The mesosphere is a relatively unexplored layer of Earth’s atmosphere, positioned directly above the stratosphere and below the thermosphere. It extends from roughly 50 to 85 kilometers above the planet’s surface. It is defined by a dramatic drop in temperature as altitude increases, making its uppermost boundary, the mesopause, the coldest naturally occurring place on Earth. Temperatures in this region can plummet below -120 degrees Celsius.
The Role of Meteors and Cosmic Dust
The most frequent objects encountered in the mesosphere are extraterrestrial: incoming meteors and the vaporized residue they leave behind. This layer acts as Earth’s primary shield, where most small space rocks (meteoroids) are incinerated long before they reach the ground. The intense heat is generated not from the thin air itself, but from the rapid compression of atmospheric gases ahead of the fast-moving space debris.
As meteoroids, which can travel at speeds between 11 and 72 kilometers per second, plunge into the mesosphere, they experience a process called ablation. Ablation is the destructive combination of melting, vaporization, and fragmentation caused by friction and collisions with the increasing density of air molecules. The bright streaks of light seen across the night sky are not the meteoroid itself, but rather the glowing, ionized gas trail left by the vaporizing material.
The continuous bombardment and ablation of these meteoroids deposit an estimated 40,000 tons of extraterrestrial material into the atmosphere each year. This material condenses into microscopic particles known as meteoric smoke or cosmic dust. These nanometer-sized particles of iron, magnesium, and other elements become a permanent population of solid objects within the mesosphere, acting as seeds for other phenomena in the region.
Noctilucent Clouds (NLCs)
One of the most striking objects in the mesosphere are noctilucent clouds (NLCs), the highest clouds in the atmosphere. Also called Polar Mesospheric Clouds (PMCs), these clouds form near the mesopause at altitudes between 76 and 85 kilometers. They possess a unique silver-blue hue, visible only at twilight when the sun is below the horizon but still illuminating the high-altitude ice crystals.
The formation of NLCs depends on the mesosphere reaching its extreme minimum temperature, which occurs during the summer months at high latitudes (typically between 50° and 70° north and south). These clouds are composed of minute ice crystals (about 0.1 microns in diameter), which precipitate onto existing meteoric smoke particles. The cosmic dust acts as the condensation nuclei for sparse water vapor to freeze in the frigid, dry environment.
The existence of these clouds requires temperatures to drop below an approximate frost point of -125 degrees Celsius. NLCs only become visible when atmospheric circulation during the summer pole creates the necessary conditions for this water ice to form. Their increasingly frequent observation has drawn scientific interest regarding their possible connection to changes in atmospheric composition.
Scientific Instruments and Temporary Visitors
The mesosphere presents a unique challenge for atmospheric scientists because it is situated in a region unreachable by most standard observational tools. Standard weather balloons cannot ascend beyond the stratosphere (peaking around 40 kilometers), while orbital satellites must maintain a minimum altitude of about 120 kilometers to avoid rapid atmospheric drag. This gap means the mesosphere is the least-understood layer of the atmosphere.
The primary man-made objects that deliberately enter this layer are scientific sounding rockets. These sub-orbital vehicles are launched directly through the mesosphere, carrying instruments to take brief, in-situ measurements of its properties. A typical flight may only provide five to twenty minutes of data collection time before the payload returns to Earth. These missions are essential for directly sampling the temperature, density, and wind structure of this region.
Invisible Atmospheric Components
Beyond solid objects like cosmic dust and ice crystals, the mesosphere contains layers of invisible chemical species resulting from continuous meteor ablation. The vaporization process releases metal atoms (such as sodium, iron, potassium, and magnesium), which collect in thin, global layers between 80 and 105 kilometers. These layers of free metal atoms are regularly studied by ground-based instruments.
The upper mesosphere partially overlaps with the lowest portion of the ionosphere, known as the D-region. This region contains a concentration of charged particles, or plasma, created primarily through the photoionization of nitric oxide molecules by solar Lyman-alpha radiation. The ionization is weak and quickly disappears after sunset, making the D-region transient.
A final phenomenon is airglow, a faint, diffuse light continually emitted by the Earth’s atmosphere. This light is a form of chemiluminescence, produced when atoms and molecules (including sodium atoms) undergo chemical reactions and return to a lower energy state. Airglow defines the chemical activity of the mesosphere at night.