comets & primitive bodies – introduction · comet – km-sized bodies ... galactic tides...
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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
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
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