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 formation interstellar 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 identified most complex species has 10 atoms still 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 moon A captured KBO Cassini/VIMS reflectance spectra H 2 O, CO 2 Organics, nitriles, CN-compounds Clark et al., Nature 435,66, 2005

Mass spectrometry Giotto/Vega results on 1P/Halley  Limited by mass resolution  Simple species and ion-molecule reaction products H 2 O, H 2 CO, H 2 S, NH 3, CH 3 OH  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 Sp.Sci. Rev, 90,3

Spectroscopic investigations : gas phase species  Visible and UV windows: essentially radicals and ions exceptions : CO and S 2 tentative detection of phenanthene and pyrene in 1P/Halley  IR 2-5  m window : fundamental bands of vibration hot bands of water (e.g., ) emission process : fluorescence  radio window (cm to submm): privileged tool cold atmospheres

Possible idendification of phenanthrene C 14 H 10 km 1P/Halley Q/Q(H 2 O) = 1.5x10 -3 Moreels et al. A&A 282, 643  Possible identification of pyrene C 16 H 10 : C 16 H 10 / C 14 H 10 = 0.04 (Clairemidi et al. PSS 52, 761, 2004)  PAHs, if present, are released from grains (Joblin et al PSS 45) Comparaison with laser-induced fluorescence spectra /jet-cooled conditions

IR spectroscopy Combes et al. (1986) IKS/VEGA Simple species : H 2 O, CO, CO 2, H 2 CO, CH 3 OH  m band : CH-bearing species in gas phase unidentified compounds at 3.42  m 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 resolution ro-vibrational lines of CH 4, C 2 H 2, C 2 H 6 CH 3 OH, HCN Unidentified lines need for detailed ro-vibrational structure and strength of CH 3 OH 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 GHz window with 3 telescopes

Bockelée-Morvan et al. A&A 353, 1101, 2000 Crovisier et al A&A 418, L35, 2004 Ethylene glycol HOCH 2 CH 2 OH 11 lines identified in 2003 when frequencies available in Cologne database 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  C 2 H 5 OH/CH 3 OH <1/25 cyanopolyynes  but CH 4 ~ C 2 H 2 ~ C 2 H 6  reduced alcohols wrt aldehydes CH 3 OH > H 2 CO OHCH 2 CH 2 OH > CH 2 OHCHO Grain surface reactions ?

Sgr B2(N) : glycol/CH 3 OH = Hale-Bopp : glycol/CH 3 OH = 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 H 2 CO, CO, HNC organic grains contribution ? H 2 CO : thermodegradation of polyoxymethylene (H 2 CO polymers) CO : extended source, if any, not identified HNC : increased production at decreasing distance from Sun; origin unkown, HCN polymers ?

Polyoxymethylene (H 2 CO)n source of H 2 CO ?  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 H 2 CO 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 Biver et al Bockelée-Morvan et al 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 sensitivity Herschel : bending modes of PAHs ?  space missions : Deep impact, Rosetta  sample return  needs for IR spectra of simple organics