3. The invisible buffer zone with space - atmospheres, magnetospheres and the solar wind

Atmospheres of the terrestrial planets

The breath of life

Our atmosphere forms an indispensable interface with nearby space, but it is often invisible. After all, you look right through the air in your room. Our atmosphere usually goes unseen on a warm, dry windless day. Yet, the slow drift of floating clouds or the sight of birds and airplanes supported by their motion proves that there is something substantial surrounding us. We can sense the touch of the wind on a stormy day, and on cold days we feel the air against our skin.

Fig. .. 

When astronauts look down at the Earth at sunrise or sunset, they detect the thin atmosphere that warms and protects us, and permits us to breathe. It is only 10 thousand meters from the ground to the top of the sky, or no further than you might run in an hour. Everything beyond that thin layer of air is the black void of space. And everything below it is what it takes to sustain life.

The major constituents of dry air on Earth are nitrogen molecules (77 percent), oxygen molecules (21 percent) that we breathe, and argon atoms (0.93 percent). Carbon dioxide is a miniscule 0.035 percent. There is no hydrogen in our air, and most of the hydrogen on Earth is found in water. The water vapor in wet air is variable in amount, usually no more than 1 percent.

Earth's weather

Fig. .. 

The temperate zones are dominated by high-altitude jet streams blowing eastward at speeds up to 40 meters per second, in a sinuous path which resembles the meandering of a river. Between the trade winds and the jet stream, the pattern is primarily a series of high pressure cells rotating clockwise in the north and counter-clockwise in the south. In the polar regions, the weather pattern is primarily a series of low pressure cells rotating counter-clockwise in the north and clockwise in the south.

Atmospheres of Venus and Mars

Russian Spacecraft Venera 7 directly measured the atmosphere of Venus, measuring the temperature and pressure all the way down to the bottom of the atmosphere, where the temperature reaches a sizzling 735 degrees kelvin. Down there the thick, heavy atmosphere produces a pressure of 92 bars that is, 90 times the sea-level pressure on Earth. It consists of 96 percent carbon dioxide. The atmosphere of Venus contains about ten thousand times as much carbon dioxide as is present in our air. This massive carbon dioxide atmosphere is responsible for the high surface temperature of Venus through the greenhouse effect.

Fig. .. 

Strong winds are blowing the highest clouds around Venus at speeds of up to 100 meters per second, racing around the planet's equator once every four Earth days. Curiously enough, Venus's surface rotates in the same westward direction but with a much longer period of 243 Earth days. So the winds blow the entire outer atmosphere around the planet much more rapidly than the planet spins. Although terrestrial jet streams move at up to half the speed of the high-flying clouds on Venus, they are limited to narrow zones high in the Earth's atmosphere.

Fig. .. 

The exact chemical composition of the atmosphere on Mars was determined by direct measurements in 1976 when the Viking 1 and Viking 2 landers arrived at the surface. Like Venus, the atmosphere on Mars is mostly carbon dioxide (95 percent) with just a whiff of nitrogen (2.7 percent). The landers confirmed that the local surface pressure is at or below 0.01 bars. They also showed that the surface temperature is almost always below the freezing point of water.

The planet breathes its atmosphere in and out as its southern polar cap grows and shrinks, producing a seasonal change in atmospheric pressure by about 30 percent. It is as if the planet was a giant lung that slowly breathes in and exhales the same gas, carbon dioxide. When the surface temperature drops during the southern winter, the atmospheric carbon dioxide condenses and freezes to enlarge the polar cap, resulting in a drop in atmospheric pressure. In the southern summer, the ice sublimates (goes directly from solid to vapor, without becoming liquid) back into the atmosphere, increasing the atmospheric pressure. The polar cap waxes and wanes due to this seasonal component of carbon dioxide ice.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University