High-resolution thermal profiles of lunar regolith over a southern high latitude location using <i>in situ</i> observations from ChaSTE/Chandrayaan-3 lander

ABSTRACT The first in situ measurements of thermal properties of lunar regolith at a high latitude location (69.36°S, 32.34°E) in the South Polar region were made by the Chandra’s Surface Thermophysical Experiment (ChaSTE) probe aboard India’s Chandrayaan-3 lander during 2023 August 24 to September 1. The probe with 10 closely spaced temperature sensors was inserted vertically into the regolith up to a depth of 10 cm to measure the lunar daytime temperature profiles. The vertical temperature gradient was independently measured by inserting the probe by 1 cm and the average value is 3.8$\pm$0.24 K cm−1 for depths ranging from 2 to 10 cm at $\sim$09:00 Lunar Local Time (lt). Sub-zero temperature (in $^{\circ }$C) exists within the 10 cm layer prior to $\sim$10 LT. The daytime temperature variations show a systematic phase delay with depth: the warmest temperature is attained at 12 LT near the surface while that is attained at 14:40 LT at 10 cm depth. The daytime temperature variations, the significant lapse rate changes with depth, and. the phase progression of the diurnal variation observed up to 10 cm depth, provide important insight into the influence of surface heat flux further deeper into the regolith layer as well as the thermal properties of the high latitude region. The daytime variations of the in situ measured temperature are affected by local topography, solar radiation reflected from the lander body, the infrared emission from the lander and lunar surface heat flux reflected by the lander, which are estimated in this study to be $\sim$40 per cent. This information is critical for planning future lander missions.