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ISU ENVI 360 - Chapter 11 Meteors, Asteroids and Comets

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Chapter 11Asteroids and CometsLeftovers of the Solar SystemMeteors and MeteoritesHeating of MeteorsMeteoritesClassificationSlide 8ChondrulesCarbonaceous ChondritesAsteroidsThe Asteroid BeltSize and Shape of AsteroidsSlide 14Asteroid CompositionOrigin of the AsteroidsSlide 17Asteroid Belt StructureApollo AsteroidsCometsStructure of CometsSlide 22Composition of CometsSpacecraft Exploration of CometsOrigin of CometsThe Oort CloudSlide 27The Comet’s TailTwo TailsA Comet’s JourneyShort Period CometsOrigin of Short-Period CometsComets and Meteor ShowersSlide 34Slide 35Slide 36Giant ImpactsGiant Meteor CratersOther Meteor CratersMass Extinction and ImpactsSlide 41Slide 42Slide 43Slide 44Chapter 11Meteors, Asteroids and CometsCopyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Asteroids and Comets•Orbiting the Sun are numerous small bodies – the asteroids and comets–Asteroids are generally rocky objects in the inner Solar System–Comets are icy bodies and spend most of their time in the outer Solar SystemLeftovers of the Solar System•Asteroids and comets are remnants of the formation of the Solar System–Some may be planetesimals–Best source of information about the Solar System’s early years•Asteroids and comets play a central role in planetary impact and in particular can have a large influence on Earth’s biological lifeMeteors and Meteorites•A “shooting star”, that streak of light that appears in the night sky for a fraction of a second, is a meteor•A meteor is the glowing trail of hot gas and vaporized debris left by a solid object heated by friction at it moves through the Earth’s atmosphere (generally, at the upper fringes)•If the solid body is in space, it is called a meteoroidHeating of Meteors•Heated to thousands of degrees Kelvin, meteors convert their kinetic energy into heating the meteor and air molecules•Meteoroids larger than a few centimeters sometimes are visible in daylight as “fireballs”Meteorites•Hundreds of tons of meteoritic material hit Earth each day•Best time to observe meteors is midnight to dawn•Most meteors are too small to reach the Earth’s surface – those that do are called meteoritesClassification•Meteorites are classified into three broad categories based on their composition: iron, stony, and stony-iron–Stony meteorites are composed mainly of silicate compounds–Iron meteorites are mostly metalsClassification•Most stony meteorites include smaller rounded chunks of rocky material called chondrules – these meteorites are called chondritic meteorites•In some chondritic meteorites, the chondrules are embedded in a black, carbon-rich, coal-like substance and are called carbonaceous chondritesChondrules•Chondrules appear to have been rapidly melted and cooled in the solar nebula•Radioactive material in chondrules allows dating back to when they first condensed from the solar nebula•Some chondrules contain ancient dust grains that have survived from before the Solar System’s birth!Carbonaceous Chondrites•The carbonaceous matter contains organic compounds, including amino acids•Raw material of life can form in space and was available from the start of the Solar SystemAsteroids•Asteroids are small, generally rocky bodies that orbit Sun•Most asteroids (thousands) lie in the asteroid belt, a region between the orbits of Mars and Jupiter•The first asteroid (Ceres) of this asteroid belt swarm was discovered as a result of a search for the “missing planet” of Bode’s law•The combined mass of all the asteroids is probably less than 1/1000 the mass of the EarthThe Asteroid BeltSize and Shape of Asteroids•Asteroids are small, so their sizes are best determined from infrared measurements: bigger bodies emit more IR than smaller ones at the same temperature•Asteroids range in size from 1000 km across (Ceres) down to kilometer-sized objects and even smallerSize and Shape of Asteroids•Most asteroids are irregularly shaped as determined from spacecraft images and their brightness fluctuations seen in telescopesAsteroid Composition•Reflection spectra show that asteroids belong to three main compositional groups: carbonaceous bodies, silicate bodies, and metallic iron-nickel bodies•Inner-belt asteroids tend to be silicate-rich and outer-belt asteroids tend to be carbon-rich•Some asteroids are loose lumps of material held together by gravityOrigin of the Asteroids•From their composition, size, and location, asteroids support the solar nebula hypothesis and are thought to be fragments of planetesimals•For this connection to be established, differentiation needed to occur in large asteroids•Fragmentation of these early large asteroids (planetesimals) through collisions created the stony and iron asteroids we see today•Asteroid belt is the result of Jupiter disturbing the accretion process in that zone and preventing a planet from formingOrigin of the AsteroidsAsteroid Belt Structure•Regions of the asteroid belt seemingly empty of asteroids are called Kirkwood Gaps–The gaps are caused by the same resonance process that causes the gaps in Saturn’s rings•Trojan asteroids are two loose swarms located along Jupiter’s orbit, 60° ahead and 60° behindApollo Asteroids•Orbits of Apollo Asteroids carry them into the inner Solar System and across the Earth’s orbit–More than 5000 have been found, which represents an Earth collision probability of once every 10,000 years–They may be “dead” comets, shifted into their orbits by Jupiter and devoid of surface ice from repeated close trips around the SunComets•Comets offer a stunning sight•Light pollution from cities distracts this view•Historically, comets held in fear and reverenceStructure of Comets•Tail - Narrow column of gas and dust, it may stretch over 100 million kilometers•Coma – Extremely rarified gaseous atmosphere that may reach a diameter of 100,000 km•Nucleus – A “dirty snowball” roughly 10 km across and containing most of the comet’s mass–Giotto spacecraft to Comet Halley determined a nucleus density of about 0.2 g/cm3 indicating that comets are “fluffy” as opposed to compacted icy material–Nucleus is odd shaped, extremely dark (dust and carbon-rich material), and emits gas in jetsStructure of CometsComposition of Comets•Spectra of coma and tail shows comets are rich in water, CO2, CO, and small amounts of other


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ISU ENVI 360 - Chapter 11 Meteors, Asteroids and Comets

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