Investigating reflectance spectra to link remote-sensed data with lab-measured data: cases of asteroid (162173) Ryugu and the Moon

Introduction.

Comparing ultraviolet (UV) -visible (Vis) -infrared (IR) spectra of terrestrial/extraterrestrial materials with asteroids and moons is important to estimate surface mineralogical and physicochemial properties, which can provide clues to reveal the formation and evolution processes of the Solar System. This presentation reviews two topics of our current findings related to spectral comparison between remote-sensing and laboratory measurements for the asteroid Ryugu and the Moon, that will be useful for future in-space small body observations and sample return missions including MMX.

Comparison between lab-measured and remote-sensed spectra of Ryugu.

C-class asteroids are thought to be primitive bodies recording the solar system formation process. Captured samples have been delivered from the near-Earth asteroid (162173) Ryugu by the JAXA's Hayabusa2 spacecraft in 2020. Based on the results obtained by initial sample analyses, captured Ryugu materials are representative of Ryugu body surface, which has been aqueously altered strongly, and most similar to CI (Ivuna-type) chondrites spectroscopically and chemically [1-5]. In a spectroscopic view, however, there is a gap between remote-sensed data and lab-measured data. Lab-measured reflectance spectra of Ryugu samples in multiple grain and powder forms show a weaker OH absorption band at 2.7 μm due to hydrous silicates. Comparison among lab Ryugu spectra and carbonaceous chondrite spectra with different grain size, porosity, and space-weathering degree reproduced by ion/laser-irradiation experiments was performed, indicating that space weathering due to micrometeoroid bombardments mainly promotes spectral change at the Ryugu's surface [6].

Lab-measured spectra of ilmenite implication for the Moon.

Clarifying the distribution and abundance of ilmenite (FeTiO₃) on the Moon is important for elucidating the formation and evolution process of the Moon and the property of the magma ocean. Ilmenite is abundant in the lunar mare basalts, and thus spectral properties of ilmenite in the UV-Vis-IR range have been investigated and known to show a positive peak at around 1 μm as a remnant of two neighbor absorption bands centered at around 630 nm and around 1300 nm due to Ti³⁺-Ti⁴⁺ charge transfer and Fe²⁺ crystal field transitions, respectively (e.g., [7] and referenses therein). Lunar spectra observed by the Spectral Profiler on board Kaguya/SELENE show a slight rise was found near the peak of 1 μm band absorption on the surface of the moon, and it was coincidently detected by the Multiband Imager [8]. Previous ground-based telescope obsevations also showed a weak rise inside 1 μm absorption band in the areas called Taurus-Littrow and Rima Bode on the lunar surface [9].  Ilmenite is thought to coexist with other minerals, therefore, Matsuoka and Yamamoto (2024) [10] performed lab spectral measurements using powdered ilmenite samples and other possible components of lunar basalts, e.g., augite and glass, with various mixing ratio. They reported that a little amount of ilmenite powder may produce absorption bands causing the 1-μm peak, and strong darkening, In contrast, significant blue slope (decreasing toward longer wavelength) in the UV range of ilmenite rapidly masked by a small amount of glass component, making the UV slope redder. It is suggested that to investigate ilmenite, using the UV feature gives very limited information on the lunar surface as considered by Sato et al. (2017) [11]. Those complex UV-Vis-IR spectral characteristics are necessary to be considered to interpret remote-sensed data based on quantitative data obtained by lab measurements.

Considering each of the component minerals has a different effect on reflectance spectra, while also unique alteration process and characteristics in space environments such as space weathering, suggests that sample return and close observations of Phobos, similar to D-type asteroids but whose surface properties are still unvailed, will provide a new property of the in-space surface characteristics of small bodies by MMX mission [12].

References:
[1] Tachibana et al. (2022) Science 375, 1011–1016.
[2] Pilorget, C. et al. (2022) Nat. Astron. 6, 221–225.
[3] Yada et al. (2022) Nat. Astron. 6, 214–220.
[4] Yokoyama et al. (2022) Science 379, eabn7850
[5] Nakamura et al. (2022) Science 379, eabn8671.
[6] Matsuoka et al. (2023) Commun Earth Environ 4, 335.
[7] Iwaza et al. (2021) Icarus, 362, 114423.
[8] Yamamoto et al. (2023) Fall Lecture of the Society of Planetary Sciences.
[9] Gaddis et al. (1985） Icarus, 61, 3, 461-489.
[10] Matsuoka and Yamamoto.(2024) Japan Geoscience Union Meeting 2024, PPS09-P01
[11] Sato et al. (2017) Icarus, 296, 216–238.
[12] Kuramoto et al. (2021) Earth, Planets and Space, 74:12.