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The Forefront of Space Science

Wind on Earth, Wind on Venus
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Living on fluidsphere

I am now looking at a video image, taken in infrared by a meteorological satellite, of clouds moving over the earth. Only a god has this point of view. At a glance, I notice that the cloud-generating areas are limited to the equatorial belt and mid- to high-latitude regions while the subtropical region is cloudless sky. In the equatorial belt, clusters of huge clouds several hundred km to several thousand km in length extend east and west. In the clusters, a number of small clouds move from east to west repeatedly emerging and disappearing. The behavior of clouds in the mid- to high-latitude region, where Japan is located, is quite a different from that in the equatorial belt. It is just like dropping ink in water. Fine streaky cloud belts several thousand km in length move from west to east while twisting and sometimes swirling. By tracking individual clouds, it can be seen that they move from the low latitude side, become thick and merge with the cloud belt. They increase their speed toward the high latitude side while snaking, and eventually disappear.

These atmospheric currents or winds simply represent features of the climate of our planet earth and are closely related to the generation of life and the development of culture. For example, a planet’s surface temperature is influenced by the amount of sunlight-reflecting cloud. The distribution of clouds is affected by winds as stated above. This is true also for water vapor, which plays a major role in greenhouse effect. Without the greenhouse effect, the mean temperature of earth would be -18 deg. C. Due to the greenhouse effect, the earth’s temperature is kept at a comfortable +15 deg. C. In addition, if it were not for heat transportation by atmospheric and ocean currents, the oceans in the low-altitude region receiving strong sunlight would not remain stable and would evaporate (runaway greenhouse effect). On the other hand, oceans in the high-altitude region would freeze. Another role of wind is to circulate water around the world.

In April, 2007, the news went around the world that a terrestrial planet which could support life had been discovered 20.5 light years away from our solar system. By observations from the European Southern Observatory in Chile, the brightness of the central star and the planet’s distance from the central star were identified. From these data, the surface temperature of the planet was estimated from zero to 40 deg. C. Liquid oceans can exist within such a temperature range. Although it really is a great discovery, I had a few questions about the news. Because, looking at our earth’s weather, I realize that the fluid sphere of planets is extremely complicated. My doubt becomes stronger when looking at Venus, a planet located on the inner side of earth in our solar system.

Blazing planet

In Kanagawa Prefecture where ISAS of JAXA is located, there is a shrine called Akahoshi Shrine after Venus. Since Venus is the brightest celestial body in the sky apart from the Sun and Moon, Venus has inspired many legends around the world. Venus is the most familiar planet to mankind, but its environment is very different from that of earth. Although its size and density are almost the same as the earth, Venus’s atmosphere is composed of mostly carbon dioxide while the earth’s is mostly nitrogen and oxygen. Venus’s atmospheric volume is huge and, due to its weight, its surface pressure is 90 atm (equal to that of a depth of 900m in water on earth). The surface temperature is up to 460 deg. C. and there are no seas. Russian Venus explorers that landed on the Venus survived no more than two hours due to the high temperature. Clouds of concentrated sulfuric acid float around at an altitude of 60km and, unlike earth, the entire sky of Venus is covered by clouds. A common misconception is that sulfuric acid falls as rain on Venus. Due to the high temperature near the surface, sulfuric acid evaporates before it reaches the surface and its molecules decompose.

The reason why Venus is so hot is not attributable, at least directly, to its close proximity to the Sun. Most sunlight is reflected by the clouds. The amount of sunlight that reaches the surface is one-tenth that of the earth. Nevertheless, because of the greenhouse effect of the huge amount of carbon dioxide, Venus heats itself very efficiently using little energy. Without its greenhouse effect, Venus would become an extremely cold world at -50 deg. C. Although Venus is closer to the Sun than earth, the energy that it receives from the Sun is less than that of earth. Nevertheless, Venus is a blazing inferno due to its thick atmosphere. From this complicated scenario, we can see how intricate is planetary climate formation. By the way, the amount of carbon dioxide in the earth’s atmosphere is maintained at a low level because, after melting in the seas, carbon dioxide transforms into calcium carbonate and is absorbed into the earth’s crust.

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