of Mars. New data is examined from the rovers all the time and adds to scientists’ understanding of Mars’s planetary processes.
Why does it matter if there is water on Mars (or any other planet)? One reason is that life on Earth requires water, which means if there is water on another planet, there may be life. Another reason scientists are interested in extraterrestrial water is that water will be necessary for any future human settlements on Mars. It may sound like a movie plot, but it’s real-life science!
Chapter 4
Home Sweet Home: Planet Earth
IN THIS CHAPTER
You could describe the earth as a ball of rock spinning through space, but the earth is made of much more than just rock. Planet Earth is multilayered. If you could travel from the moon to the earth’s core, you’d pass through layers that contain gas, liquid, rock, and metal. In this chapter, I briefly describe the various layers of the earth, including what scientists know about Earth’s interior layers (and how they learned about those layers).
Earth’s Spheres
The materials of Earth’s planetary system can be separated into spheres, or parts. These five major spheres of Earth’s system are illustrated in Figure 4-1:
Atmosphere: The atmosphere is a layer of gas that surrounds the entire planet. It serves the important role of protecting everything on Earth from being destroyed by the heat and radiation from the sun and makes life possible on Earth. Within the atmosphere, gases interact with water, forming weather systems that circulate air and clouds around the globe.
Hydrosphere: The hydrosphere includes all the water on Earth. The hydrologic cycle is the rotation of water through the hydrosphere: flowing as liquid (streams and rivers), evaporating into the atmosphere as gas (clouds), and falling to the surface as rain or snow.
Cryosphere: The cryosphere is composed of all the solid water, or ice, found on Earth’s surface. While closely tied to the hydrologic system, the cryosphere can be examined separately because of how massive amounts of surface ice affect the weather and climate systems.
Biosphere: All the organic materials on Earth — both living and dead organisms — are part of the biosphere.
Geosphere: The solid, rocky layers of the Earth, from the outermost crust to the very center, compose the planet’s geosphere. Within the geosphere, scientists have further divided the layers of rock material, which I describe in the next section of this chapter.
FIGURE 4-1: The five major spheres of Earth’s planetary system.
The earth’s spheres are connected to one another through a series of interactions. For example, rainfall from the hydrosphere causes movement of surface materials in the geosphere (a process called erosion, which I describe in Part 4). Rainfall also provides water for plants to grow in the biosphere. The interactions among spheres can be studied as subsystems of the earth. An example of a subsystem is the climate system, which is influenced by the interaction of atmosphere, biosphere, hydrosphere, and even geosphere.
Because Earth is one giant system, geologists study not only the rock materials on Earth but also how rocks in the geosphere interact with all the other spheres.
Examining Earth’s Geosphere
Many geologists study portions of the earth that can be seen. However, some of the most fascinating and still unanswered questions about the earth have to do with what is going on inside — beneath the rocks we can see and touch at the surface.
Humans do not yet have technology advanced enough to dig more than about 12 kilometers (about 7.5 miles) into the earth’s crust. So how do scientists know anything about the inside of the earth? They combine their observations of rocks on the surface with knowledge gained from laboratory experiments of temperature and pressure on different materials. Doing so gives them a pretty solid basis on which to make inferences about what occurs in places that can’t be directly observed.
Defining Earth’s layers
One way scientists separate the layers of Earth’s geosphere is by physical properties, or whether the layers are liquid or solid.
Because geologists cannot see inside the earth, they make observations about Earth’s internal properties by proxy: by interpreting information from earthquake waves that can be used to make inferences about the physical properties of Earth’s interior.
P waves travel quickly through solid materials and slow down, slightly changing direction, as they move through liquid materials. By recording where each P wave starts and how long it takes to reach the other side of the planet, scientists have recognized that it must move through regions of solid and liquid materials within the earth.
S waves travel through solid materials but cannot travel through liquid at all. When scientists record the path that S waves take through the earth, they find that some S waves never reach the other side — they simply disappear, suggesting that they have hit a section of liquid material.
Figure 4-2 illustrates how the earthquake waves would travel if the interior of the earth was made of one continuous, solid type of material. And Figure 4-3 illustrates how P waves and S waves actually travel through Earth, illustrating the different physical properties of its interior.
