Planet Earth

Basic Facts The Earth is a medium-sized planet with a diameter of 13,000 km It is one of the inner or terrestrial planets It is composed primarily of heavy elements, such as iron, silicon, and oxygen It has much less light elements, such as hydrogen and helium, than the outer planets Earth's orbit around the Sun is nearly circular The Earth is the only planet in our solar system that is neither too hot nor too cold It is warm enough to support liquid water on its surface It is “just right” to sustain life — at least life as we know it 12 July 2005 AST 2010: Chapter 7

Some Properties of the Earth 12 July 2005 AST 2010: Chapter 7

Earth's Interior (1) The interior of the Earth is difficult to study even with today's amazing technology Its composition and structure must be determined indirectly from observations made near or at the surface only Earth’s skin is a layer only a few kilometers deep The Earth is composed largely of metals and silicate rock Most of this material is in a solid state, but some of it is hot enough to be molten 12 July 2005 AST 2010: Chapter 7

Earth's Interior (2) The structure of the interior of the Earth has been probed in great detail by measuring the transmission of seismic waves through it Seismic waves are waves that spread through the interior of the Earth from earthquakes or explosion sites Seismic waves travel through Earth rather like sound waves through a struck bell A bell’s sound frequencies depend on what material the bell is made of and how it is constructed Similarly, the way seismic waves travel through a planet can reveal some information about its interior From seismic studies, scientists have learned that the Earth’s interior consists of several distinct layers with different compositions 12 July 2005 AST 2010: Chapter 7

Seismic Waves in Earth's Interior http://www.uwgb.edu/dutchs/EarthSC202Notes/quakes.htm 12 July 2005 AST 2010: Chapter 7

Earth’s Internal Layers (1) The Earth is divided into four main layers: crust, mantle, core, and inner core The crust is the top layer, the part we know best The crust under the oceans covers 55% of the surface, is typically about 6 km thick, and is composed of volcanic rocks called basalt Basalts are produced by cooling volcanic lava and made primarily of silicon, oxygen, iron, aluminum, and magnesium The continental crust covers 45% of the surface, is 20 to 70 km thick and is mainly composed of another class of volcanic rocks called granite The whole crust makes up only about 0.3% of the Earth’s total mass 12 July 2005 AST 2010: Chapter 7

Earth’s Internal Layers (2) The mantle is the largest part of the solid Earth, stretching from the base of the crust down to a depth of 2,900 km The mantle is more or less solid, but may deform and flow slowly due to the high pressures and temperatures found there Below the mantle is Earth’s dense metallic core It contains iron, and probably also nickel and sulfur, all compressed to a very high density The core is 7,000 km in diameter Its outer part is liquid The inner core is 2,400 km in diameter and is probably solid 12 July 2005 AST 2010: Chapter 7

Differentiation Scientists believe that the Earth’s layered interior resulted from differentiation This is the process by which gravity helps separate the interior of an initially molten planet into layers of different compositions and densities When much of the planet is still molten, the heavier metals sink to the center to form a dense core, while the lightest elements float to the surface to form a crust When the planet cools, this layered structure is preserved 12 July 2005 AST 2010: Chapter 7

Earth’s Magnetic Field Additional clues about the Earth's interior can be learned from its magnetic field The Earth behaves in some ways as if a giant bar magnet were inside it The magnet is roughly aligned with the rotational axis of the planet The Earth’s magnetic field is generated by moving material in Earth’s liquid metallic core The circulating liquid metal sets up an electric current, which in turn produces a magnetic field http://homepage.smc.edu/balm_simon/images/astro%201b/EARTH/magnetic_field.jpg 12 July 2005 AST 2010: Chapter 7

Earth’s Magnetosphere (1) The Earth's magnetic field extends into surrounding space and traps small quantities of charged particles, such as electrons, that roam about the solar system Within this region, called the magnetosphere, the Earth’s field dominates over the weak interplanetary magnetic field extending outward from the Sun Most of the charged particles trapped in this region originate from the hot surface of the Sun, flowing out in a stream called the solar wind This elongates the magnetosphere far beyond the Earth in the direction pointing away from the Sun The Earth’s magnetosphere was discovered in 1958 by instruments on the first U.S. Earth satellite, Explorer 1 This satellite recorded the ions (charged particles) trapped in the inner part of the magnetosphere 12 July 2005 AST 2010: Chapter 7

Earth’s Magnetosphere (2) The regions of high-energy ions in the magnetosphere are often called the Van Allen Belts after the physicist who built the instrumentation for Explorer 1 and correctly interpreted its measurements This region has a fairly complex structure Animation Cross-sectional view of Earth’s magnetosphere as revealed by spacecraft missions

What Comes to Your Mind upon Hearing “Rocks”? 

Rocks (1) Both basalt & granites are examples of igneous rock, which is any rock that has cooled from a molten state All volcanically produced rock is igneous There are two other kinds of rock Sedimentary rocks are made of fragments of igneous rocks or the shells of living organisms deposited by wind or water and cemented without melting Metamorphic rocks are produced when high temperature or pressure alters igneous or sedimentary rocks physically or chemically These are commonly found on Earth, but not on other planets 12 July 2005 AST 2010: Chapter 7

Rocks (2) A fourth kind of rock is primitive rock Its formation is believed to date back to the formation of the planet Primitive rock has largely escaped chemical modification by heating Thus, it is thought to represent the original material out of which the planetary system was made No primitive rock is left on the Earth because the planet was heated early in its history Primitive rocks may be found in comets, asteroids, or small planetary satellites 12 July 2005 AST 2010: Chapter 7

Geology & Plate Tectonics Geology is the study of the Earth’s crust and the processes that have shaped it throughout history Not until the middle of the 20th century, did geologists succeed in understanding how landforms are created Plate tectonics is a theory that explains how slow motions within the Earth’s mantle move large segments of the crust, resulting in a gradual drifting of the continents the formation of mountains and other large-scale geological features The Earth's crust and upper mantle are divided into about a dozen major plates that fit together like the pieces of a jigsaw puzzle 12 July 2005 AST 2010: Chapter 7

Plate Tectonics (1) These plates are capable of moving slowly relative to one another In some places, such as the Atlantic Ocean, the plates are moving apart, and elsewhere they are being forced together 12 July 2005 AST 2010: Chapter 7

Plate Tectonics (2) The driving power behind the plates’ motion is provided by slow convection of the mantle Convection is a process by which heat escapes from the interior through the upward flow of warmer material and the slow sinking of cooler material As the plates move slowly, they bump into one another and cause dramatic changes in the Earth’s crust over time Basically, four types of interactions between crustal plates are possible at their boundaries: They can pull apart One plate can burrow under another The can slide alongside each other They can jam together 12 July 2005 AST 2010: Chapter 7

Rift Zones Plates pull apart from each other along rift zones Most rift zones are in the oceans An example is the Mid-Atlantic ridge, which is driven by upwelling currents in the mantle A few rift zones are also found on land The best known is central African rift, an area where the African continent is slowly breaking apart Animation http://oceanexplorer.noaa.gov/explorations/03fire/logs/ridge_subd_320.mov

Subduction Zones When two plates come together, one plate is often forced down beneath another in what is called a subduction zone Continental masses cannot be subducted but thinner oceanic plates can be “easily” pushed down into the upper mantle A subduction zone is often marked by an ocean trench Subducted plates forced down into regions of high temperature and pressure eventually melt several hundred kilometers below the surface Animation

Fault Zones Crustal plates slide parallel to each other along much of their lengths Boundaries so formed lead to the formation of cracks or faults Along active fault zones, the motion of one plate relative to the other may amount to several centimeters per year basically the same as the spreading rates along rifts The creeping motions of the plates in fault zones build up stresses in the crust The stresses are eventually released in sudden, violent slippages, a.k.a. earthquakes The average motion of the plates is constant The longer the interval between earthquakes, the greater the stress and the larger the energy released when the surface finally moves 12 July 2005 AST 2010: Chapter 7

San Andreas Fault It is on the boundary between the Pacific and North American plates running from the Gulf of California to the Pacific Ocean northwest of San Francisco The Pacific plate (west side) moves north carrying along Los Angeles, San Diego, and other parts of Southern California In a few million years, LA will be an island off the coast of San Francisco 12 July 2005 AST 2010: Chapter 7

More about San Andreas The San Andreas Fault near Parkfield has slipped every 22 years during the past century moving an average of about 1 m each time In contrast, the average time interval between major earthquakes in the Los Angeles region is about 140 years the average motion is about 7 m 12 July 2005 AST 2010: Chapter 7

Mountain Building When two continental masses are brought together by the motion of the crustal plates, they are forced against each other under great pressure The surface buckles and folds, forcing some of the rock deep below the surface and others to raise to large heights (sometimes many kilometers!) This is how mountain ranges form on Earth  The Alps result from the interaction of the African plate with the European plate We will see, however, that other mechanisms lead to the formation of mountains on other planets 12 July 2005 AST 2010: Chapter 7

Volcanoes Volcanoes mark the location where molten rock, called magma, rises from the upper mantle through the crust  Volcanoes are formed numerously along oceanic rift zones where rising hot material pushes plates away from one another Volcanic activity is also observed in subduction zones In both cases, the volcanic activity brings to the surface large amount of materials from the upper mantle http://www.rocks-and-minerals.com/more-about-rocks/images/volcano.jpg 12 July 2005 AST 2010: Chapter 7

Earth’s Atmosphere It provides the air we breathe The air of the atmosphere exerts a constant pressure (on the ground) The atmospheric pressure at sea level is used to define the pressure unit called bar Humans have existed mostly at sea level and are thus accustomed to such a pressure The total mass of the atmosphere is ~5x1018 kg Although this sounds like a lot, it constitutes only one millionth of the total mass of the Earth Yet its composition is quite vital to us humans and other living creatures on the surface of this Earth 12 July 2005 AST 2010: Chapter 7

Structure of Earth’s Atmosphere 12 July 2005 AST 2010: Chapter 7

Troposphere Altitude range: Densest area of the atmosphere Sea level - 9 miles Densest area of the atmosphere Most weather occurs and almost all aircraft fly in this region Temperatures drop as elevation increases Warm air, heated on the surface, rises and is replaced by descending currents of cooler air The circulation generates clouds and other manifestations of weather As one rises through the troposphere, one finds the temperature drops rapidly with increasing elevation The temperature is near 50°C below freezing at the top of the troposphere 12 July 2005 AST 2010: Chapter 7

Stratosphere Altitude range: Dry and less dense 9 - 31 miles Dry and less dense The air in this layer moves horizontally and does not move up and down within it Temperatures here increase with elevation Near the top of the stratosphere, one finds a layer of ozone (O3) Ozone is a good absorber of ultraviolet light It thus protects the surface from the sun's ultraviolet radiation and makes it possible for life to exist on the planet 12 July 2005 AST 2010: Chapter 7

Mesosphere Altitude range: 31 - 62 miles (50 - 100 km) Temperatures fall as low as -93° Celsius in this region Chemicals are in an excited state, as they absorb energy from the sun 12 July 2005 AST 2010: Chapter 7

Ionosphere Altitude range: 62 - 124 miles This region is characterized by the presence of plasma Its boundaries vary according to solar activity 12 July 2005 AST 2010: Chapter 7

Thermosphere Altitude range: 124 - 310 miles (200 - 500 km) Temperatures increase with altitude due to the sun's energy, reaching as high as 1,727 degrees Celsius Auroras, caused by the sun's particles striking the earth's atmosphere, occur at this level 12 July 2005 AST 2010: Chapter 7

Exosphere Altitude range: 310 - 434 miles (500 - 700 km) The region begins at the top to the thermosphere and continues until it merges with interplanetary gases, or space The prime components, hydrogen and helium, are present at extremely low densities 12 July 2005 AST 2010: Chapter 7

Weather and Climate All planets with atmospheres have weather Weather is simply the name given to the circulation of air through the atmosphere The driving force behind weather is derived primarily from the sunlight that heats the Earth's surface  As the planet rotates, and orbits the Sun, the slower seasonal changes cause variations in the amount of heat received by the different parts of the planet The heat then redistributes itself from warmer to cooler areas giving rise to various weather patterns Climate is a term used to describe the evolution of weather through long periods of time (decades or centuries) Changes in climate are typically difficult to detect over short periods of time However, their accumulating effects can be sizeable and sometimes quite dramatic

Role of Carbon Dioxide (CO2) Upon striking the Earth’s surface, sunlight is absorbed by the ground heats the surface layers is re-emitted as infrared or heat radiation The CO2 in our atmosphere is transparent to visible light allowing sunlight to reach the ground However, CO2 is opaque to infrared energy acting as a blanket, trapping the heat in the atmosphere and impeding its flow back to space Such trapping of infrared radiation near a planet’s surface is called the greenhouse effect

Greenhouse Effect On average, as much heat reaches the surface from the atmospheric greenhouse effect as from direct sunlight This explains why nighttime temperatures are only slightly lower than daytime temperatures It is estimated that the greenhouse effect elevates the surface temperature by about 23°C on the average Without this greenhouse effect, the average surface temperature would be well below freezing The Earth would be locked in a global ice age Life as we know it would not be possible on Earth Animation On the other hand, increasing amounts of CO2 in our atmosphere could raise its average temperature to a much higher value and then endanger life on our planet

Global Warming Modern society increasingly depends on energy extracted from burning fossil fuels, releasing CO2 into the atmosphere The problem is exacerbated by ongoing destruction of tropical forests, which we depend on to extract CO2 and replenish our supply of oxygen (O2) Atmospheric CO2 has increased by about 25% in the last 100 years In less than 100 years, the CO2 level will likely reach twice the value it had before the industrial revolution The consequences of such an increase for the Earth are complex and not completely known The Earth’s surface and atmosphere are extremely complicated systems Scientists study how they are affected by global warming using elaborate computer models Their conclusions are not yet firm at this point

Why is there no clear evidence of craters on Earth?

Suggested Answer Geological activity! http://vulcan.wr.usgs.gov/Imgs/Jpg/Popo/popo1.jpg

Earth Craters Evidence of fairly recent impacts can be found on our planet's surface The best studied case took place on June 30, 1908, near the Tunguska River in Siberia, Russia  There was an explosion 8 km above the ground The shock wave flattened more than a thousand square kilometers of forest The blast wave spread around the world and was recorded by instruments designed to record changes in atmospheric pressure 12 July 2005 AST 2010: Chapter 7