Photometric Correction for the Thermal Channels of the Diviner Lunar Radiometer Experiment

Abstract:

The Diviner Lunar Radiometer Experiment is a nine channel multispectral imager on the Lunar Reconnaissance Orbiter currently mapping the lunar surface. Three of its channels are dedicated to locating the Christiansen Feature (CF), the mid-infrared emissivity maximum near 8 µm that is the primary indicator of surface composition as its location changes as a function of silicate polymerization. Four of its channels detect in the mid- to far-infrared (12.5-400 µm) from which temperature and thermophysical properties are derived. The shorter wavelength thermal channels (esp. 12.5-25 μm and 25-50 µm) can also be used in the determination of surface composition as silicate minerals have strong absorption features in this region.

We have found that the emissivity of these thermal channels varies as a function of solar incidence angle. This is problematic as high incidence angle measurements are necessary for obtaining spectra at high latitudes. A correction has been developed for Diviner’s 8 µm channels which normalizes all Diviner daytime data to 0° incidence angles, but a correction for the thermal channels has yet to be created. Here, we describe our method for correcting the thermal channels as a function of incidence angle.

We assume that emissivity measured at an incidence angle of 0° are correct, and calculate the change in emissivity with increasing incidence angle as a percentage of the 0° emissivity. We use these values to create a calibration function of incidence angle for each channel. Several lunar equatorial locations were used to create the calibration and locations at higher latitudes were included in testing its viability.

This correction creates a global data set that is more consistent and comparable to laboratory data, and allows the thermal channels to be used to give more detailed insight into surface mineralogy by increasing the amount of compositionally useful channels from 3 to 5. This work will allow us to create more mineral spectral indices and lead to spectral unmixing models. Future improvements to our methodology include an expanded photometric database and constraints based on emission and phase angles.