Utilization of A Meteorological Satellite for Planetary Science as A Space Telescope: The Lunar Mid-Infrared Spectrum as Seen by Himawari-8

Abstract The Japanese meteorological satellite Himawari-8 has been capturing not only the earth’s atmosphere, but also stars, planets, and the moon in its field of view and enables us to take their spectroscopy with 16 bands from visible to mid-infrared wavelengths. Especially for the lunar case, nine infrared bands equipped in the Advanced Himawari Imager (AHI) onboard Himawari-8 are unique in spaceborne observations and potentially useful for lunar science. Besides, the infrared bands of AHI cover similar wavelengths with those of other interplanetary instruments, thus are useful for their calibrations. However, the infrared AHI data have not been investigated in planetary science. To confirm the utility of AHI for planetary science, we develop the procedure to retrieve the lunar infrared spectrum and compare it with thermal conduction simulations. Our analysis shows that the lunar brightness temperature curves can be obtained in the morning, evening, and night sides at all nine bands. Particularly at 8.5 μm, they show a good agreement with the previous observation by the Diviner radiometer onboard NASA's Lunar Reconnaissance Orbiter. As pointed out previously, the brightness temperatures differ between bands, indicating the temperature mixing within a pixel. In the morning and evening, our simulation suggests that surface roughness as steep as those measured at Apollo landing sites significantly contributes to the observed brightness temperature differences. On the other hand, the nightside brightness temperatures are greatly affected by rocks that have higher thermal inertia than regolith. The rock abundances are estimated as 0.18 –0.48 and 5.6 –10.4 % at the equator and within Tycho crater, respectively. Our estimations from the AHI data are consistent with one from Diviner. These results support that AHI potentially serves as a space telescope for future lunar and planetary sciences, for example, to constrain water content on the lunar surface.