What is the Difference Between Homosphere and Heterosphere

The main difference between the homosphere and heterosphere is that the homosphere is the lower part of the atmosphere, up to about sixty miles (100 km), whereas the heterosphere is the upper part of the atmosphere.

The homosphere and heterosphere are the two layers of the atmosphere.  They have different concentrations of gases depending on the altitude. However, there is no greater variation in the composition of gases in the homosphere. But, there is a greater variation in the composition of gases in the heterosphere. Hence, the gases in the homosphere are in the form of a homogenous mixture while the gases in the heterosphere occur in the form of layers. 

Key Areas Covered 

1. What is Homosphere
     – Definition, Features, Importance
2. What is Heterosphere
     – Definition, Features, Regions, Importance
3. What are the Similarities Between Homosphere and Heterosphere
     – Outline of Common Features
4. What is the Difference Between Homosphere and Heterosphere
     – Comparison of Key Differences

Key Terms 

Altitude, Atmosphere, Composition of Gases, Heterosphere, Homosphere, Temperature

What is Homosphere 

The homosphere is the lowest part of the Earth’s atmosphere, lying between the Earth’s surface and the heterosphere, the upper part. It is the atmosphere, occurring below around 100 km (60 miles). The main feature of the homosphere is the presence of a homogenous mixture of gases as a result of the turbulent mixing or eddy diffusion. Hence, the bulk composition of air throughout the homosphere is uniform. 

Figure 1: Atmospheric Composition by Height

Moreover, the homosphere contains more than 99% of the mass of the Earth’s atmosphere. Also, its composition is usually 78% nitrogen, 21% oxygen, and trace amounts of other molecules, such as argon and carbon dioxide. Although the composition remains the same, the concentration of air decreases with the altitude. Furthermore, the three layers of the homosphere are the troposphere, stratosphere,  and mesosphere.  

Troposphere 

The troposphere is the bottom-most layer, which exists in the ground to an altitude of about 11 km. It contains 99% of the total mass of water vapor and aerosols. Besides, rising warm air within the troposphere replaces higher cooler air, which mix gases vertically. Hence, the troposphere is the part of the atmosphere where nearly all weather conditions take place. Also, commercial aircraft fly in the troposphere.

Figure 2: Layers of Homosphere (The Orange Layer: Troposphere, The White LAyer: Stratosphere, The Blue Layer: Mesosphere)

Stratosphere 

The stratosphere is the middle layer of the homosphere, lying between the altitude of roughly 15 km and 50 km. In this layer, the temperature increases with altitude. Also, the ozone layer lies in the stratosphere (above about 18-20 km). And notably, the concentration of O₃ is much higher than in the rest of the atmosphere. Consequently, this blocks most ultraviolet light from penetrating the lower layers of the atmosphere. Supersonic jets fly in this layer of the homosphere.  

Mesosphere 

The mesosphere is the top-most layer of the homosphere, extending roughly from 50 km up to 80 km. In contrast to the stratosphere, the temperature decreases with the increasing altitude in the mesosphere. This is due to the decreasing absorption of solar radiation by the rarefied atmosphere and increasing cooling by CO₂ radiative emission.  Typically, the mesosphere ends at the mesopause, the coldest part of Earth’s atmosphere with temperatures below −143 °C (−225 °F; 130 K). Meteors, which are small bodies of matter from the outer space, entering the Earth’s atmosphere, exist in this layer. 

What is Heterosphere 

Heterosphere is the uppermost layer of the Earth’s atmosphere; it begins at about 100 km altitude and extends to the outer reaches of its atmosphere. In heterosphere, the gases separate out into layers by molecular diffusion with the increasing altitude. Also, heavier molecules tend to stay in the lower layers of the heterosphere while the lighter molecules stay in the upper layers. Normally, nitrogen, oxygen, helium, and hydrogen are the major constituents of the heterosphere. However, day and night cycles, solar activity, geomagnetic activity, and seasonal cycles are the other factors affecting the density distribution in the heterosphere in addition to the diffusion.

Moreover, the heterosphere has two layers: thermosphere and exosphere. 

Thermosphere 

The thermosphere is the lower layer of the heterosphere, lying from  80km to 460km. It is directly above the mesosphere and below the exosphere, extending from about 90 km to between 500 and 1,000 km. At this layer, temperatures increase with altitude as a result of the absorption of highly energetic solar radiation.

Figure 3: Diffusion Limited Hydrogen Escape in the Earth’s Atmosphere

Exosphere  

Exosphere is the top layer of the heterosphere where the atmosphere turns into space. Therefore, it is a thin, atmosphere-like volume, surrounding the Earth or the natural satellite. Particularly, the density of this layer is too low for molecules to behave as gases by colliding with each other. However, they exist binding to the core by gravity. Also, carbon dioxide and hydrogen are gases that mostly occur in the exosphere. The lower boundary of the exosphere is called the exobase, ranging from 500 to 1,000 km.  Meanwhile, the upper boundary of the exosphere extends up to a distance at which the influence of solar radiation pressure on atomic hydrogen exceeds that of Earth’s gravitational pull. Generally, it happens at half the distance to the Moon. 

Similarities Between Homosphere and Heterosphere  

• The homosphere and heterosphere are the two layers of the atmosphere.  
• They differ by their altitude and the composition of gases. 

Difference Between Homosphere and Heterosphere 

Definition 

Homosphere refers to the lower part of the atmosphere, up to about 60 miles, in which there is no great change in its composition. Meantime, heterosphere refers to the upper part of the atmosphere, above about 60 miles, in which there is a greater variation in its composition. 

Altitude 

The homosphere occurs up to 60 miles of the atmosphere, but the heterosphere occurs above 60 miles from the earth. 

Regions 

The three regions of the homosphere are troposphere, stratosphere, and mesosphere, while the two regions of the heterosphere are thermosphere and exosphere. 

Degree of Composition of Gases 

There is no greater variation in the composition of gases in the homosphere, but there is a greater variation in the composition of gases in the heterosphere.  

Composition of Gases 

The homosphere contains a higher proportion of heavier gases such as nitrogen and oxygen, while the heterosphere contains a higher proportion of lighter gases such as hydrogen and helium. 

Significance 

The gases in the homosphere occur in the form of a homogenous mixture, but the gases in the heterosphere occur in the form of layers. 

Mixing by the Wind 

The wind highly mixes the gases up in the homosphere, while wind has less influence in mixing gases in the heterosphere. 

Reactivity 

The gases in the homosphere show a higher reactivity, but in contrast, the gases in the heterosphere show a lower reactivity. 

Conclusion 

The homosphere is one of the two layers of the atmosphere, occurring up to 60 miles from the Earth’s surface. Also, it contains a homogenous mixture of gases due to the continuous mixing by the wind. It contains mainly heavy gases such as oxygen and nitrogen. Due to high mixing, it shows a higher reactivity between gases. In contrast, the heterosphere is the second layer of the atmosphere, occurring above the homosphere. It mainly contains lighter gases in the form of layers. Therefore, it shows a lower reactivity between its gases. Hence, the main difference between the homosphere and heterosphere is their altitude and composition of gases. 

References:

1. “Homosphere And Heterosphere by Ktwenzel.” Infogram, Available Here.
2. “Layers of Earth’s Atmosphere.” Center for Science Education. UCAR, Available Here.

Image Courtesy:

1. “Msis atmospheric composition by height” By Amaurea – Own work (CC0) via Commons Wikimedia  
2. “Endeavour silhouette STS-130” By NASA/Crew of Expedition 22 (Public Domain) via Commons Wikimedia   
3. “Diffusion limited escape3” By Nickwogan – Own work (CC BY-SA 4.0) via Commons Wikimedia