Post-sunset cooling behavior of the lunar surface

Differential infrared flux scans at 22m have been made across the nighttime lunar surface over a range of phases before and after new Moon. The differential chopping technique effectively cancels atmospheric emission in the beam path but records the flux difference between adjacent resolution elements on the lunar disk. The scans show that the brightness temperature gradient across the highlands after sunset is much greater than that across the western maria. The large gradients consistently disappear approximately 3.5 days after sunset. The post-sunset enhancement could be due to surface roughness in the highlands or to a significant surface rock population with a mean size of approximately 0.5 m. The effect can be seen in the 10m measurements of other investigators, but its global nature was not detected in their limited data sets.