¹C.Pilorget,²J.Fernando
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114498]
¹Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS, Orsay 91405, France
²Independent scholar, Orsay 91400, France
Copyright Elsevier

Over the last few decades, visible and near-infrared spectroscopy has proven to be an efficient technique to characterize planetary surface mineralogy, in particular thanks to the presence of diagnostic features appropriate for the identification of most minerals of interest. A more quantitative analysis of the VIS-NIR reflectance spectra constitutes the next major step in understanding the planetary bodies’ history as the retrieval of the mineral assemblages and their relative abundances enables to constrain the chemical and physical conditions of their formation and, thus, the past and present geologic and climatic processes.

Here, we evaluate the capability to retrieve quantitative properties (abundance, grain size) of intimately mixed materials (the most common mineral mixture among planetary surfaces) from typical space VIS-NIR reflectance spectroscopic data. Such results are key to correctly assess the accuracy and relevance of the retrieved mineral information. For that purpose, we developed an inversion model based on a Monte-Carlo Markov Chains (MCMC) scheme with a Bayesian approach to invert VIS-NIR spectra. This approach allows to properly propagate the uncertainties from the data to the retrieved properties, and finally assess what such uncertainties imply for the interpretation. Different binary and ternary mixtures with minerals of interest in planetary sciences and displaying a large variety of albedos and spectral features were tested. Typical uncertainties, both for the abundance and the grain size, were derived and sensitivities on specific parameters/trends were identified. In particular, the role of absorption features in the spectra is quantified. Tests were performed using either the Hapke or the Shkuratov radiative transfer model. The case of unidentified endmembers in the mixture is also discussed. In particular, results show that if the unidentified phase does not display any significant spectral feature, the lack of knowledge about its optical properties does not significantly impact the inversion. These different results will be key in the quantitative analyses of VIS-NIR spectra from planetary bodies.

Finally, we analyze more specifically the case of phyllosilicates and carbonates, two families of minerals of high importance in understanding the Mars geologic and climate history. Typical uncertainties on their relative abundances and grain sizes are derived in various cases, providing a critical supporting dataset for the characterization of the martian mineralogy and the associated geological processes.

¹David T.Blewett,¹Brett W.Denevi,¹Joshua T.S.Cahill,¹Rachel L.Klima
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114472]
¹Planetary Exploration Group, Johns Hopkins University Applied Physics Laboratory, MS200-W230, 11100 Johns Hopkins Rd., Laurel, MD 20723, USA
Copyright Elsevier

We performed an analysis of spacecraft multispectral images for lunar swirls in order to gain an improved understanding of optical space weathering on the Moon and its causes. LROC WAC data provide information on the slope of the spectrum in the near-UV (NUV), as measured by the 321-nm/415-nm or 321-nm/360-nm reflectance ratios. Kaguya MI data were used to assess the near-infrared (NIR) continuum slope (1548-nm/749-nm reflectance ratio). Context for interpreting the spectral variations found in the remotely observed regions of the lunar surface is provided by laboratory reflectance spectra of lunar rocks and soils, as well as spectra for transparent silica gel analogs (Noble et al., 2007) containing different sizes and abundances of nanometer-sized iron (nsFe) particles. We gain additional insights into the spectral effects of sample maturity by considering the ferromagnetic resonance parameter (Is) values for mare and highland soils, as well as the number density of nsFe particles in the silica gels.

We examined a set of three mare swirls (Reiner Gamma, Ingenii, and Mare Marginis) and three highland swirls (Airy, Descartes, and Gerasimovich). The NIR continuum slopes of both mare and highland swirls are shallower than those of the nearby normal mature regolith. Bright swirl surfaces have higher NIR slopes than normal fresh material of the same albedo. The NUV ratios within mare swirls are lower than in the mature background, but for highland swirls, the NUV ratios are approximately the same as the mature background. We do not see definitive evidence for “over-maturation” (excessive darkening and reddening beyond that found in the normal background surfaces) in dark lanes at the swirls we examined, although saturation of weathering effects at a high-iron location like Reiner Gamma could prevent over-maturation from appearing – even if enhanced solar-wind bombardment related to deflection by local magnetic fields is taking place.

Evaluation of the NIR character of swirls and comparison with lab spectra of lunar soils and nsFe-bearing silica gel analogs leads to the conclusion that swirl materials contain abundances of nsFe that are lower than that of normal non-swirl background surfaces; the nsFe content of swirls corresponds to immature (though not pristine) or submature soils. However, the size distribution of nsFe in swirls is anomalous compared with normal lunar surfaces, with a deficiency in the smaller size range (< ~15 nm), as inferred from the NUV character of swirls. Because the flux of solar-wind ions reaching the surface in swirls is attenuated by shielding by crustal magnetic fields, we conclude that solar-wind exposure is the primary agent for production of small nsFe in normal lunar space weathering. Micrometeoroid bombardment, which is unimpeded by the presence of magnetic fields, is mainly responsible for production of larger nsFe in space weathering.

¹Elizaveta Kovaleva,²Monika A.Kusiak,³Gavin G.Kenny,³Martin J.Whitehouse,⁴Gerlinde Habler,⁵Anja Schreiber,²Richard Wirth
Earth and Planetary Science Letters 565, 116948 Link to Article [https://doi.org/10.1016/j.epsl.2021.116948]
¹Department of Earth Sciences, University of the Western Cape, Robert Sobukwe Road, Bellville 7535, South Africa
²Institute of Geophysics, Polish Academy of Sciences, Księcia Janusza 64, PL-01452 Warsaw, Poland
³Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
⁴Department of Lithospheric Research, University of Vienna, 1090 Vienna, Austria
⁵Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Telegrafenberg, 3.5 Surface Geochemistry, D-14473 Potsdam, Germany
Copyright Elsevier

Granular neoblastic zircon (ZrSiO4) with systematically oriented granules has been proposed as evidence for extreme shock pressures (>30 GPa) and subsequent high temperatures (>1200 °C). It is widely agreed to reflect the solid-state phase transition from zircon to its high-pressure polymorph reidite and subsequent reversion to zircon. This model is based on crystallographic relationships between granules of a single type of granular zircon and does not explain the formation of other types of granular zircon textures, for example, grains with randomly oriented granules or with large, often euhedral granules. Here we report the first nano-scale observations of granular neoblastic zircon and the surrounding environment. We conducted combined microstructural analyses of zircon in the lithic clast from an impact melt dike of the Vredefort impact structure. Zircon granules have either random or systematic orientation with three mutually orthogonal directions of their c-axes coincident with [110] axes. Each 1-2 μm zircon granule is a mosaic crystal composed of nanocrystalline subunits. Granules contain round inclusions of baddeleyite (monoclinic ZrO2) and amorphous silica melt. Tetragonal and cubic ZrO2 also occur as sub-μm-sized inclusions (<50 nm). Filament-like aggregates of nanocrystalline zircon are present as “floating” in the surrounding silicate matrix. They are aligned with each other, apparently serving as the building blocks for the mosaic zircon crystals (granules). Our results indicate shock-related complete melting of zircon with the formation of immiscible silicate and oxide melts. The melts reacted and crystallized rapidly as zircon granules, some of which experienced growth alignment/twinning and parallel growth, causing the characteristic systematic orientation of the granules observed for some of the aggregates. In contrast to the existing model, in which this type of granular zircon is considered to be a product of reversion from the high-pressure polymorph reidite, our nano-scale observations suggest a formation mechanism that does not require phase transition via reidite but is indicative of instant incongruent decomposition, melting and rapid crystallization from the melt.