Key idea: Planetary
atmospheres are composed of
gases. Each planet's
atmosphere has its own unique
The study of atmospheres and atmospheric phenomena is called
When Mars was imaged with the Hubble Space Telescope in 1995,
the planet was discovered to be cooler and drier than scientists had
expected. Much of the water vapor that does exist in the Martian atmosphere
had frozen and formed clouds of ice crystals which covered the entire planet
in a thin cloud layer. The clouds are most visible around the limb
of the planet. Large ice caps are visible at the northern and southern poles
of Mars. The southern polar cap is primarily frozen carbon dioxide (dry ice)
but the northern polar cap contains water ice in addition to dry ice.
An atmosphere is a layer of gases around a planet or natural
For the most part, atmospheric gases are heavy rather than light.
You will learn in a future lesson why this is true. Examples of heavy gases are
oxygen, nitrogen and carbon dioxide. Examples of light gases are hydrogren
and helium. The study of atmospheres is called meteorology. The
weather is the condition
of the atmosphere at a particular time and place. The typical weather
observed in a particular place over a long period of time is said to be
the climate in that
Atmospheric gases come from a variety of sources:
Nearly all of the planets in the Solar System have atmospheres. Many of
the larger satellites have atmospheres as well. The large gas
giant planets (Jupiter, Saturn, Uranus and Neptune) are composed
only of gases and do not have a distinct surface. For these planets, the
atmosphere is considered to be the outermost layer of gases. The atmospheres
of some planets and planetary satellites are very thin. Scientists suspect
that planets in other solar systems have atmospheres as well. The search
for such planets is currently a very hot astronomical topic.
- Planetary formation from the solar nebula
from material on the planetary surface
- Vaporization of impacting
- Release of gases from deep within the host body through volcanoes
- Biproducts of biological and man-made processes
Each planet's atmosphere has its own unique composition. The
atmospheric composition can help identify the host planet like a
fingerprint. Some meteors found in the Antarctic were identified as
having originated on Mars in part by the gases which were trapped inside
the rocks when they were formed billions of years ago!!!
The earth's atmosphere consists of 77% nitrogen, 21% oxygen, 1% water
vapor and 0.9% argon.
As mentioned earlier, atmospheres are composed primarily of heavy gases.
What happens when you fill a balloon with a light gas such as helium and
then release it in Earth's atmosphere? Does this help to explain why
there are only heavy gases in planetary atmospheres???
The atmosphere of Mars consists of 95% carbon dioxide, 2.7% nitrogren and
1.6% argon with trace amounts of oxygen and water vapor.
The atmosphere of Venus consists of 96% carbon dioxide and 3.5% nitrogen
with a trace amount of water vapor.
Composition alone does not determine the properties of a planetary
atmosphere. The atmospheres of Venus and Mars have similar compositions
but the properties of those atmospheres and the climates on those planets
are very different. You will learn more about atmospheres and climate in
Astronomical tidbit: Pluto is the furthest planet in the Solar
System from the Sun. The orbit of Pluto around the Sun is not at all
circular, but is highly eccentric. When Pluto is closest to the
Sun, it has a very thin atmosphere. The pressure of the atmosphere at the
surface of Pluto at that time is about one million times less than the
atmospheric pressure at the surface of the Earth. As Pluto gets further from
the Sun, the atmosphere is believed to freeze onto its surface, only to reappear
approximately 100 years later when the planet approaches the Sun once again!!!
The image of Pluto at the right was taken with the Hubble Space
Telescope in 1994. It is not a very clear picture. On the average, Pluto
is 40 times farther away from Earth than Earth is from the Sun. Pluto is
also smaller than the Moon! It will not be possible to obtain substantially
better images of Pluto until a probe is sent there with a camera. Such a
mission is in the planning stages at JPL.
Last updated: November 17, 1999
Joe Twicken /