Analysis of Chang'E-2 brightness temperature data and production of high spatial resolution microwave maps of the Moon

Abstract

The microwave radiation passively emitted from the Moon is valuable for investigating the physical properties of the lunar regolith. Microwave maps, which can reveal subsurface characteristics at depths of centimeters to meters, give clues to understanding the evolution of the lunar surface. China's Chang'E-2 (CE-2) spacecraft carried a microwave radiometer (MRM) and conducted passive microwave remote sensing of the Moon at 3, 7.8, 19.35 and 37 GHz (10, 3.84, 1.55 and 0.81 cm wavelengths) during 2010–2011. Brightness temperature (TB) data derived from the MRM orbital observations cover the entire Moon, and the time coverage is over a complete lunation. In this paper, we use spherical harmonic fits to model the TB variation as functions of local time and latitude. Using these fits, we normalized the day- and nighttime TB data measured at various local times into noon-time and midnight conditions. The spatial sampling of the normalized TB maps is ∼6 km/pixel, much higher than ground-based observations and higher resolution than the MRM maps derived from Chang'E-1. By removing the variation in TB related to local time and latitude, we produced a series of microwave maps that reveal differences in the properties of the regolith across the lunar surface. The microwave behavior of various lunar surface features can be explained primarily by differences in the microwave loss tangent of the materials. For example, we find that the high-Ti maria have high daytime TB and exhibit lower nighttime TB than the other maria as a result of the high dielectric loss tangent of ilmenite. The ilmenite-rich materials are less transparent to microwaves, and hence emissions emerge from closer to the surface, which is heated strongly in the day and cools quickly at night. Most fresh craters correspond to low-TB spots in the nighttime microwave maps, in contrast to their behavior as hot spots in nighttime infrared data, suggesting that the higher loss tangent of rocks (compared to soils) suppresses microwave emission at night. The Aristarchus plateau has a lower loss tangent relative to its surroundings because it is blanketed by fine-grained, rock-poor pyroclastic material. The pyroclastic blanket is more transparent to microwaves than the nearby maria, causing the plateau to have relatively cool daytime TB and warm nighttime TB as a consequence of microwave emissions that originate at greater depths The Orientale basin is another prominent anomaly in the microwave maps, indicating that Orientale materials differ in thermophysical and/or dielectric properties from typical highland lithologies. Brightness temperature differences among prominent rayed craters indicate that the very youngest crater rays (e.g., Giordano Bruno) have lower thermal inertia than older but still optically immature rays (e.g., Tycho).