OUR SOLAR SYSTEM & EARTH

1. OUR SOLAR SYSTEM & EARTH HOW AND WHY DO THEORIES BECOME GENERALLY ACCEPTED? 4

2. UNIT 4 OUR SOLAR SYSTEM & EARTH UNIT 4 BASICS 3Unit 4 Overview 4 Unit 4 Learning Outcomes 5 Unit 4 Lessons 6 Unit 4 Key Concepts LOOKING BACK 8 What Happened in Unit 3? KEY CONTENT 10 Threshold 4—Earth & the Solar System 11 Threshold 4: Earth & the Solar System 13 How Did Earth and the Solar System Form? 14 The Sun 15 How Our Solar System Formed 16 What Was the Young Earth Like? 17 The Early Atmosphere 18 Our Shifting Globe 19 Why We’re All Lava Surfers 20 Introduction to Geology 21 Alfred Wegener & Harry Hess 22 Eratosthenes 23 Introduction to the Geological Time Chart 24 Principles of Geology LOOKING AHEAD 26 What’s Next in Unit 5? BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

3. UNIT 4 OVERVIEW Key Disciplines: Physics, chemistry, and geology Timespan: The Sun and Solar System formed about 4.5 billion years ago Driving Question: How and why do theories become generally accepted? Threshold for this Unit: Threshold 4: Earth & the Solar System BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

4. UNIT 4 LEARNING OUTCOMES By the end of Unit 4, students should be able to: Explain why planets are more complex than stars. Use evidence to explain how the Earth and its atmosphere developed and changed over time. Explain the basic mechanisms and key pieces of evidence for plate tectonics, and how plate tectonics impacts life on Earth. Define geology, the types of questions geologists ask, and the tools they use to answer those questions. Explain why geology is important to understanding the history of the Earth. Understand how geologists can work with scientists and historians from other disciplines to form a deeper understanding of the history of the Earth. BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

5. UNIT 4 LESSONS 4.0 Earthand the Formation of Our Solar System The debris left over from the formation of the Sun experienced many collisions as it began to orbit the newly formed Sun. These collisions led to the formation of the Earth and the other planets in Solar System through a process called accretion. 4.1 What Was the Young Earth Like? As giant hunks of rock, metal, and ice slammed into the Earth’s surface, it became a planet with three layers. Despite its violent and unstable beginning, Earth slowly became the world we know today, and the interplay between the layers resulted in the Earth as we know it. 4.2 Why Is Plate Tectonics Important? Once rock formed on the early Earth, large masses of rock called plates began to appear. As a result of the movement of these plates, towering mountains and trembling earthquakes resulted. The surface of our Earth is constantly in motion, and plate tectonics is responsible for the shape and position of the Earth’s landforms. 4.3 Ways of Knowing: Our Solar System and Earth The history of our planet is written in the rock record, along with clues about the future of the Earth as well. Rock detectives—geologists—study these clues and often observe Earth’s changes first hand. Continued next slide BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

6. UNIT 4KEY CONCEPTS • accretion • Archaean eon • asteroid • atmosphere • circadian rhythm • continental drift • convergent plate boundaries • core (of the Earth) • crust (of the Earth) • differentiation (chemical) • divergent plate boundaries • Earth • exoplanet • gas giant • geology • greenhouse effect • Hadean eon • light spectrum • mantle (of the Earth) • orbit • ozone • Pangaea • planet • planetesimal • plate tectonics • protoplanetary disk • rocky planets (or terrestrial planets) • seafloor spreading • Solar System • subduction zones • Sun • tectonic plates • transform plate boundaries BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

7. LOOKING BACK

8. WHAT HAPPENED IN UNIT 3? Unit 3 focused on the formation and lifecycle of stars, as well as the emergence of new chemical elements. We learned: • How stars formed. • About the life (and death) of a star. • About the origin of heavy chemical elements in aging and dying stars. • How views of chemical elements changed over time. BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

9. KEY CONTENT

10. THRESHOLD 4—EARTH & THE SOLAR SYSTEM Video Large clouds of dust and gas were created in the aftermath of the death of stars. These chemically complex areas seeded the development of new generations of stars. The very first stars were made entirely of hydrogen and helium because those were the only elements that made up the clouds that these stars formed out of. Once a few generations of stars had lived and died, star death had created a greater variety of chemical elements, so these clouds were much more diverse. Planets formed from the small amount of material from these clouds that did not become part of the new star. The complex cloud of chemicals formed by the death of later stars enabled much greater complexity than hydrogen and helium alone. The Earth and solar system formed about 4.5 billion years ago when the universe was already over 8 billion years old, and many generations of stars had lived and died. BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

13. HOW DID EARTH AND THE SOLAR SYSTEM FORM? Video Talk / David Christian When our Sun formed out of a cloud of gas, it used over 99 percent of the matter in that cloud. The leftover matter orbited the Sun. Collisions, gravity, and the rotation of the matter around the Sun caused it to clump together. Earth and the other planets in our Solar System formed as a result. Because of the way matter was distributed, the planets that formed closest to the Sun (including Earth and Mars) are rocky. Planets further from the Sun are gaseous. BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

14. THE SUN Video Before the Sun formed, our Solar System consisted of a massive gas cloud one light-year across, with a mass equal to that of the Sun. The formation of our Solar System probably commenced when the explosion of a nearby star sent a shock wave through the gas cloud, causing the cloud to start spinning. As it spun, gravity pulled the matter in the cloud more closely together, causing the cloud to spin faster and collapse. The Sun finally lit up once the pressure and temperature had built up enough in the in the core of the forming star. The temperature needed for hydrogen to fuse into helium is around 10 million degrees Kelvin. About 99.9 percent of all of the mass contained in the original gas cloud went into making the Sun. The planets were formed from the small amount of materials that were spinning around the Sun and did not become part of it. BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

15. HOW OUR SOLAR SYSTEM FORMED Article / Cynthia Stokes Brown Planet formation is driven by accretion, a process by which larger objects grow even larger by using gravity to attract smaller, nearby objects to them. There are two types of planets in the Solar System: rocky and gas. The rocky planets are closest to the Sun, and they are composed mainly of rocks and metals. The gas planets are further from the Sun and are composed mainly of gas and ice. Not all rocky planets are the same. All of the rocky planets were very hot when they formed but cooled over time. Mercury and Mars cooled so much that they became solid. Venus may not be completely solid. The Earth did not completely solidify. Some parts of the Earth are solidified (crust and inner core) and others are not (outer core and mantle). Scientists currently believe that the Moon formed before the Earth had finished forming the layers it has today. This process began when a large object collided with the young Earth. The Moon formed from some of the material that was ejected into space after this collision and was captured in an orbit around the Earth by the Earth’s gravitational attraction. The Moon has a huge impact on life on Earth. First, the collision that resulted in the formation of the Moon gave the Earth its tilt, which is responsible for the seasons we experience. Although the Moon is smaller than the Earth, its gravitational attraction on the Earth can be seen in the tides. The Moon’s gravitational attraction also helps reduce the Earth’s wobble and slow its spin. BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

16. WHAT WAS THE YOUNG EARTH LIKE? Main Talk / David Christian The earliest period of Earth’s history is called the Hadean eon. Hades was the god of the underworld in Greek mythology, and this name was chosen to represent the “hellish” nature of the early Earth. The young Earth was very hot, its air was filled with dangerous gases, there was radioactivity, and there were many asteroid strikes. Over time, the Earth went through a process called differentiation. The heaviest materials in the Earth, particularly metals, sank to the center of the Earth to form its core. Above this layer, there formed a rocky sludge called the mantle. The thin, outer surface of the Earth became its crust. The gases and moisture that escaped from Earth formed our atmosphere. BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

17. THE EARLY ATMOSPHERE Video The Hadean eon, the most violent early period of Earth’s history, was characterized by high temperatures, collisions, and the process of differentiation. Perhaps the most important of these collisions was when a large, Mars-sized object struck the Earth, leading to the formation of the Moon. The Archaean eon saw the development of a diversity of oceans, seas, rivers, and beaches. This is also when the first life forms emerged. Some of those early life forms released oxygen during photosynthesis. The resulting buildup of oxygen in the atmosphere had a tremendous impact on life. The Phanerozoic eon, the Earth’s current eon, has seen a proliferation of plant and animal life. BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

18. OUR SHIFTING GLOBE Video Talk / David Shimabukuro The surface of the Earth is in constant motion. This motion is very slow—about the speed a human fingernail grows! —but over really long periods of time the effects can be dramatic. About 300 million years ago, the Earth consisted of a single continent, Pangaea. Since that time, plate tectonic processes have moved the Earth’s plates to form the Earth we see today. There a a few dozen tectonic plate that make up the Earth’s surface. Some are continental and some are oceanic. They can be very big (the Pacific Plate) or very small (the Juan de Fuca plate). Their movement over time gives the Earth its current appearance. Oceanic plates have an important role in the history of the Earth because they are recycled over time. New crust appears at oceanic ridges, which can be as much as 40,000 miles long. Old crust “dives down” (is subducted) into the Earth’s mantle in other parts of the ocean. Rock in the mantle is very hot and is constantly moving because of a process called convection. As the rock in the mantle moves, it moves the tectonic plates on the surface of the Earth. Earthquakes are one result of plate tectonic activity. Earthquakes occur at plate boundaries and happen when two plates sliding side by side stick to one another, building up pressure that eventually needs to be released. BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

19. WHY WE’REALL LAVA SURFERS Article / Peter Stark The author, Peter Stark, is from Montana – right in the middle of the North American plate. Continental plates, he explains, are always in motion, which makes us all lava surfers. Stark has visited places where it’s possible to experience the motion of continental plates, which is also called plate tectonics. He describes his investigations of earthquake and volcanic activity in Indonesia, which got him interested in plate tectonics and the nature of the Indonesian archipelago. He also describes a visit to Iceland, an island where two different plates move away from each other. While there, he explored a spot where “volcanic fire mingled with glacial ice.” BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

20. INTRODUCTION TO GEOLOGY Video Talk / Walter Alvarez Geology is the study of the Earth. There are two major types of geologists: Those that study Earth processes, like volcanic eruptions or the movement of glaciers, and those that study Earth history. Because liquids and gases are always moving but solids stay the same for long periods of time, rocks preserve a record of the Earth’s history. Using rocks, for example, we can see sediment from the asteroid that caused the extinction of the dinosaurs and we can see evidence of plate tectonics. Geologists rely on both simple tools, like the hammer and the compass, and more complex ones, like electron microprobes (which allow chemical analysis of grains of minerals), electron microscopes (for studying the parts of a sample that are invisible to the naked eye), and mass spectrometers (for determining the age of a sample). Geologists study rocks to better understand plate tectonic processes, to answer questions about life and why it was able to evolve on Earth, and to better understand climate, as rocks can provide clues about temperature and moisture in the past. BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

21. ALFRED WEGENER& HARRY HESS Article / Cynthia Stokes Brown Alfred Wegener was trained as a meteorologist, but he had many scientific interests and he was intrigued by the jigsaw puzzle fit of the continents. Wegener believed that plant and animal evidence and the distribution of mountain chains showed that the continents were once connected in one big land mass he called Pangaea. He proposed the idea of continental drift to explain how the continents had moved into their current configuration. Harry Hess was a geologist who used modern sonar equipment to study the ocean floor. He discovered that the ocean floor was not flat, but rather, covered with mountain ranges and trenches. He helped develop the idea of seafloor spreading, the idea that new crust is created at ocean ridges. These discoveries helped turn Wegener’s ideas about continental drift into the theory of plate tectonics. BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

22. ERATOSTHENES Article / Cynthia Stokes Brown Eratosthenes made some important contributions to our collective learning about the Earth, combining naked-eye observations and mathematics. Eratosthenes was interested in a number of questions about Earth processes. He calculated the tilt of the Earth, the distance from the Earth to the Moon and the Sun, and was also interested in mapping points on the surface of the Earth. The idea that the Earth was round was not new to educated Greeks; they had proofs for this belief. They noted that when a ship sailed over the horizon, it did not suddenly disappear from sight. Rather the ship disappeared before the top of its masts. They also noted the curved shadow of the Earth on the Moon during lunar eclipses. Eratosthenes great achievement was developing a mathematical proof that the Earth was round. Eratosthenes measured the angle of the Sun’s shadow in two wells in two Egyptian cities on the summer solstice. He knew the distance between the two cities, and he used math to calculate the circumference of the Earth from the data he collected. His calculation was close to the actual value we know today. BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

23. INTRODUCTION TO THE GEOLOGIC TIME CHART Video Talk / Walter Alvarez Geologic processes unfold over millions or billions of years. The size and variety of timescales that geologists have to deal with has led geologists to organize this information into a geologic timescale. This timescale is based on the concept of periodization, the idea of breaking history into chunks. Periodization in geology is the result of a very formal process. Geologists make decisions about periodization at the international level to ensure there is agreement about them. This process can be very different from what happens with historians, who do not always agree on periodization. Historians can disagree on when an important period like the Renaissance started, or they can argue that it started at different times in different places. Geologists use very specific terminology to refer to different geological periods. In order from the longest to the shortest, they are: Eons, eras, periods, epochs and ages. Eons are the longest periods of geological history. Each of the Earth’s eons was very different. In the Hadean period, the Earth was hot and the rate of change was very fast. In the Archaean and Proterozoic eons, the Earth had cooled and the rate of change slowed. The pace of change increased again in the Phanerozoic because of the appearance of more complex life forms. BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

24. PRINCIPLES OF GEOLOGY Article / Charles Lyell Charles Lyell (1797 –1875) was a British lawyer and the foremost geologist of his day. He is best known as the author of Principles of Geology, from which this article is excerpted. Principles of Geology popularized geologist James Hutton’s concept of “uniformitarianism” — the idea that the Earth was shaped by slow-moving forces still in operation today. Uniformitarian ideas opposed the common belief among many geologists that unique catastrophes or supernatural events, like the biblical flood in the story of Noah, shaped Earth’s surface. The motto of uniformitarianism was “the present is the key to the past.” Lyell’s friend, Charles Darwin, took the idea of uniformitarianism and extended it to biology. BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH

25. LOOKING AHEAD

26. WHAT’S NEXT? In Unit 5, we will focus on the origin and evolution of life on our planet, Earth. We will learn: About the conditions required for the emergence of life. What similarities exist across all living things. How life has changed over time, evolving from simple life forms to complex organisms. How life is affected by changes in astronomical, geological, and biological conditions. How DNA enables living things to pass adaptations to new generations. BIG HISTORY PROJECT / UNIT 4 / OUR SOLAR SYSTEM & EARTH