The exosphere is a thin, atmosphere-like volume surrounding a planetary body
where molecules are gravitationally bound to that body, but where the
density is too low for them to behave as a gas by colliding with each other. In
the case of bodies with substantial atmospheres, such as Earth's atmosphere,
the exosphere is the uppermost layer, where the atmosphere thins out and
merges with interplanetary space. It is located directly above the thermosphere.
Several moons, such as the Moon and the Galilean satellites, have exospheres
without a denser atmosphere underneath, referred to as a surface boundary
exosphere. Here, molecules are ejected on parabolic trajectories until they
collide with the surface. Smaller bodies such as asteroids, in which the
molecules emitted from the surface escape to space, are not considered to
have exospheres. Earth's exosphere
The most common molecules within Earth's exosphere are those of the lightest
atmospheric gasses. Hydrogen is present throughout the exosphere, with some
helium, carbon dioxide, and atomic oxygen near its base. Because it can be
difficult to define the boundary between the exosphere and outer space, the
exosphere may be considered a part of interplanetary or outer space.
= Lower boundary= The lower boundary of the exosphere is
called the exobase. It is also called exopause and 'critical altitude' as this
is the altitude where barometric conditions no longer apply. Atmospheric
temperature becomes nearly a constant above this altitude. On Earth, the
altitude of the exobase ranges from about 500 to 1,000 kilometres depending
on solar activity. The exobase can be defined in one of two
ways: If we define the exobase as the height
at which upward-traveling molecules experience one collision on average,
then at this position the mean free path of a molecule is equal to one pressure
scale height. This is shown in the following. Consider a volume of air,
with horizontal area and height equal to the mean free path , at pressure and
temperature . For an ideal gas, the number of molecules contained in it is:
where is the universal gas constant. From the requirement that each molecule
traveling upward undergoes on average one collision, the pressure is:
where is the mean molecular mass of the gas. Solving these two equations gives:
which is the equation for the pressure scale height. As the pressure scale
height is almost equal to the density scale height of the primary constituent,
and because the Knudsen number 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 if no action is taken to maintain
the orbit. = Upper boundary on Earth=
In principle, the exosphere covers distances where particles are still
gravitationally bound to Earth, i.e. particles still have ballistic orbits
that will take them back towards Earth. The upper boundary of the exosphere can
be defined as the distance at which the influence of solar radiation pressure on
atomic hydrogen exceeds that of Earth's gravitational pull. This happens at half
the distance to the Moon). The exosphere, observable from space as the
geocorona, is seen to extend to at least 10,000 kilometres from Earth's surface.
The exosphere is a transitional zone between Earth's atmosphere and space.
Moon's exosphere On 17 August 2015, based on studies with
the Lunar Atmosphere and Dust Environment Explorer spacecraft, NASA
scientists reported the detection of neon in the exosphere of the moon.
References External links
Gerd W. Prolss: Physics of the Earth's Space Environment: An Introduction. ISBN
3-540-21426-7