DETECTION OF AN OXYGEN ATMOSPHERE ON JUPITER MOON EUROPA

EUROPA, the second large satellite out from Jupiter, is roughly the si ze of Earth's Moon, but unlike the Moon, it has water ice on its surfa ce(1), There have been Suggestions that an oxygen atmosphere should ac cumulate around such a body, through reactions which break up the wate r molecules and form molecular hydrogen and oxygen(2,3), The lighter H -2 molecules would escape from Europa relatively easily, leaving behin d an atmosphere rich in oxygen, Here we report the detection of atomic oxygen emission from Europa, which we interpret as being produced by the simultaneous dissociation and excitation of atmospheric O-2 by ele ctrons from Jupiter's magnetosphere, Europa's molecular oxygen atmosph ere is very tenuous, with a surface pressure about 10(-11) that of the Earth's atmosphere at sea level.