Lunar Titanium and Frequency-Dependent Microwave Loss Tangent as Constrained by the Chang'E-2 MRM and LRO Diviner Lunar Radiometers

Passive microwave frequency (similar to 300 MHz to 300 GHz) observations of the Moon have a long history and have been suggested as a plausible orbital instrument for the Moon and other bodies. However, global, orbital multifrequency measurements of lunar passive microwave emission have only recently been made by the Chinese Chang'E-1 and -2 microwave radiometer (MRM) instruments. These missions carried nearly identical 4-channel (3.0, 7.8, 19.35, and 37 GHz) instruments into lunar orbit in 2007-2009 and 2010-2011, respectively. Over the same time period, the ongoing Lunar Reconnaissance Orbiter (LRO) mission carried the Diviner Lunar Radiometer, which collected surface temperature measurements in the far infrared (similar to 7.8-400 mu m) from 2009 to present. By combining these data and associated thermal models, we provide new constraints on the relationship between physical temperature and microwave brightness temperature to reveal novel information about regolith thermal and dielectric properties which can reveal unique geologic information about the Moon. Here, we describe several first-order global results to come from this combined data set, focusing primarily on the ability to detect, map, and quantify dielectric loss tangent variations of the Moon, including those from the presence of titanium-bearing ilmenite. We update the loss tangent models for both highlands and mare and identify a clear frequency dependence that differs in sign between the two. We use the correlation with visible wavelength TiO2 mapping to provide a means to separate out the loss from rocks and from that of composition. Plain Language Summary The Chang'E 2 microwave radiometer (MRM) and Lunar Reconnaissance Orbiter (LRO) Diviner infrared radiometer provide a unique look at lunar surface and subsurface temperatures. Microwave radiation observed by MRM was emitted from the upper several meters, allowing for constraint of local thermal and dielectric property changes. Using Diviner-constrained subsurface temperature models and MRM-measured brightness temperature diurnal amplitudes, we fit a frequency-dependent map of the dielectric loss properties of the lunar regolith. This is directly useful for radio-frequency studies of the Moon and allows us to map absorbing minerals such as titanium ilmenite. We also show that there is a strong correspondence between rocky areas of the Moon and microwave brightness temperature amplitude.