Number 217 (Story #2), March 10, 1995 by Phillip F. Schewe and Ben Stein EUROPA HAS AN OXYGEN ATMOSPHERE . Roughly the size of Earth's moon, Jupiter's moon Europa has been known for some time to possess exposed surface water ice. New observations by the Hubble Space Telescope (at ultraviolet wavelengths) show that Europa also has atmospheric molecular oxygen (O2), but only at a pressure of about 10**-11 that of Earth's sea-level atmosphere. The Hubble astronomers believe that the chief source of the gaseous oxygen is a process in which water molecules are broken up by incoming charged particles from Jupiter's magnetosphere. Last year oxygen was also found on the Jovian moon Ganymede, but only in surface ice form and not in gaseous form. (D.T. Hall et al., Nature, 23 February 1995.)