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 atmospheres.

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 remains high.

Like Venus, Earth is large enough to be geologically active and for its gravitational field to hold an atmosphere. Unlike Venus, it is just the right distance from the Sun so that temperature ranges allow water to exist as a liquid, a solid, and a gas. Water is thus extremely mobile and moves rapidly over the planet in a continuous hydrologic cycle. Heated by the Sun, the water moves in great cycles from the oceans to the atmosphere, over the landscape in river systems, and ultimately back to the oceans. As a result, Earth’s surface has been continually changed and eroded into delicate systems of river valleys—a remarkable contrast to the surfaces of other planetary bodies where impact craters dominate. Few areas on Earth have been untouched by flowing water. As a result, river valleys are the dominant feature of its landscape. Similarly, wind action has scoured fine particles away from large areas, depositing them elsewhere as vast sand seas dominated by dunes or in sheets of loess. These fluid movements are caused by gravity flow systems energized by heat from the Sun. Other geologic changes occur when the gases in the atmosphere or water react with rocks at the surface to form new chemical compounds with different properties. An important example of this process was the removal of most of Earth’s carbon dioxide from its atmosphere to form carbonate rocks. However, if earth were a little closer to the Sun, its oceans would evaporate, if it were farther from the Sun, the oceans would freeze solid. Because liquid water was present, self-replicating molecules of carbon, hydrogen, and oxygen developed life early in Earth’s history and have radically modified its surface, blanketing huge parts of the continents with greenery. Life thrives on this planet, and it helped create the planet’s oxygen- and nitrogen-rich atmosphere and moderate temperatures.

流体与液体或气体一样，是一种物质，其中组分颗粒（通常是分子）可以相互移动。流体可以按照其容器的形状自由地流动。与行星表面流体流动有关的地质作用可以多次完全重现行星的表面。这些地质作用可以从太阳，以及行星本身的引力中获取能量。当这些液体与表层材料相互作用时，液体会移动颗粒或与它们发生化学反应以改变或产生材料。在具有水圈和大气的固体行星上，只有很小一部分行星质量流属于表面流体。然而这些流体的流动可以彻底改变一个星球。但是金星和地球这两个类地行星都有大气。 金星和地球通常被视为是双行星，但不是同卵双胞胎。它们的大小大致相同，由大致相同的材料组成，并且可能在创建初期拥有相同数量的二氧化碳和水。然而，这对双行星之后进行了不同的演变，这主要是因为它们与太阳的距离不同。金星拥有大量的内热，地质可能会继续活跃，出现火山，裂谷和褶皱。然而，它没有任何的水文系统（水循环和分布）：没有溪流，湖泊，海洋或冰川。太空探测表明，金星最初的水量可能与地球一样多，但金星无法将其水保持在液态。因为金星从太阳获得更多热量，从内部释放的水蒸发并升至高层大气，太阳紫外线致使分子分裂。大部分游离氢逸入太空，因而金星失去了水。没有了水之后，金星变得越来越不像地球，并大气中充满了二氧化碳。二氧化碳就像一个毯子一样，产生强烈的温室效应，并致使表面温度升高，高到足以熔化铅，并禁止碳酸盐矿物的形成。火山不断向大气排放二氧化碳。在地球上，液态水可以去除大气中的二氧化碳，并将其与岩石风化的钙结合，形成碳酸盐沉积岩。没有液态水来去除大气中的碳，金星大气中的二氧化碳含量仍然很高。 与金星一样，地球足够大，因而地质活跃，并且它的引力场能够吸引住大气。与金星不同的是，正是因为地球与太阳之间非常恰当，因此地球的温度范围允许水以液体，固体和气体的形式存在。因此，水的流动性非常强，并且可以在持续的水文循环中迅速的转化。在太阳的加热下，水可以进行一个大循环，从大洋转移到大气，经过河流系统，最终回归到海洋。因此，地球表面不断变化，并侵蚀成精细的河谷系统 - 这与全是撞击坑的其他行星体的表面形成鲜明对比。地球上，很少有地区没有流动的水。因此，河谷是地球上地形的主要特征。同样的，风力作用已经从大面积的地方冲刷出了细小的颗粒，将颗粒沉积在其他地方，形成由沙丘或黄土覆盖的巨大沙滩。在太阳热量的作用下，重力流体系引发了这些流体运动。当大气或水中的气体与表面的岩石产生反应，形成具有不同性质的新化合物时，会引发其他地质变化。一个可以展现这个过程的重要例子就是，从大气中去除大部分地球的二氧化碳，形成碳酸盐岩。然而，如果地球距离太阳更近一些的话，那么地球上的海洋就会蒸发，如果地球离太阳更远一些的话，海洋就会冻成固体。因为液态水的存在，碳、氢、氧中的自我复制分子在地球形成的早期就已经发展成为生命体，并且已经从根本上改变了地球的表面，让大部分的大陆覆盖着绿色植物。这个行星上生命可以茁壮成长，有助于为地球创造一个富氧、富氮的大气，并使得地球温度适宜。