IcIcee PPhhyyssiiccss I:I: CCoommeetsts

KKaarreenn MMeeeecchh SSeessssiioonn ## 2244 –– MMoonn 11//1177//0055 TTeemmppeerraatuturree SSccaalleess

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

Scale H2O Freeze H2O Boil ΔT Fahrenheit 32 212 180 Centigrade 0 100 100 Kelvin 273 373 100 WWaatteerr –– FFeeaattuurreess RReelelevvaanntt ttoo IIccee

 Bent molecule (104.52o) – H Bond  Dipole moment  Tetrahedral Crystal structure  Expands upon freezing (less dense)  Negative slope on melting curve  Le Chatlier Principle  Triple Point [S.M.O.W.] 613  P = 611.657 Pa (0.006 bar) 612  T = 273.16 K (0.16o C)  D O: 661 Pa, 276.82K 2 611 Pressure [Pa] Pressure [Pa] 610 1 bar = 105 Pa = 105 Nt/m2 -0.02 0 0.02 1 atm = 101325 Pa Temperature [C] WaterWater IceIce PhysicsPhysics  Exists in 13 crystalline phases (diff T & P)  Phase I: P < 2700 atm  Ih – hexagonal  Ic – cubic (low T, low P)  metastable  High P forms: II to XI  Amorphous Ice  Clathrates Ice Ial and Iah formation AAmmoorrpphhoouuss IcIcee

 Forms at low T  insufficient E for crystal structure  Physical Properties  Large voids: trapped gases  Gases released between 35-120K  Annealing

 38-68K – transition from Iah  Ial  Beginning at 90K  Ic (exothermic)  Two forms

 High Density, Iah

 Low Density, Ial CCllaaththrraatete HHyyddrraatetess

 Crystalline framework of H-bonded H2O molec. trapping guest molecules  Guests don’t affect the chemistry  Released upon sublimation of water  Cages unstable without guests  Some physical properties are altered  Some clathrate formation not possible at low T and P  Stability: low T (< 273K), moderate to high P (100 atm)  Importance

 May store most of SS inventory: CO, CO2, CH4  Catastrophic destabilization due to T , P  Collapse & flow features  Outgassing  Greenhouse gas budgets CClalatthhrraattee HHyyddrraatteess –– CCoonnttdd..

 Type I  Clathrates in the Solar System

 46 H2O molecules  CH4 – marine sediments – by 2x  2 small cages, 6 large exceeds other fossil fuel sources  12- and 14- sided polygons  CO – sequester CO in ocean  Traps in ratio 1/7 2 2 seafloor from atm (climate)  Type II  Destabilization on Mars &  126 H2O molecules Europa – chaotic terrain?  16 small cages, 8 large  CH4 / N2 on Triton – geysers  14- and 16- sided polygons  Air – polar ice sheets – info on  Traps in ratio 1/17 atmosphere up to 106 yr ago IIccee RReeggimimeess –– IInntteerrioiorr PPrreessssuurreess  G = 6.67 x 10-11 Ntm2/kg2 2 2 2  Pc = 2πGρ (R – r )/3  ρ = density, kg/m3  R = radius, m Regime ρ [g/cm3] Size [km] P [bar] Dust:Ice Ices ISgrains 0.1 10-10 – High den. amorph Comet 0.5-1? 1-10 0.0003-0.1 2:1? Ih, Ic, amorphous KBO 0.5-1? 102-103 3.5-1400 2:1? Ih, II, clath, amorph Europa 2.97 1569 3x104 Ih, II, VII, VIII, clath Triton 2.07 1350 1x104 Ih, II, clathrates Mars Ice 1-1.6 3 300 <100 ppm Ih, clathrates Glacier 0.92 3 300 ppm Ih, clathrates  Phase changes: volume changes  fractures  Amorphous  Ic, exothermic  volatile release  Clathrate destabilization  outgassing CCoommeett EExxppeecctatatitioonnss

 Amorphous ice  Formation T: 30-100K  Crystalline ices  Solar heating: amorphous crystalline transition peaking at 137K  Balance with cosmic ray processing  Lack of Clathrates  Small size, low central pressures  Release of volatiles in excess of clathrate capacity  Not necessary to explain activity! CCoossmmicic SSoolalarr SSyysstteemm HHisisttoorryy

Earth in the Hadean >4.6 Gy Oceans & rocks form ISM dark cloud ~4.4 Gy ago

Planetesimals condense Planets accrete Form few x100 million years

Late planetary bombardment Comets, asteroids bring water & The Archean Epoch Comets, asteroids bring water & Organics to Earth Oldest life on Earth Organics to Earth 3.5-3.8 Gy ago CCoommeett FFoorrmmaatitioonn

100K 64K 31K

0 10 100 AU LLooww TTeemmppeerraatuturree CCoonnddeennssaatitioonn

 Ices in comets condensed T< 100K  Amorphous form  Trapped other gases  Amounts depend on r  Release of gases  90-137K amorphous  crystalline phase change  Annealing (30-120K)

 Iah  Ial

CH4  Ic  Ih ~ 150 K

N2  Sublimation start 160-180K Ar Ar  Gas release at large distances: CO controlled by Water SSuubblliimmaatitioonn ooff VVoollaatitilleess?? ] ] 1 1 - - s s

2 2 - - m m

olec olec m m Log Z [ Log Z [

Delsemme (1971)  Delsemme’s original work: albedo too high  Water-activity out beyond Jupiter CCoommppliclicaattioionnss:: HHeeaatt TTrraannssppoorrtt inin RReeaall NNuuccleleii  Solve the heat conduction equation ρ(z) c(z,T) dT/dt = d/dz [κ(z,T) dT/dz]  Boundary condition: Energy Balance Equation  Heat sources:  Solar radiation – cyclic  Radioactivity – declining  Crystallization – transient, front induced  Approximations  Ice Thermal properties very different

 κa << κc (4 orders of mag)  κa α T and κc α 1/T HHeeaatt CCoonndduuccttioionn inin PPoorroouuss MMeeddiaia

Porosity: 7.5% 13% 15% 22%  Pores reduce thermal conductivity  Conduction by radiation  Conduction by vapor sublimation  recondensation  Releases heat, warming colder areas  Reduces the thermal gradient  Sintering  decreases porosity AAccttivivitityy aatt LLaarrggee rr?? TThhee EEvvidideennccee ffoorr FFaaddiningg

 Different types of evidence  Really bright comets are all long-period  Distant comets  narrow tails (large dust)  volatile gases  New comets tend to split more frequently (more volatiles)  Non-gravitational motion (jets)  Problems  Non uniform data sets  Non-linear detectors

Great Comet 1577 Morehouse 1908 III Halley 1910 Delavan 1914 SSPP CCoommeettss 33..44--1144..55 AAUU CCoommeett AAccttivivitityy LLeevveelsls  TTrreennddss

EEvviiddeennccee ffoorr DDiiffffeerreenncceess

 Dots = All SP obs  Squares = Halley  Triangles = DN comet TThhee HHaallelleyy OOuuttbbuurrsstt

 Gas Laden amorphous ice model  Heat from perihelion penetrates to ice layer  Exothermic transformation (137K)  Released gases build up pressure  outburst CChhiirroonn’’ss BBeehhaavviioorr

 Amorphous ice model  60% dust  40% amorphous ice  0.1% trapped CO  Matches observations  Density < 0.4 g/cm3  Mass loss rates & dust  CO fluxes match obs

 Tsurface matches obs  Activity sporadic  not refreshing surface CC/1/1999955 OO11 ((HHaallee BBoopppp))

 Active at large r  Discovered in 1995 at 7.2 AU (active, inbound)  Pre-discovery image 13.0 AU (1993)  Dynamically young  Large CO fluxes seen  Molecules of different volatilities appear at similar times TThheerrmmaall mmooddeelsls:: CCoommeett HHaalele BBoopppp

 Amorphous ice crystallization model  Porosity 0.65  4% by mass trapped CO AAccttivivitityy aatt LLaarrggeerr rr??

 Distance for T ~ 137K C/2003 A2 Gleason  Beginning near 10 AU q = 11.43 AU 1/a = 42 x 10-6 AU-1  Mechanisms at r > 10 AU  Solid volatiles (e.g. CO, CO2) sublimation  Amorphous  Crystalline transition  Annealing KKBBOO11999966 TTOO66 –– AAccttivivitityy??

 Orbit  Q = 48.6, q = 38.5  q: 5/3/1910 Q: 2/1/2054  Lightcurve period  1997: 2 peak 6.25 +/- 0.03 hr, Δm = 0.12 mag  1998: single peak, Δm = 0.33  Consistent with activity  Blue colors  Vary with rotation in 1999 DDiirreecctt EEvviiddeennccee foforr IcIcee PPhhaasseess

 C/1995 O1 Hale Bopp  1.4-2.5 µm: no 1.65 µm feature  amorphous MMiriraannddaa

 1.65 µm crystalline water ice feature present .  Spectral models: NH3 H2O for 2.2 µm feature  Dark spectrally neutral component SSuummmmaarryy

 H2O ice in comets  Comets are cosmic  Ia, Ic, Ih, no clathrates “thermometers” for SS formation  Clathrates impt else where in the SS  Low heat transport (KBOs?)  Solar insolation  Radioactive decay  Gas release at large Radioactive decay  crystallization r: controlled by H2O  Annealing  Ia  Ic  Ih  sublimation

IInnaaccttivivitityy ttoo AAccttivivitityy

 Incident E depends on  Sublimation term  Heliocentric distance  Cools nucleus surface

 Surface albedo /  dms/dt = Pvapor/vth scattering  P: from lab, or thermo.  Re-radiated E  Conduction  Surface T (rotation  Often assumed small.. dependent; material prop) RRaaddiiooaacctitivvee HHeeaatt PPrroodduucctitioonn

 Mechanism  Elements spontaneously lose mass and E  Decay rate depends on # atoms  Rate characterized by “half-life”  Implications  Small comets Ia  Ih quenched by dust  Medium to large KBOs  Ih & liquid interiors

Parent Daughter Half-Life [Gy] 238U 206Pb 4.47 87Rb 87Sr 48.8 40K 40Ar 1.26 129I 129Xe 16 My 26Al 26Mg 0.72 My