Exosphere

Before we begin to understand the exosphere definition, let us discuss the atmosphere a little. The gas and aerosol envelope that stretches from a planet's ocean, soil, and ice-covered surface outward into space is known as the atmosphere. Since the gravitational attraction of the earth, which draws gases and aerosols (microscopic suspended particles of dust, soot, smoke, or chemicals) inward, is strongest close to the surface, the density of the atmosphere decreases outward. Some planets such as Mercury, have almost no atmosphere because the primordial atmosphere has escaped the planet's comparatively low gravitational attraction and has been released into space. 

The atmosphere is divided into temperature-based layers. These layers are known as the troposphere, stratosphere, mesosphere and thermosphere. Furthermore, a region at about 500 km above the Earth's surface is known as the exosphere. So, let’s discuss exosphere now. 

What is the Exosphere?

The outermost layer of the earth which is located above the thermosphere in the atmosphere is referred to as the exosphere. It thins out and merges with the interplanetary space. Thus, the exosphere is around 10,000 km and 6,200 miles thick or about as wide as Earth. It is extended about halfway towards the moon.

The exosphere is the outermost area of a planet's atmosphere, where molecular densities are low and collisions between molecules are extremely unlikely. The base of the exosphere is referred to as the critical level of escape since, in the absence of collisions, lighter, faster-moving atoms such as hydrogen and helium achieve velocities sufficient to escape the planet's gravitational field. Most molecules, on the other hand, have velocities that are significantly lower than the escape velocity, so their rate of escape to space is very limited.

The exosphere is largely composed of light gases such as hydrogen, carbon dioxide, atomic oxygen, and helium. All of the gases described above are so light that they can escape the Earth's gravitational magnetic force and scatter freely across space.

Exosphere - Characteristics

Some of the characteristics of the exosphere are listed below-

1. In the exosphere, the particles are significantly far apart.

2. It is the only layer that has a definite shape, unlike other atmospheric layers.

3. The molecules and gases present in it are available in a limited quantity.

4. The particles are not considered gas as there are no interactions and no collisions between them, since the density is too low.

5. The particles are not considered plasma as well, since there is no electric charge present in the atoms or molecules. 

6. The particles can travel along a ballistic trajectory for about hundreds of kilometres before they bump with other particles.

Exosphere - Function

Its purpose is to help as a transitional medium to a gravity-influenced zone. It also helps atoms to escape from the atmosphere and into space. It is the only layer of the atmosphere that has a distinct form, as opposed to the other layers. The molecules and gases in the exosphere are in short supply. 

Furthermore, these gaseous molecules are separated from one another. They fly at a high speed, which is why they have few, negligible, or minor collisions. It is much farther away from Earth than anybody can imagine, and therefore no meteorological phenomena can occur. Some of the functions that the exosphere layer possesses are listed below-

1. It acts as the transition layer between outer space and the earth’s atmosphere.

2. Artificial satellites, which contsantly study the outer space and the earth are carried out are present in the exosphere.

Exosphere Boundaries

The thermopause is the lower boundary of the exosphere where it interacts with the thermosphere. It begins at an altitude of approximately 250-500 km, but its height is determined by the amount of solar activity. Particles in the atmosphere have atomic collisions below the thermopause, similar to what you would see in a balloon. Above the thermopause, however, this transitions to purely ballistic collisions. 

The theoretical upper limit of the exosphere is 190,000 km (halfway to the Moon). This is the point at which the Sun's solar radiation overcomes the Earth's gravitational force on atmospheric particles. This has been observed up to 100,000 kilometres above the Earth's surface. The official limit between the Earth's atmosphere and interplanetary space, according to most scientists, is 10,000 kilometres.

Things Found in the Exosphere

The exosphere is the topmost layer of the Earth's atmosphere, containing only the faintest wisps of hydrogen and other atmospheric gases. There is almost no 'atmosphere' in this area of the atmosphere; individual particles travel hundreds of kilometres before colliding, and many of these particles drift off into space. However, there are currently a variety of objects floating around on the cold edge of Earth's atmosphere. In this frigid area of the atmosphere, a number of man-made satellites orbit, ranging from the Hubble Space Telescope to more general weather and photography satellites aimed at Earth can be found in this region.

Exosphere Facts

• The exosphere starts 311 to 621 miles above the earth's surface and finishes approximately 6200 miles above the earth's surface.

• While the exosphere is the planet's most distant layer of the atmosphere, it is also the planet's first line of protection against the sun's rays. It is the first layer to shield the Earth from meteors, asteroids, and cosmic rays.

• The temperature of the exosphere varies dramatically. The temperature is lower at night and much higher during the day.

• The exosphere's air is very thin and mainly composed of helium and hydrogen. Other gases, such as atomic oxygen and carbon dioxide, can also be present in trace amounts.

• The upper level of the exosphere is the furthest point from Earth that is still influenced by gravity. This size, however, will be halfway to the moon and is only considered valid in a technical sense. As a result, scientists disagree about the actual boundaries of the exosphere.

• If the exosphere's limit is assumed to be where it is still influenced by Earth's gravity, the exosphere will constitute the majority of the Earth's atmosphere. If the exosphere's boundary is thought to be approximately 6200 miles from the earth's surface, as many say, the thermosphere is the largest portion of the earth's atmosphere.

• The geocorona is the name given to the portion of the exosphere visible from the Earth.

• The exosphere is ideal for satellite placement because there is no friction and they can orbit reasonably quickly without being interrupted.

• Gravity pulls the majority of the molecules in the exosphere down towards the Earth's lower atmospheric levels. However, due to the exosphere's low gravity and pressure, some do make it into the outer space.

• Solar wind storms compress the exosphere, causing pressure to build up.

• Since the air in the exosphere is so thin, molecules do not collide like they do in the lower layers of the atmosphere. The majority of the molecules return to the lower layers of the atmosphere, but some escape into space.

Importance of Exosphere

The exosphere is beneath the thermosphere that shrinks and expands based on the ultraviolet radiations coming through the exospheric layer. The sun’s radiations exert pressure on the hydrogen atom in a region called geocorona. These hydrogen atoms are responsible for the scattering of ultraviolet radiation. The primary importance of the exosphere is that it absorbs the ultraviolet radiations and protects the layers underneath. If it was not present, UV rays would have been harmful to the layers underneath.

When the sun’s radiation falls into the thermosphere it expands. If the exosphere was not present, the thermosphere would have never stopped expanding because the earth’s atmosphere would not function properly, and if something like this happens, the Earth’s ecosystem would also not function properly.

Conclusion

One of the five layers of the Earth's atmosphere is the Exosphere. It is the fifth and outermost layer of the atmosphere, starting at the upper boundary of the thermosphere and extending to the lower exosphere, which is referred to as the exobase, exopause, and ‘critical altitude’. Since the upper limit of the thermosphere ranges from 311 to 621 miles, so does the starting point of the exosphere. The term "exosphere" is derived from the Greek word "Exo," which means "outside" or "external." Space starts where the exosphere stops.