112lect3 - GY 112 Lecture Notes D. Haywick (2006) 1 GY 112...

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GY 112 Lecture Notes D. Haywick (2006) 1 GY 112 Lecture Notes Origin of the Earth-Moon System Lecture Goals : A) The formation of the Earth (hot accretion vs. cold accretion) B) Origin of the Moon C) Transgressions and Regressions Textbook reference: Levin 7 th edition (2003), Chapter 6 ; Levin 8 th edition (2006), Chapter 8 Note: all decent images on this page from the Astronomy Picture of the Day webpage. A) The Formation of the Earth This is not going to be an extensive discussion about the evolution of the Earth. The rest of this course will do this. For now, let’s just talk about the very early Earth and about how it may have come to be. The Earth today is differentiated which means that it contains different layers (see adjacent image). There’s an inner and outer metallic core , the mantle and the crust ). The question is, was the Earth differentiated from the very beginning, or did it start off homogeneous and through some process(es) get differentiated later. Opinions are varied, and in fact, there are 2 distinctly different ideas about what happened to a planet a long time ago…. The Hot Accretion model: One idea is that the early solar disk was hot. Material was generally molten to start with and as the Earth grew, metals immediately separated from rock and sank toward the core. In this model, the earth was always differentiated. Each layer got bigger as the Earth grew. The Cold Accretion Model: This idea suggests that the Earth grew through accretion of cold material (it assumes that asteroids and protoplanets were solid, much like they are
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GY 112 Lecture Notes D. Haywick (2006) 2 today). The early earth was therefore “homogeneous”. Metal, rock and ice were equally distributed throughout it (much like Miranda, a moon of Uranus, is believed to be today; see image to right). In order to fit the model of the Earth today, something must have happened to heat the interior up to the point where nickel and iron would melt and sink to the core. Two possible scenarios have been proposed: 1) radioactive decay and 2) gravitational attraction. Both generate heat, and perhaps they (or some combination of them) combined to gradually heat the interior of the Earth to the melting point of Fe- Ni. Over time (millions of years?) the Earth began to differentiate with metal sinking and lighter rock and water rising toward the crust. I heard of a modification of this scenario while I was a student (it may not be currently favorable but it was a neat concept well
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112lect3 - GY 112 Lecture Notes D. Haywick (2006) 1 GY 112...

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