Terrestrial Planets
Surfaces of the terrestrial planets. For the terrestrial planets, the two major terrain-shaping processes have been impact cratering and thermal-tectonic activity. Crate red surfaces are presumably older terrain since evidence suggests that the intense period of cratering was some 4 billion years ago. The surface of Mercury is heavily cratered, resembling that of the moon but with some differences. This suggests that Mercury has had a history unlike that of the earth, which is dominated by thermal-tectonic activity. Mercury's surface features have remained relatively unchanged for 3 to 4 billion years.
Because of the obscuring cloud covers on Venus, direct observation of its surface is not possible. However, from a variety of measurements we can infer that both impact cratering and thermal-tectonic activity have shaped the surface of Venus. The younger portions of the surface may be on the order of millions of years old. Mars is more like the moon than the earth. Cratering and to some extent thermal-tectonic activity are responsible for the appearance of the Martian su rface. Almost half the surface has remained relatively stable and unchanged for 3 to 4 billion years. What appear to be channels carved by flowing liquid are present on Mars, suggesting that a warmer, denser atmosphere long ago allowed liquid water to exist on the surface.
Earth, unlike the other terrestrial planets, possesses a relatively young surface that has been shaped by thermal-tectonic activity which has erased evidence of the earlier impact-cratering phase. The oldest rocks are at most 3.8 billion years old. Earth probably never resembled the moon, Mercury, or Mars since thermal-tectonic activity has probably always been vigorous with that for Venus being less active and that for Mars, even less significant.
Internal structure of the terrestrial planets. Astronomers can infer the internal structure of a planet from knowledge of such aspects as its mean density, mass, shape, rotation rate, and surface conditions. Extensive seismic data exist only for the earth and moon. Critical assumptions must be made in order to develop a model for a planet's interior. Such models for the terrestrial planets resemble a model for the earth with three layers - core, mantle, and crust. At the time of their formation the planets' chemical composition should have been determined by their distance from the sun. Closer to the sun, the lighter gaseous and icy materials vaporize at the higher temperatures, leaving as solids the iron-rich rocky materials. Mercury has an iron-rich core comprising 65 percent of the total mass of the planet while the percentage for Venus is 38 percent, earth 33 percent, and Mars 26 percent. Mars has the smallest core for its size.
Atmospheres of the terrestrial planets. Mercu ry and the moon have almost no atmosphere. Mars and Venus have atmospheres dominated by a carbon dioxide composition with a small percentage of nitrogen. Earth's atmosphere has primarily a nitrogen composition with about 21 percent oxygen, which comes from the biological development of the planet. The composition of a planetary atmosphere depends on the planet's distance from the sun, chemical composition of the interior through outgassing and the greenhouse effect, evolution of the body, and interaction with living organisms if they exist.
Satellites of the terrestrial planets. The two satellites of Mars and the single satellite of the earth are the only satellites of the terrestrial planets. The satellites of Mars are small irregular bodies of rocky material and may be captured asteroids.
Asteroids. About 3000 asteroids have been discovered that occupy the space between 1.6 and 3.3 AU from the sun in nearly the plane of the ecliptic. They are less than 1000 kilometers in size, most are irregular in shape, and they represent two broad classes of chemical composition: either a silicate-rich one or one of carbon-rich or water-rich compounds.
Meteoroids, meteors, and meteorites. More than a thousand tons of cosmic debris pepper the earth's atmosphere everyday. The smaller meteoroids, which are the majority, evaporate in the earth's atmosphere, leaving the long visible streak characteristic of the meteor; larger ones fall to earth as meteorites. About 300 meteorite specimens have been recovered for study.
Interplanetary medium. Gas and interplanetary dust occupy the space between the planets. The dust particles, small solid grains located in the orbital planes of the planets, scatter sunlight, giving rise to the faint glow of the zodiacal light. Interplanetary gas, which is primarily protons and electrons, constitutes the solar wind rushing out from the sun through the solar system. The density of the interplanetary medium is quite low.
Surfaces of the terrestrial planets. For the terrestrial planets, the two major terrain-shaping processes have been impact cratering and thermal-tectonic activity. Crate red surfaces are presumably older terrain since evidence suggests that the intense period of cratering was some 4 billion years ago. The surface of Mercury is heavily cratered, resembling that of the moon but with some differences. This suggests that Mercury has had a history unlike that of the earth, which is dominated by thermal-tectonic activity. Mercury's surface features have remained relatively unchanged for 3 to 4 billion years.
Because of the obscuring cloud covers on Venus, direct observation of its surface is not possible. However, from a variety of measurements we can infer that both impact cratering and thermal-tectonic activity have shaped the surface of Venus. The younger portions of the surface may be on the order of millions of years old. Mars is more like the moon than the earth. Cratering and to some extent thermal-tectonic activity are responsible for the appearance of the Martian su rface. Almost half the surface has remained relatively stable and unchanged for 3 to 4 billion years. What appear to be channels carved by flowing liquid are present on Mars, suggesting that a warmer, denser atmosphere long ago allowed liquid water to exist on the surface.
Earth, unlike the other terrestrial planets, possesses a relatively young surface that has been shaped by thermal-tectonic activity which has erased evidence of the earlier impact-cratering phase. The oldest rocks are at most 3.8 billion years old. Earth probably never resembled the moon, Mercury, or Mars since thermal-tectonic activity has probably always been vigorous with that for Venus being less active and that for Mars, even less significant.
Internal structure of the terrestrial planets. Astronomers can infer the internal structure of a planet from knowledge of such aspects as its mean density, mass, shape, rotation rate, and surface conditions. Extensive seismic data exist only for the earth and moon. Critical assumptions must be made in order to develop a model for a planet's interior. Such models for the terrestrial planets resemble a model for the earth with three layers - core, mantle, and crust. At the time of their formation the planets' chemical composition should have been determined by their distance from the sun. Closer to the sun, the lighter gaseous and icy materials vaporize at the higher temperatures, leaving as solids the iron-rich rocky materials. Mercury has an iron-rich core comprising 65 percent of the total mass of the planet while the percentage for Venus is 38 percent, earth 33 percent, and Mars 26 percent. Mars has the smallest core for its size.
Atmospheres of the terrestrial planets. Mercu ry and the moon have almost no atmosphere. Mars and Venus have atmospheres dominated by a carbon dioxide composition with a small percentage of nitrogen. Earth's atmosphere has primarily a nitrogen composition with about 21 percent oxygen, which comes from the biological development of the planet. The composition of a planetary atmosphere depends on the planet's distance from the sun, chemical composition of the interior through outgassing and the greenhouse effect, evolution of the body, and interaction with living organisms if they exist.
Satellites of the terrestrial planets. The two satellites of Mars and the single satellite of the earth are the only satellites of the terrestrial planets. The satellites of Mars are small irregular bodies of rocky material and may be captured asteroids.
Asteroids. About 3000 asteroids have been discovered that occupy the space between 1.6 and 3.3 AU from the sun in nearly the plane of the ecliptic. They are less than 1000 kilometers in size, most are irregular in shape, and they represent two broad classes of chemical composition: either a silicate-rich one or one of carbon-rich or water-rich compounds.
Meteoroids, meteors, and meteorites. More than a thousand tons of cosmic debris pepper the earth's atmosphere everyday. The smaller meteoroids, which are the majority, evaporate in the earth's atmosphere, leaving the long visible streak characteristic of the meteor; larger ones fall to earth as meteorites. About 300 meteorite specimens have been recovered for study.
Interplanetary medium. Gas and interplanetary dust occupy the space between the planets. The dust particles, small solid grains located in the orbital planes of the planets, scatter sunlight, giving rise to the faint glow of the zodiacal light. Interplanetary gas, which is primarily protons and electrons, constitutes the solar wind rushing out from the sun through the solar system. The density of the interplanetary medium is quite low.
