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Comment: terms below that are printed in boldface are meant to catch your attention, and you will note that most of them are nouns. In science nouns are necessary, but verbs—the processes of the earth—are more important and these will be emphasized in Quiz 1. The quiz will be a mixture of very easy, to a few quite challenging questions. The more difficult questions will ask you to take something you al- ready know and put the pieces of information together to come up with a conclusion. LECTURE 1: ORIGIN OF THE SOLAR SYSTEM Our own sun is a typical star within the Milky Way, a galaxy that contains 2 x 10 11 , or 200 billion stars. A molecular cloud consisting of diffuse dust and gas within the galaxy, begins to collapse. This is due to the force of gravity, which causes all particles that have mass to be mutually attracted to one another, and collapse is assisted slightly by the pressure of starlight. As shrinkage continues, the cloud breaks up into a swarm of smaller clouds, each destined to become a star. The solar disk represents an early stage of star formation; it consisted of a central "bulge" surrounded by a rotating flattened disk, which itself sustains complex internal motions of dust and gas. Planets, their moons, etc. condensed from the solar disk, and the sun condensed from the central bulge. Thus the sun and planets, their moons, asteroids, comets, meteorites, etc.—the entire solar system —formed together, at the same time from the same material. Inner planets —Mercury, Venus, Earth, and Mars—are relatively small, dense bodies with small quanti- ties of volatile substances (liquids, gases at room temperature). Beyond the orbit of Mars there is a large gap populated by tens of thousands of small objects, the asteroids , and beyond the asteroids come the orbits of the outer planets : Jupiter, Saturn, Uranus, and Neptune. (Outermost Pluto, formerly re- garded as a planet, has been demoted to “dwarf planet” status.) Outer planets, known as “gas giants,” are of low density; they consist mostly of volatile substances and are relatively large bodies. We postu- late that early in the development of the solar system, the proto-sun heated up and drove away a large amount of gas in its near vicinity, leaving inner planets as small bodies consisting chiefly of rock and metal. Outer planets survived with less loss of material, and therefore they have a more sun-like compo- sition today. The earth consists of a metallic core ( 16% of its volume) overlain by a mantle ( 83%) consisting of dark, dense rock. At the surface, the earth’s crust (<1%) consists of a great variety of rock types, and it conceals the mantle almost everywhere. If the early earth had been stripped of much of its volatile substance, then there are two logical possibili- ties for the origin of the atmosphere and hydrosphere (the world ocean, glacier ice, water on the land surface, groundwater). This water could have been: (i) extracted from within the earth's interior (inter- nal source), or (ii) deposited by impact of comets (external source).
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