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Transcript
Chapter 6 A Survey of the Universe Alta High Astronomy Ancient Astronomers Had knowledge of the first 6 planets: Mercury, Venus, Earth, Mars, Jupiter, and Saturn Galileo Galilee was the first astronomer to use a telescope and publish his finding. He did not however invent it. Alta High Astronomy 19th Century Astronomers By the end of the 19th century astronomers had discovered additional planetary bodies including Uranus and Neptune They also discovered Saturn’s rings, comets, and many “minor planets” or asteroids Asteroid belt is located between Mars and Jupiter Alta High Astronomy 20th Century The 20th century brought nonoptical astronomy to the forefront, including the use of radar, and spacecraft to map our solar system. Alta High Astronomy Comparative Planetology To organize all of the data we use comparative planetology – a comparison of the different planets to the chemical and physical properties of earth. The distance of each planet from the Sun is known from Kepler’s laws once the scale of the solar system is set by radar-ranging on Venus. A planet’s (sidereal) orbital period can be measured from repeated observations of its location on the sky, so long as Earth’s own motion around the Sun is properly taken into account. A planet’s radius is found by measuring its angular size and then applying elementary geometry. Alta High Astronomy Comparative Planetology The masses of planets with moons may be calculated by application of Newton’s laws of motion and gravity, just by observing the moons’ orbits around the planets. The sizes of those orbits, like the sizes of the planets themselves, are determined by geometry. We also look at density which can be determined by d = m/v, where v = 4/3 πr3. Because the physical and chemical properties of Mercury, Venus, and Mars are somewhat similar to Earth’s, the four innermost planets are called the Terrestrial Planets. The larger outer planets—Jupiter, Saturn, Uranus, and Neptune—are all similar to one another chemically and physically They are labeled the Jovian Planets The jovian worlds are all much larger than the terrestrial planets Alta High Astronomy The Terrestrial Planets Earth like The four terrestrial planets all lie within about 1.5 A.U. of the Sun. All are small and of relatively low mass, and all have generally rocky composition and solid surfaces. Beyond that, however, the similarities end: All four terrestrial planets have atmospheres, but the atmospheres range from a near-vacuum on Mercury to a hot, dense inferno on Venus. Earth alone has oxygen in its atmosphere and liquid water on its surface. Surface conditions on the four planets range from barren, heavily cratered terrain on Mercury to widespread volcanic activity on Venus. Earth and Mars spin at roughly the same rate—one rotation every 24 (Earth) hours—but Mercury and Venus both take months to rotate just once, and Venus rotates in the opposite sense from the others. Earth and Mars have moons, but Mercury and Venus do not. Earth and Mercury have measurable magnetic fields, of very different strengths, whereas Venus and Mars have none. Alta High Astronomy Exploration of Our Solar System Mercury – Mariner 10 – 1973 – 3 flybys March & September 1974, March 1975. 45% of surface mapped. Venus – Mariner 5 and 10 did flybys but returned little data. Soviet Venera probed did most of the exploration. Venera 7 (1970) landed and returned data for 23 minutes before being crushed by atmospheric pressure. No Venera lander has lasted more than 1 hour. Venera 15 & 16 were orbiters which returned detailed radar maps. Pioneer Venus and Magellan are US probes. Magellan mapped 98% of surface. Alta High Astronomy Exploration of Our Solar System Mars – Mariner 4,6,9 photographed surface. Paving the way for the Viking landers 1,2 (1976) returned a great dal of data lasting until 1980 and 1982. Mars Global Surveyor Pathfinder (orbiter), Soujourner (rover) – 1997 return great pictures. Two failures – current spirit rovers. Alta High Astronomy Asteroids and Comets Asteroids and meteoroids reside in the asteroid belt – Between Mars and Jupiter and they have a rocky composition. Comets have an icy composition and resemble the moons of the outer planets. Since they are icy, they vaporize when they hit atmosphere. Alta High Astronomy Jovian: Jupiter like Jovian planets all lie between the Asteroid belt and the Kuiper belt They are all low density but high mass being composed mainly of Hydrogen and helium gas and have no solid surface. In fact, Saturn has a density less than water! With the exception of Uranus the Jovian planets have a differential rotation which creates a banded weather pattern on them. Both Jupiter have intense circular storms and strong winds Uranus rotates 90 on it’s side and has no internal energy. It also crosses paths with the dwarf planet Pluto. Alta High Astronomy The Outer Planets Voyager Pioneer 10 & 11 scouted for Voyager 1 &2 Voyager 1 – Jupiter and Saturn, especially Saturn’s moon Titan. Voyager 2 – Grand tour visiting all of the outer giants. Recent Missions Galileo to Jupiter Cassini to Saturn. Alta High Astronomy Comparing planets http://www.sciencenetlinks.com/interactives/messenger/psc/PlanetSize.html Alta High Astronomy