Lunar regolith grain size and density-stratification derived from LRO/Diviner and Chang’E-2/MRM data

Radiometric measurements of the (sub-)surface thermal emission are sensitive to the microphysical structure and thermophysical properties of the regolith. For the interpretation of these measurements, thermal models are required. Bürger et al. (2024) developed a microphysical thermal model for the lunar regolith, which more directly simulates regolith properties such as the grain size and volume filling factor.  In this study (Bürger et al., 2026), we derive global regolith grain size and density-stratification by matching modeled surface temperatures and microwave brightness temperatures with measurements from the infrared radiometer Diviner on board the Lunar Reconnaissance Orbiter (LRO) (Paige et al., 2010) and the Microwave Radiometer (MRM) on board Chang’E-2 (Wang et al., 2010; Zheng et al., 2019). The radiometric observations in the infrared and in the microwave range are complementary, as infrared measurements are sensitive to surface thermal emission, while the microwave measurements are sensitive to the thermal emission from subsurface layers.

It is important to note that the microwave radiometer receives the thermal emission from a range of depths within the regolith, with the overall penetration depth being controlled by measurement frequency and the dielectric properties of the regolith. Therefore, in order to derive synthetic microwave brightness temperatures, a radiative transfer model is required and this study uses the model presented in Feng et al. (2020) and Siegler et al. (2020). Furthermore, we use only the 19.35 GHz and 37 GHz channel data of MRM, because the two lowermost frequency channels of MRM are believed to suffer from a calibration issue (Feng et al., 2020; Hu et al., 2017; Hu & Keihm, 2021).

We find that the regolith grain size and density-stratification can be unambiguously constrained when fitting both datasets – Diviner and MRM. In more detail, we derive for the equatorial highlands a global regolith grain radius of 45⁺⁶_-4 µm and a deep layer bulk density of 1800⁺⁷⁰_-90 kg m^-3. These parameters describe the highland regolith well for all latitudes < 40° and are in good agreement with grain size and bulk density measurements from returned Apollo samples. Figure 1 illustrates the Diviner regolith temperatures and the MRM 37 GHz and 19.35 GHz measurements in the lunar highlands at the equator together with the simulated surface and microwave brightness temperatures resulting in the above described best fit. Figure 2 illustrates the resulting bulk-density profile together with best-estimate bulk density values inferred from Apollo data (Mitchell et al., 1974), and the range of bulk densities determined from Apollo core tube and drill core measurements (Carrier et al., 1974; Carrier et al., 1991). Finally, we also investigated the latitudinal dependence of lunar regolith properties and find that both data sets – Diviner and MRM – can be best fit with a poleward decrease in deep layer bulk density.

Figure 1: The best-fit simulations (green line) together with the measured Diviner regolith temperatures (left), MRM 37 GHz (center), and 19.35 GHz (right) brightness temperatures in the highlands at the lunar equator. The measurements are presented as a function of local time and the individual measurements (small dots) are binned (black circles) with a bin-size of 0.5 hours.

Figure 2: The resulting bulk-density profile (green line) together with constraints from Apollo measurements.

References

Bürger et al. (2024), JGR Planets, 129(3). Bürger et al. (2026), A microphysical thermal model for the lunar regolith: Determining the lunar regolith properties using a combination of LRO/Diviner and Chang’E-2/MRM data, accepted for publication in A&A. Carrier et al. (1974), The Moon, 10, 183. Carrier et al. (1991), in Heiken G. H., Vaniman D. T., French B. M., eds, Lunar Sourcebook, A User’s Guide to the Moon, 475–594. Feng, J. et al. (2020), JGR Planets, 125(1). Hu et al. (2017), Icarus, 294, 72. Hu & Keihm (2021), IEEE Geoscience and Remote Sensing Letters, 18, 1781. Mitchell et al. (1974), Apollo soil mechanics experiment S-200, Space Sciences Laboratory Series 15, Issue 7, Univ. of California, Berkeley. Paige et al. (2010), Space Sci. Rev., 150(1–4), 125–160. Siegler, M. A. et al. (2020), JGR Planets, 125(9). Wang et al. (2010), Science China Earth Sciences, 53, 1392. Zheng et al. (2019), Icarus, 319, 627-644.