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Figure 1.3 True direction of comet tails. The straight, gas tail always points directly away from the Sun. The curved, dust tail always arcs between the gas tail and the direction from which the comet is coming.

and while they are also pushed away from the Sun, they form an arc between the gas tail and the direction from which the comet came.

The only time a comet's tail points directly behind it is when the comet is heading straight for the Sun. Such comets have been observed. They are vaporized completely before reaching the Sun's photosphere. Most comets orbit the Sun, rather than heading straight for it. As you can see in the figure, for half the time the tails of such comets point ahead of them in their orbits.

Into the Frying Pan

While we are dealing with heat, suppose I were to tell you that the closest planets to the Sun are, from nearest to farthest, Mercury, Venus, Earth, and Mars, and ask you which one has the hottest surface. Mercury is nearly twice as close to the Sun as Venus, and nearly three times closer than Earth. Even if you had never thought about it before, it is entirely likely that you would choose Mercury as hottest. It does stand to reason. After all, Mercury is nearest the fire. However, this answer is incorrect.

If I were to stick a thermometer in space at Mercury's distance from the Sun, I would measure a temperature of 68o°F or 36o°C. This indicates the energy provided by the Sun at that location. Out at Venus's distance, that thermometer in space would read 375°F or i9o°C. Of course, here at the Earth, the temperature is lower still, which is why we exist. As you would correctly expect, the temperature continues to decrease as we move out through the solar system. At Pluto's distance, the average temperature is -33o°F or -2oo°C.

Let's consider now the temperatures at the planets' surfaces. Observations reveal that the surface of Mercury at noontime is a toasty 8oo°F or 425°C. This is hot enough to melt tin or lead. It is worth noting that Mercury has a very, very thin atmosphere compared to the Earth. Atmospheres affect temperatures. For example, on Earth, clouds prevent some heat from escaping at night, which is why mornings following cloudy nights are warmer than mornings following clear nights, all other things being equal. Since it is so thin, Mercury's atmosphere has virtually no effect on its surface temperature.

With the second planet, Venus, we have an entirely different situation. If, like Mercury and our Moon, Venus were an essentially airless world, its surface temperature would be about 450°F or 230°C, which is lower than Mercury's temperature. However, Venus is not airless. It is surrounded by so much gas, mostly carbon dioxide, that its surface air pressure is 100 times greater than the pressure of the air we breathe. At Venus's surface, your body would feel as though you were swimming 1,000 meters (3,300 feet) underwater here on Earth.

This incredibly thick blanket of carbon dioxide has a profound effect on Venus's surface temperature, raising it to 850°F or 450°C— higher than Mercury's. Let's see how. Most sunlight striking the top of Venus's atmosphere is scattered right back into space by the permanent clouds that completely cover the planet. Unlike Earth's clouds, those surrounding Venus are made up primarily of sulfuric acid. About a quarter of the sunlight does filter through to the planet's surface. As on Earth, this light is absorbed by the planet, heating it up, and as with any hot object, this heat is then radiated back outward. But the heat radiated by Venus does not get very far off the surface because the carbon dioxide atmosphere absorbs it very efficiently. As a result, the planet heats its own atmosphere, which acts like a hot blanket that keeps the surface much hotter than it would be otherwise. This is summarized in figure 1.4.

The process of trapping heat on Venus is exactly the same as that which occurs inside glass-encased things on Earth, like a greenhouse or your car on a hot summer day. Visible light passes through the glass and heats the objects inside. Since that heat cannot get back out through the glass, it heats the inside air and reheats the objects instead. This is why your car is so hot when you get into it after it's been closed up during the summer. This so-called greenhouse effect explains why Venus has the hottest surface of any planet with a solid surface in the solar system.

Ironic but true: the Earth's surface is also hotter than most of Mercury's surface. As you have experienced, the Earth cools down at night. It got to -5°F (-20°C) here in Maine last night. This drop in temperature occurs, as implied above, because the heat radiated from the surface is not being replaced with energy from the Sun. Furthermore, some of the heat emitted by the Earth is absorbed in our atmosphere (to a much lower degree than on Venus), which helps prevent the surface from cooling too much during the hours of darkness.

Now let's return to Mercury. Its rotation is so slow that a day there, from noon to noon, is 176 Earth days long. A year on Mercury is only

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