Introduction Cosmology: the scientific study of the Universe. Structure History Earth 4 Part 1 Opener
What Is the Structure of the Universe? Universe is made up of matter and energy. Matter substance of the universe; takes up space. Mass Density Weight Energy the ability to do work. Heat Light Pull of gravity Fig. 1.2a
Stars and Galaxies Stars are immense balls of incandescent gas. Gravity binds stars together into vast galaxies. Over 100 billion galaxies exist in the visible universe. The Solar System is on an arm of the Milky Way galaxy. Our sun is one of 300 billion stars in the Milky Way. Fig. 1.2b, c
The Nature of Our Solar System Our sun is a medium-sized star, orbited by 8 planets. The sun accounts for 99.8% of our solar system mass. Planet a planet: Is a large solid body orbiting a star (the Sun). Has a nearly spherical shape. Has cleared its neighborhood of other objects (by gravity). Moon a solid body locked in orbit around a planet Millions of asteroids, trillions of icy bodies orbit the sun.
The Nature of Our Solar System Two groups of planets occur in the solar system. Terrestrial Planets small, dense, rocky planets Mercury, Venus, Earth, and Mars Giant Planets large, low-density, gas and ice giants Gas giants: Jupiter, Saturn (hydrogen and helium) Ice giants: Uranus, Neptune (frozen water, ammonia, methane) The Solar System is held together by gravity. Fig. 1.3a
The Solar System The terrestrial planets are the four most interior. The giant planets occupy the four outermost orbits. All but two planets have moons (Jupiter has 63!). The asteroid belt lies between Mars and Jupiter. Clouds of icy bodies lie beyond Neptune s orbit. Icy fragments pulled into the inner solar system become comets. Fig. 1.3b
Forming the Universe The vastness of the Universe is staggering. Earth is a planet orbiting a star on the arm of a galaxy. The sun and over 300 billion stars form the Milky Way. Over 100 billion galaxies exist in the visible universe. Where did all this stuff come from? The Big Bang initiated the expanding universe 13.7 billion years ago. Fig. 1.2a
The Doppler Effect A moving star displays Doppler-shifted light. Approaching starlight is compressed (higher frequency): Blue shift Receding starlight is expanded (lower frequency): Red shift This observer sees light waves compressed blue-shifted. This observer sees light waves spread out red-shifted. No Doppler shift Fig. 1.4c
The Expanding Universe Light from galaxies was observed to be red-shifted. Edwin Hubble recognized the red shift as a Doppler effect. He concluded that galaxies were moving away at great speed. No galaxies were found heading toward Earth. Hubble deduced that the whole Universe must be expanding (analogous to raisin-bread dough). The expanding Universe theory. Did expansion start at some time in the past? If so, how far back? How small was the Universe before expansion? Fig. 1.5a
The Big Bang Researchers have developed a model of the Big Bang. During the first instant, only energy no matter was present. Started as a rapid cascade of events. Hydrogen atoms within a few seconds At 3 minutes, hydrogen atoms fused to form helium atoms. Light nuclei (atomic no. < 5) by Big Bang nucleosynthesis The Universe expanded and cooled. Fig. 1.5b
After the Big Bang With expansion and cooling, atoms began to bond. Hydrogen formed H 2 molecules the fuel of stars. Atoms and molecules coalesced into gaseous nebulae. Gravity caused collapse of gaseous nebulae. Collapse resulted in increases in: Temperature. Density. Rate of rotation. Earth, 4th ed., Fig. 1.7
After the Big Bang Mass in nebulae was not equally distributed. An initially more massive region began to pull in gas. This region gained mass and density. Mass compacted into a smaller region and began to rotate. Rotation rate increased, developing a disk shape. The central ball of the disk became hot enough to glow. A protostar is born. Geology at a Glance
Birth of the First Stars The protostar continued to grow, pulling in more mass and creating a denser core. Temperatures soared to 10 million degrees. At these temps, hydrogen nuclei fused to create helium. With the start of nuclear fusion, the protostar ignited. Chapter 1 Opener
Birth of the First Stars Nebulae from which first-generation stars formed consisted entirely of light elements. These first-generation stars exhausted H 2 fuel rapidly. As the stars became H 2 -starved, they initiated: Collapse and heating. Catastrophic supernova. Where did heavy elements (atomic no. > 5) come from? Fig. 1.6a
Where Do Elements Come From? Big Bang nucleosynthesis formed the lightest elements. Atomic #s 1, 2, 3, 4, and 5 (H, He, Li, Be, and B) Heavier elements are from stellar nucleosynthesis. Atomic #s 6 26 (C to Fe) Stars are element factories. Elements with atomic #s >26 form during supernovae. Fig. 1.6b
Where Do Elements Come From? First-generation stars left a legacy of heavier elements. Second-generation stars repeated heavy element genesis. Succeeding generations contain more heavy elements. The sun may be a third-, fourth-, or fifth-generation star. The mix of elements found on Earth include: Primordial gas from the Big Bang. The disgorged contents of exploded stars. We really ARE all made out of stardust!
Nebular Theory of the Solar System The nebular theory of Solar System formation A third-, fourth-, or nth-generation nebula forms 4.56 Ga. Hydrogen and helium are left over from the Big Bang. Heavier elements are produced via: Stellar nucleosynthesis. Supernovae. The nebula condenses into a protoplanetary disk. Geology at a Glance
Solar System Formation The ball at the center grows dense and hot. Fusion reactions begin; the sun is born. Dust in the rings condenses into particles. Particles coalesce to form planetesimals. Fig. 1.7 Geology at a Glance
Differentiation of Earth Planetesimals clump into a lumpy protoplanet. The interior heats, softens, and forms a sphere. The interior differentiates into: A central iron-rich core, and A stony outer shell a mantle. Geology at a Glance
Formation of the Moon ~4.53 Ga, a Mars-sized protoplanet collides with Earth. The planet and a part of Earth s mantle are disintegrated. Collision debris forms a ring around Earth. The debris coalesces and forms the moon. The moon has a composition similar to Earth s mantle. Geology at a Glance
The Atmosphere and Oceans The atmosphere develops from volcanic gases. When Earth becomes cool enough: Moisture condenses and accumulates. The oceans come into existence. Geology at a Glance
Magnetic Field Space visitors would notice Earth s magnetic field. Earth s magnetic field is like a giant dipole bar magnet. The field has north and south ends. The field grows weaker with distance. The magnetic force is directional. It flows from S pole to N pole along the bar magnet. It flows from N to S along field lines outside the bar. Fig. 1.9a
Magnetic Field Earth s magnetic field is like a giant dipole bar magnet. The N pole of the bar is near Earth s geographic S pole. A compass needle aligns with the field lines. The N compass arrow points to the bar magnet S pole. Opposites attract. Magnetic field lines: Extend into space. Weaken with distance. Form a shield around Earth (magnetosphere). Fig. 1.9b
Magnetic Field The solar wind distorts the magnetosphere. Shaped like a teardrop Deflects most of the solar wind, protecting Earth The strong magnetic field of the Van Allen belts intercepts dangerous cosmic radiation. Fig. 1.9c
91.2% of Earth s mass comprises just four elements: Iron (Fe) 32.1% What is Earth Made Of? Oxygen (O) 30.1% Silicon (Si) 15.1% Magnesium (Mg) 13.9% The remaining 8.8% of Earth s mass consists of the remaining 88 elements. Fig. 1.12
A Layered Earth The first key to understanding Earth s interior: density. When scientists first determined Earth s mass they realized: Average density of Earth >> average density of surface rocks. Deduced that metal must be concentrated in Earth s center. These ideas led to a layered model: Earth is like an egg. Thin, light crust (eggshell) Thicker, more dense mantle (eggwhite) Innermost, very dense core (yolk) Fig. 1.13
A Layered Earth Earthquakes: seismic energy from fault motion Seismic waves provide insight into Earth s interior. Seismic wave velocities change with density. We can determine the depth of seismic velocity changes. Hence, we can tell where densities change in Earth s interior. Fig. 1.14a, b Essentials of Geology, 4th edition, by Stephen Marshak 2013, W. W. Norton Chapter 1: The Earth in Context
A Layered Earth Changes with depth Pressure (P) The weight of overlying rock increases with depth. Temperature (T) Heat is generated in Earth s interior. T increases with depth. Geothermal gradient The rate of T changes with depth. The geothermal gradient varies. ~ 20-30 C per km in crust < 10 C per km at greater depths Earth s center may reach 4,700 C! Earth, 4 th ed., Fig. 2.13
The Crust The outermost skin of our planet is highly variable. Thickest under mountain ranges (70 km or 40 miles) Thinnest under mid-ocean ridges (7 km or 4 miles) Relatively as thick as the membrane of a toy balloon The Mohorovičić discontinuity (Moho) is the base. Seismic velocity change between crust and upper mantle The crust is the upper part of a tectonic plate. Fig. 1.15a
The Crust There are two kinds of crust: continental and oceanic. Continental crust underlies the continents. Average thickness 35 40 km Felsic (granite) to intermediate in composition Oceanic crust underlies the ocean basins. Average thickness 7 10 km Mafic (basalt and gabbro) in composition More dense than continental crust Fig. 1.15a
Solid rock, 2,885 km thick, 82% of Earth s volume The mantle is entirely the ultra-mafic rock peridotite. Convection below ~ 100 km mixes the mantle. Like oatmeal on a stove: hot rises, cold sinks. Convection aids tectonic plate motion. Divided into two sub-layers: Upper Mantle Transitional zone Lower Mantle The Mantle Fig. 1.15b
The Core An iron-rich sphere with a radius of 3,471 km Seismic waves segregate two radically different parts. The outer core is liquid; inner core solid. Outer core Liquid iron alloy 2,255 km thick Liquid flows Inner core Solid iron-nickel alloy Radius of 1,220 km Greater pressure keeps solid Outer core flow generates Earth s magnetic field. Fig. 1.15b
Lithosphere-Asthenosphere We can also regard layering based on rock strength. Lithosphere the outermost 100 150 km of Earth Behaves rigidly, as a nonflowing material Composed of two components: crust and upper mantle This is the material that makes up tectonic plates. Asthenosphere upper mantle below the lithosphere Shallow under oceanic lithosphere; deeper under continental Flows as a soft solid. Fig. 1.17