** Progress in Earth and Planetary Science is the official journal of the Japan Geoscience Union, published in collaboration with its 50 society members.

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    • Progress in Earth and Planetary Science
    • Progress in Earth and Planetary Science
    • Progress in Earth and Planetary Science
    • Progress in Earth and Planetary Science
    • Progress in Earth and Planetary Science
    Progress in Earth and Planetary Science

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    Space and planetary sciences

    Effects of dust layers on thermal emission from airless bodies

    Biele J, Kührt E, Senshu H, Sakatani N, Ogawa K, Hamm M, Grott M, Okada T, Arai T

    Thermophysical models, Surfaces, Small bodies, Solar system, Thermal inertia, Dust cover, Layering.

    Top: apparent thermal inertia Γeff as a function of dust cover layer thickness δ for bedrock. Γeff has been calculated using various methods (see Methods/Experimental). Top and bottom horizontal dashed lines indicate Γ for the homogeneous rock and dust, respectively. Vertical dotted lines indicate dust layer thickness in units of the diurnal thermal skin depth (9.2 mm) of the dust. Data points are connected by straight lines to guide the eye. Error bars represent the effects of worst-case modeling errors as described in the main text. Bottom: same as top, but for dust-covered porous rock/coarse regolith.

    We have investigated the influence of thin thermally opaque dust layers on the thermal emission of rocks and regolith and determined the thermal response of these dust-covered surfaces to diurnal insolation cycles. Results are computed for Hayabusa2’s target asteroid (162173) Ryugu, which was observed by thermal infrared instruments on the orbiter and in situ. We show that even a very thin (10..100 μm) fine-grained porous dust layer with thermal inertia of 25 J m−2 K−1 s−1/2 can have a significant influence on surface temperatures and alter the apparent thermal inertia of the underlying material derived under the simplified assumption of a homogenous half space by more than 20%. The masking of the underlying material is complete at about 1 diurnal skin depth, corresponding to ~ 10 mm on Ryugu. Between 0.1 and 1 diurnal skin depths, we find a thermal lag smaller than what would be predicted for a surface consisting of dust only.

    If a dust cover were present on Ryugu, this should be clearly visible in the data returned by the orbiter’s thermal infrared imager (TIR) and the MASCOT lander’s radiometer (MARA), which observed a single boulder at the landing site. However, this appears not to be the case, and dust seems to play a minor role in the thermal emission from the asteroid.