Earth

Contents

The Beginning^1,2

			cratered world covered with magma produced by planetesimal impacts. The new world was beginning to acquire a thin atmosphere. The cloud patterns are more belt-like because of the faster rotation. Figure 09-03 shows a primitive
Figure 09-01 Earth, Embryo, 4560 My ago [view large image]	Figure 09-02 Earth, Half-sized, 4550 My ago [view large image]	Figure 09-03 Earth, Primitive, 4540 My ago [view large image]	Earth in the process of solidification.

Geological and Biological Records^3,4

A more reliable history for a small part of the Earth can be found in Strathcona Park, Vancouver Island, Canada; or any places where sedimentary rocks, such as clays, shales, and limestones, are exposed. Figure 09-04 depicts the sequence of rocks (stratigraphy) that occurs within the park and immediately adjacent to it, including the names and ages of the natural rock layers or strata. The bottom layer (earliest) corresponds to the Devonian Period when earliest amphibians and first forests appeared about 400 million years ago. The Strathcona Park website carries all the information about the geology of the Park and more. While the events listed on the right column of Figure 09-04 are related locally to the Park, the Geological Periods on the left column of the Figure is global with events re-constructed by geologists and paleontologists. The periods are shown in Table 09-01 with the geological and biological events.

Figure 09-04 Earth History

[view large image]

Table 09-01 Geological Periods

Internal Structures⁵

Earth's internal structure can be separated into four layers as shown in Figure 09-05 and explained in more details in the followings.


1. Crust - The outermost part of the Earth; this is what we walk around on. It is made of cold, brittle, and relatively light material. Under the continents, the crust averages about 30-40 km thick (more under tall mountains, somewhat less in other areas) and under the ocean, it averages about 5-6 km thick.
   Continental crust is on average older, more silica-rich and thicker than oceanic crust, but is also more variable in each of these respects. The oldest parts of the continental crust, known as 'shields' or 'cratons', include some rocks that are nearly 4 billion years old. Most of the rest of the continental crust consists of the roots of mountain belts, formed at different stages in Earth history.

   Oceanic crust underlies most of the two-thirds of the Earth's surface, which is covered by the oceans. It has a remarkably uniform composition (mostly 49% ± 2% SiO2 ) and thickness (mostly 7 ± 1 km). The ocean floor is the most dynamic part of the Earth's surface. As a result, no part of the oceanic crust existing today is more than 200 million years old, which is less than 5% of the age of the Earth itself. New oceanic crust is constantly being generated by sea-floor spreading at mid-ocean ridges, while other parts of the oceanic crust are being recycled into the mantle at subduction zones.

   Figure 09-05 Earth, Structure

   [view large image]

   The boundary between the crust and the mantle is known as the 'Mohorovicic discontinuity', or 'moho'. The mantle material beneath the moho is not generally molten or even partially molten. The mantle only becomes partially molten in special circumstances such as in mid-ocean ridges, subduction zones or 'hotspots'. The crust is firmly attached to the uppermost part of the mantle and together they make up a rigid layer known as the 'lithosphere'. The rigid surface of the Earth is made up of 'plates' in the lithosphere, they move relative to one another and relative to the underlying part of the mantle, known as the 'asthenosphere'. The asthenosphere is also solid, but over millions of years it deforms in a manner similar to Plasticine (although it is actually many times more viscous).
2. Mantle - Immediately below the crust is the mantle. It is made of rocky material similar to the crust, but it is very hot and not brittle. The material of the mantle acts like a solid over timescales of a second, hour, week, and up to several thousand years. Over hundreds of thousands to millions of years, however, mantle material acts as a very viscous fluid and can flow from one place to another in a process called convection. The mantle makes up about 70% of Earth's mass and about 45% of its radius. The bulk of the lower mantle is termed the mesosphere and is stronger than the asthenosphere
3. Outer Core - Next is the outer core, which is made of very different material from the crust and mantle. The outer core is mostly iron, and is very hot. The iron mix which makes up the outer core is a fluid which moves around significantly in the course of only a few years. The fluid motions of the outer core generate Earth's magnetic field.
4. Inner Core - Finally, there is the innermost part of the Earth, called the inner core. The inner core is mostly iron, similar to the outer core, but because the pressure is so much higher near the center of the Earth, the inner core is solidified. There is some evidence that the inner core may be spinning at a faster rate than the rest of the planet.

Continental Drift

The shapes of the continents suggest that they could be joined like pieces of a jigsaw puzzle. This observation led to the suggestion, made in 1924, that in the distant past there had been one super-continent (pangaea) that broke up, with the various sections drifting apart to form the present-day continents. This concept, called continental drift is supported by the theory of plate tectonics6 - a theory that offers a comprehensive explanation of the distribution of continents, mountain chains, volcanoes, earthquake sites, and ocean trenches.

Figure 09-06a Plate Tectonics [view large image]

There are four types of plate boundaries as shown in Figure 09-06a:


1. Divergent boundaries - where new crust is generated as the plates pull away from each other. In mid-ocean, this movement results in seafloor spreading and the formation of ocean ridges; on continents, crustal spreading can form rift valleys.
2. Convergent boundaries - where crust is destroyed as one plate dives under another. In mid-ocean, this causes ocean trenches, seismic activity, and arcs of volcanic islands. Where oceanic crust is subducted beneath continental crust or when continents collide, land may be uplifted and mountains formed.
3. Transform boundaries - where crust is neither produced nor destroyed as the plates slide horizontally past each other such as the San Andreas fault. .
4. Plate boundary zones - broad belts in which boundaries are not well defined and the effects of plate interaction are unclear. Because plate-boundary zones involve at least two large plates and one or more micro-plates caught up between them, they tend to have complicated geological structures and earthquake patterns.

		For example, the Hawaiian group of volcanic islands, which lie in the middle of the Pacific plate, has been built up while the plate has been drifting over a hot spot. But volcanoes occur most commonly along the boundaries of crustal plates (Figure 09-06c). Crustal movement on continents may result in earthquakes, while movement under
Figure 09-06b Crustal Plates [view large image]	Figure 09-06c Earth Quake Zones [view large image]	the sea bed can lead to tidal waves (tsunami).

			This supercontinent broke up subsequently leading to the present geological distri- bution. The animation in Figure 09-06e shows the change starting from 740 million years ago in steps of 10 million years. To see continental positions during a particular time, click the STOP button of your
Figure 09-06d Continental Drift [view large image]	Figure 09-06e Continental Drift [view animation]	Era	browser (the on the toolbar) as the red arrow reaches the era of interest.

The slow movement of the continents over the surface of the Earth during the past 400 million years has coincided with the entire evolution of land-living animals and plants. As the tectonic plates that created the backdrop to this scene have split apart, collided and slid past one another, they have constantly rearranged the stage on which the drama of terrestrial evolution has been enacted. Continental movements can influence evolutionary pressures by altering the physical environment. A tectonic plate, for example, can move a continent from a tropical to a polar latitude, where the organisms will experience new patterns of competition. The life forms present or absent in a particular part of the world help to define the evolutionary fate of all the other organisms in the community. Land and sea barriers generated by continental drift have, by restricting movements, influenced zoogeographical distribution patterns on the face of the Earth. Organisms that arose and diversified on an ancient landmass, such as Gondwana, have been prevented by large sea barriers from colonizing other landmasses. Figure 09-06f shows the different life form corresponding to different land mass distribution over the last 560 million years.

Figure 09-06f Life and Continental Drift [view large image]

Atmosphere⁷

Earth's atmosphere can be separated into four layers as shown in Figure 09-07 and explained in more details in the followings.


1. Troposphere - Since this lowest level of the atmosphere is heated mainly by infrared radiation from the ground, its temperature decreases with increasing altitude. It is a turbulent layer within which rising plumes of moist air condense to form clouds of water droplets and ice crystals.
2. Stratosphere - The temperature rises in this layer because the presence of ozone which strongly absorbs ultraviolet radiation from the Sun. This process incidentally blocks the harmful radiation from reaching the ground level. This is a uniform layer and almost weatherless. Flying in the stratosphere is generally smooth, and the visibility is always excellent. The air is thin and offers very little resistance to a plane. Hence it is a region preferred by jet airline pilots. Weather balloon can fly up to this level before it bursts at about 30 km.
3. Mesosphere - In this layer, concentrations of ozone and water vapor are negligible. Hence the temperature is lower than that of the troposphere or stratosphere. With increasing distance from Earth's surface the chemical composition of air becomes strongly dependent on altitude and the atmosphere becomes enriched with lighter gases. At very high altitudes, the gases begin to form into layers according to molecular weight, because the force of gravity is greater on the heavier molecules. It is in this layer that foreign bodies (such as meteors and spacecraft) entering the atmosphere start to warm up.
4. Ionosphere (Thermosphere) - The temperature rises again by absorbing ultraviolet and x-ray radiation from the Sun. This process ionizes the atoms and molecules, thereby adding heat energy to this layer. The temperature can reach up to 2000 ^oK or more beyond this layer, depending on the level of solar activity. The layers of charged particles reflect radio signals around the curvature of the Earth. Major solar storms that eject large quantities of radiation and energetic particles cause dramatic changes in the ionization levels and subsequent disruption of radio communication.

		The Earth's magnetic field acts as a shield that deflects the solar wind (stream of electrically charged particles) thereby creating an elongated cavity in the wind that is called the magnetosphere as shown in Figure 09-08. The magnetosphere contains large numbers of trapped charged particles, many of which are concentrated in two doughnut-shaped belts called the Van Allen Belts8. Disturbances in the solar wind induce batches of charged particles down the field lines into the upper atmosphere around the polar region. These particles interact with atoms and ions to produce
Figure 09-07 Atmosphere [view large image]	Figure 09-08 Van Allen Belts [view large image]	auroras as shown in the top right of Figure 09-07.

Weather⁹

		Weather is defined as the atmospheric conditions at a particular time and place; climate is the average weather conditions for a given region over time. Weather conditions include temperature, wind, cloud cover, and precipitation, such as rain or snow. Good weather is generally associated with high-pressure areas, where air is sinking. Cloudy, wet, changeable weather is common in low-
Figure 09-09 Non-rotating Flow [view large image]	Figure 09-10 Air Circulation [view large image]	pressure zones with rising, unstable air. But long-term weather prediction is unreliable as shown in the Chaos Theory.

		In the northern hemisphere the two well-defined high-pressure regions are the polar high and the horse latitudes (see Figure 09-11). The formation of low-pressure systems is more complicated, however, and involves a wavelike action that occurs between two areas of high pressure with the colder air pushing under the warmer air. As the colder air keeps moving forward, it leaves behind a low-pressure cell of counterclockwise swirling air near the surface (see Figure 09-12).
Figure 09-11 Air Masses [view large image]	Figure 09-12 Weather Front [view large image]

Thus the air masses can be classified into the following types. Figure 09-11 shows the distribution of the various types over the globe.


• Continental Arctic (cA ) - very cold; very dry
• Continental Antarctic (cAA) - very cold; very dry
• Continental Polar (cP) - cold & dry
• Continental Tropical (cT) - warm & dry
• Maritime Tropical (mT) - warm & moist
• Maritime Equatorial (mE) - very warm; very moist
• Maritime Polar (mP) - cool & moist

All fronts have the following characteristics in common (see Figure 09-12):


1. Fronts form at margins of high-pressure cells.
2. Fronts form only between cells of different temperatures.
3. Warm air always slopes upward over cold air.
4. A front is found along a low-pressure trough, so pressure drops as the front approaches, rises after it passes.
5. Wind near ground always shifts clockwise (in the northern hemisphere) as the front passes because air always flows clockwise around the high and counter-clockwise around the low.

		sloping edge of a cold front may extend over several hundred kilometers horizontally, the steepness of the advancing edge means that frontal weather is limited to an extremely narrow band. Storms at a cold front are generally brief though violent. Occluded front is the result of cold front catching up with warm front. The warm air is forced up away from ground level
Figure 09-13 Types of Front [view large image]	Figure 09-14 Clouds [view large image]	(see diagram f in Figure 09-13 or its animated version).

Clouds are classified according to shape and altitude (see Figure 09-14):


1. Cirrus - thin, feathery, and usually white; high altitudes.
2. Alto - high altitude clouds.
3. Stratus - layered and usually grey; medium and low altitudes.
4. Cumulus - fluffy and lumpy; medium and low altitudes.
5. Nimbus - rain clouds; low altitudes.

Geographical, biological, and man-made factors often make local climatic conditions different from the general pattern. For examples, large lakes moderate temperature extremes; plants create microclimatic differences by their use of water and by their effect on winds; valleys and hills produce difference in temperature, wind speed, and condensations; city is warmer and less windy than countryside. All these local variations alter the movement of air as shown in Figure 09-15. They produce local weather conditions not following the general patterns.

Figure 09-15 Local Variation of Air Flow [view large image]

dramatic shifts in climate. It is generally assumed the dramatic increase in carbon dioxide concentration in the atmosphere over the past 150 years is largely due to anthropogenic (human-caused) effects. Human beings are causing the release of carbon dioxide and other greenhouse gases to the atmosphere at rates much faster than the earth can recycle them. Fossil fuels - oil, coal, natural gas, and their derivatives - are formed through the compression of organic (once living) material for millions of years, and we are burning billions of tons of these fuels per year. The CO2 expelled into the atmosphere through these activities does not disappear immediately or even over the course of a year. As a matter of fact, the residence times of greenhouse gases (how long they remain in the atmosphere) are on the order of decades to centuries. This means that the impact will be accumulated well into the future of many generations10.

Figure 09-16 Global Warm- ing [view large image]

		frequency of extreme rainfall events has increased throughout much of the United States. Figure 09-17b shows the greenhouse effect from human activities (agriculture, industrialization) warded off a glaciation that otherewise would have begun about 5000 years ago.
Figure 09-17a Rising Temp- erature [view large image]	Figure 09-17b Greenhouse Effect [view large image]

Habitable Zone¹¹

It is evident that life arose from cosmic processes just by examining the chemicals in our body. The iron in our blood and the calcium in our bones were made inside stars. All the heavy chemical elements were forged by star that exploded long ago. Terrestrial life is embedded in a cosmic web, and it seems reasonable to speculate that life is cosmically commonplace. There are three ingredients upon which life depends: water, energy, and organic molecules (or carbon). Recent discoveries inform us that these pre-requisites may exist well beyond the planets closely orbiting the sun. This area — where conditions might potentially support life — is called The Habitable Zone. Figure 09-18 shows such zone in the Milky Way and in particular a zone in the Solar System between Mars and Earth. The galactic habitable zone is envisioned as a ring around the center of our Milky Way galaxy. It may only contain about 20 percent of the galaxy's stars -- including our own sun. Near the core of the Milky Way, life may be unlikely -- comet impacts

Figure 09-18 Habitable Zone [view large image]

may be more frequent, and radiation levels are high. Meanwhile, the outer fringe of the galaxy is a difficult place to build life-supporting planets because there are fewer heavy elements.

Extra-Terrestrial Intelligence

of reports in the media indicate that radio signals (at 1420 megahertz = the hyperfine transition frequency of the hydrogen atom) have been detected three times from a point between the constellations Pisces and Aries. The transmission is very weak and shifting rapidly in frequency. It is pointed out that such drifting of frequency is too rapid to be produced by the rotation of planet and three occasions of detection is not statistically significant. The signals could be generated by a previously unknown astronomical phenomenon, or it could be something much more mundane, maybe an artefact of the telescope itself.

Figure 09-19 Search for ET [view large image]

Followings are some suggestions related to the puzzle of "where are they?"


1. It could be that primitive life may be common in the universe, but intelligent life is exceeding rare. We may be the very few that have advanced to a state where travelling in space is a possibility.
2. Another theory suggests that many plantets harbouring technologically advanced extraterrestrials exist, but space travel has fundamental limitations or inhereent dangers that we have not yet experienced. In this case, everybody is staying near home.
3. An alternate explanation about advanced extraterrestrials claims that they don't want to interfere with our development or they just don't have interest in us.
4. The absence of even radio contact may be related to the fact the artificial signals are inherently weak. It is very difficult to detect. If we have ever received anomalous radio signal from outer space, we need to verify the nature of its origin.

There are three criteria to determine the intelligence of a signal:


• A narrow-band radio signal - One of the main criteria in SETI is to look for a narrow-band radio signal of the order of 1 hz, because nature cannot produce such kind of signal as shown in Figure 09-20. It has to come from an artificial source. An anomalous signal simliar to that in Figure 09-20 has actually been intercepted by SETI. Although this strong signal was never positively identified, astronomers have found many attributes characteristic of a more mundane and ultimately terrestrial origin. In this case, a leading possibility is that the signal originates from an unusual modulation

			between a GPS satellite and an unidentified Earth-based source. Many unusual signals from space remain unidentified. No signal has yet been strong enough or run long enough to be unambiguously identified as originating from an extraterrestrial intelligence.
Figure 09-20 ET Signal [view large image]	Figure 09-21 Zipf Plot [view large image]	Figure 09-22 Entropy Order [view large image]

• Optical SETI (OSETI) programs must detect a laser signal against the glare of ET’s star. The best way for a laser to outshine a star is to do so very quickly, in a very short pulse lasting less than a billionth of a second. OSETI programs thus measure the light from a star in nanosecond intervals looking for a slight, brief excess of photons that could originate from a powerful pulsed laser.
• The Zipf Plot - Once an ET signal is detected, then the hunt for patterns will begin. The Zipf Plot counts the number of times different element of a language (such as letters, words, or characters) appearing in a text and logarithmically plots the frequency of occurrence of these elements in descending order. The resulting slope would have a gradient of "-1" (see Figure 09-21) for English, Chinese as well as most other languages and dolphin whistles. A "-1" slope is a sign of optimized communications: it is more efficient because elements that occur frequently (such as "a" and "the" in English) are coded to be shorter than those that are used less often. For an entirely random string of letters that contained no encoded information, the slope would be flat, or zero, because every letter occurs equally often. If Morse code is analysed one dot or dash at a time, the line on the Zipf plot is almost flat. At first glance, this would not seem like a promising signal. But if the dots and dashes are taken two at a time, the slope goes up. When dots and dashes are examined in groups of four, the slope approaches "-1".
• Entropy Order - This is a concept from the information theory developed by Claude Shannon to measure the complexity of a communication system. Zero-order entropy measures the diversity of the communicative repertoire - how many different types of elements (letters, words, whistles and so forth) make up the signal. In written English, for example, everything can be represented by 27 characters - 26 letters and a space. The entropy value is the logarithmic value (base 2) of the number of elements. The higher entropy levels, second order and up, relate to the notion of "conditional probabilities": the probability of predicting the next element in the series. If, for instance, you know the first and second words of a phrase, the third-order entropy tells you (in logarithmic form) the odds of guessing the third word correctly. Analyses of English and Russian suggest that these languages show evidence of 8^th or 9^th-order Shannon entropy. Figure 09-22 shows some examples, which relate more complex communications with higher entropy values and higher entropy orders.

In all the great oceans of emptiness, stars of type G are the best candidates to look for life - these are stars like the sun. They are of moderate, but comfortable brightness and remain stable for about 10 billion years - sufficient time for complex life forms to evolve. Tau Ceti is such a G-type, sunlike star, devoid of stellar companions and close enough for detailed studies. It was the first object searched for ET radio signals. Though Tau Ceti has about half the sun's luminosity, its habitable zone still comprises about one third AU - this is wide enough that a terran planet may have formed there. But we know from other stars that giant gas planets are common, and they are often very close

Figure 09-23 Tau Ceti[view large image]

The Pioneers-1015 space probe was launched on March 1972. In an attempt to contact "advanced civilizations", a plaque bearing engraving such as shown in Figure 09-24 was attached inside. The plaque depicts: an "average" man and an "average" woman standing before an outline of the spacecraft. a diagram showing the hyperfine transition of neutral hydrogen (spin flip of the electrons). a map locating the position of the Sun relative to 14 pulsars and the center of the Galaxy. some symbols representing the binary equivalent of decimal 8. all the planets in the Solar System with distance to the Sun in binary digits.

Figure 09-24 Pioneers-10 Plaque [view large image]

Starry Night¹⁶

			tourist) or study (as scientist) "heaven and hell" at the same time. Mauna Loa (Figure 09-27) in the south is consisted of an active volcanic chain, while Mauna Kea in the north lays dormant. Its summit is the location for the largest collection of modern telescopes taking advantage of the clarity of the Hawaiian night skies. The night view in Figure 09-27
Figure 09-25 Starry Night [view large image]	Figure 09-26 Big Island	Figure 09-27 Mauna Loa	shows the Southern Cross, constellation Crux, near the horizon to the left of Mauna Loa's summit, while the day view reveals a crater in the foreground.

References:

1. Formation of the Earth, Lecture Note - http://zebu.uoregon.edu/ph121/l7.html
2. Formation of the Earth - http://www.cas.muohio.edu/~mbi-ws/changethrutime/earthformationpage.htm
3. Geological Periods, Life Forms and Continental Drift -- http://www.discoveringfossils.co.uk/Earth.htm
4. Geological and Biological Events -- http://www.mun.ca/biology/scarr/Geological_Eras_Periods_&_Epochs.htm
5. Internal Structure -- http://www.citytel.net/PRSS/depts/geog12/litho/earthint.htm
6. Plate Tectonics -- http://pubs.usgs.gov/publications/text/dynamic.html
7. Earth's Atmosphere -- http://csep10.phys.utk.edu/astr161/lect/earth/atmosphere.html
8. Van Allen Belt -- http://csep10.phys.utk.edu/astr161/lect/earth/magnetic.html
9. Weather -- http://www.doc.mmu.ac.uk/aric/eae/Weather/weather.html
10. Global Warming, EPA -- http://yosemite.epa.gov/oar/globalwarming.nsf/content/index.html
11. Habitable Zone -- http://www.solstation.com/habitable.htm
12. Extra-Terrestrial Intelligence, SETI Home Page -- http://setiathome.ssl.berkeley.edu/
 
13. Extra-Terrestrial Intelligence, Drake Equation -- http://www.seds.org/~rme/drake.html
14. Extra-Terrestrial Intelligence, SETI Projects -- http://www.seti-inst.edu/seti/our_projects/Welcome.html-
15. The Pioneer Missions -- http://spaceprojects.arc.nasa.gov/Space_Projects/pioneer/PNhist.html
16. The Earth -- http://seds.lpl.arizona.edu/nineplanets/nineplanets/earth.html

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Index

Asthenosphere Atmosphere Beginning Continental crust Continental drift Crust Drake equation Entropy order Extra-Terrestrial intelligence Geological and biological records Global warming Habitable zone Inner core Internal structures Ionosphere (Thermosphere) Lithosphere Magnetosphere Mantle

Mesosphere Moho Oceanic crust Outer core Pangaea Pioneers-10 Planetesimal Plate boundaries Plate Tectonics Search for Extra-Terrestrial Intelligence (SETI) Starry night Strathcona Park Stratosphere Super-continent Troposphere Van Allen belts Weather Zipf plot

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