Sentence:  Steady <WSD>radiation</WSD> which was presumably of thermal origin was observed from Venus at 3.15 and 9.4 cm , and from Mars and Jupiter at 3.15 cm in 1956 ( Mayer , McCullough , and Sloanaker , 1958 a , b , c ) , and from Saturn at 3.75 cm in 1957 ( Drake and Ewen , 1958 ) . 
Meaning of the word marked with <WSD> tags:  the act of spreading outward from a central source