Thermal moonquakes at the lunar south pole: New evidence from Chandrayaan-3 ILSA observations

Thermal moonquakes are a series of repetitive seismic signals exhibiting nearly identical waveform patterns and amplitudes that occur periodically with the lunar diurnal cycle. India’s Chandrayaan-3 mission, which successfully landed in the south polar region of the Moon, deployed the Instrument for Lunar Seismic Activity (ILSA) to record ground accelerations at the landing site (69.37°S, 32.32°E) between August 24, 2023, and September 4, 2024. The instrument also monitored local surface temperatures, revealing extreme variations ranging from –20 °C to +60 °C.

After preliminary data processing, distinct thermal moonquakes were identified. The objective of this study is to analyze their frequency-dependent characteristics and investigate temperature-driven signatures. Based on waveform morphology, the thermal moonquakes are classified into three types: impulsive, intermediate, and emergent. Among these, emergent events are natural and occur due to the extension and contraction of lunar rocks, whereas the impulsive and intermediate events are caused by rover movement and other experiments conducted during the mission.

An additional focus of this research is to estimate the source locations of the thermal moonquakes using a chi-squared iterative single-station event-location algorithm. Assuming that seismic energy propagates along a one-dimensional path through a near-surface velocity model, we perform a grid search over latitude and longitude to identify the most probable source regions. Our results suggest that natural thermal moonquakes may originate from thermally induced stresses caused by large diurnal temperature variations in the lunar regolith, which reduce rock elasticity and lead to cracking and micro-fracturing.

The lunar south polar region remains one of the most intriguing yet least explored areas on the Moon. This study provides new insights into its near-surface mechanical behavior, offering a significant contribution toward understanding thermal stress-induced seismicity and the geophysical environment of the lunar south pole.

Keywords: Thermal moonquakes; Chandrayaan-3; ILSA; Lunar south pole; Thermal stress-induced seismicity; Single-station event location; Lunar regolith; Diurnal temperature variation.