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Summary of Key Ideas

Earth: A Dynamic, Vital World

Earth’s atmosphere is about four-fifths nitrogen and one-fifth oxygen. This abundance of oxygen is due to the biological processes of plant life on the planet.
Earth’s atmosphere is divided into layers named the troposphere, stratosphere, mesosphere, and ionosphere. Ozone molecules in the stratosphere absorb ultraviolet light rays.
The outermost layer, or crust, of Earth offers clues to the history of our planet.
Earth’s surface is divided into huge plates that move over the upper mantle. Movement of these plates, a process called plate tectonics, is caused by convection in the mantle. Also, upwelling of molten material along cracks in the ocean floor occurs during seafloor spreading. Plate tectonics is responsible for most of the major features of Earth’s surface, including mountain ranges, volcanoes, and the shapes of the continents and oceans.
Study of seismic waves (vibrations produced by earthquakes) shows that Earth has a small, solid inner core surrounded by a liquid outer core. The outer core is surrounded by the dense mantle, which in turn is surrounded by the thin, low-density crust on which we live. Earth’s inner and outer cores are composed primarily of iron. The mantle is composed of iron-rich minerals.
Earth’s magnetic field produces a magnetosphere that surrounds the planet and deflects the solar wind.
Some charged particles from the solar wind are trapped in two huge, doughnut-shaped rings called the Van Allen radiation belts. An Earthward deluge of particles from a coronal mass ejection on the Sun can pierce the belts and produce exceptional auroras.

The Moon and Tides

The Moon has heavily cratered highlands and relatively smooth-surfaced maria.
Impacts have been the only significant “weathering” agent on the Moon; the Moon’s regolith (pulverized rock layer) was formed by meteoritic action.
Lunar rocks brought back to Earth contain no water and are depleted of volatile elements.
Powdered into regolith, the anorthosite rock of the highland is brighter than the powdered basalts of the maria.
Many lunar rock samples are solidified lava formed largely of minerals also found in Earth rocks.
Anorthositic rock in the lunar highlands was formed between 4.0 and 4.3 billion years ago, whereas the mare basalts solidified between 3.1 and 3.8 billion years ago. The Moon’s surface has undergone very little geologic change over the past 3 billion years.
Frozen water has been discovered in numerous places just below the Moon’s surface.
The collision-ejection theory of the Moon’s origin, accepted by most astronomers, holds that the young Earth was struck by a huge planetesimal, and debris from this collision coalesced to form the Moon.
The Moon was molten in its early stages, and the anorthositic crust solidified from low-density magma that floated to the lunar surface. The mare basins were created later by the impact of planetesimals and were then filled with lava from the lunar interior.
Gravitational interactions between Earth and the Moon produce tides in the oceans of Earth and set the Moon into synchronous rotation. The Moon is moving away from Earth, and, consequently, Earth’s rotation rate is decreasing.

WHAT DID YOU THINK?

Can Earth’s ozone layer, which has been partially depleted, be naturally replenished? Yes. Ozone is created continuously from normal oxygen molecules by their interaction with the Sun’s ultraviolet radiation.
Who was the first person to walk on the Moon, and when did this event occur? Neil Armstrong was the first person to set foot on the Moon. He and Buzz Aldrin flew on the Apollo 11 spacecraft piloted by Michael Collins. Armstrong and Aldrin set down the Eagle Lander on the Moon on July 20, 1969.
Do we see every part of the Moon’s surface at some time during the lunar cycle? No. Because the Moon’s rotation around Earth is synchronous, we always see the same side. The far side of the Moon has been seen only from spacecraft that pass or orbit it.
Does the Moon rotate and, if so, how fast? The Moon rotates at the same rate that it revolves around Earth, once every 27.3 Earth days. If the Moon did not rotate, then, as it revolved, we would see its entire surface from Earth, which we do not.
What causes the ocean tides? The tides are created by gravitational forces from the Moon and Sun combined with Earth’s motion around the barycenters between it and those other two bodies.
When does the spring tide occur? Spring tides occur during each full and new Moon.