Utilization of a meteorological satellite as a space telescope: the lunar mid-infrared spectrum as seen by Himawari-8

Abstract

The Japanese meteorological satellite Himawari-8 has captured the Earth’s atmosphere and stars, planets, and the Moon in its field of view, enabling us to capture their spectroscopy with 16 bands from visible to mid-infrared wavelengths. The nine infrared bands in the Advanced Himawari Imager (AHI) onboard Himawari-8 are unique in spaceborne observations and are potentially useful for lunar science. In addition, infrared bands of AHI cover wavelengths similar to those of other interplanetary instruments and thus are useful for calibrations. However, infrared AHI data have not yet been investigated in planetary science. To confirm the utility of AHI for planetary science, we develop a procedure to retrieve the lunar infrared spectrum and compare it with thermal conduction simulations. Our analysis shows that lunar brightness temperature curves can be obtained in the morning, evening, and nighttime for all nine bands. Particularly at 8.5 μm, they show a good agreement with previous observations by the Diviner radiometer onboard NASA’s Lunar Reconnaissance Orbiter. As pointed out previously, the brightness temperatures differ between the bands, indicating temperature mixing within a pixel. Our simulation suggests that surface roughness as steep as those measured at the Apollo landing sites significantly contributes to the observed brightness temperature differences in the morning and evening; however, nighttime brightness temperatures are greatly affected by rocks with higher thermal inertia than the regolith. The rock abundances are estimated to be 0.18–0.48% and 6.1–10.3% at the equator and within Tycho crater, respectively. Our estimations from AHI data are consistent with those of Diviner. These results support the idea that AHI potentially serves as a space telescope for future lunar and planetary sciences, for example, for constraining water content on the lunar surface.Graphical