¹Bruce Hapke
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114105]
¹Department of Geology and Environmental Science, University of Pittsburgh, Pittsburgh, PA, USA
Copyright Elsevier

The opposition effect is the sharp, narrow surge observed in the reflectance of a scattering medium near zero phase angle. Numerous observations and experiments have shown that the primary cause of the phenomenon in particulate media is coherent backscattering, in which wavelets traveling in opposite directions along chains of scatterers interfere constructively and generate the peak. A broader opposition surge caused by shadow hiding and preferential escape is also present, but is entangled with the incoherent continuum reflectance on which the coherent peak is superposed, making it difficult to identify and isolate. Theoretical models of media of independent scatterers predict that the angular width and shape of the coherent backscatter peak depend on the wavelength, porosity and particle size. It was hoped that remote measurements of the opposition effect would give information on the latter two quantities in planetary regoliths. However, observations and laboratory studies of media of large particles in contact with one another find little dependence on any of these quantities. Instead, these studies imply that the opposition effect in regolith-like media comes from reflection by short chains only a few scatterers long located on the surfaces of the particles of the medium, and that the lengths of these chains are proportional to the wavelength. Since the angular width of the peak is controlled by the ratio of the wavelength to the mean scattering chain length, the width is independent of wavelength. Because the wavelets never enter a particle, low albedo media can exhibit a strong coherent backscatter peak. Opposition effect peaks less than a degree wide on solar system bodies can imply an immature regolith; peaks several degrees wide imply a mature regolith.

^1,2,3S.Shkolyar,⁴E.Lalla,^4,5M.Konstantindis,⁶K.Cote,⁴M.G.Daly,⁷A.Steele
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114093]
¹Universities Space Research Association, Columbia, MD 21046, USA
²NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
³Blue Marble Space Institute of Science, Seattle, WA 98154, USA
⁴Centre for Research in Earth and Space Science, York University, Toronto M3J 1P3, ON, Canada
⁵Department of Mathematics and Statistics, York University, Toronto M3J 1P3, ON, Canada
⁶Department of Physics, University of Toronto, Toronto M5S 1A7, ON, Canada
⁷Earth and Planets Laboratory, Carnegie Institution of Washington, Washington, D.C 20015, USA
Copyright Elsevier

Combined UV Raman and laser-induced fluorescence (LIF) spectroscopy instruments will soon be launched onboard missions to planetary surfaces, including Mars, to search for biosignatures. However, the rare earth element Ce3+, found in many common and Mars-relevant minerals, can produce fluorescence features within the spectral window usually attributed to organic compounds in a LIF spectrum. This study explored the detection of Ce3+ as a biosignature mimicker using UV Raman-LIF mission instruments. We assessed how LIF spectra of a suite of synthetic CePO4 samples compare with those of organics, how varying concentrations of both Ce3+ and organics in Martian regolith simulant affect this comparison, and whether two additional data sets obtainable by combined UV Raman-LIF instruments, including time-resolved fluorescence decay lifetimes and Raman spectra, can distinguish Ce3+-containing samples from organics. Results showed that the dominant LIF features of Ce3+ (320 and 338 nm) are similar to those of the aromatic amino acid tryptophan (325 and 340 nm), even when Ce3+ samples were mixed in a Martian regolith simulant at a range of concentrations. Lifetimes were revealed to be 2–9 ns in Ce3+-containing samples, typical for organic fluorophores. These results support the erroneous interpretation that LIF spectra and lifetime values obtained on these samples constitute potential organic signatures. Raman spectroscopy results suggested that with UV laser excitation, Raman is unlikely to identify Ce-bearing samples due to strong absorption of Raman scattered energy by Ce3+. We conclude that biosignature searches using UV LIF and Raman spectroscopy instrumentation may encounter challenges with unambiguously distinguishing spectra of organic compounds from Ce-bearing compounds.

¹Alex M. Ruzicka,^2,3Jon M. Friedrich,¹Melinda L. Hutson,²Juliette W. Strasser,⁴Robert J. Macke,⁵Mark L. Rivers,⁶Richard C. Greenwood,⁷Karen Ziegler,¹Richard N. Pugh
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13567]
¹Department of Geology and Cascadia Meteorite Laboratory, Portland State University, 17 Cramer Hall, 1721 SW Broadway, Portland, Oregon, 97201 USA
²Department of Chemistry, Fordham University, 441 East Fordham Road, Bronx, New York, 10458 USA
³Department of Earth and Planetary Sciences, American Museum of Natural History, 79th Street at Central Park West, New York City, New York, 10024 USA
⁴Vatican Observatory, Vatican City, V‐00120 Italy
⁵Center for Advanced Radiation Sources, University of Chicago, Argonne, Illinois, 60439 USA
⁶Planetary Sciences Research Institute, The Open University, Walton Hall, Milton Keynes, MK7 6AA UK
⁷Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico, 87131 USA
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 8709 is a rare example of a type 3 ordinary chondrite melt breccia and provides critical information for the shock compaction histories of chondrites. An L3 protolith for NWA 8709 is inferred on the basis of oxygen isotope composition, elemental composition, diverse mineral chemistry, and overall texture. NWA 8709 is among the most strongly shocked type 3 chondrites known, and experienced complete melting of the matrix and partial melting of chondrules. Unmelted phases underwent FeO reduction and partial homogenization, with reduction possibly occurring by reaction of olivine and low‐Ca pyroxene with an S‐bearing gas that was produced by vaporization. Chondrules and metal grains became foliated by uniaxial compaction, and during compression, chondrules and fragments became attached to form larger clumps. This process, and possibly also melt incorporation into chondrules to cause “inflation,” may have contributed to anomalously large chondrule sizes in NWA 8709. The melt breccia character is attributed to strong shock affecting a porous precursor. Data‐model comparisons suggest that a precursor with 23% porosity that was impacted by a 3 km/s projectile could have produced the meteorite. The rarity of other type 3 ordinary chondrite melt breccias implies that the immediate precursors to such chondrites were lower in porosity than the NWA 8709 precursor, or experienced weaker shocks. Altogether, the data imply a predominantly “quiet” dynamical environment to form most type 3 ordinary chondrites, with compaction occurring in a series of relatively weak shock events.