The Earth’s atmosphere is classified in several ways, including by its chemical makeup. A major division separates the atmosphere into two primary regions based on composition: the lower homosphere and the upper heterosphere. The homosphere is characterized by a nearly constant mixture of gases from the surface upward. The heterosphere represents the atmosphere’s outermost region, defined by its non-uniform composition where gaseous components begin to separate and layer.
Defining the Division: Uniform vs. Non-Uniform Composition
The atmosphere below the heterosphere is the homosphere, extending from the ground up through the mesosphere. Within the homosphere, strong atmospheric turbulence constantly mixes the gases, ensuring a consistent chemical composition throughout this lower region. Air in the homosphere is composed of approximately 78% nitrogen and 21% oxygen, with trace gases making up the remainder.
The heterosphere, in contrast, lacks this turbulent motion. Above a certain height, the air becomes so thin that molecular collisions are infrequent, and large-scale mixing ceases. This allows different gas types to arrange themselves according to their individual physical properties, resulting in a distinctly layered structure.
Altitude and the Turbopause Boundary
The heterosphere begins at the upper boundary of the homosphere, a transition zone known as the turbopause. This boundary represents the highest altitude where atmospheric turbulence remains an effective force for mixing gases. The turbopause is located around 80 to 100 kilometers (50 to 62 miles) above the Earth’s surface.
Below this altitude, the air is dense enough for winds to maintain mixing. The turbopause marks the shift to a fundamentally different physical environment, where the forces governing gas distribution change completely.
Gravitational Sorting and Molecular Diffusion
The heterosphere’s non-uniformity results from the shift from turbulent mixing to molecular diffusion. Above the turbopause, individual molecular movement dominates because molecular collisions are infrequent. Gravitational sorting then becomes the primary force determining gas distribution.
Each gas settles into its own layer based on its molecular weight, a process known as diffusive separation. Heavier molecules, such as molecular nitrogen (N2) and molecular oxygen (O2), are concentrated in the lower portions of the heterosphere.
As altitude increases, the concentration of these heavier gases drops rapidly, and progressively lighter species become dominant. Atomic oxygen (O) becomes increasingly abundant higher up. In the outermost reaches, the lightest elements—helium (He) and hydrogen (H)—are the predominant constituents, creating distinct shells of gas.
Temperature Layers Within the Heterosphere
The heterosphere is a compositional division that overlaps with two major thermal layers: the Thermosphere and the Exosphere. The Thermosphere, beginning around 80 to 100 kilometers, is characterized by a dramatic increase in temperature with altitude.
This temperature rise is due to the direct absorption of high-energy solar radiation, such as extreme ultraviolet light, by sparse oxygen and nitrogen atoms. Although the Thermosphere can reach temperatures over 1000 degrees Celsius, this measures the kinetic energy of the few particles present, not sensible heat. The Exosphere, the uppermost layer, is where the atmosphere gradually fades into the vacuum of space, with hydrogen being the last gas to escape Earth’s gravity.