BACKGROUND:
An atmosphere is the layer of gases that envelop a
planet. On the Earth, it is this envelope that allows organisms to live.
Atmospheric ozone protects us from ultraviolet radiation. CO2 and
other gases trap heat and keep the surface warm enough for life to thrive.
Oxygen has allowed life to evolve.
Each of the planets has a different atmosphere,
although there are clear similarities between the atmospheres of the four
terrestrial planets and the four gas giant planets. The terrestrial planets
are rich in heavier gases and gaseous compounds, such as carbon dioxide,
nitrogen, oxygen, ozone, and argon. In contrast, the gas giant atmospheres
are composed mostly of hydrogen and helium.
The atmospheres of at least the inner planets has
evolved since they formed. This is clearest for the Earth. The Earth’s
original atmosphere was probably similar to Venus in composition, consisting
of carbon dioxide and nitrogen. The evolution of photosynthesis converted
carbon dioxide in the Earth’s atmosphere to oxygen, increasing the amount
of O2 in it from an initial 0.01% to its current 22% level.
Here is basic information on the atmosphere of each
planet to guide your Post Lab discussion. Mercury has a very thin,
almost undetectable atmosphere composed of sodium and potassium gas. These
elements were likely blown from the surface of Mercury by the solar wind.
The atmosphere of Venus is composed mainly of
carbon dioxide with minor amounts of nitrogen and trace amounts of nitrogen,
helium, neon, and argon.
The Earth's atmosphere primarily composed of
nitrogen and oxygen. Minor gases include and carbon dioxide, ozone, argon,
and helium.
Mars' atmosphere is a
thin layer composed mainly of carbon dioxide. Nitrogen, argon, and small
traces of oxygen and water vapor are also present.
Jupiter's atmosphere contains mainly helium
and hydrogen with trace amounts of water, ammonia, methane, and other carbon
compounds. Three layers of clouds may exist in Jupiter’s outermost
atmosphere. The lowest are made of water ice or droplets, the next are
crystals of a compound of ammonia and hydrogen sulfide, and the highest
clouds are ammonia ice. There seems to be no solid surface under the
atmosphere, only a transition from gas to liquid metallic hydrogen. In
the top one-fourth of the planet, the pressure and temperature are so high
that the hydrogen atoms are stripped of their outer electrons, forming a
liquid metal.
Like Jupiter, Saturn has a thick atmosphere
composed of hydrogen and helium. The ratio of hydrogen to helium ratio
decreases with depth. Methane and ammonia are also present. The atmosphere
of Saturn envelops a thick layer of metallic hydrogen.
Uranus' atmosphere is composed mainly of
hydrogen and minor amounts of helium Methane is present in minor amounts,
and probably forms most of the clouds seen by space probes and telescopes.
Uranus and Neptune both appear blue because methane strongly absorbs light
of other wavelengths.
The atmosphere of Neptune consists mainly of
hydrogen and helium, but about 2.5-3% of the atmosphere is methane. Like
Uranus, clouds in Neptune's atmosphere are composed of crystals of methane.
Pluto's atmosphere seems to be very thin, and
is likely composed of nitrogen and carbon dioxide.
Note that none of the other planets or moons in the
Solar System have atmospheres similar to the Earth. This means that if
humans travel to other bodies, they will have to bring their own atmosphere
in order to survive.
PROCEDURE:
- Present the information on planetary atmospheres discussed in the
Background with your students. Have them fill out the worksheet as you
talk. If you write the information on the board, you may want to use
chemical notation as a shorthand.
- After the students record this information, have them compare and
contrast the different atmospheres. They should observe that the inner
planets all have nitrogen and carbon dioxide, except for Mercury. They
should note that the gas giant planets have abundant helium, hydrogen
and methane. You may wish to explain that planetary atmospheres have
changed through time, using the Earth as an example.
