Chapter 12: Chapter 12:

Remnants ofRemnants ofRock & IceRock & Ice

Asteroids, Comets & PlutoAsteroids, Comets & Pluto

Small Bodies in the Solar SystemSmall Bodies in the Solar System• Small bodies, the leftover Small bodies, the leftover ““scrapsscraps”” from the from the

formation of the Solar System, fall into three formation of the Solar System, fall into three distinct groups:distinct groups:asteroidsasteroids

• rocky or metallic in compositionrocky or metallic in composition• most are located between the orbits of Mars and Jupitermost are located between the orbits of Mars and Jupiter

Kuiper belt comets• made mostly of ice• orbit the Sun beyond

Neptune• orbit in same direction

and plane as the planets

Oort cloud comets• made mostly of ice• orbit at the outer fringe

of the Solar System• spherically distributed

about the Sun

Small Bodies in the Solar System

A Note on NamesA Note on Names

asteroidasteroid – a rocky leftover planetesimal – a rocky leftover planetesimal orbiting the Sunorbiting the Sun

cometcomet – an icy leftover planetesimal orbiting – an icy leftover planetesimal orbiting the Sun, regardless of its size or whether it the Sun, regardless of its size or whether it has a tailhas a tail

meteormeteor – a flash of light in the sky caused by – a flash of light in the sky caused by a particle entering the atmosphere, a particle entering the atmosphere, regardless of its originregardless of its origin

meteoritemeteorite – any piece of rock that fell to the – any piece of rock that fell to the ground from space, regardless of its originground from space, regardless of its origin

Properties of AsteroidsProperties of Asteroids

• They are small in size.They are small in size.• the largest one, the largest one, CeresCeres, is , is

only 1,000 km acrossonly 1,000 km across

► They are They are notnot spherical in spherical in shape.shape.• shaped more like shaped more like

““potatoespotatoes””• gravity not strong enough gravity not strong enough

to compress rocky to compress rocky materialmaterial

• odd shapes imply that odd shapes imply that some are fragments from some are fragments from asteroid collisions asteroid collisions

Properties of AsteroidsProperties of AsteroidsAsteroid orbits are more Asteroid orbits are more

elliptical & inclined elliptical & inclined than planetary orbits.than planetary orbits.

Most asteroids are Most asteroids are located in the located in the asteroid asteroid beltbelt..• between the orbits of between the orbits of

Mars & JupiterMars & Jupiter

Some share JupiterSome share Jupiter’’s s orbit.orbit.• two swarms at 60two swarms at 60º in º in

front of and behind front of and behind JupiterJupiter

• known as known as Trojan Trojan asteroidsasteroids

A few cross EarthA few cross Earth’’s orbit.s orbit.• they are called they are called Near-Near-

Earth asteroidsEarth asteroids

The Asteroid BeltThe Asteroid Belt

JupiterJupiter’’s gravity s gravity disrupted the orbits disrupted the orbits of those asteroids of those asteroids whose periods were whose periods were an integer fraction an integer fraction of Jupiterof Jupiter’’s.s.• orbital resonances orbital resonances

created gaps… like created gaps… like in Saturnin Saturn’’s ringss rings

• known as the known as the Kirkwood gapsKirkwood gaps

This explains why no planet formed in the asteroid belt.This explains why no planet formed in the asteroid belt.• tugs from Jupitertugs from Jupiter’’s gravity prevented the planetesimals from s gravity prevented the planetesimals from

accreting into a planetaccreting into a planet

Measuring Asteroid PropertiesMeasuring Asteroid PropertiesSizeSize

• the larger the asteroid, the more sunlight it will reflectthe larger the asteroid, the more sunlight it will reflect• measuring the brightness and knowing reflectivity & measuring the brightness and knowing reflectivity &

distance gives us the size.distance gives us the size.• reflectivity is calculated from the visual & IR brightnessreflectivity is calculated from the visual & IR brightness

MassMass• measure the effect gravity measure the effect gravity

has on a passing spacecraft has on a passing spacecraft or a moonor a moon

• use Kepleruse Kepler’’s & Newtons & Newton’’s lawss laws

DensityDensity• calculate from mass & sizecalculate from mass & size• mass mass volume volume

Measuring Asteroid PropertiesMeasuring Asteroid Properties

ShapesShapes• measure the asteroidmeasure the asteroid’’s changes in brightness as it s changes in brightness as it

rotatesrotates• the asteroidthe asteroid’’s shape can be reconstructed from thiss shape can be reconstructed from this• we can bounce radar signals off of Near-Earth asteroidswe can bounce radar signals off of Near-Earth asteroids

Measuring Asteroid PropertiesMeasuring Asteroid Properties

CompositionComposition• examine the spectrum of sunlight reflected off examine the spectrum of sunlight reflected off

the asteroidthe asteroid• look for non-Solar absorption lines in the look for non-Solar absorption lines in the

spectrumspectrum

Three categories of asteroid composition:Three categories of asteroid composition:1.1. very dark asteroids which contain Carbon-rich very dark asteroids which contain Carbon-rich

materialsmaterials► found in outer regions of the asteroid beltfound in outer regions of the asteroid belt

2.2. brighter asteroids which contain rocky materialsbrighter asteroids which contain rocky materials► found in inner regions of the asteroid beltfound in inner regions of the asteroid belt

3.3. asteroids which contain metals such as Ironasteroids which contain metals such as Iron

Rocks Falling from the SkyRocks Falling from the Sky

meteormeteor – a flash of light caused by a particle – a flash of light caused by a particle which enters Earthwhich enters Earth’’s atmosphere.s atmosphere.• most of these particles are the size of a peamost of these particles are the size of a pea• they completely burn up in Earththey completely burn up in Earth’’s atmospheres atmosphere

meteoritemeteorite – a rock which is large enough to – a rock which is large enough to have survived its fall to Earthhave survived its fall to Earth• they caused a brighter meteor…sometimes called a they caused a brighter meteor…sometimes called a

fireballfireball

► How can you tell that you have a meteorite?How can you tell that you have a meteorite? they have a higher metal content than terrestrial they have a higher metal content than terrestrial

rocksrocks they contain Iridium and other isotopes not found in they contain Iridium and other isotopes not found in

terrestrial rocksterrestrial rocks

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Meteorite ImpactMeteorite Impact

Types of MeteoritesTypes of Meteorites

► primitiveprimitive• about 4.6 billion years oldabout 4.6 billion years old• accreted in the Solar nebulaaccreted in the Solar nebula

► processedprocessed• younger than 4.6 billion younger than 4.6 billion

yearsyears• matter has differentiated matter has differentiated • fragments of a larger object fragments of a larger object

which processed the which processed the original Solar nebula original Solar nebula materialmaterial

► Each type of meteorite can be divided into two subcategories:Each type of meteorite can be divided into two subcategories:• primitive meteorites can be either primitive meteorites can be either stonystony, containing rocky minerals & , containing rocky minerals &

metals, or metals, or Carbon-richCarbon-rich, containing Carbon compounds or even water, containing Carbon compounds or even water• processed meteorites can be either processed meteorites can be either metallicmetallic, high-density Iron/Nickel , high-density Iron/Nickel

like Earthlike Earth’’s core, or s core, or rockyrocky, containing low-density material similar to , containing low-density material similar to earthearth’’s crusts crust

Based on composition, meteorites fall into two basic categories:

Origin of MeteoritesOrigin of Meteorites

Primitive meteorites condensed and accreted Primitive meteorites condensed and accreted directly from the Solar nebula.directly from the Solar nebula.• the stony ones formed closer than 3 AU from the the stony ones formed closer than 3 AU from the

SunSun• the Carbon-rich ones formed beyond 3 AU from the Carbon-rich ones formed beyond 3 AU from

the Sun, where it was cold enough for Carbon the Sun, where it was cold enough for Carbon compounds to condensecompounds to condense

Processed meteorites come from large objects Processed meteorites come from large objects in the inner Solar System.in the inner Solar System.• the metallic ones are fragments of the cores of the metallic ones are fragments of the cores of

asteroids which were shattered in collisionsasteroids which were shattered in collisions• the rocky ones were chipped off the surfaces of the rocky ones were chipped off the surfaces of

asteroids, Mars, and the Moon by impacts asteroids, Mars, and the Moon by impacts

CometsComets► One of the most beautiful sights in the sky.One of the most beautiful sights in the sky.► Throughout human history, these Throughout human history, these ““hairyhairy”” stars would stars would

appear.appear.• like planets, they moved with respect to the fixed starslike planets, they moved with respect to the fixed stars• unlike planets, they were not confined to the ecliptic and unlike planets, they were not confined to the ecliptic and

disappeared after several weeksdisappeared after several weeks► They were taken as omens of good or bad fortune. They were taken as omens of good or bad fortune.

Recent Recent Comets:Comets:

► 1986 Halley1986 Halley’’s s CometComet

► 1996 Comet 1996 Comet HyakutakeHyakutake

► 1997 Comet 1997 Comet Hale-BoppHale-Bopp

► Dozens per year Dozens per year too dim to be too dim to be seen by eyeseen by eye

Hale-Bopp

Hyakutake

CometsComets► Edmund HalleyEdmund Halley (1656 – 1742) (1656 – 1742)

• first to realize that comets orbit the Sunfirst to realize that comets orbit the Sun• predicted the return of a comet which had predicted the return of a comet which had

been seen every 76 yearsbeen seen every 76 years• the comet returned in 1758 and now bears the comet returned in 1758 and now bears

his name his name ► The Orbits of Comets:The Orbits of Comets:

Composition and Structure of Composition and Structure of CometsComets

► Comets are Comets are ““dirty snowballsdirty snowballs””…ice mixed with rock and …ice mixed with rock and dust.dust.• ices are Hices are H22O, COO, CO22, CO, NH, CO, NH33, CH, CH44 nucleusnucleus

• the the ““dirty snowballdirty snowball””• how the comet how the comet

appears far from the appears far from the SunSun

comacoma• surrounds nucleus surrounds nucleus

when near the Sunwhen near the Sun• sublimated gas & dustsublimated gas & dust

plasma tailplasma tail• ionized gas swept back ionized gas swept back

by Solar windby Solar wind

dust taildust tail• dust particles swept dust particles swept

back more slowly by back more slowly by radiation radiation

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Nucleus of CometNucleus of Comet

►A A ""dirty dirty snowballsnowball""

►Source of Source of material for material for cometcomet''s tails tail

A CometA Comet’’s Journeys Journey

A CometA Comet’’s Journeys Journey► The SOHO telescope The SOHO telescope

observed Comet NEAT observed Comet NEAT (C/2002 V1) round the (C/2002 V1) round the Sun on Feb 16, 2002.Sun on Feb 16, 2002.• courtesy of SOHO/LASCO courtesy of SOHO/LASCO

consortium. SOHO is a consortium. SOHO is a project of ESA and NASA.project of ESA and NASA.

► A comet can only visit the A comet can only visit the Sun a few hundred times Sun a few hundred times before losing all its ice to before losing all its ice to sublimation.sublimation.• the comet may then the comet may then

disintegratedisintegrate• or the rocky remains may or the rocky remains may

stick together as an asteroidstick together as an asteroid

http://sohowww.nascom.nasa.gov/

The Origin of CometsThe Origin of Comets

► We can tell where comets We can tell where comets originate by measuring originate by measuring their orbits as they visit their orbits as they visit the Sun.the Sun.

► Most approach from Most approach from random directions and do random directions and do not orbit in the same not orbit in the same sense as the planets.sense as the planets.• they come from the Oort they come from the Oort

cloud 50,000 AU distantcloud 50,000 AU distant► Others orbit along the Others orbit along the

ecliptic plane in the same ecliptic plane in the same sense as the planets.sense as the planets.• they come from the they come from the

Kuiper belt 30-100 AU Kuiper belt 30-100 AU distantdistant

Pluto and NeptunePluto and Neptune► Pluto has an eccentric and inclined orbit.Pluto has an eccentric and inclined orbit.► For 20 of its 248-year orbital period, it is actually For 20 of its 248-year orbital period, it is actually

closer to the Sun than Neptune.closer to the Sun than Neptune.• such was the case between 1979 and 1999such was the case between 1979 and 1999

► An orbital resonance between the two planets keeps An orbital resonance between the two planets keeps them from ever colliding!them from ever colliding!

Resonance

Neptune completes three orbits for every two orbits that Pluto makes.

Pluto and CharonPluto and Charon► PlutoPluto’’s moon, named Charon, was s moon, named Charon, was

discovered in 1978.discovered in 1978.• it orbits Pluto every 6.4 daysit orbits Pluto every 6.4 days

► This allows us to measure the This allows us to measure the mass of Pluto using Keplermass of Pluto using Kepler’’s Law s Law #3.#3.

► Eclipses allow us to measure the Eclipses allow us to measure the diameters of both Pluto & Charon.diameters of both Pluto & Charon.

► PlutoPluto’’s density (2 gm/cms density (2 gm/cm33) is larger ) is larger than expected for an icy world.than expected for an icy world.• Charon is less dense (1.6 gm/cmCharon is less dense (1.6 gm/cm33))

► Explanation similar to Earth/MoonExplanation similar to Earth/Moon• Charon formed from large impact Charon formed from large impact • Pluto lost lower density outer layersPluto lost lower density outer layers

► Reconstructed image of PlutoReconstructed image of Pluto

Hubble ST image of Pluto & Charon

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Hubble's View of Pluto and Its Hubble's View of Pluto and Its MoonsMoons

Planet or Kuiper Belt Comet?Planet or Kuiper Belt Comet?

► The classification of Pluto has recently come into question.The classification of Pluto has recently come into question.► Pluto has many properties in common with Kuiper belt Pluto has many properties in common with Kuiper belt

comets.comets.• it orbits in the vicinity of the Kuiper beltit orbits in the vicinity of the Kuiper belt• several Kuiper belt comets have orbital resonances with Neptuneseveral Kuiper belt comets have orbital resonances with Neptune• its composition of ice and rock is similar to cometsits composition of ice and rock is similar to comets• it has an atmosphere of Nitrogen which sublimes when Pluto is it has an atmosphere of Nitrogen which sublimes when Pluto is

closest to the Sunclosest to the Sun• some Kuiper belt comets have moonssome Kuiper belt comets have moons

► Pluto has some properties which differ from Kuiper belt Pluto has some properties which differ from Kuiper belt comets.comets.• its surface is much brighter; presumable because the Nitrogen its surface is much brighter; presumable because the Nitrogen

atmosphere refreezes on the surface rather than escapingatmosphere refreezes on the surface rather than escaping• it is much larger than most Kuiper belt cometsit is much larger than most Kuiper belt comets

► But…it is smaller than Triton, which presumably once roamed But…it is smaller than Triton, which presumably once roamed the Kuiper belt!the Kuiper belt!

► This is why Pluto is part of a new class of Dwarf PlanetsThis is why Pluto is part of a new class of Dwarf Planets

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How big can a comet be?How big can a comet be?

Comet Shoemaker-Levy 9Comet Shoemaker-Levy 9► This comet was discovered in orbit about Jupiter in 1992.This comet was discovered in orbit about Jupiter in 1992.

• a previous encounter with Jupiter broke the nucleus into a string of a previous encounter with Jupiter broke the nucleus into a string of fragmentsfragments

• the comet was on a collision course with Jupiterthe comet was on a collision course with Jupiter

Something Something similar had similar had happened to happened to Callisto.Callisto.

This crater This crater chain is chain is evidence evidence that a string that a string of nuclei of nuclei once once impacted it.impacted it.

Comet Shoemaker-Levy 9Comet Shoemaker-Levy 9► One by one, each fragment collided with Jupiter in July 1994.One by one, each fragment collided with Jupiter in July 1994.

• infrared cameras observed hot plumes ejected from the planetinfrared cameras observed hot plumes ejected from the planet• material from deep inside Jupiter was ejected, and fell… left dark material from deep inside Jupiter was ejected, and fell… left dark

spotsspots► Such impacts probably occur on Jupiter once every 1,000 years.Such impacts probably occur on Jupiter once every 1,000 years.

This was a reminder to us that impacts still occur in the present!!

Meteor ShowersMeteor Showers► Earth is impacted by an estimated 25 million Earth is impacted by an estimated 25 million

small particles each day which cause meteors. small particles each day which cause meteors. ► When the Earth passes through the trail of a When the Earth passes through the trail of a

comet, the number of particles impacting the comet, the number of particles impacting the EarthEarth’’s atmosphere increases.s atmosphere increases.

► We call this a We call this a meteor showermeteor shower..► You can see upward of 1 meteor per minute You can see upward of 1 meteor per minute

from one location.from one location.► Showers occur on the same dates each year, Showers occur on the same dates each year,

corresponding to when the Earth crosses a corresponding to when the Earth crosses a given cometgiven comet’’s orbit.s orbit.

► The meteors appear to emanate from one The meteors appear to emanate from one point in the sky.point in the sky.

Meteor ShowersMeteor Showers

Meteors appear to shoot from the point directly ahead in the direction that the Earth is moving.

Meteor ShowersMeteor ShowersMeteor showers are named after the constellation from Meteor showers are named after the constellation from

which the meteors appear to emanatewhich the meteors appear to emanate i.e., the constellation which lies in the direction of the Earthi.e., the constellation which lies in the direction of the Earth ’’s s

motion.motion.

Impacts and Mass Extinctions on Impacts and Mass Extinctions on EarthEarth

► We know that larger objects have We know that larger objects have impacted Earthimpacted Earth• Meteor Crater in northern ArizonaMeteor Crater in northern Arizona• caused by a 50-meter asteroidcaused by a 50-meter asteroid• impact occurred 50,000 years agoimpact occurred 50,000 years ago

► 65 million years ago, many 65 million years ago, many species, including dinosaurs, species, including dinosaurs, disappeared from earthdisappeared from earth

► Sedimentary rock layer from Sedimentary rock layer from that time shows:that time shows:• Iridium, Osmium, PlatinumIridium, Osmium, Platinum• grains of grains of ““shocked quartzshocked quartz””• spherical rock dropletsspherical rock droplets• soot from forest firessoot from forest fires

Impacts and Mass Extinctions on Impacts and Mass Extinctions on EarthEarth

► Elements like Iridium, rare on Earth, are found in meteorites.Elements like Iridium, rare on Earth, are found in meteorites.► Shocked quartz, found at Meteor Crater, forms in impacts.Shocked quartz, found at Meteor Crater, forms in impacts.► Rock droplets would form from molten rock Rock droplets would form from molten rock ““rain.rain.””► Forest fires would ensue from this hot rain.Forest fires would ensue from this hot rain.► All this evidence would imply that Earth was struck by an All this evidence would imply that Earth was struck by an

asteroid 65 million years ago.asteroid 65 million years ago.

► In 1991, a 65 million year In 1991, a 65 million year old impact crater was old impact crater was found on the coast of found on the coast of Mexico.Mexico.• 200 km in diameter200 km in diameter• implies an asteroid size of implies an asteroid size of

about 10 km acrossabout 10 km across• called the called the Chicxulub craterChicxulub crater

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Dinosaur fossils in lower rock layers

No dinosaur fossils in upper rock layers

Thin layer containing the rare element iridium

Iridium LayerIridium Layer

Impacts and Mass Extinctions on Impacts and Mass Extinctions on EarthEarth

► We have a plausible scenario of how the impact led to mass We have a plausible scenario of how the impact led to mass extinction.extinction.• debris in atmosphere blocks sunlight; plant die…animals starvedebris in atmosphere blocks sunlight; plant die…animals starve• poisonous gases form in atmospherepoisonous gases form in atmosphere

Could it happen again?Could it happen again?

► This chart shows how frequently objects of various sizes This chart shows how frequently objects of various sizes will impact Earth.will impact Earth.

► The odds of a large impact are small … but not zero!The odds of a large impact are small … but not zero!

Influence of the Jovian PlanetsInfluence of the Jovian Planets

► Gravity of a jovian planet (especially Jupiter) can redirect a comet.Gravity of a jovian planet (especially Jupiter) can redirect a comet.► Jupiter has directed some comets toward Earth but has ejected many more into the Jupiter has directed some comets toward Earth but has ejected many more into the

Oort cloud.Oort cloud.

PAST FUTURE ?

http://impact.arc.nasa.gov

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