The EarthThe Earth

Arny, 3Arny, 3rdrd Edition, Chapter 5 Edition, Chapter 5

The Earth 2

The Earth As A PlanetThe Earth As A Planet IntroductionIntroduction

In simple terms, the Earth is a huge, rocky sphere In simple terms, the Earth is a huge, rocky sphere spinning in space and moving around the Sun to spinning in space and moving around the Sun to the tune of about 100 miles every few secondsthe tune of about 100 miles every few seconds

Earth also has a blanket of air and a screen of Earth also has a blanket of air and a screen of magnetism that protects the surface and its life magnetism that protects the surface and its life forms from the hazards of interplanetary space forms from the hazards of interplanetary space

Shape and sizeShape and size Then Earth is large enough for gravity to have Then Earth is large enough for gravity to have

shaped it into a sphereshaped it into a sphere Earth’s spinning makes its equator bulge into a Earth’s spinning makes its equator bulge into a

shape referred to as an oblate spheroid – a result shape referred to as an oblate spheroid – a result of inertiaof inertia

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The Earth As A PlanetThe Earth As A Planet Composition of the EarthComposition of the Earth

The most common elements of the Earth’s surface The most common elements of the Earth’s surface rocks are: oxygen (45.5% by mass), silicon rocks are: oxygen (45.5% by mass), silicon (27.2%), aluminum (8.3%), iron (6.2%), calcium (27.2%), aluminum (8.3%), iron (6.2%), calcium (4.66%), and magnesium (2.76%)(4.66%), and magnesium (2.76%)

Silicon and oxygen usually occur together asSilicon and oxygen usually occur together as silicatessilicates

Ordinary sand is the silicate mineral quartz and Ordinary sand is the silicate mineral quartz and is nearly pure silicon dioxideis nearly pure silicon dioxide

Much of Earth’s interior is the mineral olivine, a Much of Earth’s interior is the mineral olivine, a iron-magnesium silicate with a olive green coloriron-magnesium silicate with a olive green color

Earth’s interior composition is determined from Earth’s interior composition is determined from analyzing seismic waves and the Earth’s densityanalyzing seismic waves and the Earth’s density

The Earth 4

The Earth As A PlanetThe Earth As A PlanetDensity of EarthDensity of Earth

DensityDensity is a measure of how much material is a measure of how much material (mass) is packed into a given volume(mass) is packed into a given volume

Typical unit of density is grams per cubic Typical unit of density is grams per cubic centimetercentimeter

Water has a density of 1 g/cmWater has a density of 1 g/cm33, ordinary , ordinary surfacesurface rocks are 3 g/cm rocks are 3 g/cm33, while iron is 8 , while iron is 8 g/cmg/cm33

For Earth, this density is found to be 5.5 For Earth, this density is found to be 5.5 g/cmg/cm33

Consequently, the Earth’s interior (core) Consequently, the Earth’s interior (core) probably is iron (which is abundant in probably is iron (which is abundant in nature and high in density)nature and high in density)

The Earth 5

The Earth’s Interior IntroductionIntroduction Probing the interior with Earthquake wavesProbing the interior with Earthquake waves

Earthquakes generateEarthquakes generate seismic waves seismic waves that that move through the Earth with speeds move through the Earth with speeds depending on the properties of the material depending on the properties of the material through which they travelthrough which they travel

These speeds are determined by timing the These speeds are determined by timing the arrival of the waves at remote points on the arrival of the waves at remote points on the Earth’s surfaceEarth’s surface

A seismic “picture” is then generated of the A seismic “picture” is then generated of the Earth’s interior along the path of the waveEarth’s interior along the path of the wave

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The Earth’s InteriorThe Earth’s Interior Probing the interior with Earthquake waves Probing the interior with Earthquake waves (continued)(continued)

Seismic waves are of two types: S and PSeismic waves are of two types: S and P P waves compress material and travel easily through liquid P waves compress material and travel easily through liquid

or solidor solid S waves move material perpendicular to the wave direction S waves move material perpendicular to the wave direction

of travel and only propagate through solidsof travel and only propagate through solids Observations show P waves but no S waves at Observations show P waves but no S waves at

detecting stations on the opposite side of the Earth detecting stations on the opposite side of the Earth from the origin of an Earthquake from the origin of an Earthquake the Earth has a the Earth has a liquid coreliquid core

Seismic studies show that the Earth’s interior has Seismic studies show that the Earth’s interior has four distinct regions:four distinct regions:

A solid, low-density and thinA solid, low-density and thin crust crust made mainly of silicatesmade mainly of silicates A hot, not-quite-liquid and thickA hot, not-quite-liquid and thick mantle mantle with silicateswith silicates A A liquid or outer core liquid or outer core with a mixture of iron, nickel and with a mixture of iron, nickel and

perhaps sulfurperhaps sulfur A A solid or inner core solid or inner core of iron and nickelof iron and nickel

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The Earth’s InteriorThe Earth’s Interior Probing the interior with Earthquake waves Probing the interior with Earthquake waves

(continued)(continued)

The Earth is layered in such a fashion that The Earth is layered in such a fashion that the densest materials are at the center and the densest materials are at the center and the least dense at the surface – this is the least dense at the surface – this is referred to asreferred to as differentiation differentiation

The Earth’s inner core is solid because it is The Earth’s inner core is solid because it is under such high pressure (from overlying under such high pressure (from overlying materials) that the temperature there is not materials) that the temperature there is not high enough to liquefy it – this is not the high enough to liquefy it – this is not the case for the outer liquid corecase for the outer liquid core

The Earth 8

The Earth’s InteriorThe Earth’s InteriorHeating the Earth’s CoreHeating the Earth’s Core

The estimated temperature of the Earth’s The estimated temperature of the Earth’s core is 6500 Kcore is 6500 K

This high temperature is probably due to at This high temperature is probably due to at least the following two causes:least the following two causes:

Heat generation from the impact of small bodies Heat generation from the impact of small bodies that eventually formed the Earth by their mutual that eventually formed the Earth by their mutual gravitationgravitation

The The radioactive decayradioactive decay of of radioactive elementsradioactive elements that occur naturally in the mix of materials that that occur naturally in the mix of materials that made up the Earthmade up the Earth

The Earth 9

The Age of the Earth Radioactive decay used to determine the Radioactive decay used to determine the

Earth’s ageEarth’s age Radioactive atoms decay intoRadioactive atoms decay into daughter atoms daughter atoms The more daughter atoms there are relative to the The more daughter atoms there are relative to the

original radioactive atoms, the older the rock isoriginal radioactive atoms, the older the rock is Radioactive potassium has a half-life of 1.28 Radioactive potassium has a half-life of 1.28

billion years and decays into argon which is a billion years and decays into argon which is a gas that is trapped in the rock unless it meltsgas that is trapped in the rock unless it melts Assume rock has no argon when originally formedAssume rock has no argon when originally formed Measuring the ratio of argon atoms to potassium Measuring the ratio of argon atoms to potassium

atoms gives the age of the rockatoms gives the age of the rock This method gives a minimum age of the Earth as 4 This method gives a minimum age of the Earth as 4

billion yearsbillion years Other considerations put the age at 4.5 billion yearsOther considerations put the age at 4.5 billion years

The Earth 10

Motions in the Earth’s InteriorMotions in the Earth’s Interior IntroductionIntroduction

Heat generated by radioactive decay in the Earth Heat generated by radioactive decay in the Earth creates movement of rock called creates movement of rock called convectionconvection

Convection occurs because hotter material will be Convection occurs because hotter material will be less dense than its cooler surroundings and less dense than its cooler surroundings and consequently will rise while cooler material sinksconsequently will rise while cooler material sinks

Convection in the Earth’s InteriorConvection in the Earth’s Interior The crust and mantle are solid rock, although The crust and mantle are solid rock, although

when heated, rock may develop convective when heated, rock may develop convective motionsmotions

These convective motions are slow, but are the These convective motions are slow, but are the cause of: earthquakes, volcanoes, the Earth’s cause of: earthquakes, volcanoes, the Earth’s magnetic field, and perhaps the atmosphere itselfmagnetic field, and perhaps the atmosphere itself

The Earth 11

Motions in the Earth’s InteriorMotions in the Earth’s Interior Plate TectonicsPlate Tectonics

RiftingRifting Hot,molten material rises from deep in the Earth’s interior Hot,molten material rises from deep in the Earth’s interior

in great, slow plumes that work their way to the surfacein great, slow plumes that work their way to the surface Near the surface, these plumes spread and drag the Near the surface, these plumes spread and drag the

surface layers from belowsurface layers from below The crust stretches, spreads, and breaks the surface in a The crust stretches, spreads, and breaks the surface in a

phenomenon called phenomenon called riftingrifting SubductionSubduction

Where cool material sinks, it may drag crustal pieces Where cool material sinks, it may drag crustal pieces together buckling them upward into mountainstogether buckling them upward into mountains

If one piece of crust slip under the other, the process is If one piece of crust slip under the other, the process is called called subductionsubduction

Rifting and subduction are the dominant forces Rifting and subduction are the dominant forces that sculpt the landscape – they may also trigger that sculpt the landscape – they may also trigger earthquakes and volcanoesearthquakes and volcanoes

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Motions in the Earth’s InteriorMotions in the Earth’s InteriorPlate Tectonics Plate Tectonics (continued)(continued)

The shifting of large blocks of the Earth’s The shifting of large blocks of the Earth’s surface is called surface is called plate tectonicsplate tectonics

The Earth 13

IntroductionIntroduction Veil of gases around Earth constitutes its atmosphereVeil of gases around Earth constitutes its atmosphere Relative to other planetary atmospheres, the Earth’s Relative to other planetary atmospheres, the Earth’s

atmosphere is uniqueatmosphere is unique However, studying the Earth’s atmosphere can tell us However, studying the Earth’s atmosphere can tell us

about atmospheres in generalabout atmospheres in general Composition of the AtmosphereComposition of the Atmosphere

The Earth’s atmosphere is primarily nitrogen (78.08% The Earth’s atmosphere is primarily nitrogen (78.08% by number) and oxygen (20.95% by number)by number) and oxygen (20.95% by number)

The remaining gases in the atmosphere (about 1%) The remaining gases in the atmosphere (about 1%) includes: carbon dioxide, ozone, water, and argon, includes: carbon dioxide, ozone, water, and argon, the first three of which are important for lifethe first three of which are important for life

This composition is unique relative to the carbon This composition is unique relative to the carbon dioxide atmospheres of Mars and Venus and the dioxide atmospheres of Mars and Venus and the hydrogen atmospheres of the outer large planetshydrogen atmospheres of the outer large planets

The Earth’s Atmosphere

The Earth 14

The Earth’s AtmosphereThe Earth’s Atmosphere Origin of the AtmosphereOrigin of the Atmosphere

Several theories to explain origin of Earth’s Several theories to explain origin of Earth’s atmosphereatmosphere

Release of gas (originally trapped when the Earth Release of gas (originally trapped when the Earth formed) by volcanism or asteroid impactsformed) by volcanism or asteroid impacts

From materials brought to Earth by comet impactsFrom materials brought to Earth by comet impacts Early atmosphere different than todayEarly atmosphere different than today

Contained much more methane (CHContained much more methane (CH44) and ammonia ) and ammonia (NH(NH33))

Solar uv was intense enough to break out H from CHSolar uv was intense enough to break out H from CH44 NHNH33 and H and H22O leaving carbon, nitrogen, and oxygen O leaving carbon, nitrogen, and oxygen behind while the H escaped into spacebehind while the H escaped into space

Ancient plants further increased the levels of Ancient plants further increased the levels of atmospheric oxygen through photosynthesisatmospheric oxygen through photosynthesis

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The Earth’s AtmosphereThe Earth’s AtmosphereThe Ozone LayerThe Ozone Layer

Oxygen in the atmosphere provides a Oxygen in the atmosphere provides a shield against solar uv radiationshield against solar uv radiation

OO22 provides some shielding, but O provides some shielding, but O33, or , or ozoneozone, provides most of it, provides most of it

Shielding is provided by the absorption of Shielding is provided by the absorption of uv photons by oxygen molecules (both Ouv photons by oxygen molecules (both O22 and Oand O33) and their resultant dissociation) and their resultant dissociation

It is doubtful that life could exist on the It is doubtful that life could exist on the Earth’s surface without the ozone layerEarth’s surface without the ozone layer

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The Earth’s AtmosphereThe Earth’s Atmosphere The Greenhouse EffectThe Greenhouse Effect

Visible light reaches the Earth’s surface and is Visible light reaches the Earth’s surface and is converted to heatconverted to heat

As a result, the surface radiates infrared energy As a result, the surface radiates infrared energy which is trapped by the blocking power of the which is trapped by the blocking power of the atmosphere at infrared wavelengthsatmosphere at infrared wavelengths

This reduces the rate of heat loss and makes the This reduces the rate of heat loss and makes the surface hotter than it would be otherwisesurface hotter than it would be otherwise

This phenomenon is the This phenomenon is the Greenhouse EffectGreenhouse Effect Water and carbon dioxide are two molecules that Water and carbon dioxide are two molecules that

create the greenhouse effect through their create the greenhouse effect through their absorption of infrared radiationabsorption of infrared radiation

Atmospheric temperatures of Mars and Venus Atmospheric temperatures of Mars and Venus directly related to COdirectly related to CO22 and the greenhouse effect and the greenhouse effect

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The Earth’s AtmosphereThe Earth’s AtmosphereStructure of the AtmosphereStructure of the Atmosphere

Atmosphere extends to hundreds of Atmosphere extends to hundreds of kilometers kilometers

The atmosphere becomes less dense with The atmosphere becomes less dense with increasing altitudeincreasing altitude

Half the mass of the atmosphere is within Half the mass of the atmosphere is within the first 4 kilometersthe first 4 kilometers

The atmosphere eventually merges with The atmosphere eventually merges with the vacuum of interplanetary spacethe vacuum of interplanetary space

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Earth’s Magnetic Field

The Earth 19

The Earth’s Magnetic FieldThe Earth’s Magnetic Field IntroductionIntroduction

The Earth acts like a magnetic as indicated by its The Earth acts like a magnetic as indicated by its affect on a compassaffect on a compass

Magnetic forces are communicated by a Magnetic forces are communicated by a magnetic magnetic field field – direct physical contact is not necessary to – direct physical contact is not necessary to transmit magnetic forcestransmit magnetic forces

Magnetic field are depicted in diagrams by Magnetic field are depicted in diagrams by magnetic lines of forcemagnetic lines of force

Each line represents the direction a compass would pointEach line represents the direction a compass would point Density of lines indicate strength of fieldDensity of lines indicate strength of field

Magnetic fields also haveMagnetic fields also have polarity polarity – a direction – a direction from a north magnetic pole to a south magnetic from a north magnetic pole to a south magnetic polepole

Magnetic fields are generated either by large-scale Magnetic fields are generated either by large-scale currents or currents on an atomic scalecurrents or currents on an atomic scale

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The Earth’s Magnetic FieldThe Earth’s Magnetic FieldOrigin of the Earth’s Magnetic FieldOrigin of the Earth’s Magnetic Field

The magnetic field of the Earth is generated The magnetic field of the Earth is generated by currents flowing in its molten iron coreby currents flowing in its molten iron core

The currents are believed to be caused by The currents are believed to be caused by rotational motion and convection (magnetic rotational motion and convection (magnetic dynamo)dynamo)

The Earth’s geographic poles and magnetic The Earth’s geographic poles and magnetic poles do not coincidepoles do not coincide

Both the position and strength of the poles Both the position and strength of the poles change slightly from year to year, even change slightly from year to year, even reversing their polarity every 10,000 years reversing their polarity every 10,000 years or soor so

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The Earth’s Magnetic FieldThe Earth’s Magnetic Field Magnetic effects in the upper atmosphereMagnetic effects in the upper atmosphere

Earth’s magnetic field screens the planet from Earth’s magnetic field screens the planet from electrically charged particles emitted from the Sun, electrically charged particles emitted from the Sun, which are often of an energy harmful to living cellswhich are often of an energy harmful to living cells

The screening entails the Earth’s magnetic field The screening entails the Earth’s magnetic field deflecting the charged particles into spiral deflecting the charged particles into spiral trajectories and slowing them downtrajectories and slowing them down

As the charged solar particles stream past Earth, As the charged solar particles stream past Earth, they generate electrical currents in the upper they generate electrical currents in the upper atmosphereatmosphere

These currents collide with and excite moleculesThese currents collide with and excite molecules As the molecules de-excite, light photons are given As the molecules de-excite, light photons are given

off resulting in off resulting in AuroraAurora

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The Earth’s Magnetic FieldThe Earth’s Magnetic FieldMagnetic effects in the upper Magnetic effects in the upper

atmosphere atmosphere (continued)(continued)

Region of the upper atmosphere where Region of the upper atmosphere where the Earth’s magnetic field affects particle the Earth’s magnetic field affects particle motion is called themotion is called the magnetosphere magnetosphere

Within the magnetosphere charged Within the magnetosphere charged particles are trapped in two doughnut particles are trapped in two doughnut shaped rings that encircle the Earth and shaped rings that encircle the Earth and are called theare called the Van Allen radiation belts Van Allen radiation belts

Van Allen belt particles are energetic Van Allen belt particles are energetic enough to be a hazard to spacecraft and enough to be a hazard to spacecraft and space travelersspace travelers

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Motions of the EarthMotions of the Earth IntroductionIntroduction

Earth variety of motions include: spinning on its axis, Earth variety of motions include: spinning on its axis, orbiting Sun, moving with Sun around the Milky Way, orbiting Sun, moving with Sun around the Milky Way, and traveling through the Universe with the Milky Wayand traveling through the Universe with the Milky Way

Rotational and orbital motions define the day and year Rotational and orbital motions define the day and year and cause the seasonsand cause the seasons

But our planet’s motions have other effectsBut our planet’s motions have other effects Air and Ocean Circulation: The Coriolis EffectAir and Ocean Circulation: The Coriolis Effect

In the absence of any force an object will move in a In the absence of any force an object will move in a curved path over a rotating objectcurved path over a rotating object

This apparent curved motion is referred to as theThis apparent curved motion is referred to as the Coriolis effectCoriolis effect

From space the Coriolis effect is a consequence of From space the Coriolis effect is a consequence of the rotating Earth moving under the direct path of a the rotating Earth moving under the direct path of a moving objectmoving object

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Motions of the EarthMotions of the EarthAir and ocean circulation Air and ocean circulation (continued)(continued)

The Coriolis effect is responsible for:The Coriolis effect is responsible for: The spiral pattern of large storms as well as their The spiral pattern of large storms as well as their

direction of rotationdirection of rotation The trade winds that move from east to west in The trade winds that move from east to west in

two bands, one north and one south of the equatortwo bands, one north and one south of the equator The direction of theThe direction of the Jet streams Jet streams, narrow bands of , narrow bands of

rapid, high-altitude windsrapid, high-altitude winds The atmospheric band structure of the rapidly The atmospheric band structure of the rapidly

rotating Jupiter, Saturn, and Neptunerotating Jupiter, Saturn, and Neptune The deflection of ocean currents creating flows The deflection of ocean currents creating flows

such as the Gulf Streamsuch as the Gulf Stream

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Motions of the EarthMotions of the Earth PrecessionPrecession

As the Earth moves around the Sun over long periods As the Earth moves around the Sun over long periods of time, the direction in which its rotation axis points of time, the direction in which its rotation axis points changes slowlychanges slowly

This changing in direction of the spin axis is calledThis changing in direction of the spin axis is called precessionprecession

Precession is caused by the Earth not being a perfect Precession is caused by the Earth not being a perfect sphere – its equatorial bulge allows the Sun and sphere – its equatorial bulge allows the Sun and Moon to exert unbalanced gravitational forces that Moon to exert unbalanced gravitational forces that twist the Earth’s spin axistwist the Earth’s spin axis

The Earth’s spin axis precesses around once every The Earth’s spin axis precesses around once every 26,000 years26,000 years

Currently the spin axis points at Polaris – in Currently the spin axis points at Polaris – in A.D.A.D. 14,000 it will point nearly at the star Vega 14,000 it will point nearly at the star Vega

Precession may cause climate changesPrecession may cause climate changes

Light and Atoms

Photographs show that the Earth is round but the asteroid Gaspra is not. Gaspra is too small for its gravity to make it spherical. (Courtesy NASA.)

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Light and Atoms

(A) Rotation makes the Earth's equator bulge. (B) Jupiter's rapid rotation creates an equatorial bulge visible in this photograph. (Courtesy NASA.)

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Olivine (the greenish crystals) in a rock sample.

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Seismic waves spread out through Earth from an earthquake.

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P and S waves move through the Earth, but the S waves cannot travel through the liquid core.

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An artist's view of the Earth's interior.

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Melting ice cream “differentiates” as the dense chocolate chips sink to the bottom of the box. So too, melting has made much of the Earth's iron sink to its core.

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Heat readily escapes from small rocks but is retained in larger bodies. (Courtesy NASA.)

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Examples of convection: (A) In our atmosphere, puffy cumulus clouds form when the Sun heats the ground and warms the air so that it rises. (B) You can see rising and sinking motions in a pan of heated soup. (C) An artist's view of convection in the

Earth's interior.

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(A) Rifting may occur where rising material reaches a planet's surface. (B) Subduction builds mountains where material sinks back toward the interior of the Earth.

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Map of the Earth, showing its plates. Smaller plates include the Cocos (Co), Caribbean (Ca), Juan de Fuca (Jf), Arabia (Ar), Philippines (Ph), and Scotia (Sc).

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Breakup of Pangea and the Earth today. Notice the close match of the African and South American coastlines.

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Artist's view of the mid-Atlantic ridge (from World Ocean Floor by Bruce C. Heezen and Marie Tharp, 1977) and the increasing age of rocks away from it.

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(A) Volcanic gas vent today. Gas from ancient eruptions built our atmosphere. (Courtesy USGS.) (B) Planetesimals collide with young Earth and release gas—another

source of our atmosphere. (C) Comets striking young Earth and vaporizing. The released gases contributed to our atmosphere.

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Light and Atoms

The greenhouse effect. Radiation at visible wavelengths passes freely through the atmosphere and is absorbed at the ground. The ground heats up and emits infrared

radiation. Atmospheric gases absorb the infrared radiation and warm the atmosphere, which in turn warms the ground.

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Light and Atoms

Schematic view of Earth's magnetic field lines and photograph of iron filings sprinkled on a toy magnet, revealing its magnetic field lines.

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Electrically charged particles from the Sun spiral in the Earth's magnetic field.

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Photographs of an aurora from (A) the ground (courtesy Eugene Lauria) and(B) from space. (Courtesy NASA.)

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Artist's view of the Van Allen radiation belts (side view).

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The Earth's many motions in space.

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Coriolis effect on a rock thrown toward the equator from the North Pole.

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Weather satellite pictures show clearly the spiral pattern of spinning air around a storm that results from the Coriolis effect. (Courtesy NOAA.)

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Cloud bands on Jupiter created in part by the Coriolis effect. (Courtesy NASA/JPL.)

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Precession makes the Earth's rotation axis swing slowly in a circle.

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