The mesosphere is the third major layer of Earth’s atmosphere, positioned directly above the stratosphere and below the thermosphere. This region acts as a transitional zone where atmospheric characteristics shift dramatically from conditions found closer to the surface. It is characterized by extreme cold and a rapidly thinning gaseous environment. It is a region of substantial scientific interest.
Location and Boundaries
The mesosphere begins at the stratopause, the boundary at the top of the stratosphere, typically around 50 kilometers (31 miles) above the Earth’s surface. The layer extends upward for about 35 kilometers, terminating at the mesopause. This upper boundary, found between 80 and 85 kilometers (50 to 53 miles) in altitude, is a distinct thermal boundary separating the mesosphere from the much hotter thermosphere. Together with the stratosphere, the mesosphere is sometimes collectively referred to as the “middle atmosphere.”
Chemical Composition
The mesosphere is part of the homosphere, meaning it contains the same primary gases found in the air we breathe. Constant turbulence and mixing keep the ratios of major gases uniform, with Nitrogen (N2) at about 78% and Oxygen (O2) at 21%. Despite this stable ratio, the air is extremely thin, and the total number of molecules decreases rapidly with altitude. Trace gases like ozone, which are concentrated in the stratosphere below, diminish quickly here.
The chemical environment is unique due to the constant influx of extraterrestrial material, which vaporizes upon entry. This ablation process introduces trace metallic elements, such as iron (Fe), magnesium (Mg), and sodium (Na), which originate from incoming meteors. These metal atoms form meteoric smoke particles that circulate within the layer, adding a distinct chemical signature.
Thermal Structure and Density
The thermal structure is defined by a rapid and continuous drop in temperature with increasing altitude. Lacking the strong solar heating source found in the stratosphere below, where ozone absorbs ultraviolet radiation, the mesosphere is the coldest region in the atmosphere. Temperatures at the mesopause regularly plummet to around -90°C (-130°F).
Air density also decreases sharply, making the air exceptionally sparse. Air pressure at the top of the layer is less than one-millionth of the pressure at sea level. Approximately 99.9% of the total mass of the Earth’s atmosphere lies below the mesopause. Although molecules are spread far apart, the remaining gaseous medium is still dense enough to create friction and influence objects passing through it.
Defining Features
The mesosphere is the atmospheric layer where most incoming space debris meets its end. As meteoroids enter the top of this layer, the friction generated by the relatively thin but high-speed air causes them to heat up and burn, creating the visible streaks known as “shooting stars.” This phenomenon makes the mesosphere a natural shield for the planet’s surface.
Another distinguishing feature is the formation of Noctilucent Clouds (NLCs), also called polar mesospheric clouds. These are the highest clouds in the atmosphere, composed of tiny ice crystals that form at the extremely cold mesopause boundary during summer months in the polar regions. NLCs reflect sunlight long after sunset, giving them a ghostly, glowing appearance against the dark sky.