The Earth’s atmosphere is a complex, layered system, with the troposphere, stratosphere, mesosphere, thermosphere, and exosphere each playing a distinct role in maintaining the planet’s environment. The mesosphere, whose name is derived from the Greek word for “middle,” sits as the third major layer, bridging the gap between the lower atmosphere and the thin, space-like conditions of the upper atmosphere. This layer is often the least understood because its altitude makes it incredibly difficult to access and study directly.
Defining the Mesosphere
The mesosphere is situated directly above the stratosphere and below the thermosphere, generally spanning an altitude range from approximately 50 kilometers to 85 kilometers above the Earth’s surface. Its lower boundary is marked by the stratopause, where temperatures cease their increase with altitude. The upper limit is the mesopause, which is the point of minimum temperature in the entire atmosphere.
The physical characteristics of the mesosphere include extremely low air pressure and density, which decrease rapidly with increasing height. While the air here is far too thin to breathe or support conventional aircraft, it is still dense enough to interact significantly with objects entering from space. This layer represents the transition zone between the atmosphere we experience daily and the near-vacuum conditions of outer space.
The Role in Planetary Protection
The most significant function of the mesosphere is its role as Earth’s primary shield against incoming space debris. Millions of meteoroids, ranging from dust-sized particles to small rocks, enter the atmosphere every day, but very few ever reach the ground. The mesosphere is the first layer dense enough to cause significant friction with these high-speed objects.
As a meteoroid speeds through the mesosphere, often traveling between 11 and 72 kilometers per second, it collides with the molecules of nitrogen and oxygen gas. This intense friction causes the object’s surface to heat up rapidly, a process known as atmospheric ablation. The resulting heat causes the meteoroid to vaporize or disintegrate entirely, creating the bright streak of light we observe as a meteor or “shooting star.”
The mesosphere effectively filters out the vast majority of small space rocks and man-made space junk before they can impact the lower atmosphere or the planet’s surface. This constant burning up of debris prevents frequent, damaging impacts. The small amount of ablated material, called meteoric smoke particles, remains suspended in this layer, often serving as condensation nuclei for high-altitude clouds.
Extreme Temperatures and Atmospheric Structure
The mesosphere is distinguished by its dramatically cold thermal profile. Within this layer, temperature decreases sharply with increasing altitude, a trend that is the reverse of the layer immediately below it, the stratosphere. The temperature decline continues until it reaches the mesopause, where the coldest temperatures in the Earth’s atmosphere are found, often dropping as low as approximately -90°C.
This severe cooling trend results from the lack of significant solar radiation absorption mechanisms at this height. The warming trend in the stratosphere, for example, is caused by the absorption of ultraviolet radiation by ozone. Since the mesosphere contains very little ozone and is far removed from the Earth’s surface, it cools rapidly.
The low density means that gas molecules are too sparse to effectively absorb and retain heat from the limited solar energy that reaches this altitude. This extreme cold at the mesopause influences atmospheric circulation patterns that transfer momentum and energy throughout the middle atmosphere.
Studying Mesospheric Phenomena
The mesosphere is home to several unique and visually striking phenomena that scientists study to better understand this remote region.
Noctilucent Clouds (NLCs)
The most well-known phenomena are Noctilucent Clouds (NLCs), also called polar mesospheric clouds, which are the highest clouds in the atmosphere. These ethereal, silvery-blue clouds are visible during deep twilight. They form when water vapor freezes onto meteoric dust particles in the extremely cold mesopause.
Airglow
Airglow is a faint emission of light caused by chemical reactions between atmospheric atoms and molecules, primarily oxygen and hydroxyl radicals, that have been excited by solar radiation. Studying the intensity and temperature of airglow provides scientists with data on the composition and thermal structure of the mesosphere.
The mesosphere is challenging to study because it is too high for weather balloons and conventional aircraft, and too dense for satellites to maintain long-term orbits. Scientists rely on specialized methods, including ground-based instruments like radar and lasers, as well as suborbital sounding rockets. These rockets fly briefly through the layer to collect direct measurements of temperature, pressure, and composition.