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(疑似)原文未缩减 gitarrelieber
节选自Global Climate Change on Venus (New Light on the Solar System; Special Editions)
Author: Mark A. Bullock and David H. Grinspoon
THE STUNNING DIFFERENCES between the climates of Earth and Venus today are intimately linked to the history of water on these two worlds. Liquid water is the intermediary in reactions of carbon dioxide and surface rocks that can form minerals. In addition, water mixed into the underlying mantle is probably responsible for the low-viscosity layer, orasthenosphere, on which Earth’s lithospheric plates slide. The formation of carbonate minerals and their subsequent descent on tectonic plates prevent carbon dioxide from building up. Models of planet formation predict that the two worlds should have been endowed with roughly equal amounts of water, delivered by the impact of icy bodies from the outer solar system. But, when the Pioneer Venus mission went into orbit in 1978, it measured the ratio of deuterium to ordinary hydrogen within the water of Venus’s clouds. The ratio was an astonishing 150times the terrestrial value. The most likely explanation is that Venus once had far more water and lost it. When water vapor drifted into the upper atmosphere, solar ultraviolet radiation decomposed it into oxygen and either hydrogen or deuterium. Because hydrogen, being lighter, escapes to space more easily, the relative amount of deuterium increased. Why did this process occur on Venus but not on Earth? In 1969 Andrew P. Ingersoll of the California Institute of Technology showed that if the solar energy available to a planet were strong enough, any water at the surface would rapidly evaporate. The added water vapor would further heat the atmosphere and set up what he called the runaway greenhouse effect. The process would transport the bulk of the planet’s water into the upper atmosphere, where it would ultimately be decomposed and lost. Later James F. Kasting of Pennsylvania State University and his co-workers developed a more detailed model of this effect. They estimated that the critical solar flux required to initiate a runaway greenhouse was about 40 percent larger than the present flux on Earth. This value corresponds roughly to the solar flux expected at the orbit of Venus shortly after it was formed, when the sun was 30 percent fainter. An Earth ocean’s worth of water could have fled Venus in the first 30 million years of its existence. A shortcoming of this model is that if Venus had a thick carbon dioxide atmosphere early on, as it does now, it would have retained much of its water. The amount of water that is lost depends on how much of it can rise high enough to be decomposed—which is less for a planet with a thick atmosphere. Furthermore, any clouds that developed during the process would have reflected sunlight back into space and shut off the runaway greenhouse. So Kasting’s group also considered a solar flux slightly below the critical value. In this scenario, Venus had hot oceans and a humidstratosphere. The seas kept levels of carbon dioxide low by dissolving the gas and promoting carbonate formation. With lubrication from water in theasthenosphere, plate tectonics might have operated. In short, Venus possessed climate-stabilizing mechanisms similar to those on Earth today. But the atmosphere’s lower density could not prevent water from diffusing to high altitudes. Over 600 million years, an ocean’s worth of water vanished. Any plate tectonics shut down, leaving volcanism and heat conduction as the interior’s ways to cool. Thereafter carbon dioxide accumulated in the air.
This picture, termed the moist greenhouse, illustrates the intricate interaction of solar, climate and geologic change. Atmospheric and surface processes can preserve the status quo, or they can conspire in their own destruction. If the theory is right, Venus once had oceans—perhaps even life, although it may be impossible to know. |
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