Brightness temperatures of the lunar surface: Calibration and global analysis of the Clementine long‐wave infrared camera data

Abstract

The scientific payload on the Clementine spacecraft included a long‐wave infrared (LWIR) camera with a single passband centered at a wavelength of 8.75 μm. The Clementine orbit deviated by ±30° from Sun synchronous, and for two lunar months, dayside nadir‐looking images were obtained near local noon. During the systematic mapping phase of the Clementine mission, approximately 220,000 thermal‐infrared images of the lunar surface were obtained. We have completed the calibration of the LWIR camera. Here we present the various steps involved in the calibration routine and the associated uncertainty analysis. The LWIR calibration routine can be outlined as follows: convert measured data number values to radiance via a calibration equation; subtract a zero‐flux background image from each lunar image; divide by a flatfield frame; identify bad pixels; smooth over only bad pixels; adjust radiances to reflect the absolute calibration; and convert radiances to brightness temperatures via the Planck function. Observed LWIR radiances can be converted to brightness temperatures, which provide information on various physical properties of the lunar surface. We also present here the LWIR global data set. The LWIR data from noontime orbits demonstrate that the Lambertian temperature model of cos1/4(i) is a fair approximation for nadir‐looking temperatures, rather than the cos1/6(i) behavior observed for ground‐based measurements of the full Moon. Deviations from the Lambertian model are likely due to surface roughness effects and variations in infrared emissivity. In addition, the LWIR global data set reveals the dayside lunar thermal emission to be largely governed by albedo and by the solar incidence angle.