Sentence:  Further , the corpuscular radiation , i.e. , the solar wind protons , must sputter away the <WSD>surface</WSD> atoms of the dust and cause a slow diminution in size , with a resultant increase in both the Poynting-Robertson effect and the ratio of the repulsive force to the gravitational force . 
Meaning of the word marked with <WSD> tags:  the extended two-dimensional outer boundary of a three-dimensional object