Thermophysical properties of the regolith on the lunar far side revealed by the in situ temperature probing of the Chang'E-4 mission

Abstract

Temperature probes onboard the Chang'E-4 (CE-4) spacecraft provide the first in situ regolith temperature measurements from the far side of the Moon. We present these temperature measurements with a customized thermal model and reveal the particle size of the lunar regolith at the CE-4 landing site to be ∼15 μm on average over depth, which indicates an immature regolith below the surface. In addition, the conductive component of thermal conductivity is measured as ∼1.53 × 10^-3 W m^-1 K^-1 on the surface and ∼8.48 × 10^-3 W m^-1 K^-1 at a depth of 1 m. The average bulk density is ∼471 kg m^-3 on the surface and ∼824 kg m^-3 in the upper 30 cm of the lunar regolith. These thermophysical properties provide important additional 'ground truth' at the lunar far side, which is critical for the future analysis and interpretation of global temperature observations.

Keywords: Chang’E-4; lunar far side; regolith; temperature; thermal conductivity.

Figure 1.

Location of the CE-4 lander and the setting of the temperature probes. (a) The mosaic of the lunar far side obtained by Chang’E-1 (CE-1) charged-coupled device (CCD) camera. The CE-4 landing site (45.4446°S, 177.5991°E) [6] is indicated by the red cross. (b) Regional context of Von Kármán crater. The regolith at the CE-4 landing site (indicated by the red cross) originates from the Finsen crater [17]. The background is the mosaic of CE-1 CCD images. (c) The CE-4 lander on the lunar surface. The yellow dashed arrow indicates the approximate movement direction of the Sun. The white dashed arrow indicates the motion of the lander's shadow along the lunar surface. The colored dots specify the positions of the four temperature probes (T1–T4). The photo was taken by the Panorama Camera on the Yutu-2 rover in the local morning. The rectangle indicates the location of (f). (d) The metallic rails in the local morning. The blue arrow indicates the direction in which the shadow moves across the surface. The positions of the four temperature probes are labeled by the colored dots. (e) The temperature probes installed at the terminals of the metallic rails. (f) Lunar regolith overflow (red arrows) is observed at the end of the rails.

Figure 2.

The temperature variation at the CE-4 landing site obtained during the third lunar day after landing. (a) The colored scatter plots represent the temperatures measured by the temperature probes on the CE-4 lander. The black curve represents the temperature during the day, simulated by the thermal equilibrium model with the assumption of direct solar illumination (see Supplementary Methods). The black scatter plots are Diviner bolometric temperatures for an area (45.25°S, 177.75°E, 0.5° in width) containing the CE-4 landing site and the vertical bars represent estimated error bounds [22]. (b) The temperature measured near the lunar noon of the third lunar day after landing.

Figure 3.

Comparisons between the modeled temperatures of various grain sizes with surface pressure and the measured temperatures of the probe T2 during the night. The colored curves specify the decrease of surface temperatures modeled for the grain sizes 10 μm, 15 μm and 20 μm with a surface pressure of 80 Pa, and the black dashed curve for the grain size 130 μm without surface pressure, whereas the black dots specify the temperatures measured by the probe T2. The best fit between the modeled temperatures and the measured temperatures is achieved with a grain size of ∼15 μm, with a surface pressure of 80 Pa.

Figure 4.

The profiles of temperature, bulk density and conductive component of thermal conductivity at the CE-4 landing site with a surface pressure of 80 Pa. (a) The minimum, average and maximum temperature profile from the surface to a depth of 1 m. (b) The bulk density profile from the surface to a depth of 1 m, corresponding to the minimum, average and maximum temperatures in Fig. 4a. (c) The conductive component of the thermal conductivity profile from the surface to a depth of 1 m, corresponding to the minimum, average and maximum temperatures in Fig. 4a.

Figure 5.

The profiles of bulk density and conductive component of thermal conductivity at the CE-4 landing site without surface pressure. (a) The bulk density profile from the surface to a depth of 1 m, corresponding to the minimum, average and maximum temperatures in Fig. 4a. (b) The conductive component of thermal conductivity from the surface to a depth of 1 m, corresponding to the minimum, average and maximum temperatures in Fig. 4a.