1. April 4, 2006Astronomy 20101 Chapter 7 Earth as a Planet What do we know about the structure, composition, and history of the planet Earth that we can use in the study of other planets? How have extraterrestrial factors influenced the Earth?

2. April 4, 2006Astronomy 20102 Chapter 7 Outline The overall structure of Earth –Below the surface –At the surface –Atmosphere Life on Earth Extraterrestrial influences

3. April 4, 2006Astronomy 20103 7.1 The Global Perspective Semimajor axis1.00 AU Period1.00 year Mass1.00 M earth = 5.98 x 10 24 kg Diameter12,756 km Escape velocity11.2 km/s Rotation periodSidereal day = 23h 56m 4s Surface area5.1 x 10 8 km 2 Atmospheric pressure1.00 bar The Earth is a terrestrial planet (composed primarily of heavy Elements) with a temperature suitable for liquid water.

4. April 4, 2006Astronomy 20104 7.1.1 Earth’s Interior The interior is difficult to study. –What’s the deepest hole bored in the Earth?

5. April 4, 2006Astronomy 20105 Seismic Waves We learn about the structure of the Earth’s interior by observing the behavior of seismic waves. –Earthquakes –Impacts –Explosions Two types of waves propagate in the Earth –P wave: compression wave (like sound) –S wave: transverse wave (like vibrating string) Evidence for a liquid core. Evidence of layers implies that the Earth differentiated.

6. April 4, 2006Astronomy 20106

7. April 4, 2006Astronomy 20107 7.1.2 Magnetic Field and Magnetosphere The Earth’s magnetic field is the result of 1.The Earth’s rotation 2.Electric charges moving within the core  core must be molten. The north and south magnetic poles lie near the north and south poles.

8. April 4, 2006Astronomy 20108 The Magnetosphere The magnetosphere is the region where the Earth’s magnetic field dominates other magnetic fields (primarily the Sun’s). Charged particles from the Sun become trapped in the magnetosphere. The flow of charged particles from the Sun is called the solar wind.

9. April 4, 2006Astronomy 20109 The Magnetosphere (cont’d)

10. April 4, 2006Astronomy 201010 Aurora Charged particles spiral down and excite atoms of gas in the upper atmosphere. The atoms then give off light of particular colors: red – hydrogen, green – helium The particles are able to spiral down at the North and South magnetic poles, hence the aurora borealis and aurora australis.

11. April 4, 2006Astronomy 201011 Aurora Borealis: The Northern Lights

12. April 4, 2006Astronomy 201012 7.2 The Crust of the Earth Igneous: rock that has cooled from a molten state. Sedimentary: rock built up by layering. Metamorphic: igneous or sedimentary rock chemically altered by pressure or temperature. Primitive: unaltered from formation of the solar sys. The Earth’s crust is the outer layer we live on, extending down about 10km. The crust is composed of 4 types of rock:

13. April 4, 2006Astronomy 201013 7.2.2 Plate Tectonics Theory that explains the motions of the Earth’s crust produced by heat-driven currents in the mantle. Also known as continental drift. The Earth’s surface can be divided into regions (plates) that tend to slide, push, or pull against each other, producing earthquakes, volcanoes, mountains, and rifts.

14. April 4, 2006Astronomy 201014 7.2.3 Rift and Subduction Zones Rift zone: where 2 plates are pulling apart, forming new crust in the gap. Subduction zone: where 2 plates push together, one sliding beneath the other.

15. April 4, 2006Astronomy 201015 7.2.4 Fault Zones and Mountain Building Fault zone: where 2 plates are sliding past each other. Plates can move at several cm per year. Usually active earthquake areas. San Andreas fault is one of the most famous fault zones.

16. April 4, 2006Astronomy 201016 Mountains When two crustal plates collide, high mountains can be formed by lifting and folding of the crust. –Alps –Himalayas –Rocky mountains –Andes On Earth, erosion will sculpt the mountains to sharp peaks.

17. April 4, 2006Astronomy 201017 7.2.5 Volcanoes Volcanoes occur when molten magma pushes up through the crust to the surface. Sometimes volcanoes involve dramatic explosions. Often they are less dramatic, involving a more or less constant release of lava. Hawaiian islands lie over a hot spot that changes position over time as the ocean floor moves.

18. April 4, 2006Astronomy 201018 7.3 The Earth’s Atmosphere We live in the troposphere. Above that are the stratosphere, mesosphere, and ionosphere. The ozone layer in the stratosphere absorbs UV radiation. “Ocean of air” in which we live. The pressure of the atmosphere is produced by the weight of the air above us. Pressure at surface is 1 bar.

19. April 4, 2006Astronomy 201019 7.3.2 Atmospheric Composition and Origin Nitrogen (N 2 )78% Oxygen (O 2 )21% Argon (A)1% Water vapor (H 2 O)Trace Carbon dioxide (CO 2 )Trace The composition is summarized in the table below. This isn’t the whole story. Water in the oceans Carbon dioxide locked in carbonaceous matter.

20. April 4, 2006Astronomy 201020 7.3.3 Weather and Climate I’ll skip this section.

21. April 4, 2006Astronomy 201021 7.4 Life and Chemical Evolution Earth is the only place we know has life. Life on Earth dates back to at least 3.5 billion years ago. Early life was plant- like, taking in CO 2 and releasing O 2. When enough oxygen was present, animals could evolve.

22. April 4, 2006Astronomy 201022 7.4.3 The Greenhouse Effect and Global Warming The surface of the Earth is heated by light from the Sun, mostly visible light. The hot surface radiates back infrared radiation, some of which escapes into space, cooling the Earth. Carbon dioxide absorbs infrared radiation. Like a blanket, CO 2 in the atmosphere can stop the infrared radiation from escaping, trapping heat and causing temperatures to rise.

23. April 4, 2006Astronomy 201023 7.5 Cosmic Influences on the Evolution of Earth If the Earth has been influenced by extra-terrestrial phenomena, where is the evidence? Why isn’t the Earth cratered like the Moon? –Could the impacts have missed the Earth and hit the Moon? –Maybe the objects burned up in the atmosphere? Possible for small meteors, but not the large objects that produce large craters.

24. April 4, 2006Astronomy 201024 7.5.2 Recent Impacts On June 30, 1908, a huge explosion occurred in the region of Tunguska River, Siberia. The shock wave flattened more than 1000 sq. km of forest and herds of animals were killed. The blast was equivalent to 15 megatons of TNT. No crater was formed. This event is believed to be due to a stony projectile of about 100,000 tons that disintegrated and exploded in the atmosphere.

25. April 4, 2006Astronomy 201025 Arizona – Meteor Crater Northern Arizona Impact 50,000 years ago Iron-nickel meteorite Weighing several hundred thousand tons Estimated size ~ 150 feet across Hurtling at about 40,000 miles per hour Explosive force greater than 20 million tons of TNT. Crater 700 feet deep, 4000 feet across. Today: crater is 550 feet deep, and 2.4 miles in circumference.

26. April 4, 2006Astronomy 201026 Impacts There is evidence that the Earth has been and continues to be hit by large objects. Over periods of about 100 million years, the Earth’s crust is (almost) completely recycled, removing evidence of old craters. Over shorter periods, erosion of craters makes them harder to spot. With modern imaging, more than 150 craters have been found.

27. April 4, 2006Astronomy 201027 Known Craters on Earth

28. April 4, 2006Astronomy 201028 7.5.3 Extinction of the Dinosaurs Very large impacts can have global consequences. 65 million years ago an impact on the Yucatan coast occurred. Evidence of the impact: –Outline of a large crater (200km in diameter) near Chicxulub, Mexico of the right age –Global sediment layers of that age with an excess of the element iridium. Hypothesis: This impact put enough material in the atmosphere to block the Sun and kill much of the plant and animal life on Earth (mass extinction). Explosive power of 5 billion Hiroshima nuclear bombs, about 50,000 trillion tons of TNT!

29. April 4, 2006Astronomy 201029

30. April 4, 2006Astronomy 201030 Chicxulub Gravity Profiles These images show the small changes in gravity (few parts per 100,000) outlining the crater.

31. April 4, 2006Astronomy 201031 7.5.5 Impacts in Our Future? Periodic large impacts may be the reason for other past mass extinctions. The evidence is that impacts have occurred in the past, and that some were large enough to cause global consequences. There is reason to believe it can happen again! –Small sized impact could destroy a city –Large impact could have global effects; could destroy human civilization.

32. April 4, 2006Astronomy 201032 What’s Being Done In the 1990’s Congress funded a program to search for and track potentially dangerous objects, objects in so-called Earth-crossing orbits. –Objects bigger than 10km have been identified. –About half of the objects bigger than 1km have been found. (Estimated total is 1100 objects.)

33. April 4, 2006Astronomy 201033 Discussion Question Rate the importance (in $) of finding objects with Earth- crossing orbits? For comparison, U.S. government spends: –Space station: $10 billion/yr –All NASA: $16 billion/yr –Medical research: $100 billion/yr –War on terrorism: $100 billion/yr How much taxpayer money should be spent?