The Rosenberg's were convicted of giving nuclear secrets to the Soviets. That is a clearcut case of aiding and abetting our nation's enemies. Scooter Libby was sentenced to 30 months for outing a covert agent (sentence commuted by Bush). John Kiriakou similarly received a 30 month sentence for leaking classified info as well as outing a covert agent. Snowden leaked a load of classified data, but did so to journalists (as with Libby and Kiriakou) and did not out any agents or put any Americans in harms way. It seems likely that he would not come close to a capital case and would probably receive less than the 30 months imposed on leakers who outed agents.