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"10.01 Overview T. Spohn, DLR Institut für Planetenforschung, Berlin, Germany 2007 Elsevier B.V. All rights reserved. 10.01.1 Introduction Humanity has always been fascinated with the wandering stars in the sky—the planets. Ancient astrologists observed and used the paths of the planets to time the seasons and predict the future. Observations of the planets helped J. Kepler formulate his laws of planetary motion, revolutionizing the perception of the world. With the advent of the space age, the planets have been transformed from bright spots in the sky into worlds that can be explored using geoscience tools such as in situ and remote sensing. The terrestrial planets are particularly interesting to geoscientists because comparison with our own planet allows a better understanding of Earth. Venus offers an example of a runaway greenhouse effect resulting in what we would call hellish conditions, with temperatures around 450°C and a corrosive atmosphere that is also optically nontransparent. This poses enormous difficulties for spacecraft exploration. Mars is much friendlier to explore but has undergone greenhouse effects and atmospheric loss processes, resulting in a cold and dusty desert. Aside from considerations of the usefulness of space exploration in terms of understanding Earth, the interested mind can visit astounding and puzzling places. Jupiter's dynamic atmosphere features a giant thunderstorm that has been raging for centuries. Saturn is known for its majestic rings, while Uranus and Neptune have complex magnetic fields. These giant planets also have moons that are similarly astounding. Io, a volcanic satellite of Jupiter, surpasses Earth in volcanic activity and surface heat flow. This activity is powered by tides that twist the satellite such that its interior partially melts. Enceladus, a much smaller moon of Saturn, has geysers possibly powered by tidal heating. Its volcanic activity releases water vapor rather than lava. Titan, another moon of Saturn, hides its surface underneath a layer of photochemical smog in a thick nitrogen atmosphere. Other moons of similar sizes lack any comparable atmosphere. Miranda, a satellite of Uranus, appears as if it has been ripped apart and reassembled. Triton, a satellite of Neptune, has geysers powered by solar irradiation. Magnetic field data suggest that icy moons orbiting the giant planets may have oceans underneath thick ice covers. These oceans can at least in principle harbor or have harbored life. Asteroids with moons and comets may still hold clues to how the solar system and life on Earth formed. This volume of the Treatise on Geophysics discusses fundamental aspects of planetary science, focusing on geophysical properties of Earth-like planets and moons—bodies consisting largely of rock, iron, and water—and processes occurring in their interiors and surfaces. It also explores giant planets and their satellites. 10.01.2 Our Planetary System The solar system contains a myriad of bodies ranging from the Sun to minuscule dust particles. Useful sources of information include Encyclopedias of Planetary Sciences (Shirley and Fairbridge, 1997), Solar System (Weissmann et al., 1999), and the Planetary Companion (Lodders and Fegley, 1998). Valuable collections of planetary data can be found on Solar System Dynamics and National Space Science Data Center websites. A collection of images of planetary surfaces is available in NASA's photojournal. The eight planets—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune—are shown in Figure 1 together with Pluto, the most prominent member of a new class of objects: dwarf planets. The International Astronomical Union (IAU) introduced this class in 2006. These are intermediate in size between terrestrial planets and small bodies such as cometary nuclei and most asteroids. Currently recognized dwarf planets include Pluto, Ceres, and Eris. According to the IAU resolution, a planet of our solar system is a celestial body that (1) orbits the Sun, (2) has sufficient mass for self-gravity to overcome rigid body forces so it assumes a hydrostatic equilibrium shape, and (3) has cleared its neighborhood. A dwarf planet satisfies conditions 1 and 2 but has not cleared its neighborhood and is not a satellite. The Sun contains 98.8% of the solar system's mass but only 0.5% of its angular momentum. Jupiter, still 300 times more massive than Earth (see Table 1), contains over 60% of the rest. Jupiter is the biggest of the giant planets, a group constituting a major subgroup of the solar system. Sun Jupiter Saturn Uranus Neptune Pluto and Charon Figure 1 The eight planets—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune—and the dwarf planet Pluto are shown in this compilation of NASA images with their correct relative sizes and ordered according to their distance from the Sun. Mercury is barely visible at left close to the arc of the surface of the Sun and Pluto is barely visible at the outer right. The dark spot on Jupiter is the shadow of Io, one of its major satellites. Overview 3 Table 1a Properties of Planets Terrestrial or Earth-like Planets Giant Planets Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Radius (km) .3302 .869 .974 .6419 Mass (10^24 kg) .46 .63 .4 Density (10^3 kg m^-3) 5.3 0 0.5 2.3 1.3 1.9 0.8 2.7 Uncompressed Density (kg m^-3) Rotational Period (da) .65 .0b .9973 .026 .4135 .4440 .7183b .6713 Inclination of rotation axis (°) 10.59 17.14 44.16 Orbital distance (AUc) .3871 .7233 5.203 .203 .572 19 .07 Orbital Period (ad) .2410 .6156 1.001 .882 .87 39 .16 0 Magnetic Moment (10^4 T radius^3) Effective Surface Temperature (K) Specific heat flow or luminosity (pW kg^-1) Table 1b Properties of Dwarf Planets Ceres Pluto Eris Radius (km) 476.2 1195 1200 Mass (10^24 kg) .01305 ? Density (10^3 kg m^-3) 2.08 2.03 ? Rotational period (da) 6.387b ? Orbital distance (semimajor axis) (AU) 2.766 39.48 67.73 Orbital period (ad) 4.6033 248.1 557.8 Orbital eccentricity 0.249 0.4400 Orbital inclination 17.14 44.16 Effective surface temperature (K) 44 30 Known satellites 1 (Charon) 1 (Champollion) Table 1c Properties of Major Satellites Moon Ganymede Titan Triton Earth Io Europa Jupiter Callisto Saturn Neptune Primary Radius (km) 1737. 1821. 1560. 2634. Mass (10^20 kg) 734.9 89 Ключевые слова: e, r, o