Level-2 processing of Chandrayaan-2 Imaging Infrared Spectrometer (IIRS) data for generation of surface reflectance

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¹Mamta Chauhan,¹Prabhakar Alok Verma,²Prakash Chauhan
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.70037]
¹Indian Institute of Remote Sensing, Indian Space Research Organization (ISRO), Department of Space, Government of India, Dehradun, India
²National Remote Sensing Centre Indian Space Research Organization (ISRO), Department of Space, Government of India, Hyderabad, India
Published by arrangement with John Wiley & Sons

Spectroscopy-based approach for remote exploration of any planetary body is significant in providing detailed understanding of surface composition, vital to any scientific exploration. Imaging Infrared Spectrometer (IIRS) is a hyperspectral imaging sensor flown over ISRO’s Chandrayaan-2 (Ch-2) orbiter for mapping mineral composition and complete characterization of hydration feature on the lunar surface. With the extended spectral range (0.8–5 μm), high-spatial resolution (80 m) and high signal-to-noise ratio, IIRS data are capable of measuring surface composition based on diagnostic spectral absorption features of known/unknown characteristic minerals present on the lunar surface. The present paper discusses for the first time the methodology to process Ch-2 IIRS data to generate photometrically corrected reflectance images after thermal correction. Spectrally and radiometrically calibrated Level-1 IIRS spectral radiance data were subjected to various data processing techniques including thermal emission correction beyond 2.5 μm, conversion to apparent reflectance, and empirical line correction for smoothing the observed reflectance spectra. The thermally corrected IIRS reflectance data in the 0.8–3.3 μm range after correction for standard geometry were calibrated with ground-based observations of the lunar surface from the Apollo 16 site to generate Level-2 product. The results generated for the selected study regions representing the dominant landforms of the Moon (Mare, Highland and Polar region) were analyzed based on overall spectral reflectance variation and prominent absorption features at particular wavelengths corresponding to their surface properties. Finally, the results were compared with observations from Chandrayaan-1 Moon Mineralogy Mapper (M3) data within the overlapping spectral range from the same region to validate the absolute reflectance of the IIRS. Overall, slight differences in reflectance have been observed in the spectral profile from both the sensors in the lower wavelength range attributed mainly due to differences in resolution and observation geometry. However, beyond 2 μm, the spectral slope variation could be clearly visible, possibly because of thermal contributions that have been removed efficiently in the case of Ch-2 IIRS spectra.

Radioisotopic age constraints of the Cambrian Ritland impact structure, Norway

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¹William R. Hyde,^2,3Steven J. Jaret,⁴Gavin G. Kenny,¹Anders Plan,⁵Elias J. Rugen,⁴Martin J. Whitehouse,¹Sanna Alwmark
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70035]
¹Department of Geology, Lund University, Lund, Sweden
²Department Physical Sciences, Kingsborough Community College, City University of New York, Brooklyn, New York, USA
³Department Earth and Environmental Sciences, CUNY Graduate Center, New York, New York, USA
⁴Department of Geosciences, Swedish Museum of Natural History, Stockholm, Sweden
⁵Department of Earth Sciences, University College London, London, UK
Published by arrangement with John Wiley & Sons

Secondary ion mass spectrometry U-Pb geochronology has been performed on zircon grains separated from impact melt rock from the 2.7 km-in-diameter Ritland impact structure, southwestern Norway. Scanning electron microscope-based imaging techniques, including electron backscatter diffraction analysis, reveal various zircon grain microtextures, including shock-recrystallization and high-temperature zircon decomposition. Analyses from unshocked zircon grains yield two distinct concordant age populations at 1.5 and ~2.5 Ga, interpreted to represent igneous crystallization ages. The former aligns with Telemarkian magmatism (1.52–1.48 Ga) which dominates the local area of the Sveconorwegian orogeny and the target sequence at Ritland. The latter indicates a more ancient zircon population in Southern Norway, representing detrital grains in cover sediments present at the time of impact in the Cambrian. Collectively, the U-Pb data form two distinct discordant arrays with poorly resolved lower intercept ages spanning the Cambro-Ordovician boundary. The melt rock at Ritland is highly altered, and significant postimpact Pb loss is observed throughout the U-Pb data, likely in response to burial-induced thermal overprinting during the Caledonian orogeny. Post-filtering and selection of the data to minimize the effects of nonimpact-specific Pb loss, the two discordia produce indistinguishable lower intercept ages of 586 ± 73 Ma (MSWD 1.6, n = 15) and 545 ± 48 Ma (MSWD = 11, n = 9) which coincide in the Cambrian–Late Ediacaran. We therefore provide radioisotopic support for previous stratigraphic age constraints for the formation of the structure (500–542 Ma).

A terrestrial rock instead of an ureilite: Caution is recommended to scientists working on material received from meteorite collections

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¹Lidia Pittarello,¹Stepan M. Chernonozhkin,¹Oscar Marchhart,¹Martin Martschini,¹Silke Merchel,¹Alexander Wieser,¹Frank Vanhaecke,¹Steven Goderis
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.70030]
¹Naturhistorisches Museum Wien (NHMW), Mineralogisch-Petrographische Abteilung, Vienna, Austria
²Departement für Lithosphärenforschung, Universität Wien, Vienna, Austria
³Atomic & Mass Spectrometry Research Unit, Department of Chemistry, Ghent University, Ghent, Belgium
⁴Faculty of Physics, Isotope Physics, University of Vienna, Vienna, Austria
⁵Archeology, Environmental Changes & Geo-Chemistry, Department of Chemistry, Vrije Universiteit Brussel, Brussels, Belgium
Published by arrangement with John Wiley & Sons

Planetary scientists heavily depend on meteorite curation facilities for the preparation and allocation of protected (e.g., Antarctic), highly valuable extraterrestrial specimens. In this work, a fragment of the Dyalpur ureilite obtained from a museum is discussed. The sample is found to contain microstructural, geochemical, and isotopic features inconsistent with any meteorite. The fragment consists of pargasitic amphibole, Ni-sulfides, and chromite grains in Fo₉₂ olivine groundmass, cut by serpentine veins. Amphibole geothermobarometry yields equilibrium conditions that are not compatible with the assumed ureilite parent body. Assuming the fragment represented a rare clast in an ureilite, further analyses were performed. Both the oxygen isotopic composition and the extremely low level of cosmogenic radionuclides confirm the terrestrial origin of the fragment; it is a partially serpentinized peridotite. This work stresses the importance of petrographic characterization of samples used for (isotope) geochemical analyses, of a well-documented sample curation, and of cosmogenic nuclide measurements for the unequivocal identification of extraterrestrial material. Finally, caution is recommended before making sensational claims in cases of anomalous results.

Measurements of three exo-planetesimal compositions: a planetary core, a chondritic body, and an icy Kuiper belt analogue

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¹Jamie T Williams,¹Boris T Gänsicke,¹Snehalata Sahu,²David J Wilson,³Detlev Koester,¹Andrew M Buchan,⁴Odette Toloza,⁵Yuqi Li,⁵Jay Farihi
Monthly Notices of the Royal Astronomical Society 541, 1377–1389 Link to Article [https://doi.org/10.1093/mnras/staf1034]
¹Department of Physics, University of Warwick, Coventry CV4 7AL, UK
²Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA
³Institut für Theoretische Physik und Astrophysik, University of Kiel, 24098 Kiel, Germany
⁴Departamento de Física, Universidad Técnica Federico Santa María, Avenida España 1680, Valparaíso, Chile
⁵Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁶Department of Physics and Astronomy, University College London, London WC1E 6BT, UK