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　　天文类Surface Fluids on Venus and Earth

　　参考阅读：

　　A fluid is a substance， such as a liquid or gas， in which the component particles （usually molecules） can move past one another. Fluids flow easily and conform to the shape of their containers. The geologic processes related to the movement of fluids on a planet’s surface can completely resurface a planet many times. These processes derive their energy from the Sun and the gravitational forces of the planet itself. As these fluids interact with surface materials， they move particles about or react chemically with them to modify or produce materials. On a solid planet with a hydrosphere and an atmosphere， only a tiny fraction of the planetary mass flows as surface fluids. Yet the movements of these fluids can drastically alter a planet. Consider Venus and Earth， both terrestrial planets with atmosphere.

　　Venus and Earth are commonly regarded as twin planets but not identical twins. They are about the same size， are composed of roughly the same mix of materials， and may have been comparably endowed at their beginning with carbon dioxide and water. However， the twins evolved differently， largely because of differences in their distance from the Sun. With a significant amount of internal heat， Venus may continue to be geologically active with volcanoes， rifting， and folding. However， it lacks any sign of a hydrologic system （water circulation and distribution）： there are no streams， lakes， oceans， or glaciers. Space probes suggest that Venus may have started with as much water as Earth， but it was unable to keep its water in liquid form. Because Venus receives more heat from the Sun， water released from the interior evaporated and rose to the upper atmosphere where the Sun’s ultraviolet rays broke the molecules apart. Much of the freed hydrogen escaped into space， and Venus lost its water. Without water， Venus became less and less like Earth and kept an atmosphere filled with carbon dioxide. The carbon dioxide acts as a blanket， creating an intense greenhouse effect and driving surface temperatures high enough to melt lead and to prohibit the formation of carbonate minerals. Volcanoes continually vented more carbon dioxide into the atmosphere. On Earth， liquid water removes carbon dioxide from the atmosphere and combines it with calcium， from rock weathering， to form carbonate sedimentary rocks. Without liquid water to remove carbon from the atmosphere， the level of carbon dioxide in the atmosphere of Venus remainshigh.

　　Origin of the Solar System

　　Comets

　　文化艺术类

　　The Origins of Writing

　　Live Performance

　　The Origins of Theater

　　The Development of Printing

　　地质类

　　Early Theories of Continental Drift

　　Attempts at Determining Earth’s Age

　　How Soil is Formed

　　Earth’s Energy Cycle

　　Thermal Stratification

　　环境类

　　The Climate of Japan

　　The Role of the Ocean in Controlling Climate

　　经济类

　　Effects of the Commercial Revolution

　　Seventeenth-Century European Economic Growth

　　考古类

　　Environmental Impact of the Anasazi

　　The Collapse of the Mays

　　The Chaco Phenomenon

　　科学类

　　The Birth of Photography

　　Early American Printing Industry

　　农业类

　　Agricultural Society in Eighteenth- Century British America

　　Water Management in Early Agriculture

　　社会类

　　Population Growth in Nineteenth-Century Europe

　　Hunting and the Setting of Inner Eurasia

　　生物类

　　Extinctions at the End of the Cretaceous

　　The Cambrian Explosion

　　The Extinction of the Dinosaurs

　　How Animals in Rain Forests Make Themselves Heard

　　Sociality in Animals

　　Dinosaurs and Parental Care

　　Habitat Selection

　　Temperature Regulation in Marine Organisms

　　Cell Theory

　　Poikilotherms

　　Forest Succession

　　The Role of Diapause

　　The Identification of the Genetic Material

　　How Plants and Animals Arrived in the Hawaiian Islands

　　Constraints on Natural Selection