Ice Physics I:Ice Physics I:CometsComets

Karen Karen MeechMeechSession # 24 Session # 24 –– Mon 1/17/05 Mon 1/17/05

Temperature ScalesTemperature Scales

Kelvin Kelvin –– absolute scale absolute scale –– atomic motion atomic motion C = 5/9 (F C = 5/9 (F –– 32) 32) K = C + 273K = C + 273

100100373373273273KelvinKelvin

10010010010000CentigradeCentigrade1801802122123232FahrenheitFahrenheit

ΔΔTTHH22O BoilO BoilHH22O FreezeO FreezeScaleScale

Water Water –– Features Relevant to Ice Features Relevant to Ice

Bent molecule (104.52Bent molecule (104.52oo) ) –– H Bond H Bond Dipole momentDipole moment Tetrahedral Crystal structureTetrahedral Crystal structure Expands upon freezing (less dense)Expands upon freezing (less dense)

Negative slope on melting curveNegative slope on melting curve Le Le ChatlierChatlier Principle Principle

Triple Point [S.M.O.W.]Triple Point [S.M.O.W.] P = 611.657 Pa (0.006 bar)P = 611.657 Pa (0.006 bar) T = 273.16 K (0.16T = 273.16 K (0.16oo C) C) DD22O: 661 Pa, 276.82KO: 661 Pa, 276.82K

-0.02 0 0.02-0.02 0 0.02 Temperature [C] Temperature [C]

612612

611611

610610

613613

Pre

ssur

e [P

a]P

ress

ure

[Pa]

1 bar = 101 bar = 1055 Pa = 10 Pa = 1055 Nt/m Nt/m22

1 1 atmatm = 101325 Pa = 101325 Pa

Water Ice PhysicsWater Ice Physics Exists in 13 crystallineExists in 13 crystallinephases (diff T & P)phases (diff T & P) Phase I: P < 2700 Phase I: P < 2700 atmatm

IhIh –– hexagonal hexagonal IcIc –– cubic (low T, low P) cubic (low T, low P)

metastablemetastable

High P forms: II to XIHigh P forms: II to XI

Amorphous IceAmorphous Ice ClathratesClathrates

Amorphous IceAmorphous Ice Forms at low T Forms at low T insufficient E insufficient E

for crystal structurefor crystal structure Physical PropertiesPhysical Properties

Large voids: trapped gasesLarge voids: trapped gases Gases released between 35-120KGases released between 35-120K

AnnealingAnnealing 38-68K 38-68K –– transition from transition from IIaahh IIaall Beginning at 90K Beginning at 90K IcIc (exothermic) (exothermic)

Two formsTwo forms High Density, High Density, IIaahh Low Density, Low Density, IIaall

Ice Ice IIaall and and IIaahh formation formation

ClathrateClathrate Hydrates Hydrates Crystalline framework of H-bonded HCrystalline framework of H-bonded H22O O molecmolec..

trapping guest moleculestrapping guest molecules Guests donGuests don’’t affect the chemistryt affect the chemistry

Released upon sublimation of waterReleased upon sublimation of water Cages unstable without guestsCages unstable without guests Some physical properties are alteredSome physical properties are altered Some Some clathrateclathrate formation not possible at low T and P formation not possible at low T and P Stability: low T (< 273K), moderate to high P (100 Stability: low T (< 273K), moderate to high P (100 atmatm))

ImportanceImportance May store most of SS inventory: CO, COMay store most of SS inventory: CO, CO22, CH, CH44 Catastrophic destabilization due to T , PCatastrophic destabilization due to T , P

Collapse & flow featuresCollapse & flow features OutgassingOutgassing Greenhouse gas budgetsGreenhouse gas budgets

ClathrateClathrate Hydrates Hydrates–– Contd. Contd.

Type IType I 46 H46 H22O moleculesO molecules 2 small cages, 6 large2 small cages, 6 large 12- and 14- sided polygons12- and 14- sided polygons Traps in ratio 1/7Traps in ratio 1/7

Type IIType II 126 H126 H22O moleculesO molecules 16 small cages, 8 large16 small cages, 8 large 14- and 16- sided polygons14- and 16- sided polygons Traps in ratio 1/17Traps in ratio 1/17

ClathratesClathrates in the Solar System in the Solar System CHCH44 –– marine sediments marine sediments –– by 2x by 2x

exceeds other fossil fuelexceeds other fossil fuelsourcessources

COCO22 –– sequester CO sequester CO22 in ocean in oceanseafloor from seafloor from atmatm (climate) (climate)

Destabilization on Mars &Destabilization on Mars &EuropaEuropa –– chaotic terrain? chaotic terrain?

CHCH44 / N / N22 on Triton on Triton –– geysers geysers Air Air –– polar ice sheets polar ice sheets –– info on info on

atmosphere up to 10atmosphere up to 1066 yr ago yr ago

Ice Regimes Ice Regimes –– Interior Pressures Interior Pressures PPcc = 2 = 2ππGGρρ22 (R (R22 –– r r22)/3)/3

ppmppm

<100 <100 ppmppm

2:1?2:1?

2:1?2:1?

Dust:IceDust:Ice

IhIh, , clathratesclathrates300300331-1.61-1.6Mars IceMars Ice

High den. High den. amorphamorph––1010-10-100.10.1ISgrainsISgrains

IhIh, , clathratesclathrates300300330.920.92GlacierGlacier

IhIh, II, , II, clathratesclathrates1x101x1044135013502.072.07TritonTriton

IhIh, II, VII, VIII, , II, VII, VIII, clathclath3x103x1044156915692.972.97EuropaEuropa

IhIh, II, , II, clathclath, , amorphamorph3.5-14003.5-1400101022-10-10330.5-1?0.5-1?KBOKBO

IhIh, , IcIc, amorphous, amorphous0.0003-0.10.0003-0.11-101-100.5-1?0.5-1?CometComet

IcesIcesP [bar]P [bar]Size [km]Size [km]ρρ [g/cm [g/cm33]]RegimeRegime

Phase changes: volume changes Phase changes: volume changes fractures fractures Amorphous Amorphous IcIc, exothermic , exothermic volatile release volatile release ClathrateClathrate destabilization destabilization outgassingoutgassing

G = 6.67 x 10G = 6.67 x 10-11-11 Ntm Ntm22/kg/kg22

ρρ = density, kg/m = density, kg/m33

R = radius, mR = radius, m

Comet ExpectationsComet Expectations Amorphous iceAmorphous ice

Formation T: 30-100KFormation T: 30-100K Crystalline icesCrystalline ices

Solar heating: Solar heating: amorphousamorphouscrystallinecrystalline transition peaking at 137K transition peaking at 137K

Balance with cosmic ray processingBalance with cosmic ray processing Lack of Lack of ClathratesClathrates

Small size, low central pressuresSmall size, low central pressures Release of volatiles in excess ofRelease of volatiles in excess of

clathrateclathrate capacity capacity Not necessary to explain activity!Not necessary to explain activity!

Cosmic Solar System HistoryCosmic Solar System History

>4.6 >4.6 GyGyISM dark cloudISM dark cloud

PlanetesimalsPlanetesimals condense condensePlanets accretePlanets accreteForm few x100 million yearsForm few x100 million years

Earth in the HadeanEarth in the HadeanOceans & rocks formOceans & rocks form~4.4 ~4.4 GyGy ago ago

Late planetary bombardmentLate planetary bombardmentComets, asteroids bring water &Comets, asteroids bring water &Organics to EarthOrganics to Earth

The The ArcheanArchean Epoch EpochOldest life on EarthOldest life on Earth3.5-3.8 3.5-3.8 GyGy ago ago

Comet FormationComet Formation

100K100K

64K64K

31K31K

00 1010 100 AU100 AU

Low Temperature CondensationLow Temperature Condensation Ices in comets condensedIces in comets condensed

T< 100KT< 100K Amorphous formAmorphous form Trapped other gasesTrapped other gases Amounts depend on Amounts depend on rr

Release of gasesRelease of gases 90-137K amorphous 90-137K amorphous

crystalline phase changecrystalline phase change Annealing (30-120K)Annealing (30-120K) IIaahh IIaall IcIc IhIh ~ 150 K ~ 150 K Sublimation start 160-180KSublimation start 160-180K

CHCH44

NN22

ArArCOCO

Gas release at large distances:Gas release at large distances:controlled by Watercontrolled by Water

Sublimation of Volatiles?Sublimation of Volatiles?

DelsemmeDelsemme’’ss original work: original work: albedoalbedo too high too high Water-activity out beyond JupiterWater-activity out beyond Jupiter

Log

Z [

Log

Z [ m

olec

mol

ec m m

-2-2 s s

-1-1]]

DelsemmeDelsemme (1971) (1971)

Complications:Complications:Heat Transport in Real NucleiHeat Transport in Real Nuclei

Solve the heat conduction equationSolve the heat conduction equation ρρ(z(z) ) c(z,Tc(z,T) ) dT/dtdT/dt = = d/dzd/dz [ [κκ(z,T(z,T) ) dT/dzdT/dz]] Boundary condition: Energy Balance EquationBoundary condition: Energy Balance Equation Heat sources:Heat sources:

Solar radiation Solar radiation –– cyclic cyclic Radioactivity Radioactivity –– declining declining Crystallization Crystallization –– transient, front induced transient, front induced

ApproximationsApproximations Ice Thermal properties very differentIce Thermal properties very different

κκaa << << κκcc (4 orders of (4 orders of magmag)) κκaa αα T and T and κκcc αα 1/T 1/T

Heat Conduction in Porous MediaHeat Conduction in Porous Media

Pores reduce thermal conductivityPores reduce thermal conductivity Conduction by radiationConduction by radiation Conduction by vapor sublimation Conduction by vapor sublimation

recondensationrecondensation Releases heat, warming colder areasReleases heat, warming colder areas Reduces the thermal gradientReduces the thermal gradient Sintering Sintering decreases porosity decreases porosity

Porosity: 7.5% 13% 15% 22%Porosity: 7.5% 13% 15% 22%

Activity at Large r?Activity at Large r?The Evidence for FadingThe Evidence for Fading

Morehouse 1908 IIIGreat Comet 1577 Halley 1910 Delavan 1914

Different types of evidenceDifferent types of evidence Really bright comets are all long-periodReally bright comets are all long-period Distant comets Distant comets narrow tails (large dust) narrow tails (large dust) volatile gases volatile gases New comets tend to split more frequently (more volatiles)New comets tend to split more frequently (more volatiles) Non-gravitational motion (jets)Non-gravitational motion (jets)

ProblemsProblems Non uniform data setsNon uniform data sets Non-linear detectorsNon-linear detectors

SP Comets 3.4-14.5 AUSP Comets 3.4-14.5 AU

Comet Activity Levels Comet Activity Levels Trends Trends

Evidence for DifferencesEvidence for Differences

Dots = All SP Dots = All SP obsobs Squares = HalleySquares = Halley Triangles = DN cometTriangles = DN comet

The Halley OutburstThe Halley Outburst

Gas Laden amorphous ice modelGas Laden amorphous ice model Heat from perihelion penetrates to ice layerHeat from perihelion penetrates to ice layer Exothermic transformation (137K)Exothermic transformation (137K) Released gases build up pressure Released gases build up pressure outburst outburst

ChironChiron’’s Behaviors Behavior

Amorphous ice modelAmorphous ice model 60% dust60% dust 40% amorphous ice40% amorphous ice 0.1% trapped CO0.1% trapped CO

Matches observationsMatches observations Density < 0.4 g/cmDensity < 0.4 g/cm33

Mass loss rates & dustMass loss rates & dust CO fluxes match CO fluxes match obsobs TTsurfacesurface matches matches obsobs Activity sporadic Activity sporadic

not refreshing surfacenot refreshing surface

C/1995 O1 (Hale C/1995 O1 (Hale BoppBopp))

Active at large rActive at large r Discovered in 1995 atDiscovered in 1995 at

7.2 AU (active, inbound)7.2 AU (active, inbound) Pre-discovery imagePre-discovery image

13.0 AU (1993)13.0 AU (1993)

Dynamically youngDynamically young Large CO fluxes seenLarge CO fluxes seen Molecules of differentMolecules of different

volatilities appear atvolatilities appear atsimilar timessimilar times

Thermal models: Comet Hale Thermal models: Comet Hale BoppBopp

Amorphous ice crystallization modelAmorphous ice crystallization model Porosity 0.65Porosity 0.65 4% by mass trapped CO4% by mass trapped CO

Activity at Larger r?Activity at Larger r?

Distance for T ~ 137KDistance for T ~ 137K Beginning near 10 AUBeginning near 10 AU

Mechanisms at r > 10 AUMechanisms at r > 10 AU Solid volatiles (e.g. CO,Solid volatiles (e.g. CO,

COCO22) sublimation) sublimation Amorphous Amorphous Crystalline Crystalline

transitiontransition AnnealingAnnealing

C/2003 A2 GleasonC/2003 A2 Gleasonq = 11.43 AUq = 11.43 AU1/a = 42 x 101/a = 42 x 10-6-6 AU AU-1-1

KBO1996 TOKBO1996 TO6666 –– Activity? Activity? OrbitOrbit

Q = 48.6, q = 38.5Q = 48.6, q = 38.5 q: 5/3/1910 Q: 2/1/2054q: 5/3/1910 Q: 2/1/2054

LightcurveLightcurve period period 1997: 2 peak 6.25 +/- 0.031997: 2 peak 6.25 +/- 0.03

hr, hr, ΔΔm = 0.12 m = 0.12 magmag 1998: single peak, 1998: single peak, ΔΔm =m =

0.330.33 Consistent with activityConsistent with activity Blue colorsBlue colors

Vary with rotation in 1999Vary with rotation in 1999

Direct Evidence for Ice PhasesDirect Evidence for Ice Phases

C/1995 O1 Hale C/1995 O1 Hale BoppBopp 1.4-2.5 1.4-2.5 µµm: no 1.65 m: no 1.65 µµmm

feature feature amorphous amorphous

MirandaMiranda

1.65 1.65 µµm crystalline water ice feature presentm crystalline water ice feature present Spectral models: NHSpectral models: NH33 .. H H22O for 2.2 O for 2.2 µµm featurem feature Dark spectrally neutral componentDark spectrally neutral component

SummarySummary HH22O ice in cometsO ice in comets IaIa, , IcIc, , IhIh, no , no clathratesclathrates ClathratesClathrates imptimpt else else

where in the SSwhere in the SS((KBOsKBOs?)?)

Gas release at largeGas release at larger: controlled by Hr: controlled by H22OO AnnealingAnnealing IaIa IcIc IhIh sublimationsublimation

Comets are cosmicComets are cosmic““thermometersthermometers”” for forSS formationSS formation

Low heat transportLow heat transport Solar Solar insolationinsolation Radioactive decayRadioactive decay crystallizationcrystallization

Inactivity to ActivityInactivity to Activity

Sublimation termSublimation term Cools nucleus surfaceCools nucleus surface dmdmss/dt/dt = = PPvaporvapor/v/vthth

P: from lab, or thermo.P: from lab, or thermo.

ConductionConduction Often assumed small..Often assumed small..

Incident E depends onIncident E depends on Heliocentric distanceHeliocentric distance Surface Surface albedoalbedo / /

scatteringscattering

Re-radiated ERe-radiated E Surface T (rotationSurface T (rotation

dependent; material prop)dependent; material prop)

Radioactive Heat ProductionRadioactive Heat Production MechanismMechanism

Elements spontaneously lose mass and EElements spontaneously lose mass and E Decay rate depends on # atomsDecay rate depends on # atoms Rate Rate characterizedcharacterized by by ““half-lifehalf-life””

ImplicationsImplications Small comets Small comets IaIa IhIh quenched by dust quenched by dust Medium to large Medium to large KBOsKBOs IhIh & liquid & liquid

interiorsinteriors

1.261.264040ArAr4040KK

0.72 My0.72 My2626MgMg2626AlAl16 My16 My129129XeXe129129II

48.848.88787SrSr8787RbRb4.474.47206206PbPb238238UUHalf-Life [Half-Life [GyGy]]DaughterDaughterParentParent