EXOSPHERE: CHARACTERISTICS, COMPOSITION, FUNCTIONS AND TEMPERATURE - ENVIRONMENT - 2025

The exosphere is the outermost layer of the atmosphere of a planet or a satellite, constituting the upper limit or border with outer space. On planet Earth, this layer extends above the thermosphere (or ionosphere), from 500 km above the earth's surface.

The terrestrial exosphere is about 10,000 km thick and is made up of gases very different from those that make up the air we breathe on the Earth's surface.

Figure 1. Layers of the Earth's atmosphere. Source: Esteban1216, from Wikimedia Commons In the exosphere both the density of gaseous molecules and the pressure are minimal, while the temperature is high and remains constant. In this layer the gases are dispersed, escaping into outer space.

characteristics

The exosphere constitutes the transition layer between the Earth's atmosphere and interplanetary space. It has very interesting physical and chemical characteristics, and it fulfills important functions of protection of the planet Earth.

Behavior

The main defining characteristic of the exosphere is that it does not behave like a gaseous fluid, like the inner layers of the atmosphere. The particles that make it up are constantly escaping into outer space.

The behavior of the exosphere is the result of a set of individual molecules or atoms, which follow their own trajectory in the Earth's gravitational field.

Properties of the atmosphere

The properties that define the atmosphere are: the pressure (P), the density or concentration of the constituent gases (number of molecules / V, where V is the volume), the composition, and the temperature (T). In each layer of the atmosphere these four properties vary.

These variables do not act independently, but are related by the gas law:

P = dRT, where d = number of molecules / V and R is the gas constant.

This law is fulfilled only if there are enough collisions between the molecules that make up the gas.

In the lower layers of the atmosphere (troposphere, stratosphere, mesosphere and thermosphere), the mixture of gases that make it up can be treated as a gas or fluid that can be compressed, whose temperature, pressure and density are related through the law of the gases.

By increasing the height or distance from the earth's surface, the pressure and the frequency of the collisions between the gas molecules decrease considerably.

At 600 km altitude and above this level, the atmosphere must be considered in a different way, since it no longer behaves like a gas or a homogeneous fluid.

Physical state of the exosphere: plasma

The physical state of the exosphere is that of plasma, which is defined as the fourth state of aggregation or physical state of matter.

Plasma is a fluid state, where practically all atoms are in ionic form, that is, all particles have electrical charges and there are free electrons present, not bound to any molecule or atom. It can be defined as a fluid medium of particles with positive and negative electrical charges, electrically neutral.

Plasma exhibits important collective molecular effects, such as its response to a magnetic field, forming structures such as rays, filaments, and double layers. The physical state of plasma, as a mixture in the form of a suspension of ions and electrons, has the property of being a good conductor of electricity.

It is the most common physical state in the universe, forming interplanetary, interstellar and intergalactic plasmas.

Figure 2. Earth's atmosphere, in the background the moon. Source: NASA, via Wikimedia Commons

Chemical composition

The composition of the atmosphere varies with altitude or distance from the Earth's surface. The composition, the state of mixing and the degree of ionization are determining factors to distinguish the vertical structure in the layers of the atmosphere.

The gas mixture due to turbulence is practically nil, and its gaseous components quickly separate by diffusion.

In the exosphere, the mixture of gases is restricted by the temperature gradient. The gas mixture due to turbulence is practically nil, and its gaseous components quickly separate by diffusion. Above 600 km altitude, individual atoms can escape from the Earth's gravitational pull.

The exosphere contains low concentrations of light gases such as hydrogen and helium. These gases are widely dispersed in this layer, with very large voids between them.

The exosphere also has other less light gases in its composition, such as nitrogen (N ₂), oxygen (O ₂) and carbon dioxide (CO ₂), but these are located near the exobase or baropause (area of ​​the exosphere that limits with the thermosphere or ionosphere).

Molecular escape velocity from the exosphere

In the exosphere the molecular densities are very low, that is, there are very few molecules per unit volume, and most of this volume is empty space.

Just because there are huge empty spaces, atoms and molecules can travel great distances without colliding with each other. The probabilities of collisions between molecules are very small, practically nil.

In the absence of collisions, the lighter and faster hydrogen (H) and helium (He) atoms can reach speeds such as to allow them to escape the planet's gravitational field of attraction and out of the exosphere into interplanetary space..

The escape into space of hydrogen atoms from the exosphere (estimated at about 25,000 tons per year), has surely contributed to major changes in the chemical composition of the atmosphere throughout geological evolution.

The rest of the molecules in the exosphere, apart from hydrogen and helium, have low average velocities and do not reach their escape velocity. For these molecules, the rate of escape to outer space is low, and escape occurs very slowly.

Temperature

In the exosphere, the concept of temperature as a measure of the internal energy of a system, that is, of the energy of molecular motion, loses meaning, since there are very few molecules and a lot of empty space.

Scientific studies report extremely high exosphere temperatures, on the order of 1500 K (1773 ° C) on average, which remain constant with height.

Features

The exosphere is part of the magnetosphere, since the magnetosphere extends between 500 km and 600,000 km from the surface of the Earth.

The magnetosphere is the area where a planet's magnetic field deflects the solar wind, which is loaded with very high-energy particles, harmful to all known life forms.

This is how the exosphere constitutes a layer of protection against the high-energy particles emitted by the Sun.

References

1. Brasseur, G. and Jacob, D. (2017). Modeling of Atmospheric Chemistry. Cambridge: Cambridge University Press.
2. Hargreaves, JK (2003). The solar-terrestrial environment. Cambridge: Cambridge University Press.
3. Kameda, S., Tavrov, A., Osada, N., Murakami, G., Keigo, K. et al. (2018). VUV Spectroscopy for terrestrial exoplanetary exosphere. European Planetary Science Congress 2018. EPSC Abstracts. Vol. 12, EPSC2018-621.
4. Ritchie, G. (2017). Atmospheric Chemistry. Oxford: World Scientific.
5. Tinsley, BA, Hodges, RR and Rohrbaugh, RP (1986). Monte Carlo models for the terrestrial exosphere over a solar cycle. Journal of Geophysical Research: Space Physics Banner. 91 (A12): 13631-13647. doi: 10.1029 / JA091iA12p13631.