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

Abstract

Passive microwave frequency (~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 (~7.8–400 μm) 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.

Reconciling the Infrared and Microwave Observations of the Lunar South Pole: A Study on Subsurface Temperature and Regolith Density

“…We also model a 2-year variation of surface temperature and high-frequency microwave T B for two specific locations. Given that there are few measurements of dielectric properties of lunar regolith at low temperatures (<100 K), a non-temperature-dependent loss tangent constrained by our previous work (Feng et al, 2020;Siegler et al, 2020) is used in these models. The modeling result indicates that both infrared and microwave observations in the PSRs are best explained by a low temperature-dependent conductivity, like suggested in Woods-Robinson et al (2019), and a low-density regolith structure.…”

“…where n is the total number of layers in the model, i E T is the physical temperature of ith layer. i E W is the weighting function determined by frequency, density, thickness and dielectric properties of each layer (Feng et al, 2020;Siegler et al, 2020). Here we assume the regolith in the polar region has the same permittivity and loss tangent as highland regolith constrained by Siegler et al (2020), then the profile of i E W at a certain frequency could be calculated only from a density profile and the thickness of each layer.…”

“…i E W is the weighting function determined by frequency, density, thickness and dielectric properties of each layer (Feng et al, 2020;Siegler et al, 2020). Here we assume the regolith in the polar region has the same permittivity and loss tangent as highland regolith constrained by Siegler et al (2020), then the profile of i E W at a certain frequency could be calculated only from a density profile and the thickness of each layer. Figure 3 gives the density structure of regolith with a global average H of 0.068 m, the loss tangents and corresponding weighting functions of four channels.…”

“…Negative gradients are likely unphysical, hinting that the 3 GHz data values are currently too low. Negative gradients would be removed by adding ∼15 K to the absolute value of the 3 GHz data, similar to the upward recalibration suggested in Siegler et al (2020). The estimation uncertainty of the gradient is discussed in Text S1.…”

“…To better understand the thermal behavior of regolith in polar region and provide a reference to future missions, we use the data from Diviner and Chang'e-2 (CE-2) microwave radiometer (MRM) to research subsurface temperatures and density structure in the lunar south polar region. These instruments combined provide a strong constraint on temperatures at the surface and at depth of several meters (e.g., Feng et al, 2020;Siegler et al, 2020)-here we examine the anomalous behavior of thermal and physical properties within the south polar cold traps that we find in comparing the data of these instruments.…”