Download 401c Notes - Early Earth Oceans Atmosphere

Chapter 12 - The Early Earth 2015

I. Chapter 12, Sec. 1 - Is Earth Unique? A. The Right Planet 1) If Earth was much larger, it would have more gravity and a thick, nasty atmosphere of lighter gases such as Ammonia, Methane, Hydrogen and Helium. 2) If Earth was much smaller, it would have less gravity – Oxygen and water vapor would escape into space and there would be no life on Earth! 3) If Earth didn’t have a plastic (“gooey”) layer of mantle below its rigid crust, there would be no plate tectonics, no continental crust and mountains. Earth’s entire surface would be fairly flat and completely covered by a shallow ocean. B. The Right Location 1) If Earth was 10 % closer to the Sun, the extra heat would drive oxygen and nitrogen into space, leaving behind mostly heavier carbon dioxide. Earth would be too hot and oxygen-poor for higher life forms to thrive. 2) If Earth was 10% farther away from the sun, it would be so cold that the ocean would freeze over. (No liquid water = no life.) 3) Our medium-sized sun has a nice long 10 billion year life span. If the Sun was a giant star, it would burn hotter and “burn out” in just a few 100 million years. There would not have been enough time for any significant evolution to occur. C. The Right Time 1) There has been enough time for the atmosphere to develop. The early atmosphere contained methane, ammonia, water vapor and carbon dioxide. There was no free oxygen (O2). 2) Photosynthetic bacteria released O2 into the atmosphere. The appearance of O2 in the atmosphere made possible the evolution of higher organisms. 3) 65 mya – Earth was struck be a 6 mile diameter asteroid, causing a mass extinction which killed off 75 % of all plant and animal life. 4) The extinction of the dinosaurs made possible the rise and diversification of mammals.

II. Chapter 12, Sec 2 – Birth of a Planet (Note: bya = billion years ago) A. Earth’s Early Evolution. The first ¾ of a billion years of Earth’s history (from 4.6 – 3.8 bya) is called the Hadean Eon: B. Proto-Earth was a chunky dust ball which weighed much less than its present mass, but was much bigger across. The sun was just warming up then, so proto-Earth was very chilly: -347 o F ( just 40 o above absolute zero). C. Proto-Earth was heated up very quickly by: a. Gravitational compression

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b. Intense radioactivity (from minerals and elements within the earth) c. Millions of meteorite impacts which terrorized the early S.S. until 3.8 bya. The intense meteorite bombardment and radioactive decay caused Earth’s crust to heat up and completely melt, resulting in a magma ocean. During this molten phase, the heavier metals Iron and Nickel sank to form an Ironrich core and a thick mantle of iron-rich silicate rocks, while the lower density (“lighter”) silicate rocks rose closer to the surface to form a thin primitive crust. This major sorting-out event is known as the Iron Catastrophe. Although it was hotter than the sun’s surface, the inner core solidified first, because the tremendously high pressures of the inner core allowed iron to crystallize and harden at 5000 o C. 50 million years later, the outer 500 miles of the earth cooled and hardened into the crust & upper mantle. Earth’s layering was completed 4.4 bya.

III. Chapter 12, Sec. 3 – Origin and Evolution of the Atmosphere and Oceans A. Earth’s original atmosphere was stripped away by the solar wind, so Earth has had to “grow” a new one! B. Earth’s early atmosphere contained Hydrogen (H), Helium (He), Methane (CH4), Ammonia, (NH3), Carbon dioxide (CO2) and water vapor (H2O). C. Earth acquired its water from two main sources: 1) Earth was bombarded by icy planetesimals and comets from beyond Mars that vaporized when they struck Earth. Earth’s gravity kept the water from escaping. 2) Worldwide volcanic eruptions released lava but also caused a lot of volcanic outgassing, releasing Carbon dioxide, Nitrogen and Sulfur oxides and a large amount of steam (boiling hot water vapor) into the atmosphere. D. The early atmosphere was about 50 % water with a lot of CO2 & NO2 : 1) The C from CO2 combined with H to make methane (CH4). 2) The N from NO2 combined with H to form ammonia (NH4). 3) Loose H and He atoms escaped into space because Earth’s gravity was too weak to hold on to these very light gases. E. The steam caused thunderstorms which raged for centuries and only collected on Earth’s surface after the rain finally solidified and cooled the magma ocean. F. Earth’s early ocean was a relatively fresh-water sea. It took many more years of acid rain to wash enough Na and Cl out of the rocks and down the rivers to the oceans to make salt water ( Seawater is a 3.5 % solution of NaCl - sea salt.) G. Most importantly, there was no free oxygen in earth’s atmosphere until 2.4 b.y.a., so early life forms were limited to those that didn’t need oxygen . H. Oxygen first appeared during the Great Oxygenation Event 2.4 by ago due to: 1) Photosynthetic bacteria (esp. the blue-green kind in the ocean)

2) Splitting of water in the sky (electrolysis) by lightning & U - V radiation I. With the addition of oxygen (and the resulting ozone layer (O3)), life on land (and Earth overall) really took off!! Planetary Impacts Airless bodies such as Mercury and the Moon offer no air resistance and friction to burn up meteors, so impacts can create microscopic cavities up to massive impact craters. From 4.6 to 3.8 bya, the “clean-up” of interstellar debris caused a period of intense meteor bombardment. Earth’s atmosphere causes smaller meteorites (