 Thermal energy causes some of the molecules at the outer edge of the atmosphere to increase their velocity to the point where they can escape from Earth 's gravity . This causes a slow but steady leakage of the atmosphere into space . Because unfixed hydrogen has a low molecular mass , it can achieve escape velocity more readily and it leaks into outer space at a greater rate than other gases . The leakage of hydrogen into space contributes to the shifting of Earth 's atmosphere and surface from an initially reducing state to its current oxidizing one . Photosynthesis provided a source of free oxygen , but the loss of reducing agents such as hydrogen is thought to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere . Hence the ability of hydrogen to escape from the atmosphere may have influenced the nature of life that developed on Earth . In the current , oxygen @-@ rich atmosphere most hydrogen is converted into water before it has an opportunity to escape . Instead , most of the hydrogen loss comes from the destruction of methane in the upper atmosphere . 
