From Anomaly to Detectability: Roof Thickness Threshold for Remote Detection of Lava Tubes Using Thermal Infrared Datasets

Lava tubes are key targets for planetary exploration due to their potential to preserve biosignatures and could serve as human habitats on the Moon. These caves form when lava flows solidify, leaving behind a tube-like void once the lava drains. Their stability is determined mainly by the thickness of the roof, a parameter that is challenging to estimate using current remote sensing methods, as visible imagery alone cannot discern the physical properties of the subsurface. Accurate characterization of roof thickness is crucial for future exploration efforts, as stable roofs are more likely to preserve potential biosignatures within the cave interior and provide safer environments for human exploration. Remote sensing is currently the primary method for studying lava tubes on other planetary bodies and in remote regions of Earth. Previous work has identified potential subsurface voids on the Moon and Mars using thermal infrared (TIR) imaging by analyzing the area's thermal inertia and temperature differences between lava tubes and surrounding terrain. Thermal inertia is an intrinsic material property that determines the material's resistance to changes in temperature and is affected by subsurface voids, which disrupt heat transfer. This study aims to constrain the maximum roof thickness that a lava tube can have to be detected with TIR remote sensing data, which can help estimate the roof thickness of lava tubes on Earth and other planetary bodies.

We present field, remote sensing, and numerical results of the thermophysical properties of lava tubes on Earth at two sites: Pisgah Crater, California, and Tabernacle Hill, Utah, with a total of 38 skylights and lava tube entrances surveyed. Satellite TIR images were acquired and compared with in-situ drone-based TIR images, both of which were used to calculate the thermal inertia of the area. To validate these observations, we utilized numerical heat transfer models to simulate thermal diffusion through basaltic roofs of varying thicknesses. The known lava tube locations were mapped, and their thermal inertia value was averaged to calculate the thermal inertia difference from the rest of the void-free terrain. These values were compared with in-situ measurements of roof thickness at each cave entrance.

Our study reveals a distinct decrease in the thermal difference from the background with increasing roof thickness, suggesting that thicker roofs behave more like the surrounding terrain. The observed data suggest that a roof thickness of at most 2 meters is required for potential detection in an Earth environment. This research helps establish a critical detection threshold, where TIR anomalies may be diagnostic of thin, potentially unstable roofs, while roofs thicker than 2 meters are likely stable but thermally indistinguishable from the background. Thermal anomalies are more distinct than visible data alone for identifying skylights in rough terrains, but larger and more stable roofs may be more challenging to detect than smaller roofs. This research reinforces the utility of TIR in identifying skylights in rough terrains. It establishes an essential constraint for the detectability and stability of lava tubes, providing a valuable framework for planetary remote sensing and future mission planning.