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.

中文翻译：

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尘埃层对无气物体散热的影响

我们研究了热不透明薄尘层对岩石和长石的热辐射的影响，并确定了这些被灰尘覆盖的表面对日照周期的热响应。计算了Hayabusa2的目标小行星（162173）Ryugu的结果，这是通过热红外仪器在轨道上和就地观测到的。我们表明，即使是非常薄的（10..100μm）细粒多孔尘埃层，其热惯性也为25 J m ^-2  K ^-1  s ^{-1 / 1/2}可以对表面温度产生重大影响，并且可以将在均匀半空间的简化假设下得出的基础材料的表观热惯性更改超过20％。底层材料的遮盖在大约1昼夜的皮肤深度处完成，相当于Ryugu上的〜10 mm。在日趋肤深的0.1到1之间，我们发现热滞后小于仅由尘埃组成的表面所能预测的滞后，如果龙宫上有防尘罩，则应该在轨道器的热红外返回的数据中清晰可见成像仪（TIR）和MASCOT着陆器的辐射计（MARA），它们在着陆点观察到一块巨石。但是，情况似乎并非如此，粉尘似乎在小行星的热辐射中起着很小的作用。