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Geology (The -I)

UNIT-1 The Planet Earth-I

1. ORIGIN OF THE AND EARTH

The Solar System The Solar System is made up of all the that orbit our . In addition to planets, the Solar System also consists of , , , minor planets, and dust and gas. Everything in the Solar System orbits or revolves around the Sun. The Sun contains around 98% of all the material in the Solar System. The larger an object is, the more gravity it has. Because the Sun is so large, its powerful gravity attracts all the other objects in the Solar System towards it. At the same time, these objects, which are moving very rapidly, try to fly away from the Sun, outward into the emptiness of . The result of the planets trying to fly away, at the same time that the Sun is trying to pull them inward is that they become trapped half-way in between. Balanced between flying towards the Sun, and escaping into space, they spend eternity orbiting around their parent star. Our solar system consists of an average star we call the Sun, the planets , , Earth, , , , , , and . It includes: the satellites of the planets; numerous comets, asteroids, and ; and the . The Sun is the richest source of electromagnetic energy (mostly in the form of heat and light) in the solar system. The Sun's nearest known stellar neighbor is a red dwarf star called Proxima Centaury, at a distance of 4.3 light years away. The whole solar system, together with the local stars visible on a clear night, orbits the center of our home galaxy, a spiral disk of 200 billion stars we call the Milky Way. The Milky Way has two small galaxies orbiting it nearby, which are visible from the southern hemisphere. They are called the Large Magellanic Cloud and the Small Magellanic Cloud. The nearest large galaxy is the Andromeda Galaxy. It is a spiral galaxy like the Milky Way but is 4 times as massive and is 2 million light years away. Our galaxy, one of billions of galaxies known, is traveling through intergalactic space. The planets, most of the satellites of the planets and the asteroids revolve around the Sun in the same direction, in nearly circular orbits. When looking down from above the Sun's north pole, the planets orbit in a counter-clockwise direction. The planets orbit the Sun in or near the same plane, called the . Pluto is a special case in that its orbit is the most highly inclined (18 degrees) and the most highly elliptical of all the planets. Because of this, for part of its orbit, Pluto is closer to the Sun than is Neptune. The axis of rotation for most of the planets is nearly perpendicular to the ecliptic. The exceptions are Uranus and Pluto, which are tipped on their sides. Composition Of The Solar System The Sun contains 99.85% of all the matter in the Solar System. The planets, which condensed out of the same disk of material that formed the Sun, contain only 0.135% of the mass of the solar system. Jupiter contains more than twice the matter of all the other planets combined. Satellites of the planets, comets, asteroids, meteoroids, and the interplanetary medium constitute the remaining 0.015%. The following table is a list of the mass distribution within our Solar System.  Sun : 99.85%  Planets : 0.135%  Comets : 0.01%  Satellites : 0.00005%  Minor Planets : 0.0000002%  Meteoroids : 0.0000001%  Interplanetary Medium : 0.0000001%

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Interplanetary Space Nearly all the solar system by volume appears to be an empty void. Far from being nothingness, this of "space" comprises the interplanetary medium. It includes various forms of energy and at least two material components: interplanetary dust and interplanetary gas. Interplanetary dust consists of microscopic solid particles. Interplanetary gas is a tenuous flow of gas and charged particles, mostly protons and electrons -- -- which stream from the Sun, called the .

Fig. : Interplanetary Space The solar wind can be measured by spacecraft, and it has a large effect on tails. It also has a measurable effect on the motion of spacecraft. The speed of the solar wind is about 400 kilometers (250 miles) per second in the vicinity of Earth's orbit. The point at which the solar wind meets the , which is the "solar" wind from other stars, is called the heliopause. It is a boundary theorized to be roughly circular or teardrop-shaped, marking the edge of the Sun's influence perhaps 100 AU from the Sun. The space within the boundary of the heliopause, containing the Sun and solar system, is referred to as the . The solar extends outward into interplanetary space; it can be measured on Earth and by spacecraft. The solar magnetic field is the dominating magnetic field throughout the interplanetary regions of the solar system, except in the immediate environment of planets which have their own magnetic fields. The Terrestrial Planets The terrestrial planets are the four innermost planets in the solar system, Mercury, Venus, Earth and Mars. They are called terrestrial because they have a compact, rocky surface like the Earth's. The planets, Venus, Earth, and Mars have significant atmospheres while Mercury has almost none. The following diagram shows the approximate distance of the terrestrial planets to the Sun.

Fig. : Mean Distances of terrestrial Planets from the Sun. Contact Us : Website : www.eduncle.com | Email : [email protected] | Call Toll Free : 1800-120-1021 2 Geology (The Planet Earth-I)

The Jovian Planets Jupiter, Saturn, Uranus, and Neptune are known as the Jovian (Jupiter-like) planets, because they are all gigantic compared with Earth, and they have a gaseous like Jupiter's. The Jovian planets are also referred to as the gas giants, although some or all of them might have small solid cores. The following diagram shows the approximate distance of the Jovian planets to the Sun.

Fig. : Mean distances of the Jovian Planets from the Sun. Origin of The Solar System Our solar system formed about 5 billion years ago, from an enormous cloud of dust and gas, a nebula. The Sun, like other stars, was formed in a nebula, an of dust and gas (mostly ). These stellar nurseries are abundant in the arms of spiral galaxies (like our galaxy, the Milky Way). In the stellar nursery, dense parts of the clouds undergo gravitational collapse and compress to form a rotating gas globule. The globule is cooled by emitting radio waves and infrared radiation. It is compressed by gravitational forces and also by shock waves of pressure from supernova or the hot gas released from nearby bright stars. These forces cause the roughly-spherical globule to collapse and rotate. The process of collapse takes from between 10,000 to 1,000,000 years. The origin of the solar system has long been a fascinating subject posing difficult questions of deep significance. It takes one to the heart of the question of our origins, of how we came to be here and why our surroundings look the way they do. Unfortunately, we currently lack a self-consistent model for the origin of the solar system and other planetary systems. The early stages of planet formation are obscure, and we have only a modest understanding of how much the orbits of planets change during and after their formation. At present, we cannot say whether terrestrial planets similar to the Earth are commonplace or highly unusual. Nor do we know the source of the that makes our planet habitable. In the face of such uncertainty, one might ask whether we will ever understand how planetary systems form. In fact, the last 10 years have seen rapid progress in almost every area of , and our understanding of the origin of the solar system and other planetary systems has improved greatly as a result. Planetary science today is as exciting as it has been at any time since the Apollo landings on the , and the coming decade looks set to continue this trend.

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Some key recent developments follow :  A decade ago, the first planet orbiting another Sunlike star was discovered. Since then, new planets have been found at an astounding rate, and roughly 200 objects are known today. Most of these planets appear to be gas giants similar to Jupiter and Saturn. Recently, several smaller planets have been found, and these may be akin to Uranus and Neptune, or possibly large analogs of terrestrial planets like Earth.  In the last 10 years, there have been a number of highly successful space missions to other bodies in the solar system, including Mars, Saturn, Titan, and several asteroids and comets. Information and images returned from these missions have transformed our view of these objects and greatly enhanced our understanding of their origin and evolution. – that  The discovery that one can physically separate and analyze star dust —has can be extracted from and that formed in the envelopes of other stars meant that scientists can for the first time test decades of theory on how stars work. The parallel development of methods for extracting isotopic information at the submicron scale has opened up a new window to the information stored in such grains.  The development of multiple collector inductively coupled plasma mass spectrometry has made it possible to use new isotopic systems for determining the mechanisms and timescales for the growth of bodies early in the solar system.  Our theoretical understanding of planet formation has advanced substantially in several areas, including new models for the rapid growth of giant planets, a better understanding of the physical and chemical evolution of protoplanetary disks, and the growing realization that planets can migrate substantially during and after their formation.  The recent development of powerful new computer codes and equations of state has facilitated realistic, high-resolution simulations of collisions between planet-sized bodies. Scientists are discovering that the resolution of their models significantly changes the outcome, and the race is on to find reliable solutions. Today, the formation of the solar system is being studied using three complementary approaches :  Astronomical observations of protoplanetary disks around young stars are providing valuable information about probable conditions during the early history of the solar system and the timescales involved in planet formation. The discovery of new planets orbiting other stars is adding to the astonishing diversity of possible planetary systems and providing additional tests for theories of how planetary systems form.  Physical, chemical, and isotopic analyses of meteorites and samples returned by space missions are generating important information about the formation and evolution of objects in the solar system and their constituent materials. This field of has taken off in several important new directions in recent years, including the determination of timescales involved in the formation of the terrestrial planets and asteroids, and constraints on the origin of the materials that make up the solar system.  Theoretical calculations and numerical simulations are being used to examine every stage in the formation of the solar system. These provide valuable insights into the complex interplay of physical and chemical processes involved, and help to fill in some of the gaps when astronomical and cosmochemical data are unavailable. Star Formation and Protoplanetary Disks –4.6 billion years (Ga) ago by collapse of a portion of a molecular The solar system formed 4.5 cloud of gas and dust rather like the Eagle or Orion Nebulae. Some of the star dust from that ancient Solar Nebula has now been isolated from primitive meteorites. Their isotopic compositions are vastly

Contact Us : Website : www.eduncle.com | Email : [email protected] | Call Toll Free : 1800-120-1021 4 Geology (The Planet Earth-I) different from those of our own solar system and provide fingerprints of nearby stars that preceded our Sun. These include red giants, asymptotic giant branch stars, supernovae, and novae. It has also become clear from studying modern molecular clouds that stars like our Sun can form in significant numbers in close proximity to each other. Such observation also provide clues as to how own solar system formed because they have —the environments in which planetary objects are born. provided us with images of circumstellar disks Observations from space-based infrared telescopes such as the Infrared Astronomical Satellite (IRAS) have shown that many young stars give off more infrared radiation than would be expected for blackbodies of the same size. This infrared excess comes from micron-sized grains of dust orbiting the star in an optically thick (opaque) disk. Dark, dusty disks can be seen with the Hubble Space Telescope surrounding some young stars in the Orion Nebula (Fig.).

Fig. : Proplyds are young stellar objects embedded in an optically dense envelope of gas and dust. The objects shown here are from the Orion Nebula These disks have been dubbed proplyds, short for protoplanetary disks. It is thought that protoplanetary disks are mostly composed of gas, and in a few cases this gas has been detected, although gas is generally much harder to see than dust. The fraction of stars having a massive disk declines with stellar age, and large infrared excesses are rarely seen in stars older than 107 years. In some cases, such as the disk surrounding the star HR 4796A, there are signs that the inner portion of a disk has been cleared of dust (Fig.), perhaps due to the presence of one or more planets.

Fig. : The circumstellar disk surrounding HR 4796A as revealed by interferometry measurements of the infrared excess. Note the area close into the star swept clear of dust, which has presumably been incorporated into planetary objects

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Roughly half of stars up to a few hundred million years old have low-mass, optically thin (nearly transparent) disks containing some dust but apparently little or no gas. In a few cases, such as the star Beta Pictoris, a disk can be seen at visible wavelengths when the light from the star itself is blocked. Dust grains in these disks will be quickly accelerated outward by radiation pressure or spiral inward due –Robertson drag caused by collisions with photons from the central star. This dust should to Poynting be either removed from the disk or destroyed in high-speed collisions with other dust grains on a timescale that is short compared to the age of the star. For this reason, the dust in these disks is thought to be second-generation material formed by collisions between asteroids or sublimation from comets orbiting these stars in more massive analogs of the in our own solar system. These are often referred to as debris disks as a result. In the solar system, the planets all orbit the Sun in the same direction, and their orbits are very roughly coplanar. This suggests the solar system originated from a disk-shaped region of material referred to as the solar nebula, an idea going back more than 2 centuries to Kant and later Laplace. The discovery of disks of gas and dust around many young stars provides strong support for this idea and implies that planet formation is associated with the formation of stars themselves. Stars typically form in clusters of a few hundred to a few thousand objects in dense regions of the interstellar medium called molecular clouds (see Fig.). The gas in molecular clouds is cold (roughly 10 K) and dense compared to that in other regions of space (roughly 104 atoms/cm3 ) but still much more tenuous than the gas in a typical “vacuum.” Stars in these clusters are typically separated by about 0.1 pc (0.3 lightyears), laboratory ’s neighborhood. much less than the distance between stars in the Sun

Fig. : This Hubble Space Telescope image of the Orion Nebula shows molecular clouds of gas and dust illuminated by radiation from young stars. Some early stars appear shrouded in dusty disks. Scientists think that our solar system formed by collapse of a portion of a similar kind of leading to formation of a new star embedded in a dusty disk. How that collapse occurred is unclear. It may have been triggered by a shock wave carrying material being shed from another star such as an AGB star or supernova.

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It is unclear precisely what causes the densest portions of a molecular cloud (called molecular cloud cores) to collapse to form stars. It may be that contraction of a cloud core is inevitable sooner or ’s own gravity, or an external event may cause the triggered collapse of a core. The later due to the core original triggered collapse theory was based on the sequencing found in the ages of stars in close proximity to one another in molecular clouds. This suggests that the formation and evolution of some stars triggered the formation of additional stars in neighboring regions of the cloud. However, several other triggering mechanisms are possible, such as energetic radiation and mass loss from other newly formed stars, the effects of a nearby, pulsating asymptotic giant branch (AGB) star, or a shock wave from the supernova explosion of a massive star. Gas in molecular cloud cores is typically moving. When a core collapses, the gas has too much angular momentum for all the material to form a single, isolated star. In many cases, a system forms. In others cases, a single protostar forms (called a T Tauri star or pre-main sequence star), while a significant fraction of the gas goes into orbit about the star forming a disk that is typically 100 astronomical units (AU) in diameter. Temperatures in T Tauri stars are initially too low for nuclear reactions to take place. However T Tauri stars are much brighter then older stars like the Sun due to the release of gravitational energy as the star contracts. The initial collapse of a molecular cloud core takes roughly 105 years, and material continues to fall onto both the star and its disk until the core is depleted. The spectra of T Tauri stars contain strong ultraviolet and visible emission lines caused by hot gas falling onto the star. This provides evidence that disks lose mass over time as material moves inward through the disk and onto the star, a process called viscous . This process provides one reason why older stars do not have disks, another reason being planet formation itself. Estimated disk accretion rates range from 10-6 to 10-9 solar masses per year. The mechanism responsible for viscous accretion is unclear. A promising candidate is magneto- rotational instability (MRI), in which partially ionized gas in the disk becomes coupled to the local magnetic field. Because stars rotate, the magnetic field sweeps around rapidly, increasing the orbital velocity of material that couples strongly to it and moving it outward. Friction causes the remaining material to move inward. As a result, a disk loses mass to its star and spreads outwards over time. This kind of disk evolution explains why the planets currently contain only 0.1% of the mass in the solar system but have retained more than 99% of its angular momentum. MRI requires a certain fraction of the gas to be ionized, and it may not be effective in all portions of a disk. Disks are also eroded over time by photo evaporation. In this process, gas is accelerated when atoms absorb ultraviolet photons from the central star or nearby, energetic stars, until the gas is moving fast enough to escape into interstellar space. T Tauri stars often have jets of material moving rapidly away from the star perpendicular to the plane of the disk. These jets are powered by the inward accretion of material through the disk coupled with the rotating magnetic field. Outward flowing winds also arise from the inner portions of a disk. It is possible that a wind arising from the very inner edge of the disk (called the x-wind) can entrain small ’s shadow. Many solid particles with it. These objects will be heated strongly as they emerge from the disk of these particles will return to the disk several AU from the star, and may drift inward again to repeat the process. Some of these particles may be preserved today in meteorites. T Tauri stars are strong emitters of X-rays, generating fluxes up to 104 times greater than that of the Sun during the strongest solar flares. Careful sampling of large populations of young solar mass stars in the Orion Nebula shows that this is normal behavior in young stars. This energetic flare activity is strongest in the first million years and declines at later times, persisting for up to 108 years. From this it has been concluded that the young Sun generated 105 times as many energetic protons as today. It is thought that reactions between these protons and material in the disk may have provided some of the short-lived isotopes whose daughter products are seen today in meteorites although the formation of nearly all of these predate that of the solar system.

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The minimum mass of material that passed through the solar nebula can be estimated from the total mass of the planets, asteroids, and comets in the solar system. However, all of these objects are depleted in hydrogen and helium relative to the Sun. Ninety percent of the mass of the terrestrial planets is made up of , magnesium, silicon, and iron (Fig.), and although Jupiter and Saturn are mostly composed of hydrogen and helium, they are enriched in the heavier elements compared to the Sun. When the missing hydrogen and helium is added, the minimum-mass solar nebula (MMSN) turns out –2% of the Sun’s mass. to be 1 The major uncertainties in this number come from the fact that the interior compositions of the giant planets and the initial mass of the Kuiper Belt are poorly known. Not all of this mass necessarily existed in the nebula at the same time, but it must have been present at some point. Current theoretical models predict that planet formation is an inefficient process, with some mass falling into the Sun or being ejected into interstellar space, so the solar nebula was probably more massive than the MMSN. Gas in the solar nebula was heated as it viscously accreted toward the Sun, releasing gravitational energy. The presence of large amounts of dust meant the inner portions of the nebula were optically thick to infrared radiation so these regions became hot. Numerical disk models show that temperatures ’s history. probably exceeded 1500 K in the forming region early in the disk Viscous heating mainly took place at the disk midplane where most of the mass was concentrated. The surfaces of the disk would have been much cooler. The amount of energy generated by viscous accretion declined rapidly with distance from the Sun. In the outer nebula, solar irradiation was the more important effect. Protoplanetary disks are thought to be flared, so that their vertical thickness grows more rapidly than their radius. As a result, the surface layers are always irradiated by the central star. For this reason, the surface layers of the outer solar nebula may have been warmer than the midplane. The nebula cooled over time as the viscous accretion rate declined and dust was swept up by larger bodies, reducing the optical depth. In the inner nebula, cooling was probably rapid. Models show that at the midplane at 1 AU, the temperature probably fell to about 300 K after 105 years. Because the energy generated by viscous accretion and solar irradiation declined with distance from the Sun, disk temperatures also declined with heliocentric distance. At some distance from the Sun, a location referred to as the ice line, temperatures became low enough for water ice to form. Initially, the ice line may have –6 AU from the Sun, but it moved inward over time as the nebula cooled. been 5 Some asteroids contain hydrated minerals formed by reactions between water ice and dry rock. This suggests water ice was present when these asteroids formed, in which case the ice line would –3 AU from the Sun at the time. Meter-sized icy bodies drifted rapidly inward have been no more than 2 through the solar nebula due to gas drag. As the collapse proceeds, the temperature and pressure within the globule increases, as the atoms are in closer proximity. Also, the globule rotates faster and faster. This spinning action causes an increase in centrifugal forces (a radial force on spinning objects) that causes the globule to have a central core and a surrounding flattened disk of dust (called a protoplanetary disk or accretion disk). The central core becomes the star; the protoplanetary disk may eventually coalesce into orbiting planets, asteroids, etc. Protostar The contracting cloud heats up due to friction and forms a glowing protostar; this stage lasts for roughly 50 million years. If there is enough material in the protostar, the gravitational collapse and the heating continue. A Newborn Star and a Solar System °F is reached, nuclear fusion begins at the core of the When a temperature of about 27,000,000 Sun. This is the nuclear reaction in which hydrogen atoms are converted to helium atoms plus energy. This energy (radiation) production prevents further contraction of the Sun.

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Young stars often emit jets of intense radiation that heat the surrounding matter to the point at which it glows brightly. These narrowly-focused jets can be trillions of miles long and can travel at 500,000 miles per hour. These jets may be focused by the star's magnetic field. Later, the Sun stabilizes and becomes a yellow dwarf, a main sequence star which will remain in this state for about 10 billion years. After that, the hydrogen fuel is depleted and the Sun begins to die. The basic premise in the understanding of our origins, and the properties of all the planets we have studied this term, is that natural forces created and shaped the Solar System. And that there is a continuity to that process, i.e. it is not a sequence of random events. Any model or theory for the formation of the Solar System must have a set of explanations for large-scale and small-scale properties. Large-Scale (i) the planets are isolated in orderly intervals (ii) orbits are nearly circular (iii) orbits are in the same plane (iv) all planets revolve prograde Small-Scale (i) most planets rotate prograde (ii) the systems of moons can be divided into regular objects (spherical) with direct orbits versus irregular objects with eccentric orbits (iii) terrestrial planets have:  high densities  thin or no atmospheres  rotate slowly  rocky, poor in ices and H/He (iv) jovian worlds have  low densities  thick atmospheres  rotate rapidly  many moons  fluid interiors, rich in ices, H/He (iv) most of outer SS objects (not just jovian worlds) are ice-rich Also note that the overall architecture of our Solar System is orderly and the ages of its members uniform. All indicators point to a single formation event about 4.6 billion years ago. The above is not to ignore the fact that a great deal of evolution occurred in the Solar System after it formed. For example, the origin secondary atmospheres of the terrestrial worlds underwent a large amount of chemical processing (Venus was baked, Mars was frozen, Earth developed ). There was also orbital evolution as well, rings were formed, moons captured, tidal locking between worlds (e.g. Pluto and Charon). So the Solar System is not a static system, it is dynamic. However, there are two major problems for a theory of this type. One is that hot gas expands, not contracts. So lumps of hot gas would not form planets. The second is that encounters between stars are extremely rare, so rare as to be improbable in the lifetime of the Universe (15 billion years).

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