Average Infrared Brightness Temperature of Lunar Rough Surface in Field of View of High-Resolution Infrared Radiation Sounder and Application to Calibration

The Moon’s surface shows long-term stability and may be a suitable candidate of thermal calibration source. When the field of view (FOV) of an Earth radiometer, e.g., the high-resolution infrared radiation sounder (HIRS), occasionally scans across the Moon, it measures the average brightness temperature (TB) of the entire nearside of the Moon. The lunar surface roughness causes changes in the surface emissivity and anisothermality. Analysis of the infrared TB (IR TB) of the lunar surface from the nadir observations of the Diviner IR radiometer showed that the IR TB difference (TB-D) between the Diviner channels is largest near sunrise and sunset, when the anisothermality is most significant. In this article, the nearside of the lunar surface is divided into
$30\times 30$
subregions, where a rough surface is constructed using small triangular meshes. The ray-tracing method is used to determine the shaded meshes. Heat conduction equations are solved for the temperatures of all meshes. The simulated IR TB and TB-D are compared with the Diviner data. The thermal emissions of Gaussian rough surfaces with different height variances, the cratered surface with digital elevation model, and two-scale rough surface are simulated to discuss the influence of topographies. Under nonnadir observation conditions, the ray-tracing method is used to determine the visibility of each mesh in the detector’s FOV. The IR TB characteristics are asymmetric with respect to the emission angle. The average IR TB of the nearside of the Moon at the wavelength of
$14~\mu \text{m}$
is obtained and compared with the IR TB derived from HIRS data. The rough surface model can more feasibly describe the average IR TB data than flat surface model and is favorable for HIRS calibration/validation.