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Atmosphere

The habitable zone around a star is defined by the area in which liquid water is available. However, a planet needs more than just liquid water to be habitable. If life is ever to survive on the planet's surface, an atmosphere is essential.

Even the smallest planets in our solar system - even dwarf planets like Pluto - have a tenuous atmosphere. The largest planets, Jupiter and Saturn, are almost entirely "atmosphere," with layers of gas clouds thousands of miles thick.

What makes a planet's atmosphere so important?

Protection from Radiation. Stars emit more than just visible light - they give off high-energy ultraviolet light and even some X-rays. High-energy light is capable of breaking apart some chemical bonds, including the bonds that hold together organic molecules such as RNA and the lipids in cell membranes. An atmosphere can help to block this high-energy light, protecting life on the surface of the planet from its damaging effects.

Boiling and Evaporation. You may know that water at high altitudes, such as Denver, Colorado, in the United States, or in the mountains of Chile or Tibet, boils at a lower temperature. (This is why it takes longer to cook things at high altitude - the water isn't as hot.) This change in boiling point is because of the difference in atmospheric pressure. At higher altitudes the atmospheric pressure is reduced. Though this is not a perfect analogy, you can think of the atmosphere "pushing" on the water to keep it in liquid form. If there is less atmosphere pushing on the water, it will more easily become steam.

Water at our normal atmospheric pressure (at sea level) boils at 100°C, but water at 1/10 of our normal atmospheric pressure boils at less than 50°C. At pressures below about 0.6% of normal atmospheric pressure, the liquid phase of water disappears entirely - only solid and gaseous water are possible! In this situation, ice transitions directly into steam, without being liquid water at any time. On a planet with such a reduced atmospheric pressure, liquid water might still exist underground where there is pressure from the weight of the rocks, but not on the surface of the planet. Clearly, if we want liquid water on the surface of our planet, it is important to have an atmosphere to keep water in its liquid form.

Temperature Regulation. Our moon, with its low gravity, has no atmosphere. However, it is about as far from the sun as the Earth, so one might expect it to have similar temperatures to Earth. Not so. The temperature on the moon ranges from a blistering 100°C when the sun shines to a freezing -173°C in the darkened areas. Such extremes are not conducive to surface life, which typically operates in the range of 0°C to 80°C. Our atmosphere helps to not only raise Earth's surface temperature, but also to moderate it, keeping us from experiencing extreme highs and lows.

The Availability of Chemicals. Liquid substances such as water can contain and spread many of the chemicals that are useful for life. Some chemicals, especially the lighter ones, can also be distributed through the atmosphere. Having an atmosphere as an additional distribution system for life-supporting chemicals increases the number of places where life might arise and thrive.

In summary, if life is to begin or survive on the surface of a planet, that planet must have an atmosphere.

Can an Atmosphere Be Too Thick?

Definitely. Gas giants like Jupiter and Saturn have such thick atmospheres that the temperature and pressure at the bottom are incredibly high. Such conditions are probably not suitable for life. However, Super-Earths are much smaller than gas giants and are unlikely to accumulate such thick atmospheres. They are in a "sweet spot" where the atmosphere can be thick, but not too thick.

Солнечная система и ее тайны