text: Every element exists in space (exept for those humans have created) and oxygen is the third most abundant after hydrogen and helium. So there is ALOT of oxygen in the universe...\nMolecular oxygen is rare though. Empty space is a vacuum and hardly any matter can be found there let alone oxygen. (Stars like the sun have a significant amount of oxygen atoms in them.) Also oxygen is highly reactive so it forms compounds with other elements any chance it gets. Almost half the moon is oxygen only it is in the form of silicone and metallic oxides and silicates (SiO4- or simply rocks).\n\nFor molecular oxygen to exist something needs to be continously producing it. On earth life is doing the job. If we find molecular oxygen in the atmosphere of an alien planet it is a strong indicator that something strange, possibly life, is happening there. There are trace amounts of oxygen in the atmospheres of both venus and mars but that oxygen can be explained by photodisassociation of water molecules by interaction with the suns ultraviolet rays. Both venus and mars have lost (and are still losing) their water through this process where water vapor is split into hydrogen and oxygen atoms. The hydrogen reach escape velocity and is blasted off into space while the oxygen reacts with other atoms in the atmosphere or with the soil.\n\nWe have found traces of oxygen in interstellar molecular clouds by studying the light from stars that pass through them. With this technique we will soon be able to study the atmospheres of extrasolar planets.