¹Shingo Kameda et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114348]
¹Rikkyo University, Japan
Copyright Elsevier

Global multiband images of the C-type asteroid (162173) Ryugu were obtained by the optical navigation camera telescope (ONC-T) onboard Hayabusa2. The 0.7-μm absorption depth of the surface reflectance spectrum, which indicates the presence of hydrous minerals, was not clearly seen on Ryugu using flat field correction data obtained in the preflight measurement. The flat field correction data were obtained in the preflight calibration test only at room temperatures (24–28 °C), whereas most observations around Ryugu were performed at a charge-coupled device (CCD) temperature of approximately −30 °C. To obtain higher accuracy measurements, we used a new flat field correction method using the Ryugu surface reflection data. We confirmed that the flat-field patterns are different in high and low temperature conditions. The 0.7-μm absorption map generated by the new method shows that the 0.7-μm absorption near the equator (5°N–5°S) is stronger than that from 30°N to 30°S. We found that the excess of the absorption depth at low latitudes was 0.072%, corresponding to 2.7σ. The spectral analysis also shows that the Ryugu surface at low latitudes is bluer than that at high latitudes and bluer materials tend to show stronger 0.7-μm absorption than redder materials, suggesting that this region has been subjected to less space weathering and less solar heating.

¹Noel A.Scudder,¹Briony H.N.Horgan,²Elizabeth B.Rampe,^1,3Rebecca J.Smith,⁴Alicia M.Rutledge
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114344]
¹Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907, USA
²Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Mailcode XI2, 2101 NASA Parkway, Houston, TX 77058, USA
³Department of Geosciences, Stony Brook University, 255 Earth and Space Sciences Building, Stony Brook, NY 11794, USA
⁴Department of Astronomy and Planetary Science, Northern Arizona University, NAU Box 6010, Flagstaff, AZ 86011, USA
Copyright Elsevier

The natural chemical and physical variations that occur within volcanic rocks (petrology) provide critical insights into mantle and crust conditions on terrestrial bodies. Visible/near-infrared (VNIR; 0.3–2.5 μm) and thermal infrared (TIR; 5–50 μm) spectroscopy are the main tools available to remotely characterize these materials from satellites in orbit. However, the accuracy of petrologic information that can be gained from spectra when rocks exhibit complex variations in mineralogy, crystallinity, and microtexture occurring together in natural settings is not well constrained. Here, we compare the spectra of a suite of volcanic planetary analog rocks from the Three Sisters Volcanic Complex, Oregon to their mineralogy, chemistry, and microtexture from X-ray diffraction, X-ray fluorescence, and electron microprobe analysis. Our results indicate that TIR spectroscopy is an effective petrologic tool in such rocks for modeling bulk mineralogy, crystallinity, and mineral chemistry. Given a library with appropriate glass endmembers, TIR modeling can derive glass abundance with similar accuracy as other major mineral groups and provide first-order estimates of glass wt.% SiO2 in glass-rich samples, but cannot effectively detect variations in microtexture and minor oxide minerals. In contrast, VNIR spectra often yield non-unique mineralogic interpretations due to overlapping absorption bands from olivine, glass, and Fe-bearing plagioclase. In addition, we find that sub-micron oxides hosted in transparent matrix material that are common in fine-grained extrusive rocks can lower albedo and suppress mafic absorption bands, leading to very different VNIR spectra in rocks with the same mineralogy and chemistry. Mineralogical interpretations from VNIR spectra should not be treated as rigorous petrologic indicators, but can supplement TIR-based petrology by providing unique constraints on oxide minerals, microtexture, and alteration processes.