Large Molecules in Comets

Dominique Bockelée-Morvan

Observatoire de Paris

What for ? Is it a dream ?

Composition of comets : from small to large species

to understand comet material origin and formationinterstellar condensates ?product of nebular chemistry ?both ?

to constrain Solar System formation to explore the role of comets in the origin of life

Present state : 25 molecules identifiedmost complex species has 10 atomsstill more complex species are suggested

Methods of investigation

Sample return : Stardust on 81P/Wild 2 ….. wait for 2006

Nucleus reflectance spectroscopy (in situ)

Comet atmosphere : parent molecules

Mass spectrometry: Giotto, Stardust missions

IR spectroscopy

Radio spectroscopy

UV

Surface reflectance spectroscopy

Comet 19P/Borrelly

Deep Space 1 mission

2.39 m feature

CN compound ?

Soderblom et al, 2000, Science 296, 1087

Phoebe Saturn’s moonA captured KBO

Cassini/VIMS reflectance spectraH2O, CO2

Organics, nitriles, CN-compoundsClark et al., Nature 435,66, 2005

Mass spectrometryGiotto/Vega results on 1P/Halley Limited by mass resolution Simple species and ion-molecule reaction products H2O, H2CO, H2S, NH3, CH3OH

High molecular mass compounds evidenced from mass spectra tentative identification of e.g, acetic acid, iminoethane, pyridine …Stardust results on Wild 2 : nitrogen rich compounds (Kissel et al. 2004)

Altwegg et al. 1999Sp.Sci. Rev, 90,3

Spectroscopic investigations :gas phase species

Visible and UV windows: essentially radicals and ions

exceptions : CO and S2

tentative detection of phenanthene and pyrene in 1P/Halley

IR 2-5 m window : fundamental bands of vibration

hot bands of water (e.g., 3-2)

emission process : fluorescence

radio window (cm to submm): privileged tool

cold atmospheres

Possible idendification of phenanthrene C14H10

TKS/Vega@450 km1P/Halley

Q/Q(H2O) = 1.5x10-3

Moreels et al. A&A 282, 643

Possible identification of pyrene C16H10 : C16H10 / C14H10 = 0.04

(Clairemidi et al. PSS 52, 761, 2004)

PAHs, if present, are released from grains (Joblin et al. 1997 PSS 45)

Comparaison with laser-induced fluorescencespectra /jet-cooled conditions

IR spectroscopy

Combes et al. (1986)

IKS/VEGA

Simple species : H2O, CO, CO2, H2CO, CH3OH3.3-3.5 m band : CH-bearing species in gas phase unidentified compounds at 3.42m 3.28 m band: PAHs ? PAHs bands at higher wavelengths not seen in Hale-Bopp ISO spectra

IR spectroscopy

C/1999 H1 (Lee) Keck/NIRSPEC Mumma et al. (2001)

High spectral resolutionro-vibrational lines ofCH4, C2H2, C2H6

CH3OH, HCN

Unidentified lines

need for detailed ro-vibrationalstructure and strength of CH3OH bands in 3 m region+ other organic species

Radio spectroscopy

19 molecules (not including isotopes, radicals, ions) detected

many first identifications in comets Hyakutake and Hale-Bopp

searching method in Meudon group

PAPSYNTHE code: input: JPL/Cologne spectroscopic databases

comet, telescope characteristics

ouput: expected intensities for all lines

optimisation of receiver tunings, ISM molecules targetted

In Hale Bopp: 10% of the 85-375 GHz window with 3 telescopes

Bockelée-Morvan et al. A&A 353, 1101, 2000

Crovisier et al. 2004A&A 418, L35, 2004

Ethylene glycol HOCH2CH2OH11 lines identified in 2003 whenfrequencies available in Colognedatabase

230.578 GHz

Molecular abundances

I glycol

Bockelée-Morvan et al.

Comets II, 2005

Upper limits for complex species

Crovisier et al. A&A 418, 1141,2004

Molecular complexity

Crovisier et al. A&A 418, 1141,2004

abundances when complexity

C2H5OH/CH3OH <1/25

cyanopolyynes

but CH4 ~ C2H2 ~ C2H6

reduced alcohols wrt aldehydes

CH3OH > H2CO

OHCH2CH2OH > CH2OHCHO

Grain surface reactions ?

Sgr B2(N) : glycol/CH3OH = 5 10-4

Hale-Bopp : glycol/CH3OH = 0.1

Analogies with ISM but material formed at high-T is present in comets

(cristalline silicates)

Other evidences for complex species

extended sources of H2CO, CO, HNC

organic grains contribution ?

H2CO : thermodegradation of polyoxymethylene (H2CO polymers)

CO : extended source, if any, not identified

HNC : increased production at decreasing distance from Sun;

origin unkown, HCN polymers ?

Polyoxymethylene (H2CO)n source of H2CO ?

Multiple observational evidences for extended distribution

Steep heliocentric evolution of production rate in comet Hale-Bopp

Laboratory experiments on polyoxymethylene (POM) photo and thermo-degradation

POM thermo-degradation: consistently explain H2CO observations with a few percent POM in grains in mass

Cottin et al. 2004, Icarus 167, 397

Large molecules, source of CO ?

Hale-Bopp : CO in the IR(Disanti et al. 2001)

Hale-Bopp: CO 1.3 and 3mm

(Bockelée-Morvan et al. 2005)

IR suggests CO extended source

Radio mapping at PDB interferometer

=> no extended source

Source of CO, if any: unidentified

Origin of HNC ?

Biver et al. 2002

Biver et al. 2005 Bockelée-Morvan et al. 2005

Hale-Bopp HNC, PdBi

- HNC/HCN increases with decreasing heliocentric distance- HNC and HCN: similar radial distributions at 3 arcsec spatial resolution- production of HNC by chemical reactions excluded- source of HNC in inner coma ?- thermo-degradation of organic material ?

Future prospects for new molecular identifications

current instrumentation : bright comets needed studies are focussing on chemical diversity/spatial distribution ALMA, Herschel Observatory

ALMA: factor 10 increase in sensitivityHerschel : bending modes of PAHs ?

space missions : Deep impact, Rosetta sample return needs for IR spectra of simple organics