Venus Atmosphere
The atmosphere of the planet consists of 97 percent carbon dioxide and is so thick that the surface pressure is 96 bars (compared with 1 bar on Earth). The surface temperature on Venus varies little from place to place and is extremely hot, about 462°C (736 K/864°F). The high surface temperature is explained by an intense greenhouse effect. Even though only a small percentage of the solar energy that falls on Venus reaches the surface, the planet stays hot because the thick atmosphere prevents the energy from escaping.
That nearly all of Venus's atmosphere is carbon dioxide is not as strange as it might seem; in fact, the crust of Earth contains almost as much carbon dioxide chemically bound in the form of limestone. About 3 percent of the Venusian atmosphere is nitrogen gas. By contrast, 78 percent of Earth's atmosphere is nitrogen. Water and water vapor are extremely rare on Venus. Many scientists argue that Venus, being closer to the Sun, was subjected to a so-called runaway greenhouse effect, which caused any oceans to evaporate into the atmosphere. The hydrogen atoms of the water molecules could have been lost to space and the oxygen atoms to the crust. Another possibility is that Venus had very little water to begin with.
Cloud particles on Venus mostly consist of concentrated sulfuric acid. Earth’s atmosphere also contains a very thin haze of sulfuric acid particles in the stratosphere. On Earth, however, sulfuric acid does not build up because rain carries it down to react with surface materials. On Venus the acid evaporates at the cloud base, which lies about 50 km (31 mi) above the surface, and so remains in the atmosphere. The upper parts of the clouds, visible from Earth and from Pioneer Venus 1, extend as haze 70 to 80 km (44 to 50 mi) above the surface. The clouds contain a pale yellow impurity, better detected at near-ultraviolet wavelengths. Variations in the sulfur dioxide content of the atmosphere may indicate active volcanism on the planet.
Certain cloud patterns and weather features that can be discerned in the cloud tops give some information about wind motion in the atmosphere. The upper-level winds circle the planet at 360 km/h (225 mph). These winds cover the planet completely, blowing toward the east at virtually every latitude from equator to pole. The motions of descending probes, however, have shown that the bulk of Venus's tremendously dense atmosphere, closer to the planet's surface, is almost stagnant. From the surface up to 10 km (6 mi) altitude, wind speeds are only about 3 to 18 km/h (2 to 11 mph). The high-speed winds probably result from the transfer of momentum from Venus's slow-moving, massive lower atmosphere to higher altitudes where the atmosphere is less massive, so that the same momentum results in a much higher velocity.
The upper atmosphere and ionosphere were studied in great detail by Pioneer Venus 1, which passed through them once each day. On Earth this region is very hot; on Venus it is not, even though Venus is closer to the Sun. Surprisingly, on the night side of Venus the upper atmosphere is extremely cold. (Day-side temperatures are 40°C/104°F, compared to night-side temperatures of -170°C/-274°F.) Scientists believe that strong winds blow from the day side toward the near vacuum that is caused by the low temperatures on the night side. Such winds would carry along light gases, such as hydrogen and helium, which are concentrated in a night-side “bulge.”
On Earth the ionosphere is isolated from the solar wind by the magnetosphere. Venus lacks a magnetic field of its own, but the solar wind seems to generate an induced magnetosphere around the planet, probably by a dynamo action involving its own magnetic field.
That nearly all of Venus's atmosphere is carbon dioxide is not as strange as it might seem; in fact, the crust of Earth contains almost as much carbon dioxide chemically bound in the form of limestone. About 3 percent of the Venusian atmosphere is nitrogen gas. By contrast, 78 percent of Earth's atmosphere is nitrogen. Water and water vapor are extremely rare on Venus. Many scientists argue that Venus, being closer to the Sun, was subjected to a so-called runaway greenhouse effect, which caused any oceans to evaporate into the atmosphere. The hydrogen atoms of the water molecules could have been lost to space and the oxygen atoms to the crust. Another possibility is that Venus had very little water to begin with.
Cloud particles on Venus mostly consist of concentrated sulfuric acid. Earth’s atmosphere also contains a very thin haze of sulfuric acid particles in the stratosphere. On Earth, however, sulfuric acid does not build up because rain carries it down to react with surface materials. On Venus the acid evaporates at the cloud base, which lies about 50 km (31 mi) above the surface, and so remains in the atmosphere. The upper parts of the clouds, visible from Earth and from Pioneer Venus 1, extend as haze 70 to 80 km (44 to 50 mi) above the surface. The clouds contain a pale yellow impurity, better detected at near-ultraviolet wavelengths. Variations in the sulfur dioxide content of the atmosphere may indicate active volcanism on the planet.
Certain cloud patterns and weather features that can be discerned in the cloud tops give some information about wind motion in the atmosphere. The upper-level winds circle the planet at 360 km/h (225 mph). These winds cover the planet completely, blowing toward the east at virtually every latitude from equator to pole. The motions of descending probes, however, have shown that the bulk of Venus's tremendously dense atmosphere, closer to the planet's surface, is almost stagnant. From the surface up to 10 km (6 mi) altitude, wind speeds are only about 3 to 18 km/h (2 to 11 mph). The high-speed winds probably result from the transfer of momentum from Venus's slow-moving, massive lower atmosphere to higher altitudes where the atmosphere is less massive, so that the same momentum results in a much higher velocity.
The upper atmosphere and ionosphere were studied in great detail by Pioneer Venus 1, which passed through them once each day. On Earth this region is very hot; on Venus it is not, even though Venus is closer to the Sun. Surprisingly, on the night side of Venus the upper atmosphere is extremely cold. (Day-side temperatures are 40°C/104°F, compared to night-side temperatures of -170°C/-274°F.) Scientists believe that strong winds blow from the day side toward the near vacuum that is caused by the low temperatures on the night side. Such winds would carry along light gases, such as hydrogen and helium, which are concentrated in a night-side “bulge.”
On Earth the ionosphere is isolated from the solar wind by the magnetosphere. Venus lacks a magnetic field of its own, but the solar wind seems to generate an induced magnetosphere around the planet, probably by a dynamo action involving its own magnetic field.
Comments