Thermophysical Properties of Lunar Regolith Revealed by Thermal-infrared Observations of the Lunar Reconnaissance Orbiter (LRO) Diviner Radiometer

Abstract

The thermophysical properties of lunar regolith are crucial for lunar sciences and engineering. Here, we evaluate the thermophysical properties of lunar regolith between 75°N and 75°S from Diviner data by treating the radius of lunar regolith grains as a proxy. By the ground-truth median/average grain radius and Diviner data at several lunar landing sites, we recalibrate the reference rolling force for a grain radius of 0.475 μm as 0.9 ± 0.1 × 10−7 N. On a global scale, the estimated grain radius values focus mostly between 40 and 60 μm and show an average value of ∼55.3 μm. Some regions of Oceanus Procellarum, Mare Imbrium, and Mare Frigoris show abnormally large grain radius 80–100 μm, which are associated with the late high-Ti volcanism during 2.3–1.2 Ga within Procellarum KREEP Terrane. Mare Humorum is dated to be as old as 3.9 Ga, but still shows large grain radius values of ∼100 μm that are far greater than those in other contemporaneous mares. Such an anomaly may indicate a distinguished evolution for the local lunar regolith. For the surficial lunar regolith on the global scale, the contact component of thermal conductivity normalized to a temperature of 250 K varies mostly within 0.0008–0.0014 W (m K)−1, whereas the radiative component normalized to 250 K varies mostly within 0.002–0.004 W (m K)−1. The thickness of the topmost loosely packed layer is estimated as 0.02–0.07 m and tends to increase with the evolution of lunar regolith, which relates to the modification of packing style with the decrease of grain radius.