Touring Our Solar System opens with an investigation of the differences between the terrestrial and Jovian planets, followed by a discussion of the evolution of the planets. Included in the chapter is a detailed study of the moon's physical characteristics and history. An inventory of the solar system presents the prominent features and peculiarities of each planet (excluding Earth). The chapter closes with a discussion of the minor members of the solar system-asteroids, comets, and meteoroids.
Learning Objectives
After reading, studying, and discussing this chapter you should be able to:
•Describe the general characteristics of the two groups of planets in the solar system.
•Discuss the evolution of the planets.
•Describe the major features of the lunar surface and discuss the moon’s history.
•List the distinguishing features of each planet in the solar system.
•List and describe the minor members of the solar system.
Chapter Summary
•The planets can be arranged into two groups: the terrestrial (Earth-like) planets (Mercury, Venus, Earth, and Mars) and the Jovian (Jupiter-like) planets (Jupiter, Saturn, Uranus, and Neptune). Pluto is not included in either group. When compared to the Jovian planets, the terrestrial planets are smaller more dense, contain proportionally more rocky material, have slower rates of rotation, and lower escape velocities.
•The nebular hypothesis (discussed in Chapter 11) describes the formation of the solar system. Due to chemical differentiation, as the terrestrial planets formed, the denser metallic elements (iron and nickel) sank toward their centers, whereas the lighter substances (silicate minerals, oxygen, hydrogen) migrated toward their surfaces. Due to their surface gravities, Earth and Venus were able to retain the heavier gases, like nitrogen, oxygen, and carbon dioxide. Because of the very cold temperatures existing far from the sun, the fragments from which the Jovian planets formed contained a high percentage of ices-water carbon dioxide, ammonia, and methane.
•The lunar surface exhibits several types of features. Most craters were produced by the impact of rapidly moving interplanetary debris (meteoroids). Bright, densely cratered highlands make up most of the lunar surface. The dark, fairly smooth lowlands are called maria (singular, mare). Maria basins are enormous impact craters that have been flooded with layer-upon-layer of very fluid basaltic lava. All lunar terrains are mantled with a soil-like layer of gray, unconsolidated debris, called lunar regolith, which has been derived from a few billion years of meteoric bombardment. Much is still unknown about the moon's origin. One hypothesis suggests that a giant asteroid collided with Earth to produce the moon. Scientists conclude that the moon evolved in three phases 1) the original crust (highlands), 2) maria basins, and 3) youthful rayed craters.
•Mercury is a small, dense planet that has no atmosphere and exhibits the greatest temperature extremes of any planet. Venus, the brightest planet in the sky, has a thick, heavy atmosphere composed of 97 percent carbon dioxide, a surface of relatively subdued plains and inactive volcanic features, a surface atmospheric pressure 90 times that of Earth's, and surface temperatures of 475°C (900°F). Mars, the "Red Planet," has a carbon dioxide atmosphere only 1 percent as dense as Earth's, extensive dust storms, numerous inactive volcanoes, many large canyons, and several valleys of debatable origin exhibiting drainage patterns similar to stream valleys on Earth. Jupiter, the largest planet, rotates rapidly, has a banded appearance caused by huge convection currents driven by the planet's interior heat, a Great Red Spot that varies in size, a thin ring system, and at least sixteen moons (one of the moons, I0, is a volcanically active body). Saturn is best known for its system of rings. It also has a dynamic atmosphere with winds up to 930 miles per hour and "storms" similar to Jupiter's Great Red Spot. Uranus and Neptune are often called "the twins" because of similar structure and composition. A unique feature of Uranus is the fact that it rotates "on its side." Neptune has white, cirrus-like clouds above its main cloud deck and an Earth-sized Great Dark Spot, assumed to be a large rotating storm similar to Jupiter's Great Red Spot. Pluto, a small frozen world with one moon (Charon), may have once been a satellite of Neptune. Pluto's noticeably elongated orbit causes it to occasionally travel inside the orbit of Neptune, but with no chance of collision.
•The minor members of the solar system include the asteroids, comets, and meteoroids. Most asteroids lie between the orbits of Mars and Jupiter. No conclusive evidence has been found to explain their origin. Comets are made of frozen gases (water, ammonia, methane, carbon dioxide, and carbon monoxide) with small pieces of rocks and metallic material. Many travel in very elongated orbits that can carry them beyond Pluto and little is known about their origin. Meteoroids, small solid particles that travel through interplanetary space, become meteors when they enter Earth's atmosphere and vaporize with a flash of light. Meteor showers occur when Earth encounters a swarm of meteoroids, probably material lost by a comet. Meteorites are the remains of meteoroids found on Earth. The three types of meteorites (classified by their composition) are 1) irons, 2) stony, and 3) stony-irons. One rare kind of meteorite, called a carbonaceous chondrite, was found to contain amino acids and other organic compounds.