Early History of the Terrestrial Planets

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1 CHAPTER 9 Early History of the Terrestrial Planets Chapter Summary According to the nebular hypothesis our solar system formed when a cloud of interstellar gas and dust condensed about 4.5 billion years ago. The planets vary in chemical composition in accordance with their distance from the Sun and with their size. Planets close to the Sun are tiny and made up of rocks and metals; the result of hot conditions that boiled many of volatiles away. The giant outer planets picked up the boiled off volatiles, and due to their great mass were able to hold them forming dense atmospheres of hydrogen, helium, and other light constituents of the original nebula. Earth probably grew by accretion of colliding chunks of matter. Very early after the Earth formed, it is thought that our Moon formed from material ejected from the Earth by the impact of a mars-size meteorite. Bolide impacts are rare events now but space is littered with debris that has occasionally hit the Earth and profoundly impacted life. Heat generated from the Moon-forming impact and the decay of radioactive elements probably caused much of the Earth to melt. Melting allowed iron and other dense matter to sink towards the Earth s center and form the core. Lower density (lighter) matter floated upward to form the mantle and crust. Release of trapped gases (mostly water) from within the Earth gave rise to the oceans and an early atmosphere. In this way, the Earth was transformed into a differentiated planet with chemically distinct zones: an iron core; a mantle of mostly magnesium, iron, silicon, and oxygen; and a crust rich in oxygen, silicon, aluminum, calcium, potassium, sodium, and radioactive elements. All of this paved the way for the eventual evolution of life. Isotopic dating of Moon rocks has yielded a crater counting time-scale that is useful in dating other planetary surfaces. Planetary probes of Mars show that water is currently present only in the form of ice at its poles and in its shallow subsurface. The presence of sedimentary structures suggest that water in liquid form may have been present at some time in the past. 110

2 Early History of the Terrestrial Planets 111 Plate tectonics has been observed on only one other planet, Venus, where the plate system is so hot they break into flakes or crumple like a rug. Flake tectonics may have occurred during Earth s early life when it was hotter. To date, over 300 planets have been discovered circling stars in other solar systems. There are no doubt many other exoplanets awaiting discovery by scientists. Learning Objectives In this section we provide a sampling of possible objectives for this chapter. No class could or should try to accomplish all of these objectives. Choose objectives based on your analysis of your class. Refer to Chapter 1: Learning Objectives How to Define Your Goals for Your Course in the Instructional Design section of this manual for thoughts and ideas about how to go about such an analysis. Knowledge Know the nebula hypothesis for the evolution of the solar system. Know when and how the Earth s interior differentiated. Know the physical characteristics and general chemical composition of the layered architecture of the Earth s interior. Know the modern ideas for the origin of the Earth s Moon. Know the role bolide impacts played in Earth and life history. Skills/Applications/Attitudes Discuss the consequences of the early Earth becoming partially molten. General Education Skills Write a short paper concerning how Earth might be different if, like most of the other planets, it lacked an active plate tectonic system. Freshman Survival Skills Reinforce the strategy of previewing before lecture by telling students to bring their answers to all preview questions to class for peer review. (Note: this reinforces assignments we suggested in earlier chapters.) Provide individual help sessions for students after the first midterm. (see teaching tips) Sample Lecture Outline Sample lecture outlines highlight the important topics and concepts covered in the text. We suggest that you customize it to your own lecture before handing it out to students. At the end of each chapter outline consider adding a selection of review questions that represent a range of thinking levels.

3 112 PART II CHAPTER 9 Chapter 9: Early History of the Terrestrial Planets Origin of our solar system Nebular hypothesis Sun forms out of the nebula Planet formation: solar nebula > cooling, condensing gases > gravity > clumping > collision and accretion Diversity of the Planets Inner terrestrial planets Giant outer gaseous planets Layered planet bombardment events > Earth melting > gravity > differentiation of Earth s core Earth s Differentiated Layers Core dense iron/nickel Inner Core solid due to pressures too high to allow melting Outer Core liquid Mantle Convecting, relative soft solid that is more mafic than the crust, less mafic (richer in iron and magnesium) than the core Most of the Earth is mantle Crust Less dense materials floated to the surface of the magma ocean, cooled into solid crust Formation of Earth s atmosphere/oceans resulted as the crust cooled allowing gases and water to escape via volcanism. Water and ice escaping from crashing planetesimals was an additional source Planetary Surfaces Isotopic dating of Moon rocks has yielded a crater counting time-scale that is useful in dating other planetary surfaces Tectonics are lacking on most other planets. Venus shows an interesting flake tectonics that is attributed to its more-vigorous convection that prevents crust from thickening Teaching Tips Cooperative/Collaborative Exercises and In-Class Activities Refer to Chapter 4: Cooperative Learning Teaching Strategies in the Instructional Design section of this manual for general ideas about conducting cooperative learning exercises in your classroom. Coop Exercise 1: Earth is Our Spaceship by Robert Butler, Department of Geosciences, University of Arizona, Tucson, Arizona Students are seated in groups of six. Starting with a short and simple trip across town, student groups are asked to produce a list of items required to accomplish a journey on increasing duration and complexity. I usually start this exercise by asking groups to list what they would need to consider to make a trip in one afternoon to a local shopping mall. Groups are given a minute to arrive at this list. After that minute has passed, I ask one group at a time to report something from their list, and I write these on the board for all to see. Obvious things which will show up on this list are transportation (by car, bus, bike, walk); money; look at the newspaper for sales notices; etc. Depending on the time allowed, the next step can be to consider a trip across state or skip directly to a trip across country. This requires a bit more time for groups to compile their lists. I usually allow two minutes for development of this list, and I

4 Early History of the Terrestrial Planets 113 will go sit with one student group and help them with their list. The question of mode of transportation becomes an important issue of discussion for this trip. Flying is fast but can be expensive; travel by car is slow but provides flexibility; if you travel by car, you need to budget money for gas; etc. Food and lodging become important considerations on a cross-country trip, so student groups begin to deal with the logistics, expense, and questions of creature comfort which surround food and lodging. The next level of complexity is to ask students to consider a trip to Mars in which they would circle Mars a few times and then return to Earth. Students are usually getting confident by the time the list of necessities for the cross-country trip are listed but they rapidly lose that confidence when asked to design a trip to Mars. You need to provide at least five minutes for construction of the required list for this space travel. While students are working on this, I usually jump around among two or three groups in different parts of the room to keep groups on task. With some prodding, the class usually does a credible job developing at least the basic list of requirements. Constructing this list forces students to think about many requirements for human survival are taken for granted and rarely explicitly considered. The list of items and systems required for a trip to Mars generally will include the obvious space vehicle (we don't get hung up on design), food, water, and air to breath. Deeper thought leads to appreciation for the importance of fuel to propel the spaceship and provide energy for the guidance systems and warmth of the travelers. Generally it will occur to one or more groups that they need to have a waste disposal system. More subtle are considerations of the potentially harmful space environment (UV radiation; cosmic rays; vacuum of space). This class discussion is concluded by showing a slide of Mother Earth taken from space. The point is made that Earth is our spaceship. It makes sense for us to understand the basics of how the Earth operates. To do that, we need to consider not just the solid Earth but also the hydrosphere, atmosphere, biosphere, and interactions within the Earth systems and of those systems with our space environment. One can bring in environmental concerns such as floods, earthquakes, and global climatic change. If time permits, I show a graph of world population growth and ask students to ponder the question of how much more complex it is to design a spaceship for six billion humans instead of the four or five passengers they were thinking of when they developed their list of requirements for their spaceship. If you want to get pointed with this discussion, you can ask students to consider that the United States consumes 40 percent of the world's resources to support six percent of the world's population. Are we a sustainable society? (A society which can continue without decreasing the options for the next generations?) This exercise will tie wonderfully with discussions at the end of the semester when Chapter 23: The Human Impact on Earth s Environment is assigned reading. Coop Exercise 2: Sense of Scale for Our Solar System A BB vs. a Basketball Use this short coop exercise to give students a sense of scale for our solar system and space beyond. You will need a BB and a basketball. Holding the BB between your fingers, present it to the class as representing the size of the Earth in a scale model of the solar system. Our Sun is about 109 times larger in diameter than the Earth, and a basketball is a good representation for the size of our Sun in this scale model. Ask a student to stand up and hold the BB (Earth) between their fingers somewhere in front of class. Then give the basketball to another student and ask the class to coach this student where the basketball should be relative to the BB such that their distance would be proportional to the actual distance between Earth and Sun. Essentially the size and distance between the BB and basketball are scaled and consistent with their relative sizes and distances.

5 114 PART II CHAPTER 9 In a scale model of our solar system where the sizes and distances between the Sun and Earth are consistent with their relative sizes (a basketball and a BB), the Earth is about 90 feet from the Sun. In a large lecture for 150 students, the basketball and BB are located approximately at the opposite corners of the room. If you maintained the proportionality of this basketball and BB model and showed the closest star, with the Sun in Tucson, Arizona, Alpha Centuri would be a basketball somewhere around Detroit. Five-Minute Write 1. What questions do you have about this lecture? 2. What did you find most interesting about this lecture? 3. How was this lecture relevant to you? Coop Exercise 3: Five-Minute Write The Five-Minute Write is done during the last five minutes of lecture. Ask students to put their names on a sheet of paper and then address the three questions on the board; see adjacent sample. Start the next lecture by discussing the answers to some of the questions students had about the previous lecture. Freshman Survival Skills Assignment In the beginning of your course it is prudent to include a few exercises to help the freshmen in your class learn how to learn and reinforce mastery of the basics of good preparation for college-level lectures. Learning skills are like critical thinking skills. They tend to be mastered slowly, over time, and with lots of practice. Even upper division students and graduate students sometimes need coaching about how to learn. (See Part I, Instructional Design for a discussion of why this is so and ideas about how to do it.) Chapter 1 discusses freshman survival as a national educational priority. Chapters 5 and 6 discuss how to develop credit assignments to encourage students to learn how to learn. To encourage students to preview this chapter announce there will be a preview quiz at the beginning of the lecture for Early History of the Terrestrial Planet. Tell them you will ask one or two of the preview questions and will accept for full credit brief answers similar to those found in the student guide. Explain that your purpose will not be to test for complete mastery of the material but rather for a degree of familiarity that will improve their understanding of your lecture. See Part I, Chapter 3 of the Student Study Guide for additional information about previewing. Topics for Class Discussion Where does matter come from? The heavy elements are created in stars, nova, and supernova by nucleosynthesis. How might the Earth be different if (like most other planets in the solar system) it lacked an active tectonic system. Comparison of planetary atmospheres. Ask your students to fill in two additional rows to this table: surface temperature range and atmospheric pressure. The Atmospheric Composition of Earth, Venus, and Mars Gas Venus Earth Mars Carbon dioxide (CO 2 ) 96% 0.03% 95% Nitrogen (N 2 ) 3.5% 78% 2.7% Oxygen (O 2 ) 0.003% 21% 0.15% Argon 0.006% 0.9% 1.6%

6 Early History of the Terrestrial Planets 115 Where does matter comes from? The heavy elements are created in stars, nova, and supernova by nucleosynthesis. Formation of Our Solar System Nucleosynthesis heavy elements are created Mass Concentrations form in interstellar dust cloud Sun and planets form by gravitational collapse Planets Begin to Cook Due to heat from gravitational collapse, decay of radioactive isotopes, and accretion (large impacts) There is good reason that the Earth s crust and mantle consist primarily of silicate minerals and the core of iron. Silicon and iron are two of the ten most abundant elements in the Universe. Ten Most Abundant Elements in the Universe Number of Atoms Per Element Symbol Atomic Number Million of Hydrogen hydrogen H 1 1,000,000 helium He 2 80,000 oxygen O neon Ne nitrogen N 7 92 carbon C magnesium Mg silicon Si iron Fe sulfur S Teaching Resources Student Study Guide Highlights (part of the Understanding Earth e-book) In Part I, chapters provide strategies for learning geology. Ideally, students would read these chapters early in the course. Chapter 1: Brief Preview of the Student Study Guide for Understanding Earth Chapter 2: Meet the Authors Chapter 3: How to Be Successful in Geology

7 116 PART II CHAPTER 9 In Part II, Chapter 9: Early History of the Terrestrial Planets Before Lecture: Preview Questions and Brief Answers During Lecture After Lecture: Check Your Notes Exam Prep: Chapter Summary Practice Exercises: The Evolving Early Earth Evidence for Water on Mars Review questions

Formation of the Earth and Solar System

Formation of the Earth and Solar System a. Supernova and formation of primordial dust cloud. NEBULAR HYPOTHESIS b. Condensation of primordial dust. Forms disk-shaped nubular cloud rotating counterclockwise.