Radical Formation in the Coma From Photodissociation of Ice Grains

Long ago visual observations of comets suggested that there are jets in comets but it has only been recently that A'Hearn and his coworkers have proven that some of these jets are due to emission from the CN radical. Recent studies in our laboratory have shown that CN radicals can be ejected directly into the gas phase from the photolysis of frozen vapors if the parent molecule has been excited to a repulsive excited state. This later observation suggest that the jets that have been observed may be due to photodissociation of icy grains in the coma. In this presentation a theory of radical formation from icy grains will be presented. It will be shown that direct formation of free radicals in the coma is an effective way to produce radicals from icy grains in the coma. The model predicts that icy grains could produce from 6 to 800,000 OH radicals/s per grain depending upon whether the radius of the grain is 0.3 to 100 μm.