Martian Soil under Tension: Visualizing Thermal Creep Gas Flow by Diffusive Wave Spectroscopy

Due to the specific atmospheric pressure of a few mbar, Mars is the only planet in the Solar System where thermal creep gas flow occurs naturally within its soil [1,2,3,4,5,6]. As with most flows, this comes with pressure variations. Especially below the top layers of grains, an overpressure develops which sets the soil under tension and supports the lifting of grains from the ground [1,2]. In de Beule et al. 2015, the thickness of an ejected dust layer was measured to be on the order of 200 µm, though upward directed (lifting) forces might also be present further down. This is also in agreement to numerical calculations [2,7]. However, pressure varies on rather small distances of sub-mm which is not easily measurable in a granular bed directly, as small disturbances or changes in the setting already influence the gas flow. So any method of verification of a sub-soil overpressure seems desirable.

Here we use diffusive waves spectroscopy to study the subtle motion of grains within the soil. Illuminating the side of a model soil with a laser beam, scattering and interference lead to a speckle pattern. This pattern changes if particles slightly move. Therefore, a correlation of the patterns over time gives positions depending on the depth where grains move more or less, showing a stratified soil with respect to motion. Among other features, we find a first minimum of particle motion below the surface which shows the characteristic pressure dependence of thermal creep. Specifically, a characteristic maximum depth of about 2 mm is found at about 4 mbar. This depth is somewhat larger than found by de Beule et al. 2015 but in view of the different methods, our measurements are yet another verification that thermal creep sub-soil overpressure is very likely a general mechanism on Mars supporting particle lift [8].

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