The Earth’s atmosphere consists of five distinct layers, each with its own unique features. In this article, we take a closer look at what precisely its outermost layer, the exosphere, is and what its defining characteristics are.

When viewed from the surface or low Earth orbit, our atmosphere seems to be one continuous layer of air. However, it consists of five different layers: the troposphere, stratosphere, mesosphere, thermosphere, and exosphere.

This article focuses on the exosphere, the outermost layer of the atmosphere. It not only examines what the exosphere is but also looks at the characteristics or facts that define it.

Before we look at specific facts about the exosphere and its defining characteristics, we first need to establish exactly what the exosphere is:

Exosphere Definition

The exosphere is the fifth and outermost layer of Earth’s atmosphere, situated above the thermosphere at altitudes of 500 to 1 000 km (311 to 620 miles), extending to 10 000 km (6 200 miles). It is the layer where any remaining properties of Earth’s atmosphere transition into outer space.

The name “exosphere” is derived from Ancient Greek, with “ἔξω éxō” meaning “outside,” and “σφαῖρα sphaĩra” meaning “sphere.” Directly translated, “outside sphere” perfectly describes the exosphere since it is the Earth’s outermost layer in a spherical shape.

Its lower boundary is also called the exobase. The exobase is also known as the “critical altitude.” Above this height, temperatures remain relatively stable, and barometric conditions are no longer relevant.

As already mentioned, the exosphere is almost completely devoid of atmospheric gases. This makes its composition resemble that of the vacuum of space very closely, more so than any other atmospheric layer.

Mainly due to its inaccessibility and the limited research that could be conducted as a result, very little is known about the precise makeup of the exosphere.

The only elements that can be found in any significant numbers in the exosphere are helium and hydrogen. They are so widely dispersed, however, that their presence is of no real importance or relevance.

The exosphere starts just above the thermosphere (and thermopause) at a height of 500 km (310 miles) and extends up to a height of around 10 000 km (6 200 miles).

Temperatures are generally very cold and relatively constant. In direct sunlight, it can get very hot, and in the shade, freezing cold, though. (There are very few particles present to conduct heat or cold, which accounts for the extreme temperatures and the generally perceived cold conditions.)

As a result, temperatures vary quite dramatically, from 0° Celsius (32° Fahrenheit) all the way up to 1700° Celsius (3092° Fahrenheit). The biggest temperature difference takes place between daytime and nighttime.

In this vast space, the last traces of elements associated with the atmosphere blend gradually and seamlessly into the vacuum of space.

Many planets and moons (like Mercury, the Moon, and the Galilean moons of Jupiter) have no atmosphere, and all have an exosphere starting at the surface level.

Since the last remnants of the atmosphere blend so gradually into outer space, no clearly defined upper boundary can be determined.

The exosphere may not play any significant role in supporting and maintaining life on Earth, but as I mentioned in the introduction, it is still relevant.

Just like the thermosphere, the exosphere forms the ideal environment for spacecraft to be placed into orbit around the Earth.

It has the added advantage of allowing satellites and other objects to be placed in a much higher orbit than communication satellites and the International Space Station, which are placed in lower orbits around the Earth. 

This higher orbit allows satellites to get a better global view of various activities on Earth. The bigger distance from the Earth’s surface also minimizes the amount of light reflecting off the planet into the atmosphere to cause any kind of interference.

This is why the Hubble Space Telescope and a variety of weather and other scientific satellites can be found orbiting the Earth at this altitude. They are also much safer in the exosphere, as there is a lot less “space traffic” to contend with, as well as more room to maneuver in.

The following list highlights the characteristics and facts of the exosphere in more detail.

1. 1. The exosphere is the fifth and outermost layer of Earth’s atmosphere.
   2. It is situated above the thermosphere at an altitude of 500 – 1 000 km (311 – 620 miles) and extends to 10 000 km (6 200 miles).
   3. It has no clearly defined upper boundary.
   4. The exosphere is situated just above the thermosphere, with a thin layer of air called the thermopause separating the two layers.
   5. The lowest part of the exosphere is also known as the exobase.
   6. In the exosphere, the air is so thin and contains so few particles that it only makes up 0.002% of the total mass of the Earth’s atmosphere.
   7. The relatively few gas molecules present in the layer basically never collide and follow a ballistic trajectory through the exosphere.
   8. Temperatures can exceed 2 000° Celsius (3 632° Fahrenheit) in the exosphere, but there are very few particles to transfer the heat, so one will never experience it.
   9. The exosphere is home to the Hubble Space Telescope and a large number of communication and weather satellites.
   10. Light atmospheric gases like hydrogen and helium are the most abundant in this layer, with a few heavier gases like carbon dioxide and oxygen present close to the exobase.
   11. A number of scientists don’t regard the exosphere as one of the atmospheric layers and view the thermosphere as Earth’s outermost layer.
   12. Other planets in our solar system, like Jupiter, Saturn, and Mars, also have exospheres.

   This is by no means a comprehensive and exhaustive list that contains all the data available about the exosphere, but it highlights the key facts and characteristics that define this layer.

As this article highlighted, the exosphere still contains small amounts of the gases present in the other four atmospheric layers, but these quickly disappear as altitude increases and the last traces of the atmosphere transition into the vacuum of space.

It is not surprising, then, that many scientists argue that the exosphere cannot be considered part of Earth’s atmosphere and that the thermosphere constitutes the atmosphere’s upper boundary.

It is technically still part of the atmosphere, though, and contains a small percentage of the gases found closer to the planet’s surface. And, yes, it still has an important role to play, as just illustrated. 

If you are interested in the complete structure and make-up of the atmosphere, this article covers all five atmospheric layers and their relation to each other in more detail.

Until next time, keep your eye on the weather!