from the perspective we get on earth.docx

8/10/2019 From the perspective we get on Earth.docx

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From the perspective we get on Earth, our planet appears to be big and sturdy with an

endless ocean of air. From space, astronauts often get the impression that the Earth is

small with a thin, fragile layer of atmosphere. For a space traveler, the distinguishing

Earth features are the blue waters, brown and green land masses and white clouds set

against a black background.

Many dream of traveling in space and viewing the wonders of the universe. In reality

all of us are space travelers. Our spaceship is the planet Earth, traveling at the speed of

108,000 kilometers (67,000 miles) an hour.

Earth is the 3rd planet from the Sun at a distance of about 150 million kilometers

(93.2 million miles). It takes 365.256 days for the Earth to travel around the Sun and

23.9345 hours for the Earth rotate a complete revolution. It has a diameter of 12,756

kilometers (7,973 miles), only a few hundred kilometers larger than that of Venus.

Our atmosphere is composed of 78 percent nitrogen, 21 percent oxygen and 1 percent

other constituents.

Earth is the only planet in the solar system known to harbor life. Our planet's rapid

spin and molten nickel-iron core give rise to an extensive magnetic field, which, along

with the atmosphere, shields us from nearly all of the harmful radiation coming from

the Sun and other stars. Earth's atmosphere protects us from meteors, most of which

burn up before they can strike the surface.

From our journeys into space, we have learned much about our home planet. The first

American satellite, Explorer 1, discovered an intense radiation zone, now called the

Van Allen radiation belts. This layer is formed from rapidly moving charged particlesthat are trapped by the Earth's magnetic field in a doughnut-shaped region

surrounding the equator. Other findings from satellites show that our planet's

magnetic field is distorted into a tear-drop shape by thesolar wind.We also now know

that our wispy upper atmosphere, once believed calm and uneventful, seethes with

activity -- swelling by day and contracting by night. Affected by changes in solar

activity, the upper atmosphere contributes to weather and climate on Earth.

Besides affecting Earth's weather, solar activity gives rise to a dramatic visual

phenomenon in our atmosphere. When charged particles from the solar wind become

trapped in Earth's magnetic field, they collide with air molecules above our planet'smagnetic poles. These air molecules then begin to glow and are known as

theaurorasor thenorthern and southern lights.

Earth Statistics

Mass (kg) 5.976e+24
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Mass (Earth = 1) 1.0000e+00

Equatorial radius (km) 6,378.14

Equatorial radius (Earth = 1) 1.0000e+00

Mean density (gm/cm^3) 5.515

Mean distance from the Sun (km) 149,600,000

Mean distance from the Sun (Earth = 1) 1.0000

Rotational period (days) 0.99727

Rotational period (hours) 23.9345

Orbital period (days) 365.256

Mean orbital velocity (km/sec) 29.79

Orbital eccentricity 0.0167

Tilt of axis (degrees) 23.45

Orbital inclination (degrees) 0.000Equatorial escape velocity (km/sec) 11.18

Equatorial surface gravity (m/sec^2) 9.78

Visual geometric albedo 0.37

Mean surface temperature 15C

Atmospheric pressure (bars) 1.013

Atmospheric compositionNitrogenOxygen

Other

77%21%

2%

The Earth's Interior

Just as a child may shake an unopened present in an attempt to discover the contents

of a gift, so man must listen to the ring and vibration of ourEarthin an attempt to

discover its content. This is accomplished through seismology, which has become the

principle method used in studying Earth's interior. Seismos is a Greek word meaningshock; akin to earthquake, shake, or violently moved. Seismology on Earth deals with

the study of vibrations that are produced by earthquakes, the impact ofmeteorites,or

artificial means such as an explosion. On these occasions, a seismograph is used to

measure and record the actual movements and vibrations within the Earth and of the

ground.
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Scientists categorize seismic movements into four types of diagnostic waves that

travel at speeds ranging from 3 to 15 kilometers (1.9 to 9.4 miles) per second. Two of

the waves travel around the surface of the Earth in rolling swells. The other two,

Primary (P) or compression waves and Secondary (S) or shear waves, penetrate the

interior of the Earth. Primary waves compress and dilate the matter they travel

through (either rock or liquid) similar to sound waves. They also have the ability tomove twice as fast as S waves. Secondary waves propagate through rock but are not

able to travel through liquid. Both P and S waves refract or reflect at points where

layers of differing physical properties meet. They also reduce speed when moving

through hotter material. These changes in direction and velocity are the means of

locating discontinuities.

Types of seismic waves(Adapted from, Beatty, 1990.)

Divisions in the Earth's Interior(Adapted from, Beatty, 1990.)
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Seismic discontinuities aid in distinguishing divisions of the Earth into inner core,

outer core, D", lower mantle, transition region, upper mantle, and crust (oceanic and

continental). Lateral discontinuities also have been distinguished and mapped through

seismic tomography but shall not be discussed here.

Inner core: 1.7% of the Earth's mass; depth of 5,150-6,370 kilometers(3,219 - 3,981 miles)The inner core is solid and unattached to the mantle, suspended in the molten

outer core. It is believed to have solidified as a result of pressure-freezing

which occurs to most liquids when temperature decreases or pressure increases.

Outer core: 30.8% of Earth's mass; depth of 2,890-5,150 kilometers (1,806

- 3,219 miles)The outer core is a hot, electrically conducting liquid within

whichconvectivemotion occurs. This conductive layer combines with Earth's

rotation to create a dynamo effect that maintains a system of electrical currents

known as the Earth's magnetic field. It is also responsible for the subtle jerking

of Earth's rotation. This layer is not as dense as pure molten iron, which

indicates the presence of lighter elements. Scientists suspect that about 10% of

the layer is composed of sulfur and/or oxygen because these elements are

abundant in the cosmos and dissolve readily in molten iron.

D": 3% of Earth's mass; depth of 2,700-2,890 kilometers (1,688 - 1,806

miles)This layer is 200 to 300 kilometers (125 to 188 miles) thick and represents

about 4% of the mantle-crust mass. Although it is often identified as part of the

lower mantle, seismic discontinuities suggest the D" layer might differ

chemically from the lower mantle lying above it. Scientists theorize that the

material either dissolved in the core, or was able to sink through the mantle but

not into the core because of its density.

Lower mantle: 49.2% of Earth's mass; depth of 650-2,890 kilometers (406

-1,806 miles)The lower mantle contains 72.9% of the mantle-crust mass and is probably

composed mainly of silicon, magnesium, and oxygen. It probably also contains

some iron, calcium, and aluminum. Scientists make these deductions by

assuming the Earth has a similar abundance and proportion of cosmic elements

as found in the Sun and primitive meteorites. Transition region: 7.5% of Earth's mass; depth of 400-650 kilometers

(250-406 miles)The transition region or mesosphere (for middle mantle), sometimes called the

fertile layer, contains 11.1% of the mantle-crust mass and is the source

ofbasalticmagmas.It also contains calcium, aluminum, and garnet, which is a

complex aluminum-bearing silicate mineral. This layer is dense when cold
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because of the garnet. It is buoyant when hot because these minerals melt easily

to form basalt which can then rise through the upper layers as magma.

Upper mantle: 10.3% of Earth's mass; depth of 10-400 kilometers (6 - 250

miles)The upper mantle contains 15.3% of the mantle-crust mass. Fragments have

been excavated for our observation by eroded mountain belts and volcaniceruptions. Olivine (Mg,Fe)2SiO4 and pyroxene (Mg,Fe)SiO3 have been the

primary minerals found in this way. These and other minerals are refractory

and crystalline at high temperatures; therefore, most settle out of rising magma,

either forming new crustal material or never leaving the mantle. Part of the

upper mantle called the asthenosphere might be partially molten.

Oceanic crust: 0.099% of Earth's mass; depth of 0-10 kilometers (0 - 6

miles)The oceanic crust contains 0.147% of the mantle-crust mass. The majority of

the Earth's crust was made through volcanic activity. The oceanic ridge system,

a 40,000-kilometer (25,000 mile) network of volcanoes, generates new oceanic

crust at the rate of 17 km3per year, covering the ocean floor with basalt.

Hawaii and Iceland are two examples of the accumulation of basalt piles.

Continental crust: 0.374% of Earth's mass; depth of 0-50 kilometers (0 -

31 miles).The continental crust contains 0.554% of the mantle-crust mass. This is the

outer part of the Earth composed essentially of crystalline rocks. These are low-

density buoyant minerals dominated mostly by quartz (SiO2) and feldspars

(metal-poor silicates). The crust (both oceanic and continental) is the surface of

the Earth; as such, it is the coldest part of our planet. Because cold rocksdeform slowly, we refer to this rigid outer shell as the lithosphere (the rocky or

strong layer).

The Lithosphere & Plate Tectonics

Oceanic Lithosphere

The rigid, outermost layer of the Earth comprising the crust and upper mantle is called

the lithosphere. New oceanic lithosphere forms through volcanism in the form of

fissures at mid-ocean ridges which are cracks that encircle the globe. Heat escapes theinterior as this new lithosphere emerges from below. It gradually cools, contracts and

moves away from the ridge, traveling across the seafloor to subduction zones in a

process called seafloor spreading. In time, older lithosphere will thicken and

eventually become more dense than the mantle below, causing it to descend (subduct)

back into the Earth at a steep angle, cooling the interior. Subduction is the main

method of cooling the mantle below 100 kilometers (62.5 miles). If the lithosphere is


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young and thus hotter at a subduction zone, it will be forced back into the interior at a

lesser angle.

Continental Lithosphere

The continental lithosphere is about 150 kilometers (93 miles) thick with a low-density crust and upper-mantle that are permanently buoyant. Continents drift laterally

along the convecting system of the mantle away from hot mantle zones toward cooler

ones, a process known as continental drift. Most of the continents are now sitting on

or moving toward cooler parts of the mantle, with the exception of Africa. Africa was

once the core of Pangaea, a supercontinent that eventually broke into todays

continents. Several hundred million years prior to the formation of Pangaea, the

southern continents - Africa, South America, Australia, Antarctica, and India - were

assembled together in what is called Gondwana.

Plate Tectonics

Crustal Plate Boundaries(Courtesy NGDC)

Plate tectonics involves the formation, lateral movement, interaction, and destruction

of the lithospheric plates. Much of Earth's internal heat is relieved through this

process and many of Earth's large structural and topographic features are consequently

formed. Continentalrift valleysand vast plateaus ofbasaltare created at plate break

up whenmagmaascends from the mantle to the ocean floor, forming new crust and

separating midocean ridges. Plates collide and are destroyed as they descend at

subduction zones to produce deep ocean trenches, strings of volcanoes, extensive

transformfaults,broad linear rises, and folded mountain belts. Earth's lithosphere

presently is divided into eight large plates with about two dozen smaller ones that are

drifting above the mantle at the rate of 5 to 10 centimeters (2 to 4 inches) per year.

The eight large plates are the African, Antarctic, Eurasian, Indian-Australian, Nazca,

North American, Pacific, and South American plates. A few of the smaller plates are

the Anatolian, Arabian, Caribbean, Cocos, Philippine, and Somali plates.

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Why Seasons Change

There are 4 major seasons in a year.The seasons keep changing. Seasons

are result of earth's movement around the Sun & earth's tilt on its axis.

Sun never changes,but the movement of the earth around it leads to thedifferences in light and darkness as well as the temperature.The orbit, theroute which the earth follows while rotating around the sun,is elliptical.Due

to this elliptical shape of the orbit,the earth's distance from the sun keepschanging.At times ,the earth is closer to the sun whereas it is far away from

it at other times.This causes in changes of temperature on the earthThe other reason is the earth's tilt on its axis.This tilt has two effects,one is

on the angle of the earth towards the sun and second is the length of theday.For half a year ,the earth is tilted in such a way that North pole is more

pointed towards the sun whereas in rest of the year South pole is pointedtowards the sun.When North pole is facing the sun,the northern hemisphere

gets more sunlight,the days are more warmer and longer and nights areshorter.There is summer in northern hemisphere.As the year progresses,the

North pole gets away from direct exposure towards the sun and winterseason sets in Northern hemisphere.Due to the same reasons,opposite effect

happens in the southern hemisphere.

various types of land forms.Landforms are natural features of earth's

landscape.There are different types of landforms like

valleys,mountains,glaciers,delta,coasts,rivers,lakes and island etc.Type of a

Landform is based upon a list of different features like slope,drainage,structure and

process.

The Greenhouse Effect can be defined as a natural process when certain gases in the atmosphere trap infrared

radiation of the environment. Like a greenhouse keeps its inside temperature warmer,the greenhouse effect makes

the planet warmer, The greenhouse effect is caused by greenhouse gases.The most important greenhouse gases

are: water vapor, carbon dioxide, and methane.
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