Mesosphere
The mesosphere is the third highest layer of Earth's atmosphere, extending from an altitude of approximately 50 to 85 kilometers (31 to 53 miles) above the planet's surface, positioned between the stratosphere below and the thermosphere above.[1] This layer, whose name derives from the Greek word "mesos" meaning middle, features a thin density of gases that are well-mixed rather than stratified by molecular weight, with temperatures plummeting to as low as -90°C (-130°F) at its upper boundary, rendering it the coldest region of the atmosphere.[2] The mesosphere plays a crucial role in atmospheric dynamics by serving as the primary site where most incoming meteors incinerate upon entry due to frictional heating from collisions with residual air molecules, preventing larger impacts from reaching lower layers.[3] Key characteristics of the mesosphere include its decreasing temperature gradient with altitude, driven by the absence of significant solar heating sources like ozone absorption, which dominates in the underlying stratosphere.[4] Composed mainly of nitrogen and oxygen similar to lower layers but at far lower pressures—about 1 millibar at its base—the region's sparse air makes it inhospitable for aircraft or balloons, limiting direct study to sounding rockets and satellite remote sensing.[5] Notable phenomena within the mesosphere encompass the formation of noctilucent clouds, also known as polar mesospheric clouds, which appear as shimmering, ice-crystal formations at high latitudes during summer months due to extreme cold and water vapor condensation, often illuminated by sunlight from below the horizon.[6] The mesosphere's importance extends to broader atmospheric and climate processes, including the deposition of meteoric dust that influences ion chemistry and potential cloud nucleation, though its full interactions with global circulation remain under investigation.[7] Recent observations, such as those from NASA's Aeronomy of Ice in the Mesosphere (AIM) mission, have highlighted seasonal variations in these clouds, linking them to climate change signals like cooling mesopause temperatures.[8] Despite challenges in measurement, the layer's role in shielding Earth from space debris underscores its protective function in the planet's atmospheric system.[9]Definition and Structure
Altitude Range and Boundaries
The mesosphere extends approximately from 50 to 85 kilometers above sea level, marking the region between the stratosphere and the thermosphere in Earth's atmosphere.[3] This altitude range positions it as the least explored layer, accessible primarily through remote sensing and high-altitude instrumentation rather than direct human presence.[4] The lower boundary, known as the stratopause, occurs at about 50 kilometers, where the temperature reaches a maximum before decreasing into the mesosphere.[4] The upper boundary, the mesopause, is situated around 85 kilometers and serves as the cold demarcation separating the mesosphere from the warmer thermosphere above.[1] These pauses are defined by thermal inversions, with the mesopause representing the temperature minimum in the atmosphere.[10] The precise altitude of these boundaries varies with seasonal and latitudinal factors; for instance, the mesopause height typically ranges from 86 to 91 kilometers in the summer hemisphere and reaches about 100 kilometers in the winter hemisphere.[11] Such variations arise from dynamic influences like planetary waves and tidal forcing that modulate the thermal structure across latitudes.[12] The mesosphere's boundaries were established in the mid-20th century through pioneering rocket soundings and radar observations starting in the 1950s, which provided the first in situ data on upper atmospheric temperatures and winds. These measurements, conducted from sites like White Sands and Wallops Island, revealed the distinct thermal layering and confirmed the mesopause's existence as a persistent feature.[13] Solar activity further influences boundary positions, causing fluctuations of up to several kilometers in mesopause height; for example, responses range from -2.57 to 3.15 kilometers per 100 solar flux units during solar cycles.[12] This variability underscores the mesosphere's sensitivity to solar forcing, which can shift the layer's extent by modulating heating and circulation patterns.[14]Key Structural Features
The mesosphere is internally divided into the lower mesosphere, spanning approximately 50 to 70 km altitude, and the upper mesosphere, from about 70 to 85 km.[15] The lower mesosphere retains residual influences from the stratosphere, such as variations in ozone concentrations that contribute to cooling trends of approximately 0.5–2 K per decade (as of 2024), primarily driven by increasing carbon dioxide levels.[15][16] Recent observations indicate ongoing cooling and contraction of the mesosphere, with temperature decreases of up to 1–2 K per decade from 2002 to 2024, affecting layer density and height.[17] In contrast, the upper mesosphere transitions toward thermospheric conditions, where the D region of the ionosphere begins to form around 60–90 km due to initial ionization from solar X-rays and ultraviolet radiation.[18] A prominent structural feature in the upper mesosphere is the sodium layer, a concentration of neutral sodium atoms extending from roughly 80 to 100 km, with a peak density near 87–90 km.[15] This layer serves as a key tracer in airglow studies, particularly through observations of sodium D-line emissions at 589 nm, which reveal atmospheric dynamics and constituent variability in the mesopause region.[15] The mesosphere's structural stability arises primarily from radiative equilibrium, where heating from ozone and oxygen photodissociation balances cooling by carbon dioxide emission in the 15 μm band, resulting in relatively uniform temperature profiles that foster a homogeneous mixing zone.[19] This radiative balance minimizes convective instabilities, promoting well-mixed conditions for trace gases across altitudes.[19] The mesosphere plays a critical role in the vertical transport of minor constituents, such as meteoric metals and water vapor, from the lower atmosphere upward through mechanisms like gravity wave dissipation and residual meridional circulation.[15] These processes redistribute species like sodium and iron atoms, influencing the chemical and thermal structure extending into the lower thermosphere.[15]Physical Properties
Temperature Profile
The mesosphere is characterized by a significant temperature decrease with increasing altitude, ranging from approximately -15°C at the stratopause to between -90°C and -120°C at the mesopause, making the latter the coldest region in Earth's atmosphere.[4] This thermal profile arises primarily from radiative cooling dominated by infrared emissions from carbon dioxide (CO₂) and water vapor (H₂O), as these molecules efficiently radiate heat to space in the absence of substantial solar heating above the ozone-rich stratosphere.[20] The minimal absorption of ultraviolet radiation in this layer further contributes to the net cooling, establishing a radiative equilibrium that drives the overall temperature gradient.[19] Seasonal variations modulate this profile, with the summer mesopause typically cooler by 5–10 K than in winter, attributable to upwelling air masses that promote adiabatic expansion and enhanced cooling.[21] This upwelling is part of the broader meridional circulation in the mesosphere, leading to a more pronounced cold summer mesopause at higher latitudes. The radiative cooling process can be approximated using a Newtonian cooling formulation: \frac{dT}{dt} \approx -\frac{\Lambda}{c_p} (T - T_{eq}) where \frac{dT}{dt} is the temperature change rate, \Lambda represents the radiative cooling coefficient dependent on molecular concentrations and optical properties, c_p is the specific heat capacity at constant pressure, T is the local temperature, and T_{eq} is the equilibrium temperature dictated by radiative balance.[22] This equation captures the relaxation toward radiative equilibrium, with cooling timescales on the order of days in the mesosphere. The resulting low temperatures profoundly influence air density and vertical stability; colder conditions increase molecular density for a given pressure via the ideal gas law (\rho = P / (R T)), contracting the atmospheric scale height and concentrating mass closer to the stratopause.[23] Furthermore, the environmental lapse rate in the mesosphere—typically 2–3 K/km, subadiabatic relative to the dry adiabatic value of ~9.8 K/km—promotes static stability, inhibiting deep convection and favoring wave-driven mixing over buoyant overturning. These thermal constraints underpin the mesosphere's role in limiting vertical transport and maintaining its distinct dynamical regime.Chemical Composition
The mesosphere's chemical composition is primarily dominated by molecular nitrogen (N₂, approximately 78%) and molecular oxygen (O₂, approximately 21%), reflecting the well-mixed conditions below the turbopause near the mesopause, where turbulent eddies maintain homogeneity similar to the lower atmosphere. Trace constituents include argon (Ar, about 0.93%), water vapor (H₂O), and ozone (O₃), with ozone levels decreasing rapidly with altitude from the stratopause due to reduced production and increased photolysis in this layer.[24] Atomic oxygen (O) and nitric oxide (NO) are present as minor species, primarily resulting from the photodissociation of molecular oxygen and other molecules by ultraviolet solar radiation, with their concentrations peaking in the upper mesosphere around 80–90 km where dissociation rates intensify. These species play key roles in energy transfer and chemical cycling, with atomic oxygen serving as a major carrier of vibrational and electronic energy in the region.[24][25] Meteoric ablation contributes trace metals such as iron (Fe) and sodium (Na) to the mesosphere, as high-speed meteoroids vaporize upon entry between 80 and 110 km, injecting neutral metal atoms that form distinct layers observable via lidar through resonance fluorescence. These metal layers exhibit Gaussian vertical profiles, with sodium influx estimated at approximately 1.6 × 10⁴ atoms/s/cm² and iron at 1.0 × 10⁵ atoms/s/cm², highlighting the significant role of extraterrestrial material in the region's chemistry.[26][27] Key photochemical processes in the mesosphere include the dissociation of molecular oxygen, represented as\ce{O2 + h\nu -> 2O}
which occurs via absorption in the 130–195 nm ultraviolet range and initiates the formation of the odd oxygen family (O, O₂, O₃). The balance within this family is maintained through subsequent reactions, such as the three-body recombination O + O₂ + M → O₃ + M, influencing ozone distributions and overall oxidative chemistry in the layer.[28][24] Research from 2020 to 2025 indicates long-term trends driven by increasing atmospheric CO₂ concentrations, which enhance radiative cooling in the mesosphere and indirectly influence minor species abundances through altered reaction rates and transport dynamics. A 2024 review synthesizes these changes, noting that CO₂-driven greenhouse cooling contributes to modifications in trace gas distributions, providing a basis for updating empirical atmospheric models.[16]