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Chapter 2 Origins Origin of a Galaxy ¾Formation of Universe, Solar System and Earth ¾Creation of Oceans 1 Formation of the Universe ¾ Huge rotating aggregation of stars, dust, gas and other debris held together by 4 gravity. Origin of the Solar System ¾Big Bang, 13*109 years ago ¾Formation of elementary particles ¾Gravitational formation of dense regions ¾1*109 yrs laterÆ laterÆfirst stars ¾ Rotating cloud of gas from which sun and planets formed ¾ Initiated by “supernova” supernova” = exploding star 2 5 Nuclear Fusion: The joining of atoms under tremendous temperatures and pressures to create atoms of a heavier element. In the Sun, four hydrogen atoms are fused to create each helium atom. Two of the hydrogen's protons become neutrons in the process Supporting Evidence for the Big Bang •Edwin Hubble discovered spreading of galaxies. •Cosmic background radiation (the glow left over from the explosion itself) discovered in 1964. 3 6 1 Moderate Size Stars (Our Sun): C & O Earth, Ocean and Atmosphere accumulated in layers sorted by density Large Stars (more, H & He): Fe The planet grew by the aggregation of particles. Meteors and asteroids bombarded the surface, heating the new planet and adding to its growing mass. At the time, Earth was composed of a homogeneous mixture of materials. Supernova: Heavier Elements Formed Earth lost volume because of gravitational compression. High temperatures in the interior turned the inner Earth into a semisolid mass; dense iron (red drops) fell toward the center to form the core, while less dense silicates move outward. Friction generated by this movement heated Earth even more. The result of density stratification: an inner and outer core, a mantle, 7 Condensation Theory A nebula (a large, diffuse gas cloud of gas and dust) contracts under gravity. As it contracts, the nebula heats, flattens, and spins faster, becoming a spinning disk of dust and gas. Planets will form in disk. Warm temperatures allow only metal/rock “seeds” to condense in the inner solar system. Cold temperatures allow “seeds” to contain abundant ice in outer solar The seeds of system. Terrestrial planets are built from metal and rock. Solid “seeds” collide and stick together. Larger ones attract others with their gravity, growing bigger still. Terrestrial planets remain in inner solar system. Gas giant planets remain in outer solar system. “Leftovers” from the formation process become asteroids (metal/rock) and comets (mostly ice). 10 How did water and water vapor form on early Earth? Star will be born in center. Hydrogen and helium remain gaseous, but other materials can condense into solid “seeds” for building planets. and the crust. gas giant planets grow large enough to attract hydrogen and helium gas, making them into giant, mostly gaseous planets; moons form in disks of dust and gas that surround the planets. ¾ The Sun stripped away Earth’ Earth’s first atmosphere ¾ Gases, including water vapor, released by the process of outgassing, outgassing, replaced the first atmosphere. ¾ Water vapor in the atmosphere condensed into clouds. ¾ After millions of years, the clouds cooled enough for water droplets to form. ¾ Hot rain fell and boiled back into the clouds. ¾ Eventually, the surface cooled enough for water to collect in basins. 8 Not to scale Early Earth 11 Sources of Water ¾ Accretion (Gaining material) ¾ Differentiation (Separating based on density Æ density stratification) stratification) ¾ Evidence of waterwater- 3.9*109 yrs ago * Mantle rocks ¾Evidence from meteorites ¾Release through volcanic activity * Outer space ¾Evidence from Dynamics Explorer 9 12 2 The evolution of our atmosphere Age and Time 1 billion = 1,000,000,000 or 109 Earth is 4.6 * 109 years old Oceans are 4.2 * 109 years old Oldest rocks date from 3.8 * 109 years ago First evidence of life dates from 3.6 * 109 years ago 1 million = 1,000,000 or 106 Ocean and atmosphere reach the state we know today 800 * 106 years ago 100 Concentration of Atmospheric Gases (%) Methane, ammonia initial rise of O2 2.7 b. y. ago – but conclusive evidence is from 2.3 b. y. ago 75 Atmosphere unknown Nitrogen 50 Water 25 Carbon dioxide 0 Oxygen 3 4 Time (billions of years ago) 4.5 1 2 Early atmosphere quite different from today’s Fig. 2-11,13p. 49 16 Radioactive Decay Series Life probably originated in the ocean Fossil of a bacteriabacterialike organism (with an artist’ artist’s reconstruction) that photosynthesized and released oxygen into the atmosphere. Among the oldest fossils ever discovered, this microscopic filament from northwestern Australia is about 3.5 billion years old. Parent Isotope 14 Billions of years ago 13 Big bang 11 5.5 4.6 Today 0 Stable Daughter Product Currently Accepted Half-Life Values Uranium-238 Lead-206 4.5 billion years Uranium-235 Lead-207 704 million years Thorium-232 Lead-208 14.0 billion years Rubidium-87 Strontium-87 48.8 billion years 17 Billions of years ago 4.6 Earth forms Millions of Ocean forms years ago First galaxies 4.2 form 3.8 Oldest dated rocks 800 First animals arise 3.6 First evidence of life Solar nebula begins to form Earth forms Today The future of Earth Past Millions of years 510 First ago 66 fishes End of dinosaurs Oxygen 2 appear revolution begins Pangaea 50 First marine breaks mammals Ocean and 210 apart 0.8 atmosphere End of reach steady dinosaurs state (as today) Humans appear 3 66 Today Today Today Age and Time – Past and Future Future 3.5 The sun swells, planets destroyed How long can Earth exist? ¾Our Sun will begin to die in 5 billion years. ¾6 billion years from now the sun will enter the red giant phase and will engulf the inner planets. ¾At that time, Earth will probably be recycled into component atoms. Sun's output too low for liquid-water ocean 5 Billions of years in the future 15 Fig. 2-15, p. 51 18 3 Summary Most of the atoms that make up Earth, its ocean, and its inhabitants were formed within stars billions of years ago. Stars spend their lives changing hydrogen and helium into heavier elements. As they die, some stars eject the elements into space during cataclysmic explosions. The sun and planets, including Earth, condensed from a cloud of dust and gas enriched by the recycled remnants of exploded stars. Earth formed by accretion – the clumping of small particles into a large mass. The mass heated as it grew and eventually melted. The heavy iron and nickel crashed toward Earth’s center to become its core; the lighter silicates and aluminum compounds rose to the surface to form a crust. Earth became density stratified – that is layered by density. The ocean formed as soon as Earth was cool enough for water to remain liquid. Life followed soon thereafter. 19 Thinking Analytically 1. A light-year is the distance light can travel in one year. Light travels at 300,000 kilometers per second. Commercial television broadcasting began in 1939. Television signals travel at the speed of light. How far away would a space probe have to be before it could no longer detect those signals? 2. Can you think of any way an astronomer could detect a large planet orbiting a star without actually seeing the planet? (Hint how would a star move as the planet orbits it?) 20 4