SUMMARY OF KEY IDEAS

• 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 absorbing carbon dioxide and giving off oxygen as a waste product. Earth’s atmosphere is divided into layers named the troposphere, stratosphere, mesosphere, and ionosphere. Ozone molecules in the stratosphere absorb ultraviolet light rays.

• 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 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.

• 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.

• All four inner planets are composed primarily of rock and metal, and thus they are classified as terrestrial.

• Even at its greatest orbital elongations, Mercury can be seen from Earth only briefly after sunset or before sunrise.

• The Mercurian surface is pocked with craters like the Moon’s, but extensive, smooth plains lie between these craters. Long cliffs meander across the surface of Mercury. These scarps formed as the planet cooled, solidified, and shrank.

• The long-ago impact of a large object formed the huge Caloris Basin on Mercury and shoved up jumbled hills on the opposite side of the planet. Several similar but smaller impact features also exist on Mercury and on our Moon.

• Mercury has an iron core, which fills more of its interior than Earth’s core fills Earth.

• Mercury has a weak, global magnetic field that partially shields it from the solar wind.

• Venus is similar to Earth in size, mass, and average density, but it is covered by unbroken, highly reflective clouds composed primarily of sulfuric acid that conceal its other features from observers using visible-light telescopes.

• Although most of Venus’s atmosphere is carbon dioxide, its clouds are composed of sulfuric acid mixed with yellowish sulfur dust. Active volcanoes on Venus are likely to be a constant source of this sulfurous veil.

• Venus’s exceptionally high temperature is caused by the greenhouse effect, as the dense carbon dioxide atmosphere traps and retains heat emitted by the planet. The surface pressure on Venus is 92 atm, and the surface temperature is 750 K (890° F). Both temperature and pressure decrease as altitude increases.

• The surface of Venus is surprisingly flat and mostly covered with gently rolling hills. It has very few impact craters compared to Mercury or our Moon. There are two major “continents” and several large volcanoes. The surface of Venus shows evidence of local tectonic activity but not the large-scale motions that play a major role in continually reshaping Earth’s surface.

• Earth-based observers found that the Martian solar day is nearly the same as that of Earth, that Mars has polar ice caps of carbon dioxide snow that expand and shrink with the seasons, and that the Martian surface undergoes seasonal color changes.

• The Martian surface has many flat-bottomed craters, several huge volcanoes, a vast equatorial canyon, and dried-up riverbeds—but no canals formed by intelligent life. River deltas and dry riverbeds on the Martian surface indicate large amounts of water once flowed there.

• Liquid water would quickly boil away in Mars’s thin present-day atmosphere, but the planet’s polar ice caps contain significant quantities of frozen water, and a layer of permafrost exists beneath parts of the regolith.

• The Martian atmosphere is composed mostly of carbon dioxide. The surface pressure is less than 0.01 atm.

• Mars has no global magnetic fields, but local fields pierce its surface in many places.

• Mars has two irregularly shaped moons, Phobos and Deimos. Both are in synchronous rotation with Mars. How they came into orbit is still under investigation.