The Earth’s atmosphere contains helium (He), but only in trace quantities. As a noble gas, helium is chemically inert and does not readily form compounds with other elements. Its lightness and non-reactivity govern its presence and ultimate fate in our atmosphere. Although continuously fed from the Earth’s interior, its unique physical properties mean it cannot be held permanently by the planet’s gravity.
Helium’s Place in Earth’s Atmosphere
Helium is a minor constituent of the lower atmosphere, accounting for approximately 5.2 parts per million (ppm) by volume. This quantity is so small that it is often measured in parts per billion, demonstrating its status as a trace gas. Unlike the heavier, well-mixed gases, helium’s distribution changes dramatically with altitude.
Because of its extremely low atomic mass, helium tends to diffuse upward. It progressively becomes a more significant component the higher one travels. At an altitude of around 800 kilometers, helium atoms become the most abundant neutral gas. This concentration occurs primarily in the thermosphere and the exosphere, which is the boundary layer merging with interplanetary space.
The Terrestrial Sources of Atmospheric Helium
The helium that replenishes the atmosphere does not originate from space but is generated deep within the Earth’s crust and mantle. The primary process responsible for the creation of new atmospheric helium is the natural radioactive decay of heavy elements. Elements such as uranium and thorium are unstable and break down over immense timescales.
During a type of decay known as alpha decay, the nucleus of the radioactive element ejects an alpha particle. This alpha particle is physically identical to the nucleus of a helium-4 atom, consisting of two protons and two neutrons. Once the particle captures two free electrons from its surroundings, it becomes a neutral helium atom.
This newly formed helium gas is extremely mobile due to its small atomic size and low mass. It migrates through rock fractures and porous layers within the crust. Eventually, this gas is released into the atmosphere through various geological conduits, including volcanic vents, tectonic fissures, and alongside natural gas deposits tapped by human activity. This constant geological release creates an ongoing supply.
The Unstoppable Escape of Helium
Despite the continuous generation of new helium from the Earth’s interior, the planet cannot retain the gas indefinitely. This inability to hold onto helium is governed by a physical process known as thermal escape, or Jeans escape. This mechanism is particularly effective against the lightest elements, like hydrogen and helium, which are the two least massive elements.
Thermal escape is dependent on the speed of the gas atoms in the upper atmosphere. In the high-altitude, low-density exosphere, individual atoms are far enough apart that they can potentially travel long distances without colliding with another particle. The temperature in this region is extremely high, causing the light helium atoms to move at very high average velocities.
Earth’s gravity holds the atmosphere in place, but any particle moving faster than a specific velocity, known as the escape velocity, will overcome the gravitational pull and fly off into space. Because helium atoms are so light, a significant fraction of them are moving fast enough to reach or exceed this escape velocity, especially in the hot exosphere. This allows them to leak away from Earth in a constant, irreversible stream.
Heavier gases like nitrogen and oxygen atoms move much slower at the same temperature and therefore rarely achieve escape velocity, remaining gravitationally bound to the planet. The continuous loss of helium, which is not offset by new supply on human timescales, means that atmospheric helium is considered a non-renewable resource. This unique physical dynamic ensures that helium is perpetually escaping into the cosmos, making its presence in our atmosphere a fleeting state of equilibrium between geological creation and atmospheric loss.