Chang'e-6 lander reveals local hour-scale temporal variations in lunar surface water

Lunar surface water primarily originates from solar wind irradiation, with diurnal variations in its content observed by orbital and ground-based remote sensing. However, low temporal resolution and the lack of accurate in-situ surface observations have limited understanding of the magnitude of these variations. Here, we report analysis results of the in-situ infrared spectra at the same location across different local times acquired by the Chang'e-6 lander. The in-situ spectra were calibrated for thermal contributions based on laboratory measurements conducted on the returned lunar soils, showing clear OH/H₂O absorption signals. The OH/H₂O contents achieved from the in-situ spectra exhibit significant temporal variations, with a notable decrease of 37%-58% from ∼10:00 a.m. to ∼11:03 a.m. local time. The discovery confirms not only the diurnal water variations detected by remote sensing but also extends such variations to the hourly range. The OH/H₂O content decreasing rate is ∼8.4 ± 0.4 ppm/K in the high temperature range (349-358 K), faster than in the low temperature period (∼5.5 ppm/K for 309 K to 319 K). This indicates that temperature exerts a dominant control on the variation of OH/H₂O content. Hydrogen exists in various forms in lunar soil grains and solar wind flux over a lunar day is insufficient to replenish the depleted OH/H₂O. Therefore, the occurrence of OH/H₂O, the only form detectable by infrared spectroscopy, is highly temperature dependent. The new discoveries shed light on the distribution and recycling of water on the lunar surface.