The Earth is certainly the most familiar planet to us cause we live on it,thought it has only been a few hunderds years since we fully realized it was a planet.
The Earth is the third planet from the sun and fifth largest.
Orbit :149.600.000 km (1.0 A.U)
Diameter :12.756,3 km
Mass :5.972e24 kg
Density :5.515 kg/m^3
Minimum Distant from Sun :146 Million km (91 million miles)
Maximum Distant from Sun :152 Million km (94.5 Million miles)
Surface Gravity :9.78 m/s^2
Rotation period with respect to sun (length of day) :24 hrs.
Rotation period with respect to stars (sidereal day) :23 hrs 56 Min.
Revoulation period about the Sun (length of a Year) :365 days 5 hrs.
Tilt of axis :23 degrees and 27"
Temperature :-89 C. degrees to 57.7 C. degrees (-128 F degrees to 136 F degrees)
Average Surface Temperature :278 K.
Satellite : (1)Moon
The Interior of the Earth:-
All the studies about Earth's surface and interior is the domain of Geology.Most of our information has come from seismic waves ,which are vibrations in the body of the Earth.
There are two main kinds of Seismic Waves.
1) P-Waves, which are longitudinal pressure waves and can propagate in both solids and liquids.
2) S-Waves, which are transvers waves that can prpagate in solids but not in liquids.
These seismic waves , which are genrated naturally by Earthquakes , by Volcanies , by impacts , and may be produced artificially by explosions and mechanical devices.
The figure on the right illustrates for a planet with varying interior density and a liquid core.
First,seismic waves have thier direction of motion changed by variations in the interior density.
Thus ,by studying the way such waves propagate in the Earth we can learn something about density variations.
Second,the fact that P-Waves propagate in liquids but S-Waves don't allow us to determine if protions of the interior are liquid.
Structure of the Interior
Accumulated detailed seismic studies , coupled with theroretical speculation , suggests the interior structure shown schematically on the left (the figure is not to scale).
The Earth is believed to have a solid inner core , made mostly of iron and nickel.This is surrounded by aliquid outer core , also mostly iron and nickel.
The diameter of the core is estimated to be 7000 Km , compared with a 12.700 Km diameterfor the entire planet.The crust is only a few tens of kilometer thick.
The region between the core and the crust is called the mantle.The upper part of the mantle and the crust together are called
Sitting just below the lithosphere is a region of plastic consistency called the aesthenosphere.
The Earth divided into several layers as we know (depth in Km)
The crust varies considerably in thickness , it's thinner under the oceans , thicker under the continents.Most of the mass of the Earth is in the mantle , most of the rest in the core ; the part we inhabit is a tiny fraction of the whole (Values below by 10^24 Kg):
Temperature at the center of the core may be as high as 7500 K , hotter than the surface of the sun .
The lower mantle is probably mostly silicon , magnesium and oxygen with some iron , calcium and aluminum.The upper mantle is mostly olivene and pyroxene (iron/magnesium sillicates) calcium and alluminum.The primarily quarts (silicon dioxide) and the other sillicates like feldspar.Taken as a whole , the Earth's chemical compositions (by mass) is :
The Earth is the densest major body in the solar system.
You have to know that Earth did not have the interior structure when it was formed.The geological process by which the Earth came to have its present interior structure is called differentiation , and is illustrated in the following figure.
The process of geologicaly differentiation
within about 1 billion years of its formation the Earth was melted by heat arising from a combination of sources :
1-Gravitational energy left from the formation of the planet,
3-Decay of radoactive material trapped in the body of the Earth.
While the Earth was molten , gravity acted to concentrate more dense material near the center and less dense material nearer the surface.
When the Earth solidified again ( except for the liquid outer core ) it was left was layered structure with more dense material like iron and nickel near the center and less dense rocks nearer the surface.
As the outer layers cooled and solidified , large cracks developed because of the thermal stress , leaving the lithosphere broken up into large blocks or plates.
As we shall see , this has enormous implications for the subsequent geolgical history of the Earth because it produces conditions favorable for Plate Tectonics.
The Erath's surface is very young.In the relatively short (by astronomical standars) period of 500 million years or so erosion and tectonics processes destroy and recreat most of the Earth's surface and thereby eliminate almost all traces of earlier geologic surface history (such as impact craters).
Thus the very early history of the Earth's has mostly been erased .The Earth is 4.5 to 4.6 billion years old , but the oldest known rocks are about 4 billion years old and rocks older than 3 billion years are rare.The oldest fossils of living organisms are less than 3.9 billion years old.Ther is no record of the critical period when life was first getting started.
71% of the Earth's surface is covered with water.Earth is the only planet on which water can exist in liquid form on the surface (though there may be liquid Ethan or Methan on Titan's surface and liquid water beneath the surface of Europa).Liquid water is , essential for life as we know it.
The heat capacity of the oceans is also very important in keeping the Earth's temperature relatively stable.
Liquid water is also responsible for most of the erosion and weathering of the Earth's continents , a process unique in the solar system today (though it may have occured on Mars in the past).
The Earth's Atmosphere:-
The present Atmosphere at the Earth is probably not its original atmosphere.
Our current atmosphere is what chemists would call an oxidiziating atmosphere , while the original atmosphere was what chemists would call a reducing atmosphere.
In particular , it probably did not contain oxygen.
Composition of the Atmosphere:
The original atmosphere may been similar to the composition of the solar nebula and close to the present composition of the gas giant planets , though this depends on the details of how the planets condensed from the solar nebula.
That atmosphere was lost to space , and replaced by compounds outgassed from the crust or (in some recent theories) much of the atmosphere may have come instead from the impacts of comet and other planetestimals rich in volatile materials.
The oxygen so characteristics of our atmosphere was almost all produces by plants.
Thus , the present composition of the atmosphere is 79% (others 78%) nitrogen , 20% (others 21%) oxygen , and 1% other gases.
Layers of the Atmosphere:-
The atmosphere of the Earth may be divided into several layers , as the following figure indicates.
The tropssphere is where all weather takes place;it is the region of rising and falling packets of air.The pressure at all the top of the troposphere is only 10% of that at sea level (0.1 atmospheres).There is a thin buffer zone between the troposphere and the next layer called the tropopause.
The Stratosphere and Ozone Layer
Above the troposphere is the startosphere , where air flow is mostly horizontal.The thin Ozone layer in the upper stratosphere has a high concentration of ozone , a particularly reactive form of oxygen.
This layer is primarily responsible for absorbing the ultraviolet radiation from the sun.The formation of this layer is a delicate matter , since only when oxygen is produced in the atmosphere can an ozone layer form and prevent an intense flux of ultraviolet radiation from reaching the surface , where it is quite hazardous to the evolution of life.
There is considerable recent concern that manmada flourocarbon compounds may be depleting the ozone layer , with dire future consequences for life on the Earth.And from our site "Our Solar System" we invite every one to protect his nature and keep it clear and don't use things which have flourocarbon compounds except for urgent matters to keep our Earth clear and beautiful.
The Mesosphere and Ionosphere
Above the stratosphere is the mesosphere and above that is the ionosphere (or thrmosphere),where many atoms are ionized (have gained or lost electons so they have a net electrical charge).
The ionnosphere is very thin ,but it is where aurora take place , and is also responsible for absorbing the most energetic photons from the Sun , and for reflecting radio waves , thereby making long-ditance radio communication possible.
The structure of ionosphere is strongly influenced by the charged particle wind from the sun (solar wind) , which is in turn governed by the level of solar activity.
One measure of the structure of the ionosphere is the free electron density , which is an indicator of the degree of ionization.
Auroras : the Northern and Southern Lights
The aurora,or northern and southern lights are often visible from the surface of the Earth at high northern or southern latitudes.
Auroras typically appear as luminous bands or streamers that can extend to altitudes of 200 miles (well into the ionosphere).
Northern and Southern Lights
The following figures shows three examples of the often spectacular visible light dispaly associated with auroras.
Sothern Aurora from the space shuttle endeavor
Northern Aurora over lkae supererior
Northern Aurora over circle,alaska
What we have to know that aurora changes with time , often looking like moving curtains of light.
The Cause of Auroras
Auroras are caused by high energy particles from the solar wind that are trapped in the Earth's magnetic field.As these particles spiral back and forth along the magnetic field lines , they come down into the atmosphere near the north and south magnetic poles where the magnetic field lines disappear into the body of the Earth.
The delicate colors are caused by energetic elecrons colliding with oxygen and nitrogen molecules in the atmosphere.
This excites the molecules and when they decay from the excited states they emit the light that we see in the aurora.
Auroras at Non-Visible Wavelengths
The collisons of trapped charged particles with atmospheric molecules causes spectacular effects in the visible spectrum , but these excited molecules can also emit radiation in other wavelength bands.
The following figures show aurora imaged in the ultraviolet (UV) and X-ray regions of the spectrum.
uv emission from northern aurora observed by polar satellite
x-ray emission from northern aurora observed by the polar satellite
These images were obtained by the NASA polar satellite.
The Earth's Magnetic Field
Structure of the Field
The field lines defining the structure of the magnetic field are similar to those of a simple bar magnet , as illustrated in the following figure.
The Earth's magnetic field and van allen radiation belts
It is well known that the axis of the magnetic field is tipped with respect to the rotation axis of the Earth.
Thus,true north (defined by the direction to the north rotational pole) does not coincide with magnetic north (defined by the direction to the north magnetic pole)and compass directions must be corrected by fixed amounts at given points on the surface of the Earth to yield true directions.
Van Allen Radiation Belts
A fudemental property of magnetic fields is that they exert forces on moving electrical charges.
Thus , a megnatic field can trap charged particles such as electrons and protons as they are forced to execute a spiraling motion back and forth along the field lines.
As illustrated in the adjacent figure,the charged particles are reflected at "mirror points" where the field lines come close together and charged the spirals tighten.
One of the first fruits of early space exploration was the discovery in the late 1950s that the Earth is surrounded by two regions of particularly high concentration of charged particles called the Van Allen radiation belts.
The inner and outer Van Allen belts are illustrated in the top figure.The primary source of these charged particles is the stream of particles emanating from the sun that we call the solar wind.
As we see the charged particles trapped in the Earth's magnetic field are responsible for the Aurora (Northern and Southern Lights).
Origin of the Magnetic Field
Magentic fields are produced by the motion of electrical charges.For example , the magnetic field of a bar magnet results from the motion of negatively charged electrons in the magnet.
The origin of the Earth's magnetic field is not completely understood , but is thought to be associated with electrical currents produced by the coupling of convective effects and rotation in the spinning liquid metallic outer core of iron and nickel.This mechanism is termed the dynamo effect.
Rocks that are formed from the molten state contain indicators of the magnetic field at the time of thier solidification.The study of such " magnetic fossil " indicates that the Earth's magnetic field reverses itself every million years or so (the north and south magnetic poles switch).
This is but one detail of the magnetic field that is not well understood.
The Earth's Magnetosphere
The solar wind mentiones above is a stream of ionized gases that blows outward from the sun at about 400 km/Second and that varies in intensity with the amount of surface activity on the sun.
The Earth's magnetic field shields it from much of the solar wind.When the solar wind encounters Earth's magnetic field it is reflected like water around the bow of a ship , as illustrated in adjacent image.
The imaginary surface at which the solar wind is first deflected is called the bow shock.The corresponding region of space sitting behind the bow shock and surrounding the Earth's is termed the magenetosphere;it represents a region of space dominated by the Earth's magnetis field in the sense that it largely prevents the solar wind from entering.
However , some high energy charged particles from the solar wind leak into the magnetosphere and are the source of the charged particles trapped in the Van Allen belts.
Our knowledge of the inerior of the Earth is derived almost entirely from highly indirect evidence.How can we get more information?
Despite substantial increases in the solar "constant" the average temperature on the surface of the Earth has remained very stable for several billion years.The best theory is that this has been accomplished by varying the amount of carbon dioxide in the atmosphere to regulate the Greenhouse effect.But just how is this done? The Gaia Hypothesis asserts that the biosphere actively regulates it.
More detailed information about Venus and Mars may provide clues.
How much more Carbon dioxide can we dump into the Earth's atmosphere before it ends up like Venus?
And from our site "Our Solar System" invite everyone to protect his nature to make it clear and