Exosphere

The exosphere is the uppermost layer of the atmosphere

Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low...

. In the exosphere, an upward travelling molecule can escape to space if it is moving fast enough to attain escape velocity

Escape velocity

In physics, escape velocity is the speed where the kinetic energy of an object is equal to the magnitude of its gravitational potential energy, as calculated by the equation,...

; otherwise it will be pulled back to the celestial body by gravity. In either case, such a molecule is unlikely to collide with another molecule due to the exosphere's low density

Density

The density of a material is defined as its mass per unit volume. The symbol of density is ρ .- Formula :Mathematically:where: is the density, is the mass, is the volume....

.

The main gas

Gas

This page is about the physical properties of gas as a state of matter. For the uses of gases, and other meanings, see Gas .A gas is one of four states of matter. Near absolute zero, a substance exists as a solid...

es within the Earth

Earth

Earth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...

's exosphere are the lightest gases, mainly hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H₂...

, with some helium

Helium

Helium is the chemical element with atomic number 2, and is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table...

, carbon dioxide

Carbon dioxide

Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...

, and atomic oxygen near the exobase.

The exosphere is the uppermost layer of the atmosphere

Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low...

. In the exosphere, an upward travelling molecule can escape to space if it is moving fast enough to attain escape velocity

Escape velocity

In physics, escape velocity is the speed where the kinetic energy of an object is equal to the magnitude of its gravitational potential energy, as calculated by the equation,...

; otherwise it will be pulled back to the celestial body by gravity. In either case, such a molecule is unlikely to collide with another molecule due to the exosphere's low density

Density

The density of a material is defined as its mass per unit volume. The symbol of density is ρ .- Formula :Mathematically:where: is the density, is the mass, is the volume....

.

Earth's exosphere

The main gas

Gas

This page is about the physical properties of gas as a state of matter. For the uses of gases, and other meanings, see Gas .A gas is one of four states of matter. Near absolute zero, a substance exists as a solid...

es within the Earth

Earth

Earth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...

's exosphere are the lightest gases, mainly hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H₂...

, with some helium

Helium

Helium is the chemical element with atomic number 2, and is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table...

, carbon dioxide

Carbon dioxide

Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...

, and atomic oxygen near the exobase. The exosphere is the last layer before space. Since there is no clear boundary with space and the exosphere, the exosphere is sometimes used synonymously with outer space.

Lower boundary

The altitude of its lower boundary, known as the thermopause

Thermopause

The thermopause is the atmospheric boundary of Earth's energy system, located at the top of the thermosphere.Below this, the atmosphere is defined to be active on the insolation received, due to the increased presence of heavier gases such as monoatomic oxygen. The solar constant is thus expressed...

or exobase, ranges from about depending on solar activity. Its lower boundary at the edge of the thermosphere

Thermosphere

The thermosphere is biggest of all the layers of the earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. The International Space Station has a stable orbit within the upper part of the thermosphere, between...

 has sometimes been estimated to be above the Earth's surface. The exobase is also called the critical level, the lowest altitude of the exosphere, and is formally defined in one of two ways:

1. The height above which there are the negligible atomic collisions between the particles and
2. The height above which constituent atoms are on purely ballistic trajectories.

At the exobase, the mean free path

Mean free path

In physics the mean free path of a particle is the average distance covered by a particle between successive impacts.. Alternatively, it is the distance at which the intensity of particles drops by 1/e.-Derivation:...

 of a molecule is equal to one pressure scale height

Scale height

A scale height is a term often used in scientific contexts for a distance over which a quantity decreases by a factor of e...

. As the pressure scale height is almost equal to the density scale height of the primary constituent, and since the Knudsen number

Knudsen number

The Knudsen number is a dimensionless number defined as the ratio of the molecular mean free path length to a representative physical length scale. This length scale could be, for example, the radius of a body in a fluid...

 is the ratio of mean free path and typical density fluctuation scale, this means that the exobase lies in the region where .

The fluctuation in the height of the exobase is important because this provides atmospheric drag on satellites, eventually causing them to fall from orbit

Orbit

In physics, an orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a planet around a star....

 if no action is taken to maintain the orbit.

Upper boundary

The upper boundary of the exosphere can be defined theoretically by the altitude about , half the distance to the Moon) at which the influence of solar radiation pressure on atomic hydrogen velocities exceeds that of the Earth’s gravitational pull. The exosphere observable from space as the geocorona

Geocorona

The geocorona is the luminous part of the outermost region of the Earth's atmosphere, the exosphere. It is seen primarily via far-ultraviolet light from the Sun that is scattered from neutral hydrogen. It extends to at least 15.5 Earth radii...

is seen to extend to at least from the surface of the Earth. The exosphere is a transitional zone between Earth’s atmosphere and interplanetary space.