|A DESCENT INTO HELL|

Various probes have provided very important information about the atmosphere of Venus. This had always been a problem for astronomers due to the thick blanket of clouds. By now, we are quite familiar with the features of Earth's clouds as seen from space: rings, swirls, wads; there are really many different types. Instead, the clouds on Venus are absolutely uniform: as if there were no movements of any kind. The fact is that the clouds on Earth show their movements quite clearly because they are made from water vapor, which absorbs the visible light differently than the surrounding areas. Already several observations made with telescopes have shown that the Venusian clouds, seen in the ultraviolet spectrum instead of visible light, do seem to have some structures. However, no one expected such spectacular results like what was obtained on February 5, 1974 by the Mariner 10 probe, which had been equipped with an instrument to take ultraviolet pictures: strips of clouds in the north and the south enveloped the planet and moved at such high speeds that they completed a full revolution around Venus in just four Earth days. Mariner 10 then continued its journey towards Mercury, but four years later, the Pioneer Venus probes, penetrating directly into the Venusian atmosphere, had to confirm all its measurements. But this is what they discovered. The planet is shrouded by a thick fog of small particles high above the visible layer of clouds, at an altitude of about 70 kilometers. A few thousand meters below, at a height of 60 kilometers, is where the layer of real clouds begins: and this was a surprise, considering that on the Earth there are no cloud formations above 10 kilometers. Here we quickly enter a very thick ocean of small concentrated sulfuric acid droplets, at the mercy of winds which blow at a hundred meters per second or 360 kilometers per hour. The temperature is very low, about -43 degrees Celsius at the top of the cloud layer, but at a height of 55 kilometers it begins to rise. The clouds contain an infinite number of particles which seem to be solid, though they have not yet been identified chemically, which mix with the drops of sulfuric acid. At an altitude of 48 kilometers the clouds begin to tear themselves apart. Now it is so hot that the sulfuric acid breaks down into water, oxygen, sulfur dioxide and other sulfur compounds. These lighter substances rise towards the upper cloud layers and the cycle starts again, just like on the Earth when the rain evaporates after coming in contact with the ground: only that on Venus the drops never reach the surface. When we reach the base of the cloud layer, the winds are not so destructive: they have a speed of "just" 175 kilometers per hour. On the other hand, the temperature has increased to 90 degrees Celsius and lightening flashes everywhere at a rate of 25 per second: the crash of thunder is practically continuous. Finally we emerge from the clouds: after a thin layer of finer particles, the atmosphere is surprisingly clear for 30 kilometers until reaching the surface. Now the heat is unbearable: the temperature has increased to 300 degrees Celsius and will continue to increase gradually as we descend until reaching a maximum of 460 degrees. But where is the ground? It is impossible to see: the atmosphere has become so dense that it is like looking through a layer of water, and even the descent is just like what occurs when a flat object zig-zags through the water on its way down to the bottom of a pond. Even the wind, which by now has a speed of only 4 or 5 kilometers per hour, "blows" lightly but furiously like the current of an unstoppable swollen river. But why are the higher layers of the clouds so cold and the ground so hot? One possible explanation involves carbon dioxide, which is the main component of almost the entire atmosphere. The light from the Sun can reach the ground because the clouds are semi-transparent to the high-frequency radiation emitted by the star. However, when it does reach the ground, this radiation is transformed into heat, in other words into a form of low-frequency radiation, called infrared, which then tries to return into space. But the carbon dioxide is almost completely opaque to infrared radiation and pushes almost all of it to the bottom. This is why the surface of Venus is so hot even during the long night: the planet acts like a gigantic greenhouse whose "glass walls" consists of carbon dioxide. But a greenhouse that is so hot and deadly that no plant could ever grow there. What a difference from the delicate thermal balance that has allowed life to develop on our planet! Now, like Pilgrims in space, let's take a look at that hospitable world called Earth.
Various probes have provided very important information about the atmosphere of Venus. This had always been a problem for astronomers due to the thick blanket of clouds. By now, we are quite familiar with the features of Earth's clouds as seen from space: rings, swirls, wads; there are really many different types. Instead, the clouds on Venus are absolutely uniform: as if there were no movements of any kind. The fact is that the clouds on Earth show their movements quite clearly because they are made from water vapor, which absorbs the visible light differently than the surrounding areas. Already several observations made with telescopes have shown that the Venusian clouds, seen in the ultraviolet spectrum instead of visible light, do seem to have some structures. However, no one expected such spectacular results like what was obtained on February 5, 1974 by the Mariner 10 probe, which had been equipped with an instrument to take ultraviolet pictures: strips of clouds in the north and the south enveloped the planet and moved at such high speeds that they completed a full revolution around Venus in just four Earth days. Mariner 10 then continued its journey towards Mercury, but four years later, the Pioneer Venus probes, penetrating directly into the Venusian atmosphere, had to confirm all its measurements. But this is what they discovered. The planet is shrouded by a thick fog of small particles high above the visible layer of clouds, at an altitude of about 70 kilometers. A few thousand meters below, at a height of 60 kilometers, is where the layer of real clouds begins: and this was a surprise, considering that on the Earth there are no cloud formations above 10 kilometers. Here we quickly enter a very thick ocean of small concentrated sulfuric acid droplets, at the mercy of winds which blow at a hundred meters per second or 360 kilometers per hour. The temperature is very low, about -43 degrees Celsius at the top of the cloud layer, but at a height of 55 kilometers it begins to rise. The clouds contain an infinite number of particles which seem to be solid, though they have not yet been identified chemically, which mix with the drops of sulfuric acid. At an altitude of 48 kilometers the clouds begin to tear themselves apart. Now it is so hot that the sulfuric acid breaks down into water, oxygen, sulfur dioxide and other sulfur compounds. These lighter substances rise towards the upper cloud layers and the cycle starts again, just like on the Earth when the rain evaporates after coming in contact with the ground: only that on Venus the drops never reach the surface. When we reach the base of the cloud layer, the winds are not so destructive: they have a speed of "just" 175 kilometers per hour. On the other hand, the temperature has increased to 90 degrees Celsius and lightening flashes everywhere at a rate of 25 per second: the crash of thunder is practically continuous. Finally we emerge from the clouds: after a thin layer of finer particles, the atmosphere is surprisingly clear for 30 kilometers until reaching the surface. Now the heat is unbearable: the temperature has increased to 300 degrees Celsius and will continue to increase gradually as we descend until reaching a maximum of 460 degrees. But where is the ground? It is impossible to see: the atmosphere has become so dense that it is like looking through a layer of water, and even the descent is just like what occurs when a flat object zig-zags through the water on its way down to the bottom of a pond. Even the wind, which by now has a speed of only 4 or 5 kilometers per hour, "blows" lightly but furiously like the current of an unstoppable swollen river. But why are the higher layers of the clouds so cold and the ground so hot? One possible explanation involves carbon dioxide, which is the main component of almost the entire atmosphere. The light from the Sun can reach the ground because the clouds are semi-transparent to the high-frequency radiation emitted by the star. However, when it does reach the ground, this radiation is transformed into heat, in other words into a form of low-frequency radiation, called infrared, which then tries to return into space. But the carbon dioxide is almost completely opaque to infrared radiation and pushes almost all of it to the bottom. This is why the surface of Venus is so hot even during the long night: the planet acts like a gigantic greenhouse whose "glass walls" consists of carbon dioxide. But a greenhouse that is so hot and deadly that no plant could ever grow there. What a difference from the delicate thermal balance that has allowed life to develop on our planet! Now, like Pilgrims in space, let's take a look at that hospitable world called Earth.