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 (fine-grained soil deposits). 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 Earths 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.

流体是一种物质，如液体或气体，流体的组成粒子（通常是分子）可以相互移动。流体很容易移动，形状会随容器的形状而变。与行星表面流体运动有关的地质过程可以多次使行星表面发生彻底的改变。这些地质过程从太阳和行星自身的引力中获得能量。当这些流体与表面物质相互作用时，它们的粒子发生交换或者发生化学反应以改变或者创造新的物质。在一个拥有水圈和大气的固体行星上，行星表面的流体只占其重量的一小部分。然而，这些流体的运动可以极大地改变一颗行星。比如金星和地球，这两颗类地行星都拥有大气层。金星和地球通常被认为是双胞胎行星，但它们却又不完全相同。它们的大小大致相同，大体上由相同的材料组成，并且相比其他行星，它们可能自诞生之时起就含有二氧化碳和水。然而，因为它们与太阳之间距离的不同，这对双胞胎行星的演变方式也大为不同。金星内部含有大量热能，所以在金星上，火山、裂陷、折叠等地质活动可能会一直很活跃。然而，它没有任何水文系统（水循环和分布）的迹象：没有溪流、湖泊、海洋或冰川。航天探测器显示，金星和地球的含水量在开始的时候可能是一样的，但是金星无法将水以液态的形式保存。因为金星从太阳那里接收到更多的热量，从内部释放的水分会被蒸发，上升到上层大气中，在那里太阳的紫外线会将水分子分解。大部分被释放出来的氢逃逸到太空中，而金星便失去了水。没有了水，金星和地球变得越来越不相似，在金星的大气层中充满了二氧化碳。二氧化碳起到毛毯的作用，产生强烈的温室效应，使金星的表面温度变得高到足以熔化铅，并阻止碳酸盐矿物的形成。火山不断向大气中排放更多的二氧化碳。在地球上，液态水从大气中带走二氧化碳，并使之与钙结合，从岩石风化，到形成碳酸盐沉积岩（都是二氧化碳与钙结合的实例）。金星上没有液态水去除大气中的碳，所以金星大气层中二氧化碳的含量一直很高。和金星一样，地球足够大，地质活动很活跃，它的引力也能维持大气层。而与金星不同的是，地球与太阳之间的距离刚刚好，因此这一温度范围能够使水以液体、固体和气体的形式存在。所以水极易流动，以连续的水文循环的形式在地球上快速地流动。在太阳的加热作用下，水从海洋循环到大气中，经过河流系统后，又再次回到海洋中。因此，地球的表面不断地发生变化，被侵蚀成复杂的河谷系统——这与其他行星表面布满的陨石坑形成了鲜明的对比。地球上只有极少数地区没有流动的水。因此，河谷是地球地貌的主要特征。同样，风力作用也将细颗粒从大片的区域吹走，使这些细颗粒在其他地方堆积成为由沙丘和黄土（精细的土壤沉积物）组成的茫茫沙海。这些流体运动是由太阳热能作用下产生的重力流系统导致的。当大气中的气体或水与地表的岩石发生反应形成性质不同的新化合物时，就会发生其他地质变化。这个过程中的一个重要例子就是从大气中除去地球上大部分的二氧化碳，形成碳酸盐岩。然而，如果地球离太阳稍微近一点，地球上的海洋就会蒸发；如果离太阳更远一些，海洋就会结冰。因为液态水的存在，自我复制的碳、氢、氧分子形成了地球早期的生命，并从根本上改变了地表，使大片陆地被绿色覆盖。生命在这个星球上蓬勃发展，而生命的产生也有利于为地球创造富氧和富氮的大气层和适宜的温度。