<P> The atmospheres of the planets Venus and Mars are primarily composed of carbon dioxide, with small quantities of nitrogen, argon, oxygen and traces of other gases . </P> <P> 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 . </P> <P> The low temperatures and higher gravity of the Solar System's giant planets--Jupiter, Saturn, 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 compounds . </P> <P> 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 . </P>

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