Earth

Contents

The Beginning^1,2

The Solar system started with the collapse of a cloud of interstellar gas and dust. Gravity caused the cloud to fragment and condense into ball of heated gas that eventually became the Sun. Meanwhile, whirling disks around the nascent star gave birth to the planets. About 4.5672 billion years ago bits of dust around the growing Sun started sticking together to form small, inch-long clumps (reproducible in the laboratories). The next step was amalgamation of the small bits into mile-wide objects call planetesimals. Figure 09-00 illustrates the successive stages in the earlier growth of planet Earth.

Figure 09-00 The Beginning [view large image]

			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

		(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 and objects listed in Figure 09-04a are related locally within the Park, the history of the Geological Periods in Figure 09-04b is supposed to be global with events re-constructed by geologists and paleontologists. Table 09-01 depicts the geological and biological events in each of the period.
Figure 09-04a Earth History, Local [view large image]	Figure 09-04b Earth History, Global [view large image]

Table 09-01 Geological Periods

Internal Structures⁵

Earth's internal structure can be separated into four layers as shown in Figure 09-05a 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 09-05a Earth, Structure [view large image]	Figure 09-05b Earth, Structure (new)

   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
   New study in 2010 reveals that there is a relatively thin layer at the bottom of the lower mantle, which has perovskite (MgSiO₃) as its main composition. This thin layer underwent a phase transition to another crystal form at the specific temperature and pressure prevalent in that location (Figure 09-05b). According to computer simulations, it makes the mantle more dynamic and carries heat more efficiently than previously thought and thus explains the fast growth rate of the continents in the last 2 billion years. It may also be responsible for the hot spot in Hawaii, the evolution of the Earth's magnetic field, and the periodic precession of the Earth's axis of rotation.
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]

	Earth is the only planet that has plate tectonics. Models of the Earth have shown that the lithosphere (crust + mantle) is too thick for smaller planet, while the gravitational force for larger planet would squeeze any plates together. Even when the size criterion is met, it needs a way to crack the lithosphere. Numerous computer models fail to simulate conditions in
Figure 09-06b Plate Tectonics Theory [view large image]	which a break in the crust would spontaneously occur. It is suggested that perhaps asteroid or comet strikes may have led to the creation of the subduction process as shown in Figure 09-06b.

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 (Figure 09-06c). Such movement produces earthquakes.
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.

		The movement of the Earth is induced by the convection currents of molten magma deep down in a zone called the mantle. These currents rise, then turn sideways below the solid crust. The crust is divided into eight major plates in the lithosphere (Figure 09-06e). Slowly, at rates of a few centimeters per year, the rising current moves these plates. If the plate moves over a localized hot spot (Figures 09-06a and c) in the mantle, volcano will form until the plate carries it away from this source of magma.
Figure 09-06c San Andreas Fault [view large image]	Figure 09-06d Hawaiian Volcanos [view large image]

		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 (Figaure 09-06c). But volcanoes occur most commonly along the boundaries of crustal plates (Figure 09-06f). Crustal movement on continents may result in earthquakes, while
Figure 09-06e Crustal Plates [view large image]	Figure 09-06f Earth Quake Zones [view large image]	movement under the sea bed can lead to tidal waves (tsunami).

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

Plate tectonics recycles water, carbon and nitrogen, creating an environment that is perfect for life. It makes oceans open and close, mountains rise and fall and continents gather and split. Every 500 to 700 million years, plate tectonics brings the continents together to form a supercontinent. When these supercontinents slowly break up, separating landmasses and forming shallow seas, evolution goes into overdrive, forming countless new species which colonise the new habitats. 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 09-06i shows the different life forms living in different land mass over the last 560 million years.

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

The Earth's climate is remarkably stable, and has remained in a narrow, live-able, range for almost 4 billion years. The key appears to lie in the interplay between plate tectonics, carbon dioxide and the oceans (see Figure 09-06j). Carbon dioxide is released into the atmosphere by volcanic activities. Too much of CO2 will warm up the air, and cause more seawater to evaporate. Acidic rain reduces the amount of CO2 by producing carbon-containing minerals, which is carried into the mantle by plate tectonics, and eventually returns to the

Figure 09-06j Earth's Thermostat [view large image]

atmosphere through volcanoes to repeat the cycle again. This mechanism of climate regulation may not work very well if the carbon dioxide released by human activities becomes too much for the slow process of plate tectonics.

Rocks, Minerals, and Gemstones

Table 09-02 Evolution of Minerals

	Rocks are aggregates of minerals - usually several, but sometimes only one or two. Minerals are either free, uncombined native elements (such as gold, silver, and copper), or elemental compounds (such as silicates - metallic elements combine with the Si-O tetrahedral radical). Gemstones are minerals suitable for use in jewelry after cutting and polishing.
Figure 09-06k Rocks, Minerals, and Gemstones

		Since the Earth's crust composed mainly of Oxygen (46.6%) and Silicon (27.7%) for a total of 75%, the predominant compositions in minerals and thus in rocks are compounds such as quartz (SiO2), feldspars (XAl1-2Si3-2O8 where X can be either the elements Na, K, or Ca), and Mica (...Si3O10...) (see Figure 09-06l). There are three types of rocks according to the formation process (see Figure
Figure 09-06l Minerals in Rocks [view large image]	Figure 09-06m Types of Rock	09-06m, and Table 09-03). They are further sub-divided into different grain sizes and colors (light, medium, dark, not shown in the figure).

Type	Formation	Characteristic	Composition	Examples
Igneous	Solidified from molten magma either at the Earth's surface (extrusive) or underneath (intrusive).	The crystals can be very large (via slow cooling), and mostly have random distribution		Basalt, Granite
Metamorphic	Created when existing rock is chemically or physically modified by intense heat or pressure, e.g., in collision of crustal plates	Have either wavy foliation (layer) or more random arrangement		Gneiss, Schist
Sedimentary	Formed from erosion, transportation and subsequent deposition of pre-existing rocks or other kinds of sediments	May occur in layers, grains may be poorly held together		Shale, Sandstone

Table 09-03 The Three Types of Rock

1a,b. Magma (molten rock) inside the Earth's crust rises through cracks and cools slowly underground forming igneous rocks composed of minerals with fairly large crystal sizes, these are known as intrusive igneous rocks. When the magma erupts onto the surface, as through a volcano, it is termed lava, the rapid rate of cooling makes the extrusive igneous rocks to form with medium to very small mineral crystals. 2. Once on the surface, the forces of erosion and weathering produce smaller particles (sands), which accumulate and compactify by pressure from upper layers to become sedimentary layers (rocks).

Figure 09-06n Rock Cycle [view large image]

3. When sedimentary and igneous rocks are subjected to intense heat and pressure such as in the collision of the crustal plates, they turn into metamorphic rocks. Some of these are uplifted back to the surface by tectonic action.

	1. SolutionPrecipitation - Near surface water becomes weak acid solution (with CO2 dissolved in it) in which many minerals are soluble. Gems will form as the water evaporates (Figure 09-06o (1)). Hot water from hydrothermal deep under are sometimes highly acidic or alkaline, making an even better solvent for more types of minerals. The slower rates of
Figure 09-06o Gem Formation [view large image]	cooling and/or evaporation allow for larger crystals to form. Many of the world's highest quality specimens and metal ores have come form such environments (Figure 09-06o (2)). It may also appear as veins in the cracks (Figure 09-06o (3)).

Class	Composition	#	Location	Examples
Silicate	Metallic elements + Si-O	> 500	95% of all rocks	Quartz SiO2, Garnet Mn3Al2(SO4)3
Carbonate	Metallic elements + (CO3)-2	200	Marine and evaporitic settings	Calcite CaCO3, Dolomite CaMg(CO3)2
Sulfate	Metallic elements + (SO4)-2		Evaporitic settings, hydrothermal veins	Gypsum Ca(SO4)H2O, Barite Ba(SO4)
Halide	Metallic elements + halogen	100	Evaporitic settings	Halite NaCl, Fluorite CaF2
Oxide	Metallic elements + oxygen	> 250	Precipitates on Earth's surface	Ice H2O, Hematite Fe2O3
Sulfide	Metallic/Semi-metallic elements + sulfur	> 300	Metal ores	Pyrite FeS2, Chalcopyrite CuFeS2
Phosphate	Metallic elements + (AO4)-3, where A can be P, As or V		Phosphate in teeth and bones	Pyroxmangite Pb5(PO4)3Cl, Bayldonite (Cu,Zn)3Pb(AsO4)2(OH)2H2O
Element	Chemical elements and alloys		Mines	Gold Au, Sulfur S, Silicides Fe3Si
Organic	Carbon + hydrogen		Fossil fuels	Hydrocarbons CnH2n+2, CnH2n, CnHn

Table 09-04 Mineral Classification by Chemical Properties

Table 09-05 Gem Identification by Physical Properties

Atmosphere⁷

		Earth's atmosphere can be separated into four layers as shown in Figure 09-07a and explained in more details in the followings. Figure 09-07b shows the atmospheric layers taken by astronaut on board the International Space Station as the Space Shuttle Endeavour coming in to dock. Several layers of Earth's atmosphere were visible. Directly behind the shuttle is the mesosphere, which appears blue. The white layer is the stratosphere, while the troposphere is in orange color. The ionosphere is transparent except producing auroras occasionally.
Figure 09-07a Atmosphere	Figure 09-07b Atmospheric Layers [view large image]

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 auroras as shown in the top right of Figure 09-07a.

Figure 09-08 Van Allen Belts [view large image]

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 09-09a Non-rotating Flow [view large image]	Figure 09-09b Air Circulation [view large image]	pressure zones with rising, unstable air. But long-term weather prediction is unreliable as shown in the Chaos Theory.

The movements of the large air masses are controlled primarily by the strong winds that blow continually at high altitudes. Around the polar low, the atmospheric circulation is counterclockwise. This upper-air westerlies do not move along circular paths; instead, they meander north and south in a wavy patterns. They are particularly well developed in the altitude range from 10 to 12 km, where a narrow band of air moves with speeds of 350 to 450 km/hr. This high-speed river of air is called the jet stream (Figure 09-09c), which separates the cold and warm air masses. Weather condition is very different from one side of the jet stream to the other.

Figure 09-09c Jet Stream

An air mass is a vast body of air (often covering several thousands of km2 wide and several km thick) in which the conditions of temperature and moisture are much the same at all points in a horizontal direction. It takes on these characteristics of the surface over which it forms. Air masses that affect the weather move across the country and carry with them the temperature and moisture of their origin. An air mass is modified by the surface over which it moves, but its original characteristics tend to persist. High-pressure ridges may develop

Figure 09-10 Air Masses [view large image]

any place where air cools, compresses, and sinks. Conversely, low-pressure cells form under the opposite conditions.

Thus the air masses can be classified into the following types. Figure 09-10 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

		relatively high, central-pressure zone. As the air diverges from the central region, it is deflected by the Coriolis force in a clockwise circulation (Figure 09-11a). Thus, most Highs are generally elliptical in shape following their formation. But as they interact with other air masses and topography, and are distorted by forces of the upper atmosphere, high pressure cells often become long and
Figure 09-11a High Pressure Ridge	Figure 09-11b Low Pressure Cell [view large image]	narrow in shape, and is referred to as high pressure ridge in the weather map. Since the air at high altitude is dry, the High is usually associated with fair weather.

Fronts form at the boundary when air masses collide. The colder air mass pushes under the warm one and lifts it. Then, if the boundary doesn't move, the front becomes stationary. Usually, it does move, and one air mass pushes the other along. If the cold mass pushes the warm air back, it is called a cold front. If the cold air retreats with the warm air pushing over it, it is called a warm front. In either case, frontal weather is either unsettled or stormy. Fronts usually bring bad weather.

Figure 09-12 Weather Front

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.

		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 (see diagram f in Figure 09-13 or its
Figure 09-13 Types of Front [view large image]	Figure 09-14a Types of Clouds [view large image]	animated version).

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


1. Cirrus - They are usually thin, white, and feathery. Its presence indicates significant amount of moisture at high altitudes, and serves as a warning that a warm front is on its way, bringing steady rain.
2. Alto - These are middle-level clouds. The air at these altitudes is generally stable and without vertical currents. The two principal types are altocumulus (fluffy) and altostratus (layered). Small and temporary altocumulus usually do not associate with an organized weather system or flow of moisture into the area. Only when altocumulus cover a larger area for a longer period of time, then it indicates a more significant source of moisture is present.
3. Stratus - They are layered and usually gray occurring at medium and low altitudes. Stratus clouds usually yield no precipitation other than drizzle, ice crystals, steady rain, or snow grains. Sometimes they resemble fog, except the clouds are above the ground with the base as low as just above the treetops. There are many variations: opacus nebulosus and opacus uniformis (dark and without features); undulatus (simple linear structure); and translucidus (thin stratus).
4. Cumulus (pile in Latin) - They are fluffy and lumpy clouds at medium and low altitudes. The presence of cumulus clouds is a sign of fine weather in the region. Shallow, vertical cumulus clouds are made when warm air raised, reaches its dew point, and then condenses. They appear light in color and have flat bases and rounded tops.
5. Nimbus - Nimbus are rain clouds at low altitudes. Nimbostratus are dark gray to pale blue water-droplet precipitation clouds with noticeable blurring in the area below cloud base (caused by falling precipitation). They occur all year round. Cumulonimbus are highly organized clouds make up of water droplets in lower portions, and ice particles in upper portions, with dark bases and with precipitation falling from them. They carry the thunder storms in the summer.

As mentioned above, further division is made into types such as cirrostratus and cumulonimbus in order to give a more detailed description of the cloud features. The highest clouds - those of the cirrus group - are composed chiefly of ice crystals. They are thin and wispy, and do not block the sunlight. The layered stratus clouds, on the other hand, tend to be much more dense and usually obscure the Sun. The fluffy, white low-altitude cumulus clouds are associated with good weather (see Figure 09-14b and hear cloud lyrics). The nimbostratus clouds, which also occur at low altitudes, are rain-bearing clouds. The most spectacular of all cloud formations are the towering cumulonimbus

Figure 09-14b Fluffy Clouds [view large image]

clouds, which develop during thunderstorm activity and rise to great heights.

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]

been observed since the end of the last ice age and have only previously been observed in association with 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]

		over the past century. Worldwide precipitation over land has increased by about one percent. The 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 otherwise would have begun about 5000 years ago.
Figure 09-17a Rising Temp- erature [view large image]	Figure 09-17b Greenhouse Effect [view large image]	For more details on global warming, go to the 2007 IPCC (Intergovernmental Panel on Climate Change) website.

There is no unanimous agreement on the cause of glaciation. One explanation involves plate tectonics. The movement of landmass to higher latitudes from tropical regions is responsible. Another explanation is known as the Milankovitch cycle (Figure 09-17c). It describes the way Earth's orbit gradually changes shape from a circle to a slight ellipse (eccentricity, curve a) and back again roughly every 100,000 years in exact agreement with the period between ice ages. Other causes include the tilt of the Earth's axis (obliquity, curve b), which takes 41,000 years to complete a cycle; and the top-like wobble (precession, curve c) of the Earth's axis, which follows a 23,000-year cycle. These other effects generate the secondary variations within the main cycles.

Figure 09-17c Ice Age Cycles [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 09-18a 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 and in between spiral arms. 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

Figure 09-18a Habitable Zone [view large image]

be unlikely -- comet impacts 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. The habitable zone in the Solar System is restricted by the Sun's radiation. If it is too close, the heat from the Sun would boil off waters and break down organic molecules. If it is too far, then water would freeze to ice. The habitable zone around a star depends on its mass. Stars with higher mass will provide more heat to its surrounding. Figure 09-18b shows the relationship between the habitable zone and the stellar mass. The inhabitant in the habitable zone is rather broadly defined to include perhaps just a strand of RNA (a primitive version of DNA).

Figure 09-18b Habitable Zone & Stellar Mass [view large image]

The criterions for a good parent star include: It remains at approximately the same luminosity for billions of years to offer a chance for the development of life. It forms out of an interstellar cloud with enough metal content to build the terrestrial planets. It stays in between galactic spiral arms for as long as possible to minimize the chance of collision with other stars. It does not emit gigantic flares that singe the surfaces of nearby planets. It does not form multiple system with companion stars that swoop in and out of the habitable zone. Figure 09-18c shows the four types of terrestrial planets at different location of the habitable zone. The corresponding temperatures could be too scorched (with no water), too humid, too cold, or just right. The best location for an earth-like world is in the habitable zone's center.

Figure 09-18c Types of Terrestrial Planets [view large image]

Scientists are using the European Space Agency's (ESA) Venus Express to search for life on Earth. It may sound obvious and silly (and merit for an Ig Nobel Prize), but they are actually looking for the kind of signatures that might be present on other habitable planets. The signals would not be some artificial structures such as the Great Wall of China, which is indiscernible by the naked eye from about 100 km up. It is the form of molecular spectrum (Figure 09-18d) at both the visible and near-infrared regions that is unique from the life-support planet. The presence of water and molecular oxygen is not a good enough evidence. More subtle signals, such as the so-called red edge caused by photosynthetic life (in the near infrared) are more reliable. The analysis to see whether this red edge is visible is just at the beginning in mid 2008.

Figure 09-18d Life on Earth [view large image]

Extra-Terrestrial Intelligence

In order to communicate with the other worlds, it requires that both sides should be highly evolved to an advanced technological stage. In 1961, Frank Drake (Figure 09-19a), now President of the SETI12 (Search for Extra-Terrestrial Intelligence) Institute, proposed a formula for estimating the existence of communicating Intelligent Life elsewhere in our galaxy. This is known as The Drake Equation13, which states that N = R x Fp x Ne x Fl x Fi x Fc x L

Figure 09-19a Frank Drake [view large image]

where

		currently broadcasting detectable signals. These chosen numbers are the lower limits and worse case scenarios. For example, a techno-logical civilization could last for 1000 years or more, dramatically increasing the value of N. Figure 09-19b depicts the parameters of the Drake equation in a pictorial form. The diagram in Figure 09-19c illustrates the stringent requirements in limiting the number of
Figure 09-19b Drake Parameters [view large image]	Figure 09-19c Drake Diagram [large image]	communicating civilization into a small fraction of the number of stars in the Milky Way.

In the summer of 2004, a flurry 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 09-19d 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 radio contact may be related to the fact that artificial signals are inherently weak. It is very difficult to detect. If we have ever received anomalous signal from outer space, we still have to uncover 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.

The End

The habitable zone on Earth will not last forever. Like most stars, the Sun exists in a stable configuration balanced between gravity pulling inward and pressure (maintained by heat from nuclear burning) pushing outward. This balance is self-regulating. Any slight change in one force will be off set by the other as long as there is enough fuel to maintain the burning. The solar system will undergo drastic change with the depletion of the hydrogen fuel. At the end of the evolution, the Earth may be still around, but any evidence that a biosphere once existed on the planet will have long since been melted and re-crystallized into oblivion. As illustrated in Figure 09-28, evolution of the Sun can be divided into three phases.

Figure 09-28 The End [view large image]

1. Main Sequence - Since the contraction from the pre-main sequence evolution, the Sun has settled into the main sequence for about 4.5 billion years. There will be another 7.5 billion years before the hydrogen fuel becomes exhausted. Meanwhile, its core grows denser, the central temperature climbs high enough that hydrogen fuses all the faster. During the next 1.2 billion years its luminosity will increase another 10% as it swells slightly and its surface temperature raised about 150^oC hotter. The warmer climate will actually be a boon for life in the next 500 million years until the temperature reaches to the boiling point of water. Eventually, the Earth will reach a runaway moist greenhouse in the next 1.2 billion years, when very hot temperatures lead to greatly increased evaporation from the oceans, more water vapor in the atmosphere, thus even hotter temperatures, and so on until the oceans boil dry. The water vapor is then subject to photolysis (breaking up of the water molecule by ultraviolet light), and lost to space for good.
2. Red Giant 1 - By the end of the main sequence phase, hydrogen burning will shift to a shell outside the Sun's core and happen faster and faster. Energy production will increase dramatically, causing the Sun's outer layers to balloon in size. As the Sun nears the apex of its red giant 1 phase its size will increase than 200 times to engulf the present orbit of Venus and almost to the present orbit of Earth. Detailed study shows that the planets may spiral outward by tidal force. Thus, it is not yet clear whether the Earth will escape being engulfed by the red giant Sun. It is conceivable that the habitable zone would expand together with the Sun. Those now frozen moons such as Europa, and Titan may provide a shelter for life in the dying phase of the Solar system. The first red giant phase ends abruptly when the core temperature reaches 100 million degrees Celsius and helium begins to fuse into carbon, providing a fresh new energy source. The Sun responds by shrinking drastically and cutting its overall luminosity by a factor of nearly 100, providing a respite from a long string of disasters if the Earth is still intact.
3. Red Giant 2 - This phase is the repeat of the previous one except that it is the exhaustion of helium instead of hydrogen. Once again the Sun becomes a serious threat to Earth's physical survival. During the second red giant phase it experiences several epochs of enormous energy output (helium shell flashes) that lead to massive, roughly 10,000 year long pulsations in size. It is distinctly possible that Earth could be briefly engulfed during these pulses. Then, roughly 100 million years after the red giant 2 phase begins, the Sun will throw off its massive outer layers completely to form a planetary nebula leaving behind a brilliantly hot but very tiny white dwarf. The surviving planets, including perhaps the cinders of Earth and Mars, will orbit the white dwarf quietly and stably for hundreds of billions of years as they and the Sun's little remnant cool ever closer to absolute zero - awaiting the bizarre ending of the cosmic expansion when everything is cut off from everything else (according to one of the many pre-bang theories).

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

[Top]

---

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

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

---

[Home Page]