5-1 Most of Earth’s surface is covered with flowing water that radically changes the landscape

Upon first glance, Earth is unquestionably a blue planet. Unique among all known planets and moons discovered so far, Earth’s surface is mostly covered by liquid water. Other objects in our solar system have surprisingly large amounts of water, as shown in Figure 5-1, but rarely so obviously covering much of the surface. This chapter’s opening image showing our home planet is famously known as the “blue marble.” It was painstakingly created by stitching together months and months of satellite images, giving us an unprecedented view of every square mile of Earth’s surface. The water covering our planet has tremendous impacts on how the landscape is shaped, how rocks change, how our air moves, and how the surface temperature is maintained. This same water also influences what life-forms can live here.

Figure 5-1: RIVUXG Earth’s Water Amount in Comparison If you were to collect all of Earth’s water, it would be a ball about 860 mi in diameter. If you were to burst this imaginary ball of water, the resulting flow would cover the contiguous United States (lower 48 states) to a depth of about 107 mi. This is similar to how much water is suspected to be hiding on Mars and Saturn’s moon Enceladus and far less than exists beneath the surfaces of Jupiter’s moon Europa or Saturn’s moon Titan.

Nearly all of Earth’s water is in the form of liquid salt water like that found in the oceans, amounting to about 97%. The remaining 3% is largely locked up underground in the tiny pore spaces within rocks or as frozen ice near the poles. Only a very small fraction of Earth’s water is easily accessible, found as a liquid in rivers and lakes with even a smaller fraction as gaseous water vapor in Earth’s atmosphere. All of this water makes us wonder how it might impact the landscape and even how we live on our planet.

Earth’s Water Cycle

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One of the most important principles to know about living on a planet where the surface is dominated by liquid water is that water moves in various ways. One way that water moves is by flowing from one place to another, largely due to gravity. Water at the top of a mountain can flow downhill in the forms of streams and rivers, water on the ground can seep deep into the soil and even into and through rocks, and water in the air can fall downward as rain.

Liquid water can also move from one part of the ocean to another. One way this can occur is in the form of water waves driven by the wind or storms. Another way water can move in the ocean is in giant rivers driven by differences in water temperature or by differences in the amount of salt in seawater. Still another way water moves is by Earth’s spinning and the gravitational influences with our Sun and Moon.

A quite different, but still important, way that water moves is by changing in and out of its liquid form. When water freezes into a solid, it is called ice. Ice can move around the ocean as icebergs, or even fall from the sky as snow or hail. Alternatively, when water turns into a gas, it is known as water vapor—or steam—and can sometimes be seen as clouds moving across the sky, as seen in Figure 5-2. This water vapor can move around Earth quite quickly. If the water vapor in the sky turns back into a liquid, it can fall to Earth as rain. If this water vapor in the air freezes, it can fall to Earth as snow or hail. There are other ways water moves around our planet, but the important point is that when water changes forms among its liquid, solid, and gaseous states, it can quickly transfer vast amounts of energy around our spinning planet. This energy drives our short-term weather and long-term climate systems.

This movement of water around our planet in different forms through rising evaporation and falling precipitation—including flows in the atmosphere, across the surface, under the ground, and along ocean currents—is known as the water cycle. As illustrated in Figure 5-2, the water cycle is dominated by an exchange of water between the oceans and the atmosphere. This is because our planet is largely covered by oceans, rather than covered by land. Energy from the Sun evaporates water from the oceans, which rises in the form of water vapor. This water vapor then cools as it moves higher into the sky and, under the right conditions, changes back into liquid and precipitates down into the oceans as rain. Driven by the Sun’s energy, the water cycle is the continuous flow of water among the oceans, the atmosphere, and the land.

Figure 5-2: Earth’s Water Cycle The water cycle is the continuous movement of water through Earth’s crust, atmosphere, oceans, lakes, and streams. The numbers indicate the amount of water (in thousands of cubic kilometers per year) that flows among these reservoirs annually.

This process of evaporation of liquid into water vapor and precipitation downward as rain or snow also occurs over land, but in much smaller amounts, mostly because there is more oceanscape than landscape. Some of the rain that falls on land runs off downhill into streams, rivers, lakes, and the oceans. Much rain soaks into the ground and enters rock through cracks or tiny—nearly invisible—pore spaces within rocks. This underground water can then slowly flow toward rivers, lakes, or oceans. Alternatively, this water could evaporate back into the atmosphere. Another portion of this water is taken up by plants and eventually returned to the atmosphere. Snow that falls layer after layer and does not melt could stay locked up in giant ice fields for nearly a million years.

An important point to remember is that water has tremendous influences as it continuously moves through the water cycle. What we are probably most aware of is that this movement of water results in rain that helps crops grow. Perhaps less obvious to us is that flowing water can rapidly change the landscape. As water moves across the surface, whether as waves relentlessly pounding into a cliff or as a river digging deep canyons like the Grand Canyon, it continuously carves away at Earth’s surface, reshaping the landscape. Nearly invisible to us, but perhaps the most important, is that the water cycle helps move energy around our planet, impacting our short-term weather system and influencing the nature of our long-term climate.

Water’s Unique Properties

You probably remember that water is a molecule composed of two small hydrogen atoms and one larger oxygen atom, called H₂O. What you might not remember is that water has all sorts of fascinating properties. One is that when water gets cold enough to freeze, it expands in size. Perhaps this has unexpectedly happened to you when you put a plastic bottle of water in the freezer. This is a unique property, because most other substances get smaller and take up less space when they get colder. This is important for fish living in lakes that freeze in the winter, because ice takes up more space than the same amount of water. Ice also floats to the top of a lake, rather than sinking to the bottom, leaving space for fish to swim all winter long. This also impacts the landscape because when water seeps into rocks and expands upon freezing it breaks rocks apart.

Another important property of water is that the amount of heat needed to change the temperature of water is larger than many other substances. You might already know that swimming pools and lakes are quite slow to heat up in the summer and equally slow to cool down in the autumn. Water’s great resistance to changing temperature compared to other substances makes it a stable substance that life can depend upon even when short-term weather changes the temperature quickly and dramatically. This same ability to hold tremendous amounts of energy allows water vapor to move energy around our planet quickly wherever the wind takes it.

Different sides of an individual water molecule have different characteristics, giving water additional unique properties. One is that water molecules tend to cling to each other, making them flow from one place to another in continuous streams while sticking together. This property accounts for water collecting together and falling from the sky in the form of spherical drops. Water also tends to stubbornly stick to other surfaces. This same property allows it to spread out evenly across a surface, like a lake, or even within a substance, like inside a rock. Another property, compared to other liquids, is that water has a wide ability to break apart and dissolve a large variety of substances, including common substances such as salt, sugar, and ammonia. When mixed with acid, such as pollution-forming acid rain, water can even dissolve solid rock, like marble and limestone. As far as we know, life cannot exist without water. It is these unique properties taken together that make water a highly useful substance for life to thrive on our planet.