planet earth 12 july 2005ast 2010: chapter 72 basic facts the earth is a medium-sized planet with a...

Planet Planet EarthEarth

12 July 2005 AST 2010: Chapter 7 2

Basic FactsThe Earth is a medium-sized planet with a diameter of 13,000 kmIt is one of the inner or terrestrial planets

It is composed primarily of heavy elements, such as iron, silicon, and oxygenIt has much less light elements, such as hydrogen and helium, than the outer planets

Earth's orbit around the Sun is nearly circularThe 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 surfaceIt is “just right” to sustain life — at least life as we know it


Some Properties of the Earth


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 deepThe 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


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 sitesSeismic 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 constructedSimilarly, 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


Seismic Waves in Earth's Interior


Earth’s Internal Layers (1) The Earth is divided into four main layers: crust, mantle, core, and inner coreThe 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


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 coreIt contains iron, and probably also nickel and sulfur, all compressed to a very high density The core is 7,000 km in diameterIts outer part is liquid The inner core is 2,400 km in diameter and is probably solid


DifferentiationScientists 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 densitiesWhen 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 crustWhen the planet cools, this layered structure is preserved


Earth’s Magnetic FieldAdditional clues about the Earth's interior can be learned from its magnetic fieldThe 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


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 systemWithin this region, called the magnetosphere, the Earth’s field dominates over the weak interplanetary magnetic field extending outward from the SunMost 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

12AST 2010: Chapter 712 July 2005

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 measurementsThis region has a fairly complex structureAnimation

Cross-sectional view of Earth’s magnetosphere Cross-sectional view of Earth’s magnetosphere as revealed by spacecraft missionsas revealed by spacecraft missions

http://www.digitalradiance.com/sng/magnetosphere.htm


What Comes to Your Mind upon Hearing “Rocks”?

http://hal.physics.wayne.edu/~jtandean/ast2010s2005/pw.html


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 igneousThere 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 meltingMetamorphic rocks are produced when high temperature or pressure alters igneous or sedimentary rocks physically or chemicallyThese are commonly found on Earth, but not on other planets


Rocks (2)A fourth kind of rock is primitive rock

Its formation is believed to date back to the formation of the planetPrimitive rock has largely escaped chemical modification by heatingThus, it is thought to represent the original material out of which the planetary system was madeNo primitive rock is left on the Earth because the planet was heated early in its historyPrimitive rocks may be found in comets, asteroids, or small planetary satellites


Geology & Plate TectonicsGeology is the study of the Earth’s crust and the processes that have shaped it throughout historyNot 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 continentsthe 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


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


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 timeBasically, four types of interactions between crustal plates are possible at their boundaries:

They can pull apartOne plate can burrow under anotherThe can slide alongside each otherThey can jam together


Rift ZonesPlates 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 ZonesWhen 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 mantleA 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 surfaceAnimation

http://oceanexplorer.noaa.gov/explorations/03fire/logs/subduction.html


Fault ZonesCrustal plates slide parallel to each other along much of their lengthsBoundaries so formed lead to the formation of cracks or faultsAlong 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 riftsThe creeping motions of the plates in fault zones build up stresses in the crustThe stresses are eventually released in sudden, violent slippages, a.k.a. earthquakesThe 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


San Andreas FaultIt 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 CaliforniaIn a few million years, LA will be an island off the coast of San Francisco


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


Mountain BuildingWhen 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


VolcanoesVolcanoes 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 anotherVolcanic activity is also observed in subduction zonesIn both cases, the volcanic activity brings to the surface large amount of materials from the upper mantle


Earth’s AtmosphereIt provides the air we breatheThe 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 barHumans have existed mostly at sea level and are thus accustomed to such a pressureThe 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 EarthYet its composition is quite vital to us humans and other living creatures on the surface of this Earth


Structure of Earth’s Atmosphere


Troposphere Altitude range:

Sea level - 9 miles

Densest area of the atmosphereMost weather occurs and almost all aircraft fly in this regionTemperatures 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 weatherAs 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


StratosphereAltitude range:

9 - 31 miles

Dry and less denseThe air in this layer moves horizontally and does not move up and down within itTemperatures here increase with elevationNear the top of the stratosphere, one finds a layer of ozone (O3)

Ozone is a good absorber of ultraviolet lightIt thus protects the surface from the sun's ultraviolet radiation and makes it possible for life to exist on the planet


Mesosphere

Altitude range: 31 - 62 miles (50 - 100 km)

Temperatures fall as low as -93° Celsius in this regionChemicals are in an excited state, as they absorb energy from the sun


Ionosphere Altitude range: 62 - 124 milesThis region is characterized by the presence of plasmaIts boundaries vary according to solar activity


Thermosphere


Temperatures increase with altitude due to the sun's energy, reaching as high as 1,727 degrees CelsiusAuroras, caused by the sun's particles striking the earth's atmosphere, occur at this level


Exosphere


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

Weather and ClimateAll 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 planetThe 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 groundheats the surface layersis 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 energyacting 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 EffectOn 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 averageWithout this greenhouse effect, the average surface temperature would be well below freezing

The Earth would be locked in a global ice ageLife 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

http://perso.wanadoo.fr/prof.danglais/animations/greenhouse/greenhouse.htm

Global WarmingModern 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 systemsScientists study how they are affected by global warming using elaborate computer models Their conclusions are not yet firm at this point

Craters

Why is there no clear evidence of craters on Earth?

planet earth 12 july 2005ast 2010: chapter 72 basic facts the earth is a medium-sized planet with a...

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