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 loss 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.
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 loss 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.