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
Mercury as seen by the Messenger probe January 14 2008 model of the heavens in the 16th century explained why Mercury and Venus always appear in close proximity to the Sun expressed a deathbed regret that he had never set eyes on the planet Mercury himself AN INTRODUCTION TO MERCURY Until the last part of the 20th century Mercury was one of the least-understood planets and even now the shortage of information about it leaves many basic questions unsettled unsettled Indeed the length of its day was not determined until the 1960s and Mercury’s nearness to the Sun gave scientists 10
Mercury scientists bound to Earth many observational hurdles which were overcome only by the Messenger MErcury Surface Space ENvironment GEochemistry and Ranging probe Messenger was launched in 2004 flew past the planet twice in 2008 and once in 2009 and settled into orbit in 2011 It mapped the entire surface of Mercury before crashing into the planet in 2015 Mercury’s proximity to the Sun has also been exploited to confirm predictions made by relativity theory about the way gravity affects space and time At first glance the surface of the planet looks similar to the cratered terrain of the Moon an impression reinforced reinforced by the roughly comparable size of the two bodies Mercury is far denser however having a metallic core that takes up about 61 percent of its volume compared with 4 percent for the Moon and 16 percent for Earth Moreover its surface shows significant differences from lunar terrain including a lack of the massive dark-coloured lava flows known as maria and the presence of buckles and scarps that suggest Mercury is shrinking Mercury is an extreme planet in several respects Because of its nearness to the Sun its average orbital distance is 58 million km 36 million miles it has the shortest year a revolution period of 88 days and receives the most intense solar radiation of all the planets With a radius of about 2,440 km 1,516 miles Mercury is the smallest major planet smaller even than Jupiter’s largest 11
