Stephen P. Broker
The last three decades have seen a tremendous growth in our understanding of Solar System structure and dynamics. The space program has sent manned missions to the Moon and unmanned missions to the terrestrial and the gaseous planets. We have learned that the Solar System is incredibly diverse, with no two bodies (major planets or their satellites) very similar to each other.
Distance from the Sun to each of its planets is measured in astronomical units (AU), one AU being defined as the average distance from the Sun to Earth, or 149,597,870 kilometers (about 93 million miles). The innermost planet, Mercury, is 0.3871 AU from the Sun, Earth is by definition 1.00 AU away, and the most distant planet, Pluto, with a highly elliptical orbit of the Sun, averages 39.44 AU. There are periods in its annual orbit when Pluto moves within the orbit of Neptune, as is the case from 1979 to 1999. Neptune thus is the planet farthest from the Sun today. Check the National Audubon Society field guide for details on the planets.
I include here brief information on Mars and on the four Galilean satellites (Io, Europa, Ganymede, and Callisto) which applies to this unit. Mars (D = 6786 km; 1.5237 AU; escape velocity 5.0 km/sec) has a widely varied topography colored a distinctive red, huge volcanic mountains including Olympus Mons, the largest known volcano in the Solar System, deep valleys, a very thin atmosphere of carbon dioxide with some nitrogen and argon, huge dust storms, polar ice caps of carbon dioxide and water, evidence of liquid surface water early in its history, meteorite impacts, and two small satellites (Phobos and Deimos) which are believed to be captured asteroids. Mars has been explored by Mariner 4 (1965) and by Viking 1 and 2 landers in 1975.
The Galilean moons of Jupiter are extraordinarily diverse in their composition and dynamics: Io is the most volcanically active object in the Solar System; Europa is covered by an ocean of deep ice and may be geologically active and harbor life forms; Ganymede is the largest satellite of the Solar System; Callisto is completely cratered. Four smaller outer satellites of Jupiter probably are captured asteroids. Uranus (D = 51,118; 19.1914 AU; escape velocity 21.3 km/sec), discovered by Herschel in 1781, rotates in reverse direction from the other planets and has a severely tilted axis suggesting it was knocked into its present orientation by a Solar System body. Pluto (D—2300 km; 39.5294 AU; escape velocity 1.1 km/sec), discovered by Tombaugh in 1930, is an anomaly of the outer Solar System—smallest of the planets, farthest from the Sun, least dense, not gaseous like its outer neighbors but rocky, with a strongly elliptical orbit, a thin atmosphere, extreme cold, and one known satellite, Charon (discovered in 1978). Pluto may be an exhausted comet captured from the trans-Neptunian belt of comets called the Kuiper Belt (see below), and Charon may be a comet captured by Pluto. Careful examination of meteorites originating from the asteroid belt, the Moon, and Mars has provided further information on the diversity of the Solar System.
