Simultaneous Physical Retrieval of Daytime Lunar Surface Temperature and Spectral Emissivity in the 3–5 Mm Range from Chandrayaan-2 IIRS Observations

The Imaging InfraRed Spectrometer (IIRS) on board Chandrayaan-2 has been providing high spatial and spectral resolution observations of the lunar surface in 256 spectral bands (0.7–5 µm) since September 2019. It is primarily intended for mineral mapping and identifying hydration features on the lunar surface using reflectance spectra in the range of 0.7–3.2 µm. Here, we have used the IIRS observations in the 3–5 µm range to retrieve daytime lunar surface temperature and spectral emissivity using an optimal estimation theory-based retrieval algorithm. The surface temperature is retrieved at every pixel, while spectral emissivity is retrieved at every third pixel of the hyperspectral image. The mean uncertainty of the retrieved spectral emissivity varies from 0.04 to 0.08, while for surface temperature, it is about 3.5 K. The retrieved spectral emissivity is found to be in close agreement with the emissivity of the Apollo-16 return soil samples.