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

The scientific payload on the Clementine spacecraft included a long-wave in frared (LWIR) camera with a single passband centered at a wavelength of 8.7 5 mu m. The Clementine orbit deviated by +/-30 degrees 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 ob tained. We have completed the calibration of the LWIR camera. Here we prese nt the various steps involved in the calibration routine and the associated uncertainty analysis. The LWIR calibration routine can be outlined as foll ows: convert measured data number values to radiance via a calibration equa tion, subtract a zero-flux background image from each lunar image; divide b y a flatfield frame; identify bad pixels; smooth over only bad pixels; adju st radiances to reflect the absolute calibration; and convert radiances to brightness temperatures via the Planck function. Observed LWIR radiances ca n be converted to brightness temperatures, which provide information on var ious physical properties of the lunar surface. We also present here the LWI R global data set. The LWIR data from noontime orbits demonstrate that the Lambertian temperature model of cos(1/4)(i)(i) is a fair approximation for nadir-looking temperatures, rather than the cos(1/6)(i) behavior observed f or ground-based measurements of the full Moon. Deviations from the Lamberti an model are likely due to surface roughness effects and variations in infr ared emissivity. Tn addition, the LWIR global data set reveals the dayside lunar thermal emission to be largely governed by albedo and by the solar in cidence angle.