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Lecture 10

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SCI 238
Mike Fich

Mars Mars lacks an ozone layer, so much of the Sun’s damaging ultra- violet radiation passes unhindered to the surface. Moon The lunar maria are generally circular because they are essentially flooded craters (and craters are almost always round). Their dark color comes from the dense, iron-rich rock (basalt) that rose up from the lunar mantle as molten lava. Mercury The most surprising features of Mercury are its many tremendous cliffs—evidence of a type of past tectonics quite different from anything we have found on any other terrestrial world. They probably formed when tectonic forces compressed the crust, causing the surface to crum- ple. Because crumpling would have shrunk the portions of the surface it affected, Mercury as a whole could not have stayed the same size unless other parts of the surface expanded. Early in its history, Mercury’s larger size and greater iron content allowed it to gain more internal heat from accre- tion and differentiation than did the Moon. This heat caused the large iron core to swell. Later, as the core cooled, it contracted by perhaps as much as 20 kilometers in radius. The mantle and lithosphere must have contracted along with the core, generating the tectonic stresses that created the great cliffs. The contraction probably also closed off any remaining volcanic vents, ending Mercury’s period of volcanism. Why is Venus so hot? It’s tempting to attribute Venus’s high surface temperature solely to the fact that it is closer than Earth to the Sun, but Venus would actually be quite cold without its strong greenhouse effect. Venus absorbs less sunlight than Earth, despite being closer to the Sun, because its clouds reflect so much sunlight back to space. Venus has a far thicker atmosphere than Earth—its surface pressure is about 90 times that on Earth—and this atmosphere is about 96% carbon dioxide. The total amount of carbon dioxide in Venus’s atmosphere is nearly 200,000 times that in Earth’s atmosphere, and it creates an extremely strong greenhouse effect. Global Winds and Storms Storms around low-pressure regions tend to circulate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Earth’s rotation causes these circulation patterns by diverting north- or south-flowing air. (More technically, Earth’s rota- tion produces something called the Coriolis effect, Uranus weather The greatest surprise in jovian weather comes from Uranus. When Voyager 2 flew past Uranus in 1986, photographs revealed virtually no clouds and no banded structure like those found on the other jovian plan- ets. Scientists attributed the lack of weather to the lower internal heat of Uranus. However, subsequent observations from the Hubble Space Tele- scope and ground-based adaptive optics telescopes showed storms raging in Uranus’s atmosphere. The storms were brewing because of the changing seasons: Thanks to Uranus’s extreme axis tilt and 84-year orbit of the Sun, its northern hemisphere was seeing sunlight for the first time in decades. Lecture 10. The formation of the SS and Extrasolar planets 1. Patterns of motion among large bodies. The Sun, planets, and large moons generally orbit and rotate in a very organized way: a. All planetary orbits are nearly circular and lie nearly in the same plane. b. All planets orbit the Sun in the same direction: counterclockwise as viewed from high above Earth’s North Pole. c. Most planets rotate in the same direction in which they orbit, with fairly small axis tilts. The Sun also rotates in this direction. d. Most of the solar system’s large moons exhibit similar properties in their orbits around their planets, such as orbiting in their planet’s equatorial plane in the same direction that the planet rotates. 2. Two major types of planets. The eight planets divide clearly into two groups: the small, rocky planets that are close together and close to the Sun, and the large, gas-rich planets that are farther apart and farther from the Sun. a. Terrestrial: small and dense, with rocky surfaces and an abundance of metals deep in their interiors. They have few moons, if any, and no rings. We often count our Moon as a fifth terrestrial world, because its history has been shaped by the same processes that have shaped the terrestrial planets. b. Jovian: much larger in size and lower in average density than the terrestrial planets, and they have rings and many moons. They lack solid surfaces and are made mostly of hydrogen, helium, and hydrogen compounds—compounds containing hydrogen, such as water (H2O), ammonia (NH3), and methane (CH4). Because these substances are gases under earthly conditions, the jovian planets are sometimes called ―gas giants.‖ 3. Asteroids and comets. Between and beyond the planets, vast numbers of asteroids and comets orbit the Sun; some are large enough to qualify as dwarf planets. The locations, orbits, and compositions of these asteroids and comets follow distinct patterns. a. Asteroids are rocky bodies that orbit the Sun much like planets, but they are much smaller. Even the largest asteroids are much smaller than our Moon. Most known asteroids are found within the asteroid belt between the orbits of Mars and Jupiter. b. Comets: made of ices (water ice, ammonia ice, methane ice) mixed with rock. The vast majority of comets never visit the inner solar system. Instead, they orbit the Sun in one of the two distinct regions  region beyond the orbit of Neptune that we call the Kuiper belt. Contains at least 100,000 icy objects, of which Pluto and Eris are the largest known. The second cometary region, called the Oort cloud, is much farther from the Sun and may contain a trillion comets. These comets have orbits randomly inclined to the ecliptic plane, giving the Oort cloud a roughly spherical shape. 4. Exceptions to the rules. The generally orderly solar system also has some notable exceptions. For example, only Earth has a large moon among the inner planets, and Uranus is tipped on its side. A successful theory must make allowances for exceptions even as it explains the general rules. Venus rotates ―backwards‖ Nebular model of the SS Formation - formed from the gravitational collapse of an interstellar cloud of gas. = nebular hypothesis. - 20 century – Nebular theory - universe as a whole is thought to have been born in the Big Bang, which essentially produced only two chemical elements]: hydrogen and helium. Heavier elements were produced later by massive stars and released into space when the stars died. The heavy elements then mixed with other interstellar gas to form new generations of stars - Heating, Spinning, and Flattening As the solar nebula shrank in size, three important processes altered its density, temperature, and shape, changing it from a large, diffuse (spread-out) cloud to a much smaller spinning disk (Figure 6.15): • Heating. The temperature of the solar nebula increased as it collapsed. Such heating represents energy conservation in action [Section 4.3]. As the cloud shrank, its gravitational potential energy was converted to the kinetic energy of individual gas particles falling inward. These particles crashed into one another, converting the kinetic energy of their inward fall to the random motions of thermal energy (see Figure 4.12b). The Sun formed in the center, where temperatures and densities were highest. • Spinning. Like an ice skater pulling in her arms as she spins, the solar nebula rotated faster and faster as it shrank in radius. This increase in rotation rate represents conservation of angular momentum in action. The rotation of the cloud may have been imperceptibly slow before its collapse began, but the cloud’s shrinkage made fast rotation inevitable. The rapid rotation helped ensure that not all the material in the solar nebula collapsed into the center: The greater the angular momentum of a rotating cloud, the more spread out it will be. • Flattening. The solar nebula flattened into a disk. This flattening is a natural consequence of collisions between particles in a spinning cloud. A cloud may start with any size or shape, and different clumps of gas within the cloud may be moving in random directions at random speeds. These clumps collide and merge as the cloud collapses, and each new clump has the average velocity of the clumps that formed it. The random motions of the or
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