The thermal constraint behind methane emissions into the Martian atmosphere from Earth-based studies

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A significant observation at Mars' Gale Crater by Curiosity's Tunable Laser Spectrometer involves repeatable methane fluctuations with distinct seasonal and sub-diurnal variability. After a decade of data, these methane emissions clearly require robust geophysical explanations rooted in thermodynamics. On Earth, extensive field and laboratory research demonstrated that subsurface gas flows, particularly radon-222, are primarily driven by surface temperature gradients. This thermally induced transport exhibits exponential dependence, verified through long-term field measurements over years, and also in controlled laboratory conditions where oscillating vertical gas flow closely matches surface heating cycles, from the natural one per day to an experimental one per nine days. The field monitoring has shown that radon gas flows downward throughout all daylight hours within the bedrock to a measured depth of 100 m and responds inversely to atmospheric temperatures at night, which fall below subsurface temperatures, creating an inverted surface temperature gradient that enforces nocturnal exhalation upward . While gases on Earth's ground also respond linearly to semi-diurnal barometric pressure changes (barometric pumping), streaming within porous media, cracks, voids, or fractures between geological layers and structures, our experience indicated that such effects become negligible when the pressure gradient is less than 2 millibars. Specifically, on Mars, where barometric pressure is two orders of magnitude lower than Earth's, the resulting gradient is insufficient to drive significant gas transport, even through sand on Earth's surface. Therefore, temperature-gradient-driven gas migration, validated under terrestrial conditions, emerges as the primary mechanism likely responsible for the observed methane emissions on Mars. Plain language summary The detection of methane temporal variations at the surface of Gale crater on Mars, by the Curiosity rover, and the mechanism behind the temporal variability of methane near the surface, poses a challenge to understanding the process. On Earth, long-term laboratory and field measurements have confirmed that gases such as radon move through soil and rock primarily due to temperature differences between the surface and the geological media. This heat-driven movement creates a seasonal daily cycle in which gases flow downward during the daylight hours and return to the surface at night when the air is cooler than the ground. The daily downward temperature gradient at the surface is a strong driving force that constrains the exponential downward movement of radon up to depths of 100 m. At night, the hotter surface layer leads to upward nocturnal radon emissions (exhalation). Additionally, based on our experience, changes in the extremely low air pressure on Mars can not cause methane to be pumped from the ground. In light of our experience and understanding of underground gas flow, we propose that surface temperatures, driven by solar radiation rather than pressure changes, are the most likely cause of the methane variations detected on Mars.

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