7. Venus: The veiled planet

Penetrating the clouds

A hot and heavy atmosphere

Venus's thick, carbon-dioxide atmosphere traps the Sun's heat, raising the ground temperature by the greenhouse effect to almost three times what it would be without an atmosphere, and to about as hot as a self-cleaning oven. An airless body at Venusís distance from the Sun would be warmed by solar radiation to a surface temperature of only about 230 degrees kelvin, below the freezing point of water, but the greenhouse effect raises the surface temperature to a sizzling 735 degrees kelvin. That is hot enough to boil the ground dry, and to incinerate any humans that might visit the planet. The massive atmosphere imposes a pressure that is 92 times that at sea level on Earth. It would crush you out of existence. The surface pressure is comparable to that experienced by a submarine 500 fathoms, or 1,000 meters, below the surface of our terrestrial oceans.

Fig. .. 

Some scientists thought that the high temperatures and pressures would melt, flatten and chemically weather the surface into a featureless plain. However, the surface photographs showed fresh-appearing rock without eroded edges.

Clouds of concentrated sulfuric acid

What accounts for the unbroken layer of pale yellow clouds that covers Venus? A detailed study of the sunlight reflected from the uppermost clouds indicates that the reflecting cloud particles have a spherical shape, implying that the particles are liquid droplets rather than ice crystals. Water and other plausible liquids were ruled out because they have the wrong reflecting and refracting properties. Baffled astronomers found the answer in the 1970s. A combination of spectroscopy and polarimetry, or how the cloud droplets polarize light, showed that the clouds of Venus are composed of concentrated sulfuric acid! That is the same sulfuric acid that is commonly used in car batteries.

Circulation of the atmosphere

Fig. .. 

The wind speed increases with altitude, rising to about 100 meters per second in the clouds at about 70 thousand meters in height. The high-flying clouds race around the planet once every four Earth days, from east to west in the same backwards direction that the planet rotates. So, the top of the atmosphere is blown around Venus more than 50 times faster than the planet rotates; such a rapid motion is sometimes called super-rotation. These high-speed zonal (east-west) winds are driven by the rotation of the solid planet beneath them, but the exact mechanism for maintaining the flow is not well understood.

Fig. .. 

The atmosphere and winds have transformed the impact craters on Venus, which are unlike those seen on any other world. The dense atmosphere affects the impact debris, changing it into fluid-like flows, and the material ejected during impact is moved by the winds. Some fresh craters are surrounded by radar-bright haloes, streamlined hoods and tail-like wind streaks that act like wind vanes, pointing downwind at the time of impact. The wind streaks indicate that the winds just above the surface were blowing toward the equator from the northern and southern hemisphere.

Fig. .. 

The atmosphere redistributes heat from one part of Venus to another, thereby moderating temperature differences. Most of the sunlight falling on Venus is either reflected by the clouds or absorbed in them. And because the Sun's rays fall directly on the equator and obliquely at the poles, the equatorial clouds are initially warmer than the polar ones. But this temperature difference generates winds that transfer heat in a single large Hadley cell.

Fig. .. 

Energetic ultraviolet sunlight ionizes some of the atoms and molecules in the outer atmosphere above the clouds, forming an electrified layer similar to the Earth's ionosphere, and this layer helps shield the ground from the solar wind. The ions provide conduction paths for electrical currents that produce forces that counter the wind. As a result, the solar wind slows down and is deflected around the planet in a bow shock, and the interplanetary magnetic field is draped back to form a magnetotail. The Pioneer Venus Orbiter found that the solar windís interaction with Venus changes on times scales of hours to years, depending on the vagaries of the wind, with a bow shock that expands and contracts in step with the 11-year cycle of solar magnetic activity.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University