Of all the planets, Mars is our closest relation in terms of makeup (not distance -- Venus is closer), but that's not saying much. And it certainly doesn't mean that it is hospitable. The atmosphere of Mars differs from Earth's in many ways, and most of them don't bode well for humans living there.
- It's composed mostly of carbon dioxide (95.3 percent compared to less than 1 percent on Earth).
- Mars has much less nitrogen (2.7 percent compared to 78 percent on Earth).
- It has very little oxygen (0.13 percent compared to 21 percent on Earth).
- The red planet's atmosphere is only 0.03 percent water vapor, compared to Earth, where it makes up around 1 percent.
- On average, it exerts only 6.1 millibars of surface pressure (Earth's average sea-level atmospheric pressure is 1,013.25 millibars) [source: NASA].
Because the "air" on Mars is so thin, it holds little of the heat that comes from the ground after it absorbs solar radiation. The thin air also is responsible for the wide, daily swings in temperature (almost 100 degrees Fahrenheit or 60 degrees Celsius). Martian atmospheric pressure changes with the seasons. During the Martian summer, carbon dioxide sublimes from the polar ice caps into the atmosphere, increasing the pressure by about 2 millibars. As found by NASA's Mars Reconnaissance Orbiter, during the Martian winter, carbon dioxide refreezes and falls from the atmosphere as carbon dioxide snow! This snowfall causes the pressure to decrease again. Finally, because the Martian atmospheric pressure is so low and the average temperature is so cold, liquid water cannot exist; under these conditions, water would either freeze, evaporate into the atmosphere or, as seen by NASA's 2008 Phoenix Lander mission, fall as snow [source: NASA].
The weather on Mars is pretty much the same each day: cold and dry with small daily and seasonal changes in temperature and pressure, plus a chance of dust storms and dust devils [source: NASA]. Light winds blow from one direction in the morning and then from the reverse direction in the evening. Clouds of water ice hover at altitudes of 12 to 18 miles (20 to 30 kilometers), and clouds of carbon dioxide form at approximately 30 miles (50 kilometers). Because Mars is so dry and cold, it never rains. That's why Mars resembles a desert, much like Antarctica on Earth.
During the spring and early summer, the sun heats up the atmosphere enough to cause small convection currents. These currents lift dust into the air. The dust absorbs more sunlight and heats the atmosphere further, causing more dust to lift into the air. As this cycle continues, a dust storm develops. Because the atmosphere is so thin, great speeds (60 to 120 mph or 100 to 200 kph) are required to stir up the dust. These dust storms spread across large regions of the planet and can last for months. All that dust can be bad for the rovers traversing the surface, but the storms can also clear off dirt caked on their solar panels.
Dust storms are also thought to be responsible for the variable dark regions on Mars that are seen from ground-based telescopes, which were mistaken for canals and vegetation by Percival Lowell and others. The storms are also a major source of erosion on the Martian surface.
Is all that dust making you thirsty? Read on to find out about water on Mars.