The Inner Planets

The Inner Planets — page 1 — The Surface of Venus 44 Venus’s Surface Composition 47 Surface Features 48 Mountain Belts of Venus 50 Rifts 51 Coronae 52 Tesserae 53 Impact Craters 54 The Interior of Venus 56 Venus’s Volcanoes 58 Chapter3 Earth 63 Earth’s Atmosphere 67 Earth’s Hydrosphere 74 Earth’s Outer Shell 77 The Interior of Earth 79 Earth’s Geomagnetic Field and Magnetosphere 82 Earth’s Development 85 Impacted by Planetesimals 86

The Inner Planets — page 2 — Chapter4 Mars 90 Basic Features of Mars 92 Mars’s Atmosphere 95 Composition and Surface Pressure 97 The Surface of Mars 101 Valleys and Lakes 105 Outflow Channels and Oceans 106 Polar Sediments Ground Ice and Glaciers 107 Searching for Life on Mars 108 The Interior of Mars 112 The Moons of Mars 114 Conclusion 116 Glossary 118 Bibliography 121 Index 126

The Inner Planets — page 3 — INTRODUCTION Broadly a planet is defined as any relatively large natural body that revolves in an orbit around the Sun or around some other star and that is not radiating energy from internal nuclear fusion reactions In addition to the above description some scientists impose additional constraints constraints regarding characteristics such as size e g the object should be more than about 1,000 km 600 miles across or a little larger than the largest known asteroid Ceres shape it should be large enough to have been squeezed by its own gravity into a sphere i e roughly 700 km 435 miles across depending on its density or mass it must have a mass insufficient for its core to have experienced experienced even temporary nuclear fusion As the term is applied to bodies in Earth’s solar system the International Astronomical Union IAU which is charged by the scientific community community with classifying astronomical objects lists eight planets orbiting the Sun in order of increasing increasing distance they are Mercury Venus Earth Mars Jupiter Saturn Uranus and Neptune The planets can be divided into two groups The inner planets Mercury Venus Earth and Mars lie between the Sun and the asteroid belt The outer planets Jupiter Saturn Uranus and 6

The Inner Planets — page 4 — This illustration shows the approximate sizes of the planets rela tive to each other The inner planets are the first four from the Sun left noticeably smaller than the outer planets Neptune lie beyond the asteroid belt The inner planets are dense rocky and small Since Earth is a typical inner planet this group is sometimes called the terrestrial or Earth-like planets The inner planets along with the Moon have average densities in the range of 3 9 5 5 grams per cubic cm setting setting them apart from the four outer giant planets whose densities are all close to 1 gram per cubic cm the density of water The compositions of these two groups of planets must therefore be significantly different This dissimilarity is thought to be attributable to conditions that prevailed during the early development of the solar system The surfaces of the terrestrial planets and many satellites satellites show extensive cratering produced by high-speed impacts On Earth with its large quantities of water and an active atmosphere many of these cosmic footprints have eroded but remnants of very large craters can be seen in aerial and spacecraft photographs of the terrestrial surface surface On Mercury Mars and Earth’s Moon the absence of water and any significant atmosphere has left the craters unchanged for billions of years apart from disturbances 7

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