Earth's atmosphere extends past the Moon
Earth does not simply end where space begins. Its outermost layer of hydrogen gas, known as the geocorona, glows under ultraviolet light and extends about 630,000 kilometers into space. That is nearly twice the distance to the Moon. In a literal sense, the Moon orbits right inside Earth's diffuse atmospheric envelope.
The Luminous Outer Boundary of Earth
Human perception of Earth's atmosphere is typically tied to the dense, breathable layer hugging the planetary surface. However, the true physical boundary where Earth's gaseous envelope tapers off into interplanetary space is far more expansive. The outermost region of the atmosphere, known as the exosphere, consists of extremely sparse neutral atoms that are gravitationally bound to the planet. Within this outer fringe lies a vast, glowing cloud of neutral hydrogen known as the geocorona.
The geocorona is not visible to the naked human eye because it radiates predominantly in the far-ultraviolet spectrum. When neutral hydrogen atoms in the exosphere interact with incoming solar radiation, they absorb and re-emit ultraviolet light through a process called resonant scattering. This interaction produces a faint, diffuse glow that envelopes the planet and stretches hundreds of thousands of kilometers across space, far beyond where conventional boundaries of the atmosphere are drawn.
Solar Radiation and the Mechanism of the Glow
The primary emission responsible for the geocorona is the Lyman-alpha line of hydrogen, a specific wavelength in the far-ultraviolet region at approximately 121.6 nanometers. Solar photons at this exact wavelength strike the neutral hydrogen atoms drifting in Earth's exosphere, exciting their electrons. As these electrons drop back to their ground state, they re-radiate Lyman-alpha photons in all directions, illuminating the tenuous cloud against the darkness of space.
Solar influence also shapes the physical structure of the geocorona. Radiation pressure exerted by sunlight pushes against the hydrogen atoms, creating a pronounced asymmetry between the side of Earth facing the Sun and the side facing away. On the day side, the hydrogen envelope is compressed closer to the planet, while on the night side, the atoms are swept outward, forming an extended tail that resembles the faint tail of a comet stretching away into deep space.