An atmosphere (from Greek ἀτμός (atmos), meaning 'vapour', and
σφαῖρα (sphaira), meaning 'sphere') is a layer or a set
of layers of gases surrounding a planet or other material body, that
is held in place by the gravity of that body. An atmosphere is more
likely to be retained if the gravity it is subject to is high and the
temperature of the atmosphere is low.
The atmosphere of
Earth is composed of nitrogen (about 78%), oxygen
(about 21%), argon (about 0.9%) with carbon dioxide and other gases in
Oxygen is used by most organisms for respiration;
nitrogen is fixed by bacteria and lightning to produce ammonia used in
the construction of nucleotides and amino acids; and carbon dioxide is
used by plants, algae and cyanobacteria for photosynthesis. The
atmosphere helps to protect living organisms from genetic damage by
solar ultraviolet radiation, solar wind and cosmic rays. The current
composition of the
Earth's atmosphere is the product of billions of
years of biochemical modification of the paleoatmosphere by living
The term stellar atmosphere describes the outer region of a star and
typically includes the portion above the opaque photosphere. Stars
with sufficiently low temperatures may have outer atmospheres with
2 Atmospheric escape
5.2.1 In the Solar System
5.2.2 Outside the Solar System
8 See also
10 Further reading
11 External links
Main article: Atmospheric pressure
Atmospheric pressure at a particular location is the force per unit
area perpendicular to a surface determined by the weight of the
vertical column of atmosphere above that location. On Earth, units of
air pressure are based on the internationally recognized standard
atmosphere (atm), which is defined as 101.325 kPa (760
14.696 psi). It is measured with a barometer.
Atmospheric pressure decreases with increasing altitude due to the
diminishing mass of gas above. The height at which the pressure from
an atmosphere declines by a factor of e (an irrational number with a
value of 2.71828...) is called the scale height and is denoted by H.
For an atmosphere with a uniform temperature, the scale height is
proportional to the temperature and inversely proportional to the
product of the mean molecular mass of dry air and the local
acceleration of gravity at that location. For such a model atmosphere,
the pressure declines exponentially with increasing altitude. However,
atmospheres are not uniform in temperature, so estimation of the
atmospheric pressure at any particular altitude is more complex.
Main article: Atmospheric escape
Surface gravity differs significantly among the planets. For example,
the large gravitational force of the giant planet
light gases such as hydrogen and helium that escape from objects with
lower gravity. Secondly, the distance from the Sun determines the
energy available to heat atmospheric gas to the point where some
fraction of its molecules' thermal motion exceed the planet's escape
velocity, allowing those to escape a planet's gravitational grasp.
Thus, distant and cold Titan, Triton, and
Pluto are able to retain
their atmospheres despite their relatively low gravities.
Since a collection of gas molecules may be moving at a wide range of
velocities, there will always be some fast enough to produce a slow
leakage of gas into space. Lighter molecules move faster than heavier
ones with the same thermal kinetic energy, and so gases of low
molecular weight are lost more rapidly than those of high molecular
weight. It is thought that
Mars may have lost much of their
water when, after being photo dissociated into hydrogen and oxygen by
solar ultraviolet, the hydrogen escaped. Earth's magnetic field helps
to prevent this, as, normally, the solar wind would greatly enhance
the escape of hydrogen. However, over the past 3 billion years Earth
may have lost gases through the magnetic polar regions due to auroral
activity, including a net 2% of its atmospheric oxygen.
Other mechanisms that can cause atmosphere depletion are solar
wind-induced sputtering, impact erosion, weathering, and
sequestration—sometimes referred to as "freezing out"—into the
regolith and polar caps.
Atmospheres have dramatic effects on the surfaces of rocky bodies.
Objects that have no atmosphere, or that have only an exosphere, have
terrain that is covered in craters. Without an atmosphere, the planet
has no protection from meteoroids, and all of them collide with the
surface as meteorites and create craters.
Most meteoroids burn up as meteors before hitting a planet's surface.
When meteoroids do impact, the effects are often erased by the action
of wind. As a result, craters are rare on objects with
Wind erosion is a significant factor in shaping the terrain of rocky
planets with atmospheres, and over time can erase the effects of both
craters and volcanoes. In addition, since liquids can not exist
without pressure, an atmosphere allows liquid to be present at the
surface, resulting in lakes, rivers and oceans.
Earth and Titan are
known to have liquids at their surface and terrain on the planet
Mars had liquid on its surface in the past.
Earth's atmospheric gases scatter blue light more than other
Earth a blue halo when seen from space
A planet's initial atmospheric composition is related to the chemistry
and temperature of the local solar nebula during planetary formation
and the subsequent escape of interior gases. The original atmospheres
started with the radially local rotating gases that collapsed to the
spaced rings that formed the planets.[clarification needed] They were
then modified over time by various complex factors, resulting in quite
The atmospheres of the planets
Mars are primarily composed
of carbon dioxide, with small quantities of nitrogen, argon, oxygen
and traces of other gases.
The atmospheric composition on
Earth is largely governed by the
by-products of the life that it sustains. Dry air from Earth's
atmosphere contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04%
carbon dioxide, and traces of hydrogen, helium, and other "noble"
gases (by volume), but generally a variable amount of water vapor is
also present, on average about 1% at sea level.
The low temperatures and higher gravity of the Solar System's giant
Uranus and Neptune—allow them more
readily to retain gases with low molecular masses. These planets have
hydrogen–helium atmospheres, with trace amounts of more complex
Two satellites of the outer planets possess significant atmospheres.
Titan, a moon of Saturn, and Triton, a moon of Neptune, have
atmospheres mainly of nitrogen. When in the part of its orbit closest
to the Sun,
Pluto has an atmosphere of nitrogen and methane similar to
Triton's, but these gases are frozen when it is farther from the Sun.
Other bodies within the
Solar System have extremely thin atmospheres
not in equilibrium. These include the
Moon (sodium gas), Mercury
(sodium gas), Europa (oxygen), Io (sulfur), and Enceladus (water
The first exoplanet whose atmospheric composition was determined is HD
209458b, a gas giant with a close orbit around a star in the
constellation Pegasus. Its atmosphere is heated to temperatures over
1,000 K, and is steadily escaping into space. Hydrogen, oxygen,
carbon and sulfur have been detected in the planet's inflated
Atmosphere of Earth
Earth's atmosphere consists of a number of layers that differ in
properties such as composition, temperature and pressure. The lowest
layer is the troposphere, which extends from the surface to the bottom
of the stratosphere. Three quarters of the atmosphere's mass resides
within the troposphere, and is the layer within which the Earth's
terrestrial weather develops. The depth of this layer varies between
17 km at the equator to 7 km at the poles. The stratosphere,
extending from the top of the troposphere to the bottom of the
mesosphere, contains the ozone layer. The ozone layer ranges in
altitude between 15 and 35 km, and is where most of the
ultraviolet radiation from the Sun is absorbed. The top of the
mesosphere, ranges from 50 to 85 km, and is the layer wherein
most meteors burn up. The thermosphere extends from 85 km to the
base of the exosphere at 690 km and contains the ionosphere, a
region where the atmosphere is ionised by incoming solar radiation.
The ionosphere increases in thickness and moves closer to the Earth
during daylight and rises at night allowing certain frequencies of
radio communication a greater range. The Kármán line, located within
the thermosphere at an altitude of 100 km, is commonly used to
define the boundary between
Earth's atmosphere and outer space. The
exosphere begins variously from about 690 to 1,000 km above the
surface, where it interacts with the planet's magnetosphere. Each of
the layers has a different lapse rate, defining the rate of change in
temperature with height.
Other astronomical bodies such as these listed have known atmospheres.
In the Solar System
Graphs of escape velocity against surface temperature of some Solar
System objects showing which gases are retained. The objects are drawn
to scale, and their data points are at the black dots in the middle.
Atmosphere of the Sun
Atmosphere of Mercury
Atmosphere of Venus
Atmosphere of Earth
Atmosphere of the Moon
Atmosphere of Mars
Atmosphere of Ceres
Atmosphere of Jupiter
Atmosphere of Io
Atmosphere of Callisto
Atmosphere of Europa
Atmosphere of Ganymede
Atmosphere of Saturn
Atmosphere of Titan
Atmosphere of Enceladus
Atmosphere of Uranus
Atmosphere of Titania
Atmosphere of Neptune
Atmosphere of Triton
Atmosphere of Pluto
Outside the Solar System
Main article: Extrasolar atmosphere
HD 209458 b
Main article: Atmospheric circulation
The circulation of the atmosphere occurs due to thermal differences
when convection becomes a more efficient transporter of heat than
thermal radiation. On planets where the primary heat source is solar
radiation, excess heat in the tropics is transported to higher
latitudes. When a planet generates a significant amount of heat
internally, such as is the case for Jupiter, convection in the
atmosphere can transport thermal energy from the higher temperature
interior up to the surface.
From the perspective of a planetary geologist, the atmosphere acts to
shape a planetary surface.
Wind picks up dust and other particles
which, when they collide with the terrain, erode the relief and leave
deposits (eolian processes). Frost and precipitations, which depend on
the atmospheric composition, also influence the relief. Climate
changes can influence a planet's geological history. Conversely,
studying the surface of the
Earth leads to an understanding of the
atmosphere and climate of other planets.
For a meteorologist, the composition of the
Earth's atmosphere is a
factor affecting the climate and its variations.
For a biologist or paleontologist, the Earth's atmospheric composition
is closely dependent on the appearance of the life and its evolution.
Atmospheric sciences portal
International Standard Atmosphere
^ ἀτμός Archived 2015-09-24 at the Wayback Machine., Henry
George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
^ σφαῖρα Archived 2017-05-10 at the Wayback Machine., Henry
George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
^ Seki, K.; Elphic, R. C.; Hirahara, M.; Terasawa, T.; Mukai, T.
(2001). "On Atmospheric Loss of
Oxygen Ions from
Magnetospheric Processes". Science. 291 (5510): 1939–1941.
PMID 11239148. Archived from the original on 2007-10-01.
^ Weaver, D.; Villard, R. (2007-01-31). "Hubble Probes Layer-cake
Structure of Alien World's Atmosphere". Hubble News Center. Archived
from the original on 2007-03-14. Retrieved 2007-03-11.
Sanchez-Lavega,, Agustin (2010). An Introduction to Planetary
Atmospheres. Taylor & Francis. ISBN 978-1-4200-6732-3.
Properties of atmospheric strata - The flight environment of the
Atmosphere - an Open Access journal
HD 209458 b