Long-lived lunar volcanism sustained by precession-driven core-mantle friction.

Core-mantle friction induced by the precession of the Moon's spin axis is a strong heat source in the deep lunar mantle during the early phase of a satellite's evolution, but its influence on the long-term thermal evolution still remains poorly explored. Using a one-dimensional thermal evolution model, we show that core-mantle friction can sustain global-scale partial melting in the upper lunar mantle until ∼3.1 Ga, thus accounting for the intense volcanic activity on the Moon before ∼3.0 Ga. Besides, core-mantle friction tends to suppress the secular cooling of the lunar core and is unlikely to be an energy source for the long-lived lunar core dynamo. Our model also favours the transition of the Cassini state before the end of the lunar magma ocean phase (∼4.2 Ga), which implies a decreasing lunar obliquity over time after the solidification of the lunar magma ocean. Such a trend of lunar obliquity evolution may allow volcanically released water to be buried in the lunar regolith of the polar regions. As a consequence, local water ice could be more abundant than previously thought when considering only its accumulation caused by solar wind and comet spreading.

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

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.