Mesosphere

The mesosphere is the third layer of the Earth’s atmosphere, situated above the stratosphere and below the thermosphere. It extends from roughly 50 kilometres to 85 kilometres above the Earth’s surface. The mesosphere is characterised by decreasing temperature with altitude, extremely low air pressure, and the presence of spectacular phenomena such as meteors burning up and noctilucent clouds.
The name mesosphere is derived from the Greek words mesos (meaning “middle”) and sphaira (meaning “sphere”), indicating its position as the middle layer of the atmosphere. Despite being one of the least accessible regions, the mesosphere plays a crucial role in protecting the Earth and maintaining atmospheric balance.

Position and Extent

The atmosphere is divided into layers based on temperature gradients:

1. Troposphere: Surface to about 12 km (where weather occurs).
2. Stratosphere: 12 km to about 50 km (contains the ozone layer).
3. Mesosphere: 50 km to about 85 km.
4. Thermosphere: 85 km to about 600 km.
5. Exosphere: Beyond 600 km, gradually merging into outer space.

The lower boundary of the mesosphere is called the stratopause, and its upper boundary is known as the mesopause, which marks the coldest point in the Earth’s atmosphere, with temperatures dropping as low as −90°C.

Temperature and Pressure

• Temperature:
  • Decreases steadily with altitude, from about −5°C at 50 km to −90°C at 85 km.
  • The cooling trend is due to the absence of ozone and other gases capable of absorbing solar radiation.
• Pressure and Density:
  • Air pressure drops to less than 0.01% of surface pressure.
  • The air density is extremely low—though still sufficient for the burning of meteoroids.

The sharp temperature decline distinguishes the mesosphere from the warmer stratosphere below and the thermosphere above, where temperatures rise again due to solar radiation absorption by atomic oxygen.

Composition of the Mesosphere

The chemical composition of the mesosphere is similar to that of the lower atmosphere but far more rarefied. The major constituents are:

• Nitrogen (N₂): ~78%
• Oxygen (O₂): ~21%
• Argon (Ar), Carbon Dioxide (CO₂), and Trace Gases: ~1%

However, molecular dissociation begins to occur at these altitudes, leading to an increase in individual atomic oxygen and nitrogen atoms at higher levels. Water vapour content is extremely low, though ice crystals occasionally form, giving rise to noctilucent clouds.

Features and Phenomena of the Mesosphere

1. Meteor Ablation (Shooting Stars):
   • Most meteors entering Earth’s atmosphere burn up in the mesosphere due to frictional heating.
   • This protects the Earth’s surface from meteor impacts.
2. Noctilucent Clouds:
   • These are thin, wispy clouds composed of ice crystals that form near the mesopause at about 80–85 km.
   • They are visible during twilight and are the highest clouds in the atmosphere.
3. Airglow:
   • Faint emissions of light caused by chemical reactions between oxygen, nitrogen, and other gases.
   • Contributes to the subtle illumination of the night sky.
4. Sounding Rocket Experiments:
   • Since neither aircraft nor satellites operate efficiently in this region, scientists use sounding rockets to study temperature, density, and composition.
5. Sprites and Elves (Upper-Atmospheric Lightning):
   • Transient luminous events caused by powerful thunderstorms below.
   • They occur in the mesosphere and lower thermosphere, appearing as flashes of red or blue light.

Importance of the Mesosphere

Although remote and thin, the mesosphere plays several vital roles in Earth’s atmospheric system:

1. Protection from Meteoroids:
   • Acts as a shield where meteoroids burn due to friction, preventing them from striking the surface.
2. Thermal Regulation:
   • Helps maintain global temperature balance by radiating energy from the upper atmosphere back into space.
3. Atmospheric Circulation:
   • Contains large-scale atmospheric waves (such as gravity waves) that influence weather and climate patterns in lower layers.
4. Chemical Interactions:
   • Plays a part in ozone and hydroxyl radical chemistry that affects the energy dynamics of the atmosphere.

Challenges in Study

The mesosphere is often referred to as the “ignorosphere” because it is difficult to study:

• It lies too high for aircraft and balloons to reach.
• It is too low for satellites to orbit effectively.
• Consequently, data must be obtained through rocket probes, radar observations, and remote sensing techniques.

Modern research missions, such as NASA’s Sounding Rocket Program and lidar measurements, have improved our understanding of mesospheric dynamics and composition.

The Mesopause

The mesopause marks the upper boundary of the mesosphere and the transition to the thermosphere. It is the coldest region in Earth’s atmosphere, with temperatures dropping below −90°C. The mesopause is significant because:

• It defines the limit of the atmospheric temperature decrease with altitude.
• It acts as a transition layer where molecular collisions become extremely rare.
• It influences the formation of noctilucent clouds and airglow phenomena.

Comparison with Other Atmospheric Layers

Scientific and Environmental Relevance

1. Climate Research:
   • Variations in mesospheric temperature and circulation help track climate change and global atmospheric dynamics.
2. Aerospace Studies:
   • Understanding mesospheric drag and density variations is vital for spacecraft re-entry and satellite trajectory planning.
3. Atmospheric Chemistry:
   • Plays a role in studying upper-atmospheric ozone and reactions involving hydroxyl (OH) and atomic oxygen.
4. Communication and Navigation:
   • Certain radar and radio wave interactions occur within or just below the mesosphere, influencing ionospheric signal transmission.