The mesosphere is the third major layer of Earth’s atmosphere, situated between the stratosphere below and the thermosphere above. This atmospheric region extends approximately from 50 kilometers up to about 85 kilometers above the planet’s surface. It is defined by a characteristic temperature profile where the air temperature sharply decreases with increasing altitude, a trend that culminates in the mesopause, the coldest point in the entire atmosphere.
This atmospheric layer is notoriously difficult to study directly. It exists at an altitude too high for weather balloons and conventional aircraft to reach. Conversely, the mesosphere is too low for satellites to maintain a stable orbit due to atmospheric drag. These physical constraints mean that the objects found within the mesosphere are either transient celestial bodies, unique atmospheric formations, or fleeting scientific instruments.
Celestial Visitors: The Source of Shooting Stars
The most common and dramatic objects encountered in the mesosphere are extraterrestrial fragments that create the visual phenomenon known as a shooting star. These space rocks, called meteoroids, streak into Earth’s atmosphere at tremendous speeds, typically ranging from 11 to 72 kilometers per second. Upon entering the mesosphere, they encounter enough residual air density to generate intense frictional heating.
The immense heat causes the surface material of the meteoroid to melt and vaporize in a process called ablation. The bright streak of light visible from the ground, the meteor, is not the space rock itself but the glowing, superheated air and the vaporized material trailing behind it. Most smaller meteoroids entirely fragment and turn into vapor in this layer, effectively shielding the Earth’s surface from constant bombardment. The peak rate of this ablation process often occurs near the mesopause, around 90 kilometers in altitude.
The vaporization of these celestial visitors deposits a significant amount of material into the mesosphere, primarily in the form of cosmic dust and metallic ions. This fine material, often termed “meteoric smoke,” consists of tiny particles that remain suspended in the atmosphere. The ablated vapor includes elements like iron, magnesium, and sodium, which form layers of neutral metal atoms and ions that persist for a time in the upper atmosphere.
The Highest Clouds: Noctilucent Ice Formations
A distinct and visible natural object found exclusively in this layer is the Noctilucent Cloud (NLC), also referred to as Polar Mesospheric Clouds (PMCs). These are the highest clouds in Earth’s atmosphere, forming near the mesopause at an altitude of about 76 to 85 kilometers. They are composed of exceedingly tiny water ice crystals.
The formation of these unique clouds requires two conditions: the presence of water vapor and the atmosphere’s coldest temperatures. Temperatures must drop below approximately -120°C for the ice crystals to begin forming. The water ice crystals condense around minute dust particles, many of which are believed to be the meteoric smoke left behind by the vaporizing space rocks.
NLCs are too faint to be seen during the day and are only visible from the ground during twilight hours, primarily in the summer months at high latitudes. Their spectacular appearance is due to their high altitude, allowing them to catch sunlight from below the horizon after the ground is already in darkness. This illumination causes them to glow with a wispy, ethereal, and often electric blue or silvery sheen.
Instruments and Trace Particles
Since the mesosphere is inaccessible to long-term observation platforms, the primary man-made objects found within it are specialized research vehicles. These are suborbital sounding rockets, designed to briefly travel into and through the layer before falling back to Earth. These rockets carry scientific payloads to take direct measurements of atmospheric properties like temperature, density, and wind structure. They offer a temporary way to collect data that cannot be obtained by ground-based or space-based instruments.
The mesosphere also acts as a filter for the smallest natural and artificial particles. Micrometeorites, which are too small to cause a visible light streak or significant ablation, slowly filter down through the layer. This constant rain of cosmic dust contributes to the meteoric smoke that helps nucleate the noctilucent clouds. The mesosphere also contains fragments of man-made space debris, though the low altitude means any larger objects are quickly dragged down and vaporized due to the residual atmosphere.