¹A. Goodwin,¹R. Tartèse,¹R. J. Garwood,²N. V. Almeida
Journal of Geophysical Research (Planets)(in Press) Open Access Link to Article [https://doi.org/10.1029/2023JE007916]
¹Department of Earth and Environmental Sciences, The University of Manchester, Manchester, UK
²Natural History Museum, London, UK
Published by arrangement with John Wiley & Sons

The Martian regolith breccia Northwest Africa (NWA) 11220 and paired stones represent the only known meteorites that sample a clastic sub-surface lithology from Mars. By applying X-ray computed microtomography to monomineralic clasts, we identify three phases that can be automatically segmented by thresholding X-ray attenuation greyscale values: (A) feldspars, (B) pyroxene and apatite, and (C) iron-rich oxides and sulfides, confirmed via scanning electron microscopy and Raman spectroscopy. For these three phases, we demonstrate scale invariance in size and shape for sand-sized clasts and smaller, a characteristic commonly observed for clast populations generated by fragmentation without further sorting from sedimentary transport (e.g., Aeolian or fluvial processes). Additionally, by assessing the preferred orientation of fitted ellipses and ellipsoids to manually segmented proto-breccia clasts in two and three dimensions, we identified a weak planar fabric that likely resulted from compaction rather than impact transport. Combining clast size distribution with evidence for nested textures inside proto-breccia clasts, we propose that NWA 11220 has experienced a minimum of two hypervelocity impact events and should be considered a lithified impact ejecta lithology with little to no reworking via surface regolith processes.

^1,2G. Massa,²E. Palomba,²A. Longobardo,^1,2M. Angrisani,^1,2C. Gisellu,²F. Dirri,²M.C. De Sanctis,²A. Raponi,²F.G. Carrozzo,²M. Ciarniello
Icarus (in Press) Open Access Link to Article [https://doi.org/10.1016/j.icarus.2023.115870]
¹University of Rome “Sapienza”, Piazzale Aldo Moro 5, Rome 00185, Italy
²INAF Istituto di Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere, Rome 00133, Italy
Copyright Elsevier

NASA’s Dawn mission was launched in September 2007 and orbited asteroids Vesta (2011−2012) and Ceres (2015–2018). Vesta shows surface dark units that have been suggested to be linked to exogenous materials and are therefore useful to understand the initial stages of the Solar System.

This work takes advantage of the newly calibrated data of the VIR spectrometer, which are characterized by a better signal to noise (S/N) ratio, giving us the opportunity to search for spectral features that were never seen before due to noise. Considering that hydroxyl has been shown to be present in every dark unit on Vesta and also in carbonaceous chondrites, the goals of this work are the search for and characterization of carbonates that are present in carbonaceous chondrites, i.e., the supposed darkening agents of Vesta.

The estimate of the abundances of carbonates is fundamental to identify which carbonaceous chondrite fell on Vesta; this can be crucial for the definition of an evolutionary history of Vesta and the Solar System. The study of a possible feature at 3.9 μm related to the presence of carbonates was analyzed and found to be noise-induced. Although spectral features related to carbonates were not observed, the 3.4 μm absorption band was analyzed anyway in order to fix an upper limit to the abundance of carbonates in carbonaceous chondrites on Vesta. This value is consistent with petrochemical analyses, i.e., no more than 0.2% of carbonates in carbonaceous chondrites.