Sentence:  The results of present observations of the thermal radio emission of the moon are consistent with the very low thermal conductivity of the <WSD>surface</WSD> layer which was derived from the variation in the infrared emission during eclipses ( e.g. , Garstung , 1958 ) . 
Meaning of the word marked with <WSD> tags:  the outer boundary of an artifact or a material layer constituting or resembling such a boundary