Profile of Venus Help (page 2)
The Year And The Day
Mercury’s mean orbital radius is 72 percent that of Earth—roughly 108 million kilometers (67 million miles). The perihelion and aphelion of Venus differ by less than one percent, so the orbit of the planet is nearly a perfect circle. At times, Venus comes within 38 million kilometers (24 million miles) of Earth. Venus takes 225 Earth days to orbit the Sun.
The day on Venus is strange because it is longer than the year. Venus rotates once on its axis, relative to the distant stars, every 243 Earth days. The peculiarity goes further: Venus spins from east to west rather than from west to east.
The tilt of Venus’ equatorial plane, relative to that of its orbit around the Sun, can be interpreted in either of two ways. If we consider the rotation of the planet to be retrograde (that is, from east to west), then the tilt is only about 3 degrees. This is the conventional view. However, we also can imagine the tilt as being 177 degrees (3 degrees short of a complete flipflop), so the north pole points in the opposite direction from the north poles of all the other planets. No one on the surface would care about details like this. They would be preoccupied with surviving the heat, comparable with the temperature of Mercury’s Caloris Basin at midday, and the pressure, which would necessitate that housing be designed as if it were for the deep sea. The daylight on Venus is comparable with that of a gloomy winter afternoon in London or Seattle.
Venus, like Mercury, is moonless. The planet is almost the same size as Earth (12,100 kilometers or 7,500 miles in diameter), and the internal composition is similar to that of our own planet. However, from the surface upward, Venus and Earth could hardly be more different.
The surface of Venus remained a mystery until it was mapped by radar. The microwave signals of radar equipment can penetrate Venus’s thick clouds, which maintain an unbroken overcast at an altitude several times higher than the highest clouds on Earth. These clouds reflect light very well, and this accounts for the brilliance of the planet as we see it in our evening or morning sky. When the surface was finally observed using radar and computer-graphics programs, huge volcanoes were discovered. The largest of these would make Hawaii’s Big Island seem tiny in comparison.
In the 1970s, Russian Venera probes landed on Venus and survived long enough to take a few photographs. The surface appeared strewn with rocks that cast no shadows on account of the overcast. The landscape and sky were bathed in rusty light, produced by filtration of sunlight through the sulfurous clouds. There was no sign of life, and it was surmised that life as we know it could not exist there. Along with the searing heat, the pressure at the surface of Venus is 90 times as great as the pressure at Earth’s surface.
The surface of Venus is not cratered in the same way as the Moon or Mercury because the atmosphere of Venus has eroded all minor impact marks that might have been formed long ago. Small meteors are burned up by the atmosphere; Venus’s shroud of heavy carbon dioxide provides better protection than does Earth’s atmosphere. Gigantic meteors and, of course, asteroids can survive even the thickest atmosphere, and it is certain that Venus, like all the other planets, has been struck many times. However, the more notable source of cratering on Venus is volcanic activity.
When astronomers used spectroscopes to analyze the atmosphere of Venus by looking at the light reflected from the planet’s clouds, they knew that the air on that world consists largely of carbon dioxide. This led them to suspect that Venus must be much hotter than Earth because carbon dioxide is well known for its ability to trap heat. It was not until space probes gave the planet close examinations that the truly Hades-like nature of the atmosphere was determined. While the air itself contains mainly carbon dioxide, especially near the surface, the clouds are laden with sulfuric acid.
The clouds that surround Venus are different from those in Earth’s atmosphere in almost every possible way. Much of Earth’s surface is obscured by overcast, but there are always clear spots through which the surface can be seen from space and through which direct sunlight can pass. This is not the case on Venus, at least not at the visible wavelengths. As seen through the most powerful Earth-based telescopes, Venus looks like a cue ball. At ultraviolet wavelengths, details in the clouds can be seen, and from analyzing such views it has been determined that the upper equatorial clouds race around the planet approximately once in every 100 Earth hours.
The clouds on Venus exist in a single layer several kilometers thick and much higher above the surface than any clouds on Earth (except the noctilucent clouds sometimes seen in Earth’s stratosphere). No place on the surface of Venus is ever subjected to direct sunlight. The sky appears angry because of the orange color produced by the sulfur compounds. The winds at the surface of Venus are light, at least in an absolute sense, never moving more than a few meters per second. However, because the air at the surface is thick, a breeze that would be a whisper on our planet carries considerable force on Venus. Perhaps the landscape on Venus is subjected to an ongoing severe thunderstorm with sulfuric acid “rain” that evaporates before it can get down to the surface. Figure 5-3 provides a comparison between Earth’s atmosphere and that of Venus.
Practice problems of this concept can be found at: Mercury and Venus Practice Problems
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