02_03_Geology_Origin of the universe and solar system.pdf

7/30/2019 02_03_Geology_Origin of the universe and solar system.pdf

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ORIGIN OF THE UNIVERSE AND SOLAR

SYSTEM, AND EARTHS PLACE IN THEM Most scientists think that the universe originated about 14

billion years ago in what is popularly called theBig Bang.The Big Bang is a model for the evolution of the universe inwhich a dense, hot state was followed by expansion,cooling, and a less dense state.

According to modern Cosmology (the study of the origin,evolution, and nature of the universe), the universe has noedge and therefore no center. Thus, when the universebegan, all matter and energy were compressed into an infinitely small high-temperature and high-density state inwhich both time and space were set at zero. Therefore,there is no before the Big Bang, only what occurred afterit. As demonstrated by Einsteins theory of relativity, spaceand time are unalterably linked to form a spacetimecontinuum, that is, without space, there can be no time.


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The study of the universe its nature, origin, andevolution is called cosmology. The mathematical basisfor cosmology is general relativity, from which equationswere derived that describe both the energy and mattercontent of the universe. These equations, combined withobservations of density and acceleration, led to the mostaccurate model so farthe Big Bang model. The fact thatthe universe is expanding implies that it had a beginning.The theory that the universe began as a point and has beenexpanding since is called the Big Bang theory. Although the

name might seem to imply explosion into space, the theorydescribes an expansion of space itself while gravity holdsmatter in check.


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Two fundamental phenomena indicate that the BigBang occurred. First, the universe is expanding, andsecond, it is permeated by back-ground radiation.

When astronomers look beyond our own solar system,they observe that everywhere in the universe galaxiesare moving away from each other at tremendousspeeds. Edwin Hubble first recognized thisphenomenon in 1929. By measuring the optical spectra

of distant galaxies, Hubble noted that the velocity atwhich a galaxy moves away from Earth increasesproportionally to its distance from Earth


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Arno Penzias and Robert Wilson of Bell TelephoneLaboratories made the second importantobservation that provided evidence of the Big

Bang in 1965. In 1965, scientists discovered apersistent background noise in their radio antenna,shown in Figure 30.23. This noise was caused byweak radiation, called the cosmic backgroundradiation. They discovered that there is a pervasivebackground radiation of 2.7 Kelvin (K) above

absolute zero (absolute zero equals -273C; 2.7 K =270.3C) everywhere in the universe. Thisbackground radiation is thought to be the fadingafterglow of the Big Bang.

Currently, cosmologists cannot say what it was likeat time zero of the Big Bang because they do notunderstand the physics of matter and energyunder such extreme conditions.


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However, it is thought that during the first second followingthe Big Bang, the four basic forces - gravity (the attractionof one body toward another), electromagnetic force(combines electricity and magnetism into one force andbinds atoms into molecules), strong nuclear force (bindsprotons and neutrons together), and weak nuclear force(responsible for the break-down of an atoms nucleus,producing radioactive decay)separated and the universeexperienced enormous expansion. By the end of the fi rstthree minutes following the Big Bang, the universe was cool

enough that almost all nuclear reactions had ceased, andby the time it was 30 minutes old nuclear reactions hadcompletely ended and the universes mass consisted almostentirely of hydrogen and helium nuclei.


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As the universe continued expanding and cooling, stars andgalaxies began to form and the chemical makeup of theuniverse changed. Initially, the universe was 100%hydrogen and helium, whereas today it is 98% hydrogenand helium and 2% all other elements by weight. How didsuch a change in the universes composition occur?Throughout their life cycle, stars undergo many nuclearreactions in which lighter elements are converted intoheavier elements by nuclear fusion. When a star dies,often explosively, the heavier elements that were formed

in its core are returned to interstellar space and areavailable for inclusion in new stars. In this way, thecomposition of the universe is gradually enhanced byheavier elements.


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Outward expansion

Similar to a stars internal fusionpressure opposing the effort of agravitational force to collapse thestar, the universe has two opposingforces. In the Big Bang model, themomentum of the outwardexpansion of the universe is opposedby the inward force of gravity actingon the matter of the universe to slowthat expansion, as illustrated in the

figure. What ultimately will happendepends on which of these twoforces is stronger.


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Possible outcomes Based on the Big Bang theory, there are three

possible outcomes for the universe, as shownin the figure 30.22. The average density of theuniverse is an observable quantity with vastimplications to the outcome.

Open universe An open universe is one inwhich the expansion will never stop. Thiswould happen if the density of the universe isinsufficient for gravity to ever halt theexpansion.

Closed universe A closed universe will resultif the expansion stops and turns into acontraction. That would mean the density ishigh enough that eventually the gravitycaused by the mass will halt the expansion ofthe universe and pull all of the mass back tothe original point of origin.

Flat universe A flat universe results if theexpansion slows to a halt in an infiniteamount of time, but never contracts. Thismeans that while the universe wouldcontinue to expand, its expansion would beso slow that it would seem to stop.


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Contents of the Universe All the evidence is now pointing in the same direction, and

astronomers can say with a high degree of precision of whatthe universe is composed. Their best clue comes from theradiation left in space from the universes beginning. Theripples left during the time of cooling of the universesbeginning radiation set the density at that point of time anddictated how matter and energy would separate. This in turnlaid the groundwork for future galaxies. The figure gives one

view into the universe. Dark matter and energy Cosmologists estimate that the

universe is composed of dark matter (21 percent), dark energy(75 percent), and luminous matter. If you compare theuniverse to Earth, dark energy is like the water covering thesurface of Earth. That would be like saying that 70 percent ofEarth is covered with something that is not identified. What isunknown today is the nature of the dark matter and darkenergy. Dark matter is thought to consist of subatomicparticles, but of the known particles, none display the rightproperties to explain or fully define dark matter. And althoughscientists recognize the effects of dark energy, they still do notknow what it is.


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Our Solar SystemIts Origin and

Evolution

Theories of the origin of the solar system rely

on direct observations and data from probes.

Scientific theories must explain observed

facts, such as the shape of the solar system,

differences among the planets, and the nature

of the oldest planetary surfaces asteroids,

meteorites, and comets.


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A Collapsing Interstellar Cloud

Stars and planets form from interstellar clouds, which exist in spacebetween the stars. These clouds consist mostly of hydrogen andhelium gas with small amounts of other elements and dust. Dustmakes interstellar clouds look dark because it blocks the light fromstars within or behind the clouds. Often, starlight reflects off of the

dust and partially illuminates the clouds. Also, stars can heat clouds,making them glow on their own. This is why interstellar cloudsoften appear as blotches of light and dark, as shown in the figure.This interstellar dust can be thought of as a kind of smog thatcontains elements formed in older stars, which expelled theirmatter long ago.


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At first, the density of interstellar gas is lowmuch lower than thebest vacuums created in laboratories. However, gravity slowly drawsmatter together until it is concentrated enough to form a star andpossibly planets. Astronomers think that the solar system beganthis way. They have also observed planets around other stars, and

hope that studying such planet systems will provide clues to howour solar system formed.


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Collapse accelerates. At first, thecollapse of an interstellar cloud is

slow, but it gradually acceleratesand the cloud becomes muchdenser at its center. If rotating, thecloud spins faster as it contracts,for the same reason that ice

skaters spin faster as they pulltheir arms close to their bodiescentripetal force. As the collapsingcloud spins, the rotation slows thecollapse in the equatorial plane,

and the cloud becomes flattened.Eventually, the cloud becomes arotating disk with a denseconcentration of matter at thecenter.


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Matter condenses

Astronomers think our solar system began in this manner. The Sunformed when the dense concentration of gas and dust at the centerof a rotating disk reached a temperature and pressure high enoughto fuse hydrogen into helium. The rotating disk surrounding theyoung Sun became our solar system. Within this disk, the

temperature varied greatly with location; the area closest to thedense center was still warm, while the outer edge of the disk wascold. This temperature gradient resulted in different elements andcompounds condensing, depending on their distance from the Sun.This also affected the distribution of elements in the formingplanets. The inner planets are richer in the higher melting point

elements and the outer planets are composed mostly of the morevolatile elements. That is why the outer planets and their moonsconsist mostly of gases and ices. Eventually, the condensation ofmaterials into liquid and solid forms slowed.


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Planetesimal Next, the tiny grains of condensed

material started to accumulate andmerge, forming larger particles. Theseparticles grew as grains collided and stucktogether and as gas particles collected ontheir surfaces. Eventually, collidingparticles in the early solar system mergedto form planetesimals objectshundreds of kilometers in diameter.Growth continued as planetesimalscollided and merged. Sometimes,collisions destroyed planetesimals, butthe overall result was a smaller numberof larger bodies the planets. Theirproperties are given in the next table.

The protoplanetary disk HH-30 in Taurus, about

450 light years away. The disk emits the reddishstellar jet, a common structure of these

formations.


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Gas giants form. The first large planet to develop wasJupiter. Jupiter increased in size through the merging of icyplanetesimals that contained mostly lighter elements. Itgrew larger as its gravity attracted additional gas, dust, and

planetesimals. Saturn and the other gas giants formedsimilarly, but they could not become as large becauseJupiter had collected so much of the available material. Aseach gas giant attracted material from its surroundings, adisk formed in its equato-rial plane, much like the disk of

the early solar system. In this disk, matter clumpedtogether to form rings and satellites.

Terrestrial planets form. Planets also formed by themerging of planetesimals in the inner part of the main disk,near the young Sun. These were composed primarily of

elements that resist vaporization, so the inner planets arerocky and dense, in contrast to the gaseous outer planets.Also, scientists think that the Suns gravitational force sweptup much of the gas in the area of the inner planets andprevented them from acquiring much of this material fromtheir surroundings. Thus, the inner planets did not developsatellites.


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Debris. Material that remainedafter the formation of the planetsand satellites is called debris.Eventually, the amount of

interplanetary debris diminishedas it crashed into planets or wasdiverted out of the solar system.Some debris that was not ejectedfrom the solar system became icy

objects known as comets. Otherdebris formed rocky planetesimalsknown as asteroids. Mostasteroids are found in the areabetween Jupiter and Mars knownas the asteroid belt, shown in thefigure. They remain there becauseJupiters gravitational forceprevented them from merging toform a planet.


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Modeling the Solar System

Heliocentric model. In 1543, Polish scientistNicolaus Copernicus suggested that the Sun wasthe center of the solar system. In this Sun-

centered, or heliocentric (hee lee oh SEN trihk)model, Earth and all the other planets orbit theSun. In a heliocentric model, the increased gravityof proximity to the Sun causes the inner planets

to move faster in their orbits than do the outerplanets. It also provided a simple explanation forretrograde motion.


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Keplers first law Within a century, the ideas of Copernicus were confirmed by other

astronomers, who found evidence that supported the heliocentric model. For

example, Tycho Brahe, a Danish astronomer, designed and built very accurateequipment for observing the stars. From 15761601, before the telescope wasused in astronomy, he made accurate observations to within a half arc minuteof the planets positions. Using Brahes data, German astronomer JohannesKepler demonstrated that each planet orbits the Sun in a shape called anellipse, rather than a circle. This is known as Keplers first law of planetarymotion. An ellipse is an oval shape that is centered on two points instead of a

single point, as in a circle. The two points are called the foci (singular, focus).The major axis is the line that runs through both foci at the maximum diameterof the ellipse, as illustrated in the figure.


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Each planet has its own elliptical orbit, but the Sun isalways at one focus. For each planet, the averagedistance between the Sun and the planet is its

semimajor axis, which equals half the length of themajor axis of its orbit, as shown in Figure 28.5. Earthssemimajor axis is of special importance because it is aunit used to measure distances within the solar system.Earths average distance from the Sun is 1.496 108km, or 1 astronomical unit (AU).


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Eccentricity. A planet in an elliptical orbit does notorbit at a constant distance from the Sun. The shape ofa planets elliptical orbit is defined by eccentricity,which is the ratio of the distance between the foci tothe length of the major axis. The orbits of most planetsare not very eccentric; in fact, some are almost perfectcircles. The eccentricity of a planet can change slightly.Earths eccentricity today is about 0.02, but the

gravitational attraction of other planets can stretch theeccentricity to 0.05, or cause it to fall to 0.01.


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Keplers second law In addition to discovering the shapes of planetary orbits,

Kepler showed that planets move faster when they arecloser to the Sun. He demonstrated this by proving that animaginary line between the Sun and a planet sweeps outequal amounts of area in equal amounts of time, as shownin the figure. This is known as Keplers second law.


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Keplers third law

The length of time it takes for a planet or otherbody to travel a complete orbit around the Sun iscalled its orbital period. In Keplers third law ofplanetary motion, he determined the

mathematical relationship between the size of aplanets ellipse and its orbital period. Thisrelationship is written as follows:

P2 = a3

P is time measured in Earth years, and a is lengthof the semimajor axis measured in astronomicalunits


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Gravity Newton first developed an understanding of gravity by

observing falling objects. He described falling as downwardacceleration produced by gravity, an attractive force betweentwo objects. He determined that both the masses of and thedistance between two bodies determined the force betweenthem. This relationship is expressed in his law of universalgravitation, illustrated in the figure, and that is stated

mathematically as follows:

F is the force measured in newtons, G is the universalgravitation constant (6.6726 1011 m3/(kg s2)), m1 and m2are the masses of the bodies in kilograms, and r is thedistance between the two bodies in meters.


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Gravity and orbits Newton realized that this attractiveforce could explain why planets move according to Keplerslaws. He observed the Moons motion and realized that itsdirection changes because of the gravitational attraction of

Earth. In a sense, the Moon is constantly falling towardEarth. If it were not for this attraction, the Moon wouldcontinue to move in a straight line and would not orbitEarth. The same is true of the planets and their moons,stars, and all orbiting bodies throughout the universe.


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Center of mass Newtonalso determined thateach planet orbits a point

between it and the Suncalled the center of mass.For any planet and theSun, the center of mass is

just above or within the

surface of the Sun,because the Sun is muchmore massive than anyplanet. The figure shows

how this is similar to thebalance point on aseesaw.


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Present-Day Viewpoints

Astronomers traditionally divided the planets into twogroups: the four smaller, rocky, inner planets, Mercury,Venus, Earth, and Mars; and the four outer gas planets,Jupiter, Saturn, Uranus, and Neptune. It was not clear

how to classify Pluto, because it is different from thegas giants in composition and orbit. Pluto also did notfit the present-day theory of how the solar systemdeveloped. Then in the early 2000s, astronomersdiscovered a vast number of small, icy bodies

inhabiting the outer reaches of the solar system,thousands of AU beyond the orbit of Neptune. At leastone of these is larger than Pluto.


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These discoveries have led many astronomersto rethink traditional views of the solarsystem. Some already define it in terms ofthree zones: Zone 1, Mercury, Venus, Earth,

Mars; Zone 2, Jupiter, Saturn, Uranus,Neptune; and Zone 3, everything else,including Pluto. In science, views change asnew data becomes available and new theories

are proposed. Astronomy today is a rapidlychanging field.


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02_03_Geology_Origin of the universe and solar system.pdf