodies. Our results showed that sample preparation influences the shape of the spectra, and thus asteroid spectral matching, especially for carbonaceous chondrites. Overall, the petrographic groups defined initially coincide with reflectance spectral groups, with only few exceptions. The meteorite spectra were then compared with reference end-member spectra of asteroids taxonomy. We matched the 25 studied meteorites to asteroids types, using a qualitative match of the shape of the spectra, as well as a quantitative comparison of spectral parameters (bands positions, bands depths and slopes at 1 and 2 μm). We define links between some of the studied ungrouped chondrites and asteroid types that had no meteorite connection proposed before, such as some very primitive type 3.00 ungrouped chondrites to B-type or Cg-type asteroids. We also matched metamorphosed ungrouped carbonaceous chondrites to S-complex asteroids, suggesting that this complex is not only composed of ordinary chondrites or primitive achondrites, as previously established, but may also host carbonaceous chondrites. Conversely, some ungrouped chondrites could not be matched to any known asteroid type, showing that those are potential samples from yet unidentified asteroid types.

¹Renbiao Tao,¹Yingwei Fei
Earth and Planetary Science Letters 562, 116849 Link to Article [https://doi.org/10.1016/j.epsl.2021.116849]
¹Earth and Planets Laboratory, Carnegie Institution for Science, 5251 Broad Branch Road, NW, Washington, DC 20015, USA
Copyright Elsevier

The partitioning of light elements between liquid and solid at the inner core boundary (ICB) governs compositional difference and density deficit between the outer and inner core. Observations of high S and low Fe concentration on the surface of Mercury from MESSENGER mission indicate that Mercury is formed under much more reduced conditions than other terrestrial planets, which may result in a Si and S-bearing metallic Fe core. In this study, we conducted high-pressure experiments to investigate the partitioning behavior of Si and S between liquid and solid in the Fe-Si-S system at 15 and 21 GPa, relevant to Mercury’s ICB conditions. Experimental results show that almost all S partitions into liquid. The partitioning coefficient of Si (D_Si) between liquid and solid is strongly correlated with the S content in liquid (XSliquid) as: ⁎log10⁡(DSi)=0.0445+5.9895⁎log10⁡(1−XSliquid). Within our experimental range, pressure has limited effect on the partitioning behavior of Si and S between liquid and solid. For Mercury with an Fe-Si-S core, compositional difference between the inner and outer core is strongly dependent on the S content of the core. The lower S content is in the core, the smaller compositional difference and density deficit between the liquid outer core and solid inner core should be observed. For a core with 1.5 wt% bulk S, a model ICB temperature would intersect with the melting curve at ∼17 GPa, corresponding to an inner core with a radius of ∼1600 km.