Mars’s atmosphere is far thinner than Earth’s and has a very different mix of gases: it is about 95% carbon dioxide, while Earth’s is about 78% nitrogen and 21% oxygen. Mars’s low air pressure means its atmosphere retains less heat, but it still supports winds and dust storms. Those differences shape the planet’s cold, variable surface conditions and make it difficult for liquid water to persist there.
What are the atmospheres made of?
Mars’s atmosphere is dominated by carbon dioxide. NASA’s Mars Fact Sheet lists its composition by volume as approximately 95.1% carbon dioxide, 2.59% nitrogen, 1.94% argon and 0.16% oxygen, along with smaller amounts of other gases. Earth’s atmosphere, by comparison, is about 78% nitrogen and 21% oxygen, according to NASA’s Earth facts page.
The small amount of oxygen in Martian air is not comparable to the abundant oxygen in Earth’s atmosphere. Mars’s composition figures are reference values, not a claim that the air is identical at every location or altitude. See NASA’s Mars Fact Sheet for the reported composition.
How much thinner is Mars’s atmosphere?
At Mars’s mean radius, NASA lists a surface pressure of 6.36 millibars, with a seasonal range of 4.0 to 8.7 millibars. Pressure varies with season and location, so 6.36 millibars is a reference value rather than a universal surface reading. Elevation also matters.
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For a broad comparison, NASA describes Mars’s surface pressure as less than one percent of Earth’s average. The difference is consequential: Mars has much less air pressing on its surface than Earth does. The pressure figures are from NASA’s Mars Fact Sheet; the less-than-one-percent comparison appears in NASA’s 2018 explanation of Mars’s atmosphere and terraforming.
How does the difference affect temperature?
Mars’s sparse atmosphere does not hold heat as effectively as Earth’s, and solar heat escapes readily. That contributes to a colder, more variable surface environment. Temperature is not one fixed value across Mars: it changes with location and time, so a single average would not describe conditions everywhere. NASA explains these atmospheric effects in its Mars atmosphere overview.
Does Mars still have weather?
Yes. Thin air can move and raise dust: Mars has winds, dust devils and dust storms. Some storms spread across much of the planet, and dust from major events can take months to settle. Martian atmospheric conditions also vary by time and place, as described by NASA and NASA’s Mars facts page.
So “thin” does not mean motionless. Mars’s atmosphere can drive striking weather, even though its low pressure and different composition make it unlike Earth’s air.
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What does this mean for liquid water—and was Mars always this way?
Today, Mars’s atmosphere is too thin for liquid water to persist on the surface for long; exposed water quickly evaporates or freezes. NASA also reports evidence that Mars was warmer and wetter billions of years ago and once had a thicker atmosphere. The planet’s present atmosphere is therefore not a complete picture of its climate history. See NASA’s Mars atmosphere overview and NASA’s Mars facts page.
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