Comets & PrimitiveComets & PrimitiveBodies Bodies –– Introduction Introduction

Karen Karen MeechMeech,,Session # 22 Session # 22 –– Mon 1/17/05 Mon 1/17/05

Overview Overview –– Solar SolarSystem IcesSystem Ices

Water in many formsWater in many forms Material delivery toMaterial delivery to

terrestrial planetsterrestrial planets Chemical reactionsChemical reactions SS & climate history recordsSS & climate history records Potential life environmentPotential life environment

DefinitionsDefinitions CometComet –– km-sized bodies with km-sized bodies withvolatiles & volatiles & refractoriesrefractories

AsteroidAsteroid –– small planetary bodies small planetary bodiesorbiting the sunorbiting the sun

MeteoroidMeteoroid –– small (<< km) small (<< km)extraterrestrial body orbiting sunextraterrestrial body orbiting sun

MeteorMeteor –– meteoroid passing meteoroid passingthrough Earth through Earth atmatm

MeteoriteMeteorite –– meteoroid which hits meteoroid which hitsthe groundthe ground

FireballFireball –– very bright meteor very bright meteor FindFind –– meteorite found on ground, meteorite found on ground,

not associated with a fallnot associated with a fall Parent bodyParent body –– comet or asteroid- comet or asteroid-

like body where meteoroid formedlike body where meteoroid formed

Archaeological RemnantsArchaeological Remnants

Icy debris left fromIcy debris left fromformationformation

Keys to chemistry &Keys to chemistry &physics in nebulaphysics in nebula

Preservation of inter-Preservation of inter-stellar material?stellar material?

Sources of organicsSources of organics necessary for life necessary for life

Comets Inspire TerrorComets Inspire Terror

Sudden appearance in skySudden appearance in sky Only a few bright naked-eye comets / centuryOnly a few bright naked-eye comets / century Tail physically large Tail physically large millions of km millions of km Early composition: toxic chemicalsEarly composition: toxic chemicals

Historical HighlightsHistorical Highlights10661066 Halley Halley Wm conquerorWm conqueror1456 1456 Halley Halley ExcommunicatedExcommunicated1531 1531 HalleyHalley ObsObs by by KeplerKepler1744 1744 De De CheseauxCheseaux 6 tails6 tails1858 1858 DonatiDonati Most beautifulMost beautiful18111811 FlaugergeusFlaugergeus comet winecomet wine18611861 TebbuttTebbutt Naked eye, Naked eye, auroraeaurorae19011901 Great SGreat S Daytime visibilityDaytime visibility

Comet of 1577Comet of 1577

HistoricalHistoricalUnderstandingUnderstanding

TychoTycho BraheBrahe 1577 1577 Parallax Parallax –– outside outside atmatm..

Edmund HalleyEdmund Halley 1531, 1607, 16811531, 1607, 1681 Orbit determinationOrbit determination Newton Newton –– Principia Principia

19501950’’s s –– Models Models Whipple Whipple ‘‘Dirty SnowballDirty Snowball’’ LyttletonLyttleton ‘‘SandbankSandbank’’

The Whipple ModelThe Whipple Model Sublimation from a jet on aSublimation from a jet on a

rotating nucleusrotating nucleus Exerts a torqueExerts a torque Delays / accelerates arrival at qDelays / accelerates arrival at q da/dtda/dt = 2a = 2a22v / v / µµ [ v . a [ v . ad d ]] E = - E = - µµ / 2a / 2a If aIf add and v have same and v have same

direction, a increases (so doesdirection, a increases (so doesE) E) arrival at q delayed arrival at q delayed

Model works because of lowModel works because of lowthermal conductivity thermal conductivity lag in lag inoutgassingoutgassing..Direction of orbitDirection of orbit

Direction of rotation axisDirection of rotation axis

[a][a]

[b][b]

Retrograde rotationRetrograde rotation

ProgradePrograde rotation rotation

The The OortOort Cloud Cloud 1717thth century physics: century physics:

BraheBrahe, , KeplerKepler & Newton & Newton EEorbitorbit = - = -µµ/2a/2a

Distribution of 1/aDistribution of 1/aoriginaloriginal 22 long-period comets22 long-period comets Strongly peaked Strongly peaked

Source 50,000-150,000 AUSource 50,000-150,000 AU Contains 10Contains 101111 comets comets

Oort, J. (1950) B.A.N. 408, 91-110.Oort J. H. & M. Schmidt (1951) B.A.N. 419,259-270

OortOort Cloud Issues Cloud Issues

CompositionComposition ΔΔvvTOTTOT = 100 = 100 m/sm/s ~ ~ vvcirccirc at at

50,000 AU50,000 AU Random perturbations Random perturbations

randomize randomize OortOort orbits orbits Width of peak of incomingWidth of peak of incoming

comets too narrowcomets too narrow Proposed Fading?Proposed Fading?

Comet showers, impacts &Comet showers, impacts &extinctionsextinctions

Galactic tides cause periodicGalactic tides cause periodicOortOort cloud perturbations cloud perturbations

Correlation w/ largeCorrelation w/ largeimpacts? No.impacts? No.

MateseMatese et al (2001), in et al (2001), in CollisionalCollisional Proc. Proc. In the Solar System, In the Solar System, MarovMarov & Rickman, & Rickman,EdsEds, , VolVol 261, 91. 261, 91.

The Modern The Modern OortOort Cloud Cloud

Outer Outer OortOort Cloud 15,000-10 Cloud 15,000-1055 AU AU Stellar perturbations > 10Stellar perturbations > 1044 AU AU

Inner Inner OortOort Cloud 2000-15,000 Cloud 2000-15,000 Galactic TidesGalactic Tides

Dynamically inert 50-2000 AUDynamically inert 50-2000 AU KuiperKuiper Belt 35-50 AU Belt 35-50 AU

Stable, dynamically activeStable, dynamically active Classical, 3:2, scatteredClassical, 3:2, scattered

Dynamically newDynamically new 1/a1/aorigorig < 100x10 < 100x10-6-6 AU AU-1-1

Long PeriodLong Period P > 200 yrP > 200 yr Short Period P < 200 yrShort Period P < 200 yr

Halley family Halley family –– OortOort cloud origin cloud origin Jupiter family Jupiter family –– KBO origin KBO origin

Centaurs transition objectsCentaurs transition objects

Other Other OortOort Clouds? Clouds? IRC+10216IRC+10216 1.5-4M1.5-4Msunsun

L ~ 3000LL ~ 3000Lsunsun

Radius ~ 5AURadius ~ 5AU

C-rich star, so O should beC-rich star, so O should bebound as CObound as CO

Large HLarge H22O signature: few xO signature: few x101099 OortOort comets vaporizing comets vaporizing

Modern Comet Modern Comet –– Terminology Terminology

NucleusNucleus Solid body, few kmSolid body, few km HH22O ice + dust + otherO ice + dust + other

volatiles: CO, COvolatiles: CO, CO22organicsorganics

ComaComa Gas & dustGas & dust 101066 km km

TailsTails Ion tail (blue: CO, CN)Ion tail (blue: CO, CN) Dust tail (yellow)Dust tail (yellow)

Comet Nucleus SizesComet Nucleus Sizes HST: 5 comets (35 orbits), Keck: 21HST: 5 comets (35 orbits), Keck: 21

cometscomets Median RMedian RNN = 1.6 km = 1.6 km Differential Differential powerlawpowerlaw

aa-3.5-3.5, truncated between 0.3-2.0 km, truncated between 0.3-2.0 km Matches outer satellite Matches outer satellite crateringcratering record record

((ZahnleZahnle; Phillips, 2001); Phillips, 2001) Small comets more prone to break upSmall comets more prone to break up

((SamarasinhaSamarasinha, 2001), 2001)

Physical ProcessesPhysical Processes Sublimation of gasesSublimation of gases Drags dust from nucleusDrags dust from nucleus

Gravity lowGravity low Most dust escapesMost dust escapes Solar radiation pressureSolar radiation pressure coma coma dust tail dust tail

photodissociationphotodissociation

Ionization Ionization gas tail gas tail Energy BalanceEnergy Balance

Sunlight Sunlight Scattered light + Heating/Sublimation + Conduction Scattered light + Heating/Sublimation + ConductionUsually very smallUsually very small

Energy needed depends on iceEnergy needed depends on iceInverse square law: 1/rInverse square law: 1/r22

Comet SpectraComet Spectra

Reflected sunlight fromReflected sunlight fromdust (blackbody radiation)dust (blackbody radiation)

Emitted Emitted ““heatheat”” FluorescenceFluorescence

Dust Coma DevelopmentDust Coma Development

q-620 dy; Afρ = 14 cm

q+590 dy; Afρ = 13 cmq+300 dy; Afρ = 83 cmq+210 dy; Afρ = 87 cm

q-150 dy; Afρ = 105 cmq-350 dy; Afρ = 31 cm

01/19/99 r=3.1AU

11/12/01 r=4.4 AU09/30/00 r=2.8 AU08/22/00 r=2.6 AU

04/06/98 r=4.3 AU 07/15/99 r=2.2 AU

Comet CompositionsComet Compositions

1.51.5HH22SS0.010.01NHNH22CHOCHO

0.030.03HCHC33NN0.10.1HNCOHNCO

0.020.02CHCH33CNCN0.050.05HCOOHHCOOH

0.20.2HCNHCN1-71-7CHCH33OHOH

0.60.6NHNH330.1-10.1-1HH22COCO

0.30.3CC22HH663-203-20COCO22

0.10.1CC22HH221-201-20COCO

0.60.6CHCH44100100HH22OO

Comet Tail FormationComet Tail Formation Dust TailDust Tail

Reflected sunlightReflected sunlight(yellow)(yellow)

Follows orbitFollows orbit

Plasma Tail Plasma Tail –– ions ions Caught by solar B fieldCaught by solar B field Anti-solarAnti-solar Fluorescing Fluorescing blue blue

(CO(CO++))

1P/Halley, 19101P/Halley, 1910

West, 1986West, 1986

Plasma Tail FormationPlasma Tail Formation

Solar magnetic field initially radialSolar magnetic field initially radial Wraps up as the sun rotatesWraps up as the sun rotates When the comet encounters field, charged particlesWhen the comet encounters field, charged particles

cannot cross field lines cannot cross field lines drape over comet drape over comet Plasma (ionized gas), must then follow the field linesPlasma (ionized gas), must then follow the field lines Sector boundary crossings Sector boundary crossings disconnection events disconnection events

Comet DensitiesComet Densities

Light curve brightening Light curve brightening activity activity Physical propertiesPhysical properties

RRNN ~ 90 km; nearly spherical ~ 90 km; nearly spherical Gravity should control comaGravity should control coma RRexpexp = r [M = r [MNN/M/MSSββ]]0.50.5

2060 Chiron2060 Chiron

Hubble Space Telescope ObservationsHubble Space Telescope Observations

ExopauseExopause detected detected

ρρ < 0.5 g cm < 0.5 g cm-2-2

RRexpexp = r [M = r [MNN/M/MSSββ]]0.50.5

ββ = ratio of solar radiation = ratio of solar radiationPressure to gravity (grainPressure to gravity (graindensity & scattering prop.)density & scattering prop.)

MMNN = nucleus mass = nucleus mass

MMss = solar mass = solar mass

HST ChironHST Chiron

Comet Tensile StrengthComet Tensile Strength

Iron meteoritesIron meteorites4 x 104 x 1088

Snow, snow packs (Snow, snow packs (ρρ ~ 0.1-0.5 g cm ~ 0.1-0.5 g cm33))1-20 x 101-20 x 1044

ChondritesChondrites6 x 106 x 1066

Shock strength Tunguska; Shock strength Tunguska; sungrazerssungrazers101088

Laboratory meas. Artificial nucleiLaboratory meas. Artificial nuclei101033 –– 10 1055

Ram pressure from Ram pressure from outgassingoutgassing101044

Comet rotation periodsComet rotation periods101022 –– 10 1044

SL9 Breakup at JupiterSL9 Breakup at Jupiter2.7 x 102.7 x 1022

GiottoGiotto –– fragmentation comet dust fragmentation comet dust101022-10-1033

Comet nucleus modelsComet nucleus models101033

Brooks 2 splittingBrooks 2 splitting4.3 x 104.3 x 1022

Tidal stresses on Tidal stresses on IkeyaIkeya Seki Seki101033

DraconidsDraconids meteor shower meteor shower1-5 x 101-5 x 1033

IDP IDP measurmentsmeasurments1-5 x 101-5 x 1033

TechniqueTechniqueStrength [Strength [NtNt m m-2-2]]

CometaryCometary Fates? Fates?

Surface turns off (clogged)Surface turns off (clogged) Rocky body with no iceRocky body with no ice Completely sublimatesCompletely sublimates Breaks upBreaks up CollisionCollision

Aging ProcessesAging Processes Build up of surface dustBuild up of surface dust

Lower Lower albedoalbedo Large grains cannot leaveLarge grains cannot leave Uneven surface Uneven surface jets jets Non gravitationalNon gravitational

accelerationacceleration

Maximum Dust SizesMaximum Dust Sizes Equation of motion of a dust grainEquation of motion of a dust grain mmgg a = -GMm a = -GMmgg/r/r22 + + FFdragdrag

Drag force is momentum per molecules x collisions/sDrag force is momentum per molecules x collisions/s aacritcrit = const Q = const Q vvthth / / ρρgg ρρnucnuc R R33

nucnuc

vvthth

mmggaa

Dust grainDust grain

Nucleus + gas flowNucleus + gas flow

Fireballs / DebrisFireballs / Debris

Radiation PressureRadiation Pressure Radiation pressure on dustRadiation pressure on dust

Force Force –– time rate change of time rate change ofmomentum (p = E/c)momentum (p = E/c)

dE/dtdE/dt = L = Lsunsun/(4 /(4 ππ r r22) [) [QQrprp ππ a a22]] FFrprp = = QQrprp a a22 LLsunsun / (4 c r / (4 c r22)) ββ = = FFrprp/F/Fgravgrav = 5.74 x 10 = 5.74 x 10-4-4 QQrprp//ρρaa

QQrprp obtained from obtained from MieMie scattering scattering Strongly peaked near 0.1 Strongly peaked near 0.1 µµmm Particles a < Particles a < λλ don don’’t interact wellt interact well Depends on Depends on albedoalbedo Depends on backscatteringDepends on backscattering

Most strongly effects sub- Most strongly effects sub- µµm tom toµµm sized dust (m sized dust (e.g.e.g. comets) comets)

SummarySummary Comets are primordialComets are primordial

remnants of SS formremnants of SS form Surfaces may be heavilySurfaces may be heavily

alteredaltered

Composed of HComposed of H22O ice,O ice,other volatiles, organics &other volatiles, organics &dustdust

““ActivityActivity”” driven by solar driven by solarradiationradiation

Comets reside in Comets reside in OortOortcloud and cloud and KuiperKuiper Belt Belt

Physical propertiesPhysical properties Few km radiusFew km radius Possibly low density, porousPossibly low density, porous Low heat conductivityLow heat conductivity Low tensile strengthLow tensile strength

Comet dustComet dust Meteor showersMeteor showers Zodiacal lightZodiacal light

OortOort Physics PhysicsPerturbations from starsPerturbations from stars d > r; m << d > r; m << MMstarstarGravGrav force felt by comet: force felt by comet: F = F = GMGMstarstar m / d m / d22

F = m a = m [ F = m a = m [ΔΔvv / / ΔΔtt]]Star is within a distance d forStar is within a distance d for ΔΔtt = 2 d / = 2 d / vvstarstar

Setting the forces equalSetting the forces equal ΔΔvv = 2 = 2 GMGMstarstar / / dvdvstarstar = 0.4 = 0.4 m/sm/sNo. of stars passed in SS age:No. of stars passed in SS age: dn/dtdn/dt = = ππdd22vvstarstarN = 2 x 10N = 2 x 1044

Adding Adding ΔΔvv randomly: randomly: ΔΔvvTOTTOT = N = N0.50.5 ΔΔvv = 100 = 100 m/sm/s

mmcometcomet

MMstarstar

dd

rr

vvstarstar

Comet ParadigmsComet Paradigms

““Comets are theComets are themost pristine thingsmost pristine thingsin the Solar Systemin the Solar System””

““Comets tell us aboutComets tell us aboutthe formation of thethe formation of theSolar SystemSolar System

Meteorites/MeteorsMeteorites/Meteors

Earth passes thru dustEarth passes thru dust Burns in Earth Burns in Earth atmatm

Larger chunks result inLarger chunks result inmeteorites frommeteorites from MoonMoon MarsMars AsteroidsAsteroids

PoyntingPoynting Robertson Drag Robertson Drag

Interaction of light withInteraction of light with~cm-sized dust~cm-sized dust

““AberrationAberration”” of light of light Particles in circular orbitsParticles in circular orbits Solar photons movingSolar photons moving

radiallyradially strike leading side strike leading side Photon momentum onPhoton momentum on

leading sideleading side Re-radiation causes netRe-radiation causes net

loss of E loss of E particle spirals particle spiralsin to sunin to sun