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
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
The Inner PlaneTs produced by infrequent later impacts Volcanic activity has been an important force in the shaping of the surfaces of the Moon and the terrestrial planets We can learn a lot about Earth from our inner-planet neighbors Mercury is too hot to retain an atmosphere but Venus’s brilliant white appearance is the result of it being completely enveloped in thick clouds of carbon dioxide impenetrable at visible wavelengths Below the upper clouds Venus has a hostile atmosphere containing clouds of sulfuric acid droplets The cloud cover shields the planet’s planet’s surface from direct sunlight but the energy that does filter through warms the surface which then radiates at infrared wavelengths The long-wavelength infrared radiation radiation is trapped by the dense clouds such that an efficient greenhouse effect keeps the surface temperature near 465 C 870 F 740 K Radar which can penetrate the thick Venusian clouds has been used to map the planet’s surface In contrast the atmosphere of Mars is very thin and is composed mostly of carbon dioxide 95 percent with very little water vapour the planet’s surface pressure is only about 0 006 that of Earth The outer planets have atmospheres atmospheres composed largely of light gases mainly hydrogen and helium The inner planets are similar to Earth in many ways and in other ways very different This gives scientists great opportunities to study the nature and origin of Earth 8
CHAPTER 1 Mercury The planet that orbits closest to the Sun is Mercury It is also the smallest of the eight planets in the solar system These features make Mercury difficult to view from Earth as the small planet rises and sets within about two hours of the Sun Observers on Earth can only ever see the planet during twilight when the Sun is just below the horizon The difficulty in seeing it notwithstanding Mercury was known at least by Sumerian times some 5,000 years ago In Classical Greece it was called Apollo when it appeared as a morning star just before sunrise and Hermes the Greek equivalent of the Roman god Mercury when it appeared as an evening star just after sunset Hermes was the swift messenger of the gods and the planet’s name is thus likely a reference to its rapid motions relative to other objects in the sky Even in more recent eras many sky observers passed their entire lifetimes without ever seeing Mercury It is reputed that Nicolaus Copernicus whose heliocentric 9
