Oxygen is found throughout the universe. It is formed in the nuclear reactions that take place in stars. Much of the universe's oxygen is contained in water.
Many forms of life on Earth depend on oxygen, and mammals need a significant quantity of it. Atmospheric oxygen primarily consists of two oxygen molecules paired together, which is the form mammals need to breathe. After life began to arise on Earth, little oxygen existed. Early life forms would create it by breaking down carbon dioxide. After the concentration of oxygen in the atmosphere grew, more advanced forms of life began to flourish
However, much of the universe's oxygen is combined with other molecules. One of the most common, and important, of these molecules is water, which consists of two hydrogen atoms bonded to a larger oxygen atom. While water might seem rare on other planets, it is actually fairly common. Comets are covered with water, Neptune contains a significant amount of water. The Jovian moon Europa is covered with ice, and experts believe that it has a large subsurface sea of liquid water. Since rockets depend on oxygen, some futurists believe that future space travelers will use the oxygen and hydrogen contained in it to make fuel after they have left Earth.