| Lunar regolith is a mix of rock fragments,mineral grains,and impact-generated glass deposited on the Moon’s surface.Its formation is a lengthy,complex process influenced by meteorite impacts,cosmic rays,and temperature fluctuations.The properties of lunar regolith are vital for space exploration and astronaut tasks.The tidal lock between Earth and the Moon allows only the nearside to be seen from Earth.China’s Chang’E-4（CE-4）mission is the first successful exploration of the lunar farside.CE-4 landed at the base of the Von Kármán crater in the Moon’s South Pole-Aitken Basin（SPA）farside（45.4446°S,177.5991°E）.The landing site’s surface regolith comes from Finsen impact crater ejecta（～3.0-3.1 Ga）.This study analyzed fresh concentric impact craters（diameter<250 m）in the landing area and found the lunar regolith thickness to be around 2.5-7.5 meters.Four thermistor-based temperature sensors were installed on the CE-4 lander’s metal rails,directly contacting lunar regolith.Measuring temperatures every 900seconds within 77-523 K,they have an accuracy of±0.3 K.The CE-4’s high-resolution,precise regolith temperature data provided the first in-situ measurements for the lunar farside,spanning a full day-night cycle.This data is vital for understanding the farside regolith’s thermal properties,formation,and spatial weathering processes.This study is the first to systematically examine temperature measurements and thermal properties of lunar regolith at the CE-4 landing site.The Apollo program sparked widespread interest in lunar regolith’s thermal properties,such as thermal conductivity,heat capacity,and density.Early measurements mainly relied on Apollo mission samples,but their reliability was altered by sample disturbance during interstellar transportation and Earth’s stronger gravitational influence.Compared to early laboratory experiments,numerical simulation methods allow for analysis without the returned lunar samples and do not alter the original depositional manner of the regolith.The innovative aspect of the study method presented in this thesis is expressing the density and thermal conductivity of lunar soil as a function of particle size.By retaining the grain size as the sole free parameter affecting the thermal properties,the computational process avoids the influence of parameter interdependence while taking into account the effects of temperature variation on each parameter.In the mathematical model of this study,we represent lunar regolith density and thermal conductivity as particle size functions.We describe regolith compaction by the lander’s metal rails in relation to pressure,considering the lander’s shadow shielding effect.These parameters are integrated into a temperature model using the heat conduction equation.We fit simulated nighttime surface temperatures to measured values and use the least squares method to determine optimal regolith particle size,resulting in temperature,density,and thermal conductivity profiles.Additionally,we compare and discuss our findings with Apollo and Chang’E-5 lunar sample measurements.The research results indicate that the optimal particle size of lunar regolith at the CE-4 landing site is 15μm,representing an average condition from the lunar surface to a depth of 0.5 m.This is similar in physical significance to the number-weighted average particle size.The depth of influence of radiation received by the lunar surface on subsurface temperatures is 0.5 m,providing a reference for the deployment depth of heat flow probes.The depth of influence of radiation received by the lunar surface is correlated with the duration of the day-night cycle.The average lunar regolith surface density at the CE-4 landing site is 717 kg/m3,with a range between 651 and 865 kg/m3.Without surface pressure,the bulk density is only 471 kg/m3,significantly lower than earlier measurements.The regolith density rapidly increases within the topmost 0.3 m layer and converges to 1838 kg/m3at a depth of 5 m,which is consistent with previous findings.The thermal conductivity of the regolith at the CE-4 landing site is approximately2.30×10-3 W/（m·K）,which decreases to 1.53×10-3 W/（m·K）without pressure,indicating an older region on the Moon.As the depth increases,the thermal conductivity rapidly rises within 0.3 m and eventually converges to 8.86×10-3 W/（m·K）at a depth of 1 m.These data are consistent with the local lunar regolith age of～3.0-3.1 Ga.These thermal properties offer invaluable"ground truth"for the lunar farside,which is essential for future global temperature data analysis.Due to its extremely low thermal conductivity properties,lunar regolith can serve as an insulating material for lunar base surfaces. |
PDF Full Text Request