¹Michael S.Bramble,¹Ralph E.Milliken,²William R.PattersonIII
Icarus (in Press) Link to Journal [https://doi.org/10.1016/j.icarus.2020.114251]
¹Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI, USA
²School of Engineering, Brown University, Providence, RI, USA
Copyright Elsevier

We investigate a suite of synthetic mineral mixtures designed to act as bulk mineralogical analogs to H, L, and LL ordinary chondrite meteorites in order to probe how the thermal emission characteristics of such materials change between ambient and simulated asteroid environmental conditions. Due to the parent body link with certain S-type asteroids, studying these analog mixtures in an environment that is relevant to actual asteroid surfaces advances our understanding of the thermal emission properties of one of the most common regolith types among the main-belt and near-Earth asteroid populations. The observed changes in spectral emissivity features due to cold, vacuum conditions are not as large as previously observed for single mineral (silicate) samples. We interpret this difference to be the result of metallic and opaque components weakening near-surface thermal gradients in the mixtures. As such, we predict that near-surface thermal gradients on ordinary chondrite parent bodies (e.g., S-type asteroids) are likely much weaker than would be inferred from cold, vacuum measurements of individual mineral components. We tested whether the increased spectral contrast observed in fine-grained (<25 μm) samples measured in a cold, vacuum environment increases the efficacy of least squares linear unmixing methods. It is found that the accuracy of such models does not improve relative to measurements made under ambient conditions, thus linear unmixing models are not expected to yield accurate estimates of the modal mineralogy of airless planetary surfaces if they are dominated by fine-grained regolith. Mixtures with coarse particle sizes (125–250 μm) that were modeled using the coarse particle size endmembers yielded results that were largely independent of the environmental conditions, but with larger errors in spectral fits for samples measured in a simulated asteroid environment. At simulated asteroid environmental conditions, the bulk silicate composition and metal content play a more important role in determining the thermal state and brightness temperature of the sample than at ambient conditions. Modest changes in metal content (10–25 wt%) lead to large differences in the brightness temperature of a sample. Under simulated asteroid conditions, an ~10 K increase in maximum brightness temperature that tracks with increased iron content is observed at fine particle sizes (<25 μm) between each of the analog ordinary chondrite groups. Based on these results, it may be difficult to distinguish H, L, and LL compositions of suspected ordinary chondrite parent bodies using only Earth- or space-based thermal emission spectra. This is inferred from the spectral similarity of the analog mixtures and the absence of significant variation in spectral emissivity associated with reported differences in bulk metal content for ordinary chondrites.

¹L.Riu et al. (>10)
Icarus (in Press) Link to Journal [https://doi.org/10.1016/j.icarus.2020.114253]
¹Institut of Space and Astronautical Science (ISAS), Japanese Aerospace eXploration Agency (JAXA), Sagamihara, Japan
Copyright Elsevier

C-type rubble pile asteroid (162173) Ryugu was observed and characterized up close for a year and a half by the instruments on-board the Japanese Aerospace eXploration Agency (JAXA) Hayabusa2 spacecraft. The asteroid exhibits relatively homogeneous spectral characteristics at near-infrared wavelengths (~1.8–3.2 μm), including a very low reflectance factor, a slight positive (“red”) slope towards longer wavelengths, and a narrow absorption feature centered at 2.72 μm that is attributed to the presence of OH− in phyllosilicate minerals. Numerous craters have been identified at the surface that provide good candidates for identifying and studying younger and/or more recently exposed near-surface material to further assess potential spectral/compositional heterogeneities. We present here the results of a spectral survey of all previously identified and referenced craters (Hirata et al. 2020) based on reflectance data acquired by the NIRS3 spectrometer, with an emphasis on the spectral characteristics between different craters as well as with their surrounding terrain. At a global scale, the spectral properties inside and outside of craters are found to be very similar, indicating that subsurface material is either compositionally similar to material at the surface that has a longer exposure age or that material at Ryugu’s optical surface is spectrally altered over relatively short timescales by external factors such as space weathering. Although, the imaging data from ONC camera suites show more morphological and color diversity in craters at a smaller scale than the resolution provided by the NIRS3 instrument, which could indicate a wider compositional diversity on Ryugu than that observed in the near-infrared and discussed in this paper. The 2.72 μm band depth exhibit a slight anti-correlation with the reflectance factor selected at 2 μm, which could indicate different surface properties (e.g., grain size and/or porosity) or different alteration processes (e.g., space weathering, shock metamorphism and/or solar heating). Four different spectral classes were identified based on their reflectance factor at 2 μm and 2.72 μm absorption strength. The most commonly spectral behavior associated with crater floors, is defined by a slightly lower reflectance at 2 μm and deeper band depth. These spectral characteristics are similar to those of subsurface material excavated by the Hayabusa2 small carry-on impactor (SCI) experiment, suggesting these spectral characteristics may represent materials with a younger surface exposure age. Alternatively, these materials may have experienced significant solar heating and desiccation to form finer grains that subsequently migrated towards and preferentially accumulated in areas of low geopotential, such as craters floors. It is believed that the Hayabusa2 mission successfully collected typical surface material as well as darker material excavated by the SCI experiment, and detailed analyses of those samples upon their return will allow for further testing of these formation and alteration hypotheses.