EARTH : The Blue Planet
The planet Earth is the third planet out from the Sun. We begin our discussion of the planets with this one, however, because it is our home, and because we will use knowledge of the other planets to help us better understand the Earth itself. The Earth has a fragile environment. Using a Comparative Planetology approach to this study will help us to know ourselves better in a realistic manner.
As a planet the study of the interior and the surface of the Earth is called Geology. Our purpose here is twofold, to better understand the Earth as our home and secondly, to recognize and appreciate what a planet is as part of a star system, part of a galaxy, part of the universe as a whole. We begin by exploring the basic makeup of the planet Earth and then look at those features which make it sort of unique in our solar system and how it is a special place to support life as we know it.
We have grown up knowing that the Earth is "round", rather, that it is spherical in shape. Actually, it is not quite a perfect sphere, but is somewhat oblate in that its equatorial radius is slightly larger than that drawn about the poles. While the Greeks thought the Earth was spherical for aesthetic reasons (spherical objects being "perfect") and constructed various "proofs" that it was spherical, we know today that it "bulges" about 21 km or 13 miles more at the equator than at the poles. The measured sizes are 6378 km radius at the equator and 6357 km at the poles. This is due to the rotation of the Earth. Since the equator is about 25,000 miles in circumference then the rotational rate at the equator is more than 1,000 mph. (25,000 miles in 24 hours) Since it rotates faster at the equator than at the poles the centripetal force of rotation results in a slight bulge.
The density is defined as the mass divided by the volume. Using the above value for the radius and knowing that the volume of a sphere is merely:
4/3 pi r3 = Volume
and determining the mass of the Earth from orbital calculations (mass = 5.97 X 1027 kg) we get for the density:
p = 5.5 gm/cm3
On the whole, this is about twice that of normal rocks. Thus we can conclude that other parts of the Earth must be significantly denser. This permits us to explore what the constiuents of the Earth are and what its general makeup must be.
The Interior
We can infer that the interior parts of the Earth must be denser than the materials on the surface. How do we know that with some confidence? We use the science of Seismology to help understand the interior. Basically, seismic waves are sound waves that travel through the Earth. Much like ultrasound is used in a clinic or hospital to image certain internal organs with high frequency sound waves, by closely studying the reflective and refractive properties of the waves as they travel throught the earth, we can get a pretty good idea of the chemical composition of the interior.
The interior of the Earth looks like it has a generally molten core. It is probable that part of the core is solid and part of it molten. It is suspected that this core is more than likely iron and nickel with small amounts of sulfur. The basic structure, then, is from the solid core, surrounded by a liquid core, and then the Mantle. The mantle is considered to be liquid in that is is viscous and "flows". If you were a giant, holding the Earth in your hand like a big ball, it would feel like a heavy ball, or you'd note that it was pretty dense. (Recall the density of 5.5 g/cm3) If you dropped it, the mantle which is liquid, would be more like toothpaste in consistency rather than like water as we know it. The final, outer layer, is the Crust This is the surface we walk on.
The surface, or crust is technically called the Lithosphere It "floats" on the Asthenosphere in a series of plates. This basic description of the surface consisting of moveable plates is called Plate Tectonics. Tectonics comes from the Greek word meaning "to build". This is often referred to as the theory of Continental Drift. As the plates move about the surface of the Earth, earthquakes and volcanoes occur at the boundaries of two plates.
The plates can move together, away from each other or some other way relative to their positions. When they move apart, molten material from the interior of the Earth comes up. One of the most classic examples of this is the mid-Atlantic ridge. When two plates come together and push upwards, mountain ranges can be formed. If one plate moves under another, volcanoes typically form. The basic movement is slow, perhaps a few cm per year. Over a hundred years that's a few metres. Over thousands of years the distance becomes significant. Fault lines such as the famous San Andreas Fault in California, are the geographic boundaries of such plates.
Tides
It is known that the moon generally moves to the east across the sky. Since it takes roughly 29 days beyween successive New Moons, 360° divided by 29 yields about 13° per day or about 53 minutes. The moon generally rises about 53 minutes later each day. (This is not exact as the moon's orbit is elliptical, not perfectly circular.) Coincident with this is the fact that tides also take place close to an hour later each day. Tides are caused by the difference in gravitational force oof the moon on different parts of the Earth. Because it tugs differently on different parts of the Earth, the differential force causes tides.
The sun is gravitationally stronger than the moon, but it is so far away that it basically pulls the same on all parts of the Earth. It is the differential force that causes the waters of the earth to move to high and low tides respectively. When the sun and moon are lined up (new or full moon) we have very high tides, called spring tides. When the moon is at first or third quarter the effects are smaller, called neap tides.
The Atmosphere
The earth's atmosphere protects the Earth and sustains life. Again, if you were a giant, the atmosphere would make the "ball" seem slippery. The atmosphere, containing the air we breath (78% nitrogen and 21% oxygen with less than 1% of other elements) is essentially within the first 15 - 20 miles of the surface. Comparing this with a radius of about 4000 miles, one can recognize that the atmosphere is basically part of the surface. It consists of thin spherical shells, much like an onion skin. The basic level where weather takes place and the bulk of the mass of the atmosphere exists is called the troposphere. It extends only about 8 - 10 km in altitude. The upper atmosphere where ozone which protects us from the harmful ultraviolet rays of the sun, is called the stratosphere. It extends upward to about 50 km. Above this is the ionosphere, the layer up to several hundreds of km and then the exosphere The ionosphere has layers of ionized gases of oxygen and nitrogen. These layers of ions and electrons from the ionized atoms reflect radio waves back to the surface of the Earth. The exosphere is the outer layer where gases are still gravitationally held to the Earth. It goes up to about 500 km and is quite tenuous.
Atmospheric Level Characteristics
| Atmospheric Level | Characteristics | Troposphere |
Sea Level to 8-10 km
Most of weather occurs here Nearly 7/8 of atmosphere within it |
| Stratosphere |
Extends up to about 50 km
Ozone Layer exists at about 25 km Absorbs UV radiation from the SUN |
| Ionosphere |
Extends irregularly up to 200 km
Ionized O2 and N2 |
| Exosphere |
Extends up to about 500 km
Very tenuous border to Outer Space |
The gases that make up the atmosphere thin out, decreasing by a factor of 2 every 18,000 feet in altitude until space is reached. i.e., at 18,000 feet the density and pressure decrease to one half the sea level value, at 36,000 feet it is one fourth, at 54,000 feet it is down to one eighth and so on. By 90,000 feet it has decreased to only 3% of sea level values. And so on. Even so, at altitudes of several hundred miles where the space shuttle orbits (since the aerodynamic forces are not measureable we would be incorrect to call this "flying") there are some atoms and molecules of the atmosphere (albeit very very small amounts) leading to friction which ultimately causes satellites to decay in orbit until they burn up. These small numbers of particles result in ablative friction which becomes significant at the high speeds characterizing orbiting vehicles.
Environmental Concerns of the Atmosphere
We study the Earth before the other planets so as to be able to reference the others to our "home". As we examine the atmospheres of other planets, we'll look at Venus with a "runaway" Greenhouse Effect and ask ourselves how it evolved differently. We could ask similar questions about Mars which is considerably less dense. The ultimate question comes back to us in the form of are we doing anything which may impact negatively and if so, what might we do, if anything, to change the course of events?
At a height of about 25 km above sea level the stratosphere contains the ozone layer. Ozone is a molecule of oxygen, O3 formed when ultraviolet light interacts with a single oxygen atom, O and a normal oxygen molecule, O2. Excess ultraviolet photons break down ozone. Essentially the complex process enters an equilibrium state where the amount created equals the amount decomposed. If more decomposes than is formed, we have a state of ozone depletion. When this happens life forms on the Earth are threatened. Since the early 1980s scientists have carefully measured ozone levels in the atmosphere. Curiously, on or about September 21st when the rays of the sun first reach the south pole, an ozone hole is formed above the south pole of the Earth. While the chemical balance seems to be understood, the complex dynamics of the Earth's rotation and the atmosphere leave many questions about the future evolution of the atmosphere and its impact on life. There seems to be a strong link between use of aerosol sprays and air conditioning refrigerants released into the atmosphere and this depletion. World-wide concern and world governments have agreed to limit production and use of such chemicals.
Greenhouses seem nice in the winter. They allow the sun's rays to enter a room, heat the room and then the room radiates as a black body at a longer wavelength. The glass then "traps" the heat, raising the internal temperature of the room. The earth's atmosphere also acts like a greenhouse. Some of the sunlight gets reflected, some (about 15 %) gets absorbed in the upper atmosphere, and basically 50 % strikes the ground. When the atmosphere is transparent to the incoming sunlight, but opaque to the infrared, the resultant heating is called a Greenhouse Effect. For the most part, this is beneficial to life forms. Without an atmosphere the temperature of the surface of the Earth would probably be about 250 K. But if the balance gets out of control, like it has on Venus, the temperature could become too high to sustain life. A major concern is that there has been a messureable increase in the temperature of the Earth. Many scientists have attributed this to the burning of fossil fuels and other greenhouse gases. Some have correlated the growth of some gases with the numbers of beef cattle of the planet. While the temperature is increasing, the causes are unclear and more careful research needs to be done.
Magnetic Field
A child with a compass can tell us that the Earth has a magnetic field. From the Earth we know a compass needle tells us which way is north. But to a scientist, a compass needle points from the north pole of a magnet to the south pole of the magnet. This can be verified with a simple experiment. Comparing this with the Earth may confuse some, but the conclusion is obvious, the magnetic south pole of the Earth is located near the geologic north pole and vice versa. That's a piece of trivia that becomes obvious after some careful thought.
Before the advent of space probes beginning with the first US Explorer in 1958 (after Sputnik in Oct 1957), scientists have been able to study the extent and strength of the Earth's magnetic field. It is generally believed that the source of the field is the molten iron-nickel core which can conduct a current and generate a magnetic field. The magnetic Field protects the Earth from high energy charged particles emitted in solar flares. An extension of this field into space shows doughnut shaped belts surrounding the Earth which literally trap charged particles emitted by the Sun. These are called the Van Allen Belts named after their discoverer, Dr. James Van Allen. When charged particles come close to the surface of the Earth, as they spiral around the field lines back and forth between the poles, they interact with the molecules of oxygen and Nitrogen. The resultant excited particles emit radiation in the form of reddish and greenish and flashing lights. This effect is called the Aurora Borealis in the northern hemisphere and the Aurora Australis in the southern hemisphere.
Just outside this region the magnetic field lines are "bowed" back by the solar wind Many of the charged particles are deflected away and into space. This point is called the Magnetosphere.
Studying the magnetic fields of other planets too, can help us to better understand our own. We may be able to learn how they are formed and how they interact with their environment.
Chronological History of the Earth
We know from carbon dating of materials that the Earth is 4.6 billion years old. The following table of events is a possible scenario of its evolution:
| Time[years ago] | Stage of Development |
| 4.6 Billion | Formation of the planet by accretion |
| 4.5 Billion | Outgassing of water, CO2, formation of Oceans and surface crust |
| 3.7 Billion | Cooling and thickening of the crust and early plate tectonic movements |
| 600 Million to present | Breakup of plates to form continents and oceans as known today |
The above dates come to us from geologists best explanation of the planets evolution. Life as we know it today is a recent phenomena and the Earth itself has been fairly stable over the last several hundred million years. What we need to do now, is compare the planet with the others in our solar system, see what is different and what is similar or the same, and ask ourselves why? That's our challenge.
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