The basic properties of the Earth are summarized in Table 7.1. The Earth is composed primarily of heavy elements such as iron, silicon, and oxygen. This is very different from the composition of the Sun and other stars which are primarily hydrogen and helium. Earth is the only planet (in our solar system) with significant amounts of water, and seems to be the only one with surface conditions that can support life as we know it.
We divide the interior of the Earth by layers into the crust, mantle, core, and inner core. The outer part of the core of the Earth is liquid, and the inner part is probably solid.
The circulation of material in the liquid core produces a magnetic field in and around the Earth. Above the atmosphere, the magnetic field traps electrons and other charged particles in a region known as the magnetosphere. Most of the charged particles come from the Sun, their collective effect known as the solar wind. The magnetic field is important for protecting the Earth from the direct onslaught of the solar wind. The region of space where the solar wind and the magnetosphere meet is called the magnetopause.
The material in the Earth's crust has undergone significant modification due to heating, erosion, movement through different depths of the Earth. None of the rock on Earth is left in its original, "primitive" state. To study the primordial material from which the planets formed, we must go to comets, asteroids, and small moons.
Heat from the Earth's interior drives currents within the mantle (imagine a pot of water on a stove). These currents in turn cause the regions of the crust called plates to move (see Figure 7.6). The force of two plates colliding can raise mountains or produce earthquakes. Where two plates are separating, material from within the mantle can rise to the surface and produce new crust.
In the early twentieth century, Alfred Wegener developed the theory of plate tectonics which proposed, for the first time, continental drift. Not understanding how a force large enough to move a continent could be produced, most scientists initially rejected the idea. (See p. 154 for a discussion.)
Life on Earth depends on the atmosphere.
The atmosphere is divided in layers. I won't ask you to memorize the layers.
The atmosphere is mostly nitrogen (N2). The most abundant components of the atmosphere are:
| component | symbol | mass percentage |
|---|---|---|
| nitrogen | N2 | 78% |
| oxygen | O2 | 21% |
| argon | A | 1% |
| water vapor | H2O | trace |
| carbon dioxide | CO2 | trace |
The source of the material in the Earth's atmosphere is still an open issue. It could have been here when the Earth originally formed, it could have been released from the interior (by volcanos) at a later time, or it could come from the impact of icy comets. Our current evidence and understanding favors the comet hypothesis. Note: The comet impacts would also account for the water on Earth!
Earlier, I raised the hypothesis that the gases of the Earth's atmosphere have come from comet impacts after the formation of the Earth. In essence then, life on Earth depends on influences from outside the Earth. These influences continue to this day, some of them being quite dramatic.
One of the most