The atmosphere of Earth does not end abruptly at any particular altitude. Instead, it gradually fades into the surrounding interplanetary medium. The outermost layer where this fading happens is called the exosphere. It begins at about 500-600 kilometres altitude (at the "exobase," the top of the thermosphere) and extends up to 10,000 kilometres or more — to the altitude at which the atmospheric particles become indistinguishable from the surrounding solar wind. In the exosphere, the gas is so thin that individual atoms can travel for long distances without colliding with each other. They follow ballistic trajectories — long arcs influenced by Earth's gravity but not by collisions with other gas molecules. Some atoms with high enough speeds escape Earth's gravity entirely and disappear into space. The exosphere is therefore the boundary between Earth's atmosphere and the rest of the universe — a transitional region with properties unlike anywhere else in the atmospheric system.
The Definition
The exosphere is defined by a particular kinetic property: it is the region where the mean free path of atmospheric particles (the average distance traveled between collisions) is longer than the local scale height of the atmosphere. In simpler terms: in the exosphere, particles are more likely to escape gravitationally than to collide with another particle. The exobase — the bottom of the exosphere — is the altitude at which this transition occurs. At the exobase, the atmospheric density is about 10⁷-10⁸ particles per cubic centimetre. By comparison, sea-level density is about 10¹⁹ particles per cubic centimetre — a difference of more than 11 orders of magnitude. The exobase altitude varies with solar activity, ranging from about 500 km during solar minimum to 1,000 km during solar maximum.
The Composition
The exosphere has a distinctive composition, dominated by the lightest atmospheric species. Atomic hydrogen is the most abundant atom by far in the upper exosphere. Helium is the second most abundant. Atomic oxygen is present at the lower altitudes but becomes proportionally less important higher up. Heavier species (nitrogen, molecular oxygen, etc.) are essentially absent from the exosphere because they have settled out to lower altitudes due to gravitational stratification. The dominance of light atoms is a consequence of escape physics: light atoms have higher average speeds at any given temperature and are therefore more likely to reach escape velocity at the exobase. Earth's gravity holds onto the heavier gases (nitrogen, oxygen, argon, water vapor) at lower altitudes, but lets the lighter gases (hydrogen, helium) leak away into space.
Atmospheric Escape
Some atoms in the exosphere are moving fast enough to escape Earth's gravity entirely. The escape velocity at the exobase is about 11 kilometres per second. Atoms moving faster than this in an outward direction will leave the Earth-atmosphere system entirely and not return. The mechanism of escape is called Jeans escape (after the British physicist James Jeans, who described it in 1916). The Maxwell-Boltzmann distribution of speeds at any temperature has a "high-velocity tail" extending to very fast speeds. For each gas, some small fraction of particles will always be in this tail with velocities above escape velocity. The lighter the gas and the higher the temperature, the larger this fraction. Earth loses about 3 kilograms per second of hydrogen and 50 grams per second of helium through Jeans escape — modest rates but cumulative over geological time. Earth has lost most of its primordial hydrogen and helium over the past 4.5 billion years through this and related mechanisms.
The Geocorona
The exosphere of Earth has a particularly visible signature in ultraviolet light: the geocorona. Hydrogen atoms in the exosphere absorb and re-emit solar UV at the Lyman-alpha wavelength (121.6 nm), producing a faint glow that extends out to enormous distances from Earth. NASA's SOHO solar observatory has imaged the geocorona extending over 600,000 kilometres from Earth — beyond the Moon's orbit. This means, technically, that the Moon orbits inside Earth's extended atmosphere. The total amount of gas at these distances is tiny — just a few atoms per cubic centimetre — but it is detectable because of the resonant scattering of solar UV. The Apollo 16 mission in 1972 captured the first far-UV photographs of the geocorona from the Moon.
Hydrogen Loss and Mars
The continuous loss of hydrogen from Earth's exosphere has been ongoing throughout Earth's history. Most of this hydrogen comes from water vapour broken apart by UV in the upper atmosphere. Because hydrogen escapes preferentially while oxygen stays behind, this process oxidises the atmosphere over time — it has been one of the major mechanisms producing free oxygen on Earth (the other being photosynthesis). Mars provides a counterpoint. Mars has a similar but more severe hydrogen escape process. Without a strong magnetic field to deflect the solar wind, Mars has lost much of its early atmosphere — including most of its water — to space. Mars' current low atmospheric pressure (less than 1 percent of Earth's) is partly the result of billions of years of atmospheric escape, mostly through processes similar to those operating in Earth's exosphere today but more efficient because of Mars' lower gravity and weaker magnetic field.
Non-Thermal Escape
Jeans escape is not the only escape mechanism. "Non-thermal" escape processes include: charge exchange (an atom acquires a positive charge from a passing ion and is then accelerated by electric fields out of the atmosphere); photochemical escape (a molecule is split by UV into atoms with high kinetic energies, some of which exceed escape velocity); and atmospheric sputtering (high-energy particles from the solar wind directly knock atmospheric particles out of the planet's gravitational well). These non-thermal processes can be much more efficient than Jeans escape, particularly for heavier species. On planets without strong magnetic fields, such as Mars and Venus, non-thermal escape has played the dominant role in atmospheric loss over geological time.
The Interplanetary Boundary
At some altitude, the exosphere fades into the surrounding interplanetary medium — primarily the solar wind streaming outward from the Sun. The boundary is not sharp. The solar wind has a density of about 5 particles per cubic centimetre at Earth's distance from the Sun, similar in magnitude to the very low densities of the outer exosphere. The atmospheric particles slow with distance from Earth (due to gravitational pull), while the solar wind particles move at hundreds of kilometres per second. The transition zone where atmospheric particles still bound to Earth gradually become indistinguishable from the unbound solar wind constitutes the effective boundary between Earth's atmosphere and the broader heliosphere.
Satellites in the Exosphere
Many satellites operate in or above the exosphere. Geostationary satellites (at 36,000 km altitude) are well within the exosphere. Many science satellites studying the magnetosphere operate at exospheric altitudes. The relative absence of atmospheric drag at these altitudes is what makes long-duration orbits possible. By contrast, low Earth orbit satellites at 300-600 km altitude are still in the upper thermosphere or lower exosphere and experience measurable drag. The International Space Station at 400 km altitude experiences enough drag to require monthly reboosts. The Hubble Space Telescope at 540 km altitude similarly experiences gradual orbit decay (it has been reboosted several times by Space Shuttle visits, though the orbit will continue to decay until eventual atmospheric re-entry).
Space Weather Effects
The exosphere responds to solar activity, but less dramatically than the thermosphere below. During major solar events, the upper thermosphere can heat substantially and expand into the lower exosphere, temporarily raising the exobase altitude. This affects satellite drag at altitudes near the exobase. The exosphere itself does not warm much in the conventional sense (it has too few particles to have meaningful thermal mass), but the boundary between the thermosphere and exosphere does move and the composition of the lower exosphere can change in response to space weather. Atmospheric loss rates can also vary substantially with solar activity, with hydrogen escape rates increasing during periods of high UV output.
The Long View
The exosphere is in some ways the boundary of Earth — the place where our atmosphere finally gives way to the universe beyond. The slow, continuous leak of gas from the exosphere into space is one of the long-term processes shaping the planet. Earth has held onto most of its atmosphere over 4.5 billion years partly because of its mass (enough gravity to retain heavier gases) and partly because of its magnetic field (deflecting the solar wind that would otherwise strip away the atmosphere). Mars, Venus, and the Moon have all suffered greater atmospheric losses to their own exospheric escape processes. As we look outward to other planets in our solar system and beyond, the dynamics of exospheres and atmospheric escape are central to understanding which planets can retain atmospheres long enough to be habitable. Earth's exosphere, faint and distant as it is, plays its role in the broader story of why our planet has remained livable while others have not.