Search

We present the results of a molecular survey of comet 46P/Wirtanen undertaken with the IRAM 30-m and NOEMA radio telescopes in December 2018. Observations at IRAM 30-m during the 12-18 Dec. period comprise a 2 mm spectral survey covering 25 GHz and a 1 mm survey covering 62 GHz. The gas outflow velocity and kinetic temperature have been accurately constrained by the observations. We derive abundances of 11 molecules, some being identified remotely for the first time in a Jupiter-family comet, including complex organic molecules such as formamide, ethylene glycol, acetaldehyde, or ethanol. Sensitive upper limits on the abundances of 24 other molecules are obtained. The comet is found to be relatively rich in methanol (3.4 percent relative to water), but relatively depleted in CO, CS, HNC, HNCO, and HCOOH., Comment: 35 pages, 16 figures, to be published in Astronomy & Astrophysics

Context. The Microwave Instrument on the Rosetta Orbiter (MIRO) on board the Rosetta spacecraft was designed to investigate the surface and gas activity of the comet 67P/Churyumov-Gerasimenko. The MIRO spectroscopic measurements carry information about the velocity of gas emanating from the nucleus surface. Knowledge of the terminal velocity of the H2O gas is valuable for interpretation of in situ measurements, calibrating 3D coma simulations, and studying the physics of gas acceleration. Aims. Using a neural network technique, we aim to estimate the gas terminal velocity from the entire MIRO dataset of nadir geometry pointings. The velocity of the gas is encoded in the Doppler shift of the measured rotational transitions of o-H216O and o-H218O even when the spectral lines are optically thick with quasi or fully saturated line cores. Methods. Neural networks are robust nonlinear algorithms that can be trained to recognize underlying input to output functional relationships. A training set of about 2200 non-LTE simulated spectra for the two transitions is computed for known input cometary atmospheres, varying column density, temperature, and expansion velocity profiles. Two four-layer networks are used to learn and then predict the gas terminal velocity from the MIRO nadir measured o-H216O and o-H218O spectra lines, respectively. We also quantify the accuracy, stability, and uncertainty of the estimated parameter. Results. Once trained, the neural network is very effective in inverting the measured spectra. We process the entire dataset of MIRO measurements from August 2014 to July 2016, and investigate correlations and temporal evolution of terminal velocities derived from the two spectral lines. The highest terminal velocities obtained from H218O are higher than those from H216O with differences that evolve in time and reach about 150 m s−1 on average around perihelion. A discussion is provided on how to explain this peculiar behavior.

We analyze 2–5μm spectroscopic observations of the dust coma of comet 67P/Churyumov-Gerasimenko obtained with the Visible InfraRed Thermal Imaging Spectrometer (VIRTIS-H) instrument on board Rosetta from 3 June to 29 October 2015 at heliocentric distancesrh= 1.24–1.55 AU. The 2–2.5μm color, bolometric albedo, and color temperature were measured using spectral fitting. Data obtained atα= 90° solar phase angle show an increase in bolometric albedo (0.05–0.14) with increasing altitude (0.5–8 km), accompanied by a possible marginal decrease in color and color temperature. Possible explanations include dark particles on ballistic trajectories in the inner coma and radial changes in particle composition. In the phase angle range 50°–120°, phase reddening is significant (0.031%/100 nm deg−1) for a mean color of 2%/100 nm atα= 90°, which might be related to the roughness of the dust particles. Moreover, a decrease in color temperature with decreasing phase angle is also observed at a rate of ~0.3 K deg−1, consistent with the presence of large porous particles, with low thermal inertia, and showing a significant day-to-night temperature contrast. Comparing data acquired at fixed phase angle (α= 90°), a 20% increase in bolometric albedo is observed near perihelion. Heliocentric variations in dust color are not significant in the time period we analyzed. The measured color temperatures vary from 260 to 320 K, and follow arh−0.6variation in therh= 1.24–1.5 AU range, which is close to the expectedrh−0.5value.

We present spatially and spectrally-resolved observations of CH$_3$OH emission from comet C/2012 K1 (PanSTARRS) using The Atacama Large Millimeter/submillimeter Array (ALMA) on 2014 June 28-29. Two-dimensional maps of the line-of-sight average rotational temperature ($T_{rot}$) were derived, covering spatial scales $0.3''-1.8''$ (corresponding to sky-projected distances $\rho\sim500$-2500 km). The CH$_3$OH column density distributions are consistent with isotropic, uniform outflow from the nucleus, with no evidence for extended sources of CH$_3$OH in the coma. The $T_{rot}(\rho)$ radial profiles show a significant drop within a few thousand kilometers of the nucleus, falling from about 60 K to 20 K between $\rho=0$ and 2500 km on June 28, whereas on June 29, $T_{rot}$ fell from about 120 K to 40 K between $\rho=$ 0 km and 1000 km. The observed $T_{rot}$ behavior is interpreted primarily as a result of variations in the coma kinetic temperature due to adiabatic cooling of the outflowing gas, as well as radiative cooling of the CH$_3$OH rotational levels. Our excitation model shows that radiative cooling is more important for the $J=7-6$ transitions (at 338 GHz) than for the $K=3-2$ transitions (at 252 GHz), resulting in a strongly sub-thermal distribution of levels in the $J=7-6$ band at $\rho\gtrsim1000$ km. For both bands, the observed temperature drop with distance is less steep than predicted by standard coma theoretical models, which suggests the presence of a significant source of heating in addition to the photolytic heat sources usually considered., Comment: Accepted for publication in ApJ, January 2017

The VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument on board the Rosetta spacecraft has provided evidence of carbon-bearing compounds on the nucleus of the comet 67P/Churyumov-Gerasimenko. The very low reflectance of the nucleus (normal albedo of 0.060 ± 0.003 at 0.55 micrometers), the spectral slopes in visible and infrared ranges (5 to 25 and 1.5 to 5% kÅ −1 ), and the broad absorption feature in the 2.9-to-3.6–micrometer range present across the entire illuminated surface are compatible with opaque minerals associated with nonvolatile organic macromolecular materials: a complex mixture of various types of carbon-hydrogen and/or oxygen-hydrogen chemical groups, with little contribution of nitrogen-hydrogen groups. In active areas, the changes in spectral slope and absorption feature width may suggest small amounts of water-ice. However, no ice-rich patches are observed, indicating a generally dehydrated nature for the surface currently illuminated by the Sun.

International audience; Interferometric observations are essential to probe the molecular emission in the inner cometary atmospheres and study the outgassing from the nucleus. Mapping the continuum emission can provide information about the dust and/or nucleus properties. We present here a summary of the observations of the dust and gas coma of comet 17P/Holmes and nuclear observations of 8P/Tuttle, both carried out with the IRAM interferometer at Plateau de Bure (PdBI) in 2007-2008. The observations of these two comets demonstrate the ability of the PdBI in terms of cometary science. In the near future, several improvements will be made (new receivers at 0.8 mm, a new wide-band correlator) allowing more frequent and more detailed studies of comets. On the long term, NOEMA, an expansion project, may add up to six antennas to the Plateau de Bure Interferometer, and increase the baseline lengths. Such an instrument would offer a complement to ALMA to track comets of the northern hemisphere with about half the sensitivity of ALMA for continuum studies.

Production and Kinematics of CO in Comet 1995 O1 (Hale-Bopp) at Large Post-Perihelion Distances

Since the apparition of comet Kohoutek 1973 XII, the 18-cm lines of the OH radical have been systematically observed in a number of comets with the Nancay radio telescope. Between 1973 and 1999, 52 comets have been successfully detected. This allowed an evaluation of the cometary water production rates and their evolution with time, as well as a study of several physical processes such as the excitation mechanisms of the OH radio lines, the expansion of cometary atmospheres, their anisotropy in relation to non-gravitational forces, and the Zeeman effect in relation to the cometary magnetic field. Part of these observations and their analysis have already been published. The bulk of the results are now organized in a data base. The present paper is a general presentation of the Nancay cometary data base and a more specific description of the observations of 53 cometary apparitions between 1982 and 1999. Comets observed before 1982 are only partly incorporated in the data base. Observations of comets since 2000 have benefited from a major upgrade of the telescope; they will be presented in forthcoming publications.

Volatile abundances in Jupiter-family Comet 81P/Wild 2 were measured on four dates in February and March 2010 using high-dispersion ( λ Δ λ ∼ 2.5 × 10 4 ) infrared spectroscopy with NIRSPEC at the W.M. Keck Observatory. H 2 O was detected on all dates, including measurements on UT March 29 of lines from the ν 2 + ν 3 − ν 2 hot-band not previously targeted in comets. C 2 H 6 and HCN were detected on three dates, and CH 3 OH was detected on one date. Tentative detections or upper-limits are reported for CH 3 OH on other dates, as well as for C 2 H 2 , H 2 CO, and NH 3 . Abundances of all species relative to H 2 O are in the typical range with the exception of CH 3 OH, which is depleted compared to other comets. Gas production was significantly higher in late February than in late March. Rotational temperatures were determined for H 2 O on UT February 22 and 23 and found to be about 30–40 K. The spatial distributions of H 2 O, C 2 H 6 , and CH 3 OH are all symmetric and similar to the spatial distribution of the dust continuum. H 2 O abundances from this work are compared to other measurements from both the 1997 and 2010 apparitions. There is no clear evidence of a change in overall gas productivity between the two apparitions within measurement accuracy. Abundances of C 2 H 2 , C 2 H 6 , HCN and NH 3 are consistent with these species being the primary parents of C 2 , CN, NH and NH 2 as measured at optical wavelengths. Although optically classified as carbon-chain depleted, Wild 2 appears more chemically similar in parent volatile chemistry to carbon-chain typical comets; however, we note that in the small sample of Jupiter-family comets measured to date, each comet is chemically distinct.

Comet Hale-Bopp was observed five times with ISOPHOT, the photometer on board ESA's Infrared Space Observatory (ISO) between 4.6 and 2.8 AU. Each time, broadband photometry was performed using 4 different detectors, 5 apertures and 10 filters covering the range between 3.6 and 170 μm. Background observations were performed with identical instrument settings at the same positions on the sky several days after the comet observations. The observation strategy and the data reduction steps are described in some detail, including the techniques to correct for variable detector responsivity. The resulting inband power values of the Hale-Bopp observations and their uncertainties are given. The mean uncertainty is 25%. The final fluxes were computed, taking into account the zodiacal background, possible offset of the comet's position from the center of the aperture, the brightness distribution within the coma, and the spectral energy distribution of the comet's emission. Strong thermal emission from a broad size distribution of dust particles was detected in all of the data sets, even at r = 4.6-4.9 AU pre-perihelion and 3.9 AU post-perihelion; the total thermal energy varied as r-3. The 7.3-12.8 μm color temperature was ~1.5 times the blackbody temperature, higher than that observed in any other comet. Silicate features at 10 and 25 μm were prominent in all 5 data sets, the largest heliocentric distances that silicate emission has been detected in a comet. The presence of crystalline water ice grains is suggested from the 60 μm excess emission at 4.6-4.9 AU, consistent with the observed QOH if the icy grains were slightly warmer than an equilibrium blackbody. The average albedo of the dust is higher than that of comet P/Halley, but lower than other albedo measurements for Hale-Bopp nearer perihelion. There is no evidence for a component of cold, bright icy grains enhancing the scattered light at 4.6 AU. Simple models for a mixture of silicate and absorbing grains were fit to the ISO spectra and photometry at 2.8 AU. The observed flux at λ >100 μm requires a size distribution in which most of the mass is concentrated in large particles. Dust production rates of order 1.5 x 105 kg s-1 at 2.8 AU and 3 x 104 kg s-1 at 4.6 AU have been found. They correspond to dust to gas mass ratios of 6 to 10.

The Infrared Space Observatory (ISO) offered us the opportunity to observe celestial bodies from 2.4 to 196 μm, which is of particular interest for comets. We present here spectroscopic observations with ISO of comets C/1995 O1 (Hale-Bopp) and 103P/Hartley 2. Fluorescence emissions of H2O, CO2 and (for comet Hale-Bopp only) CO and CH4 are observed, yielding the production rates of these species. High-resolution spectra of water permit the study of the rotational and spin temperatures of this molecule. In comet Hale-Bopp, the thermal infrared region of the spectra shows specifically, in addition to continuum-like emission, the signatures of crystalline, Mg-rich olivine (forsterite), and signs of crystalline pyroxenes, amorphous silicates and water ice. The presence of crystalline silicates is also suggested in the Jupiter-family comet P/Hartley 2.

1996 and 1997 years were very prolific years for cometary studies. In March 1996, C/1996 B2 (Hyakutake), a medium-bright comet, passed at only 0.10 astronomical units (AU) from the Earth. On 1st April, 1997, C/1995 O1 (Hale–Bopp) passed at perihelion. This comet was 10 to 100 times brighter than comet Halley when compared at the same distance. It could be studied and monitored from its discovery in July 1995 as far as 7 AU from the Sun until its perihelion at 0.9 AU, where it was the object of extensive observations. It was thus possible to study these two exceptional comets with modern instrumentation covering the electromagnetic spectrum from radio to X-ray domains. Our knowledge of the composition and nature of comets has been transformed. New cometary molecules were observed, as well as isotopic species. New physical processes could be studied, owing to the large amounts of matter escaping from these objects, to the proximity of Earth allowing a detailed investigation of the region close to the nucleus (comet Hyakutake), or to the unusual physical conditions far from the Sun (comet Hale–Bopp). Selected results of these observations, relevant to the chemistry and physics of molecules and grains, are discussed. It is shown that whereas some old problems are now solved, new ones have emerged. The needs for laboratory data and theoretical work are pointed out.

We present OH 18-cm observations of comet Hale-Bopp (C/1995 O1) at the Nancay radio telescope. On nucleus and offset position observations allowed us to obtain both OH production rates and quenching radii. The maximum OH production rate was reached around perihelion, at about1031 s-1.

Near-infrared emissions of the SL9 impact sites of Jupiter have been recorded on July 16-18, 1994, at ESO (La Silla, Chile). A very strong emission of methane was recorded between 3.50 and 3.56 μm, shortly after impact H, showing evidence for a temporary increase of the Jovian stratospheric temperature. Emissions of H 2 (2.12 μm) and H 3 + (3.53 μm) were also detected above some of the impact sites, several hours after the impacts. The observed H 3 + emissions, however, seem to be at least partly contaminated by the southern aurora. A strong continuum was also detected at 2.12 μm over most of the impact sites, presumably due to intense scattering of reflected sunlight by stratospheric haze.

High resolution spectroscopy of the impacts and impact sites of comet P/Shoemaker-Levy 9 in Jupiter has been performed at the Pic-du-Midi observatory, France. The transient emission of seven emission lines from five different metals (Na, Fe, Ca, Li, K) and of the Hα line have been identified in spectra from the fresh impact sites of fragments L and Q1.

In July 1994, the collisions of the fragments of comet Shoemaker-Levy 9 with Jupiter resulted in dramatic changes in the planet's atmosphere. Observations of the events suggest that the composition and thermal properties of the atmosphere were considerably modified at the impact sites, with the changes persisting for times lasting from minutes to weeks (see, for example, refs 1-4). Here we report observations of the impact sites at millimetre wave-lengths, which reveal strong emission lines associated with carbon monoxide, carbonyl sulphide and carbon monosulphide. The abundance of carbon monoxide in the jovian atmosphere is normally very low; carbonyl sulphide and carbon monosulphide, on the other hand, have not hitherto been detected. We find that the largest fragments (G and K) each produced approximately 10(14) g of carbon monoxide, 3 x 10(12) g of carbonyl sulphide and 3 x 10(11) g of carbon monosulphide, most probably by shock-induced chemical reactions. Our observations also place firm constraints on the thermal response of Jupiter's stratosphere to the impacts.

The Jupiter-family comet 10P/Tempel 2 was observed during its 2010 return with the Herschel Space Observatory. We present here the observation of the (J, K) = (1, 0)-(0, 0) transition of ammonia at 572 GHz in this comet with the Heterodyne Instrument for the Far Infrared (HIFI) of Herschel. We also report on radio observations of other molecules (HCN, CH3OH, H2S and CS) obtained during the 1999 return of the comet with the CSO telescope and the JCMT, and during its 2010 return with the IRAM 30-m telescope. Molecular abundances relative to water are 0.09%, 1.8%, 0.4%, and 0.08% for HCN, CH3OH, H2S, and CS, respectively. An abundance of 0.5% for NH3 is obtained, which is similar to the values measured in other comets. The hyperfine structure of the ammonia line is resolved for the first time in an astronomical source. Strong anisotropy in the outgassing is present in all observations from 1999 to 2010 and is modelled to derive the production rates., 6 pages and 8 figures. Accepted for publication in Astronomy & Astrophysics

The OH radio lines at 18 cm were monitored in six comets with the Nancay radio telescope in 1992 and early 1993: Zanotta-Brewington 1991g 1 , Bradfield 1992b, Mueller 1991h 1 , Shoemaker-Levy 1991a 1 , P/Swift-Tuttle 1992t and P/Schaumasse 1992x. Neither Bradfield 1992b nor Mueller 1991h 1 could be detected. Zanotta-Brewington 1991g 1 , Shoemaker-Levy 1991a 1 and P/Schaumasse 1992x were relatively weak comets with OH production rates of the order of 10 28 s −1 and required several hours of integration to be detected. P/Swift-Tuttle was observed and detected as early as mid-October 1992. The signal was much stronger than for the other comets, especially at the beginning of December 1992, when P/Swift-Tuttle was observed against galactic background radiation. The OH production rate was several 10 29 s −1 . All along the observations, the OH radio lines exhibited unusually high blueshifts, corresponding to gas bulk motions of 0.5–1.0 km s −1 towards the observer. This undoubtedly corresponds to the strong jets observed in optical images and to the important nongravitational forces that affect the orbit of this comet. P/Swift-Tuttle was still observed at Nancay at the beginning of January 1993 when the small solar elongation precluded optical observations. A summary of these observations and of the estimations of the OH production rates is presented.

Little is known about the physical properties of cometary nuclei. Apart from space mission targets, measuring the thermal emission of a nucleus is one of the few means to derive its size, independently of its albedo, and to constrain some of its thermal properties. This emission is difficult to detect from Earth but space telescopes (Infrared Space Observatory, Spitzer Space Telescope, Herschel Space Observatory) allow reliable measurements in the infrared and the sub-millimetre domains. We aim at better characterizing the thermal properties of the nucleus of comet 8P/Tuttle using multi-wavelentgh space- and ground-based observations, in the visible, infrared, and millimetre range. We used the Plateau de Bure Interferometer to measure the millimetre thermal emission of comet 8P/Tuttle at 240 GHz (1.25 mm) and analysed the observations with the shape model derived from Hubble Space Telescope observations and the nucleus size derived from Spitzer Space Telescope observations. We report on the first detection of the millimetre thermal emission of a cometary nucleus since comet C/1995 O1 Hale-Bopp in 1997. Using the two contact spheres shape model derived from Hubble Space Telescope observations, we constrained the thermal properties of the nucleus. Our millimetre observations are best match with: i) a thermal inertia lower than ~10 J K-1 m-2 s-1/2, ii) an emissivity lower than 0.8, indicating a non-negligible contribution of the colder sub-surface layers to the outcoming millimetre flux., Comment: 7 pages. Accepted for publication in Astronomy & Astrophysics

Comet C/2006 W3 (Christensen) was observed in November 2009 at 3.3 AU from the Sun with Herschel. The PACS instrument acquired images of the dust coma in 70- and 160-micrometers filters, and spectra covering several H2O rotational lines. Spectra in the range 450-1550 GHz were acquired with SPIRE. The comet emission continuum from 70 to 672 micrometers was measured, but no lines were detected. The spectral energy distribution indicates thermal emission from large particles and provides a measure of the size distribution index and dust production rate. The upper limit to the water production rate is compared to the production rates of other species (CO, CH3OH, HCN, H2S, OH) measured with the IRAM 30-m and Nancay telescopes. The coma is found to be strongly enriched in species more volatile than water, in comparison to comets observed closer to the Sun. The CO to H2O production rate ratio exceeds 220%. The dust to gas production rate ratio is on the order of 1., Accepted for publication in Astronomy & Astrophysics (special issue on Herschel first results)

International audience; "Water and related chemistry in the Solar System" is a Herschel Space Observatory Guaranteed-Time Key Programme. This project, approved by the European Space Agency, aims at determining the distribution, the evolution and the origin of water in Mars, the outer planets, Titan, Enceladus and the comets. It addresses the broad topic of water and its isotopologues in planetary and cometary atmospheres. The nature of cometary activity and the thermodynamics of cometary comae will be investigated by studying water excitation in a sample of comets. The D/H ratio, the key parameter for constraining the origin and evolution of Solar System species, will be measured for the first time in a Jupiter-family comet. A comparison with existing and new measurements of D/H in Oort-cloud comets will constrain the composition of pre-solar cometary grains and possibly the dynamics of the protosolar nebula. New measurements of D/H in giant planets, similarly constraining the composition of proto-planetary ices, will be obtained. The D/H and other isotopic ratios, diagnostic of Mars' atmosphere evolution, will be accurately measured in H2O and CO. The role of water vapor in Mars' atmospheric chemistry will be studied by monitoring vertical profiles of H2O and HDO and by searching for several other species (and CO and H2O isotopes). A detailed study of the source of water in the upper atmosphere of the Giant Planets and Titan will be performed. By monitoring the water abundance, vertical profile, and input fluxes in the various objects, and when possible with the help of mapping observations, we will discriminate between the possible sources of water in the outer planets (interplanetary dust particles, cometary impacts, and local sources). In addition to these inter-connected objectives, serendipitous searches will enhance our knowledge of the composition of planetary and cometary atmospheres.

Observations of the CO J(1-0) 115 GHz and J(2-1) 230 GHz lines in comet C/1995 O1 (Hale-Bopp) were performed with the IRAM Plateau de Bure interferometer on 11 March, 1997. The observations were conducted in both single-dish (ON-OFF) and interferometric modes with 0.13 km s-1 spectral resolution. Images of CO emission with 1.7 to 3" angular resolution were obtained. The ON-OFF and interferometric spectra show a velocity shift with sinusoidal time variations related to the Hale-Bopp nucleus rotation of 11.35 h. The peak position of the CO images moves perpendicularly to the spin axis direction in the plane of the sky. This suggests the presence of a CO jet, which is active night and day at about the same extent, and is spiralling with nucleus rotation. The high quality of the data allows us to constrain the characteristics of this CO jet. We have developed a 3-D model to interpret the temporal evolution of CO spectra and maps. The CO coma is represented as the combination of an isotropic distribution and a spiralling gas jet, both of nucleus origin. Spectra and visibilities (the direct output of interferometric data) analysis shows that the CO jet comprises ~40% the total CO production and is located at a latitude ~20 degrees North on the nucleus surface. Our inability to reproduce all observational characteristics shows that the real structure of the CO coma is more complex than assumed, especially in the first thousand kilometres from the nucleus. The presence of another moving CO structure, faint but compact and possibly created by an outburst, is identified., 20 pages, 26 figures. Accepted for publication in Astronomy & Astrophysics

As part of the international observing campaign in support of the NASA Deep Impact mission to comet 9P/Tempel 1, we conducted a monitoring of its outgassing at radio wavelengths. OH observations conducted at the Nancay radio telescope provided a 4-month monitoring of the comet from March to July, followed by the observation of H2O with the Odin satellite from June to August 2005. The peak of outgassing was found to be around 1028 molec. s-1 between May and July. Observations of HCN conducted with the IRAM 30 m radio telescope in May 2005 showed a variation of the production rate with a period of 1.73±0.10 days, consistent with the rotation period of the nucleus.

International audience; We report on simultaneous optical and infrared observations of the Halley Family comet 8P/Tuttle performed with the ESO Very Large Telescope. Such multi-wavelength and coordinated observations are a good example of what can be done to support space missions. From high resolution optical spectroscopy of the CN (0,0) 388 nm and NH2 (0,9,0) 610 nm bands using UVES at UT2 we determined 12C/13C = 90 ± 10 and 14N/15N = 150 ± 20 in CN and we derived a nuclear spin temperature of NH3 of 29 ± 1 K. These values are similar to those found in Oort-Cloud and Jupiter Family comets. From low resolution long slit spectroscopy with FORS1 at UT2 we determined the CN, C3 and C2 production rates and the parent and daughter scale lengths up to 5.2 105 km tailward. From high resolution IR spectroscopy with CRIRES at UT1 we measured simultaneously the production rates and mixing ratios of H2O, HCN, C2H2, CH4, C2H6, and CH3OH.

We detected hydrogen sulfide through observations of its 1 10 -1 01 rotational line at 2 mm in Comets Austin (1989c1) and Levy (1990c) using the IRAM 30-m radiotelescope. In Comet Levy, the 2 20 -2 11 line of H 2 S was also observed; the relative intensities of the two lines suggest a cold rotational distribution for H 2 S. We also unsuccessfully searched for other sulfur molecular species in Comet Levy: SO 2 , OCS, H 2 CS. From these observations, we derive molecular production rates using an out-of-equilibrium excitation model which takes into account collisions and vibrational radiative excitation. The infereed hydrogen sulfide production rate is about 0.002 that of water for both comets. H 2 S and the parent of the CS radical appear to be the main depositories of sulfur in cometary volatiles. This element seems to be depleted in cometary volatiles with respect to cosmic abundance. The presence in cometary nuclei of H 2 S, which is a highly volatile molecule, is a clue that comets formed and remained stored at very cold temperatures.

International audience; The VIRTIS (Visual IR Thermal Imaging Spectrometer) experiment has been one of the most successful experiments built in Europe for Planetary Exploration. VIRTIS, developed in cooperation among Italy, France and Germany, has been already selected as a key experiment for 3 planetary missions: the ESA-Rosetta and Venus Express and NASA-Dawn. VIRTIS on board Rosetta and Venus Express are already producing high quality data: as far as Rosetta is concerned, the Earth-Moon system has been successfully observed during the Earth Swing-By manouver (March 2005) and furthermore, VIRTIS will collect data when Rosetta flies by Mars in February 2007 at a distance of about 200 kilometres from the planet. Data from the Rosetta mission will result in a comparison – using the same combination of sophisticated experiments – of targets that are poorly differentiated and are representative of the composition of different environment of the primordial solar system. Comets and asteroids, in fact, are in close relationship with the planetesimals, which formed from the solar nebula 4.6 billion years ago. The Rosetta mission payload is designed to obtain this information combining in situ analysis of comet material, obtained by the small lander Philae, and by a long lasting and detailed remote sensing of the comet, obtained by instrument on board the orbiting Spacecraft. The combination of remote sensing and in situ measurements will increase the scientific return of the mission. In fact, the “in situ” measurements will provide “ground-truth” for the remote sensing information, and, in turn, the locally collected data will be interpreted in the appropriate context provided by the remote sensing investigation. VIRTIS is part of the scientific payload of the Rosetta Orbiter and will detect and characterise the evolution of specific signatures – such as the typical spectral bands of minerals and molecules – arising from surface components and from materials dispersed in the coma. The identification of spectral features is a primary goal of the Rosetta mission as it will allow identification of the nature of the main constituent of the comets. Moreover, the surface thermal evolution during comet approach to sun will be also studied.

A simplified algorithm to compute the infrared fluorescence cascade of molecules in cometary conditions is presented. This method could be applied to observations of 9P/Tempel 1 by Deep Impact and to future cometary infrared observations such as those of comet 67P/Churyumov-Gerasimenko by Rosetta/VIRTIS. Results are presented for water and carbon dioxide.

Spectra of comet C/1995 O1 (Hale-Bopp) were obtained with the Infrared Space Observatory (ISO) at medium resolution with the grating spectrometer in the photometer (PHT-S) and/or at high resolution with the short wavelength spectrometer (SWS) and long wavelength spectrometer (LWS) in April 1996 (Crovisier et al., 1996), September–October 1996 (Crovisier et al., 1997a, b) and December 1997, at distances from the Sun of 4.6, 2.9 and 3.9 AU, respectively. For the first time, high-resolution spectra of a comet covering the entire 2.4 to 200 μm spectral range were obtained.

International audience; Comets are among the most primitive bodies of the solar system, and their chemical composition is rich in information on the protosolar nebula and its possible connection with interstellar cloud chemistry. Comets are also a source of light atoms and, possibly, prebiotic organic molecules for the early Earth. We know better and better the abundances of cometary volatiles through spectroscopy, mainly at infrared and radio wavelengths. Another crucial component of cometary matter - organic refractories - is still poorly characterized, however.We summarize here the ˜30 abundances and ˜20 upper limits obtained on cometary volatiles and highlight a few species and problems: ethylene glycol, NS, HNC/HCN, ^{14}N/^{15}N, the origin of CN, CS_{2}, PAHs, and H_{2}O. Comet-to-comet variations and comet internal heliocentric variations can now be studied, and cometary comas can be mapped with a variety of techniques. We list a number of temperature indicators, for they can help understand the relation between IS and cometary matters, which present both a global similarity but also marked differences, such as the high ethylene glycol content of comets.We conclude by outlining a few key problems to be addressed by future ground-based and space instruments or by cometary sample analysis. For many species, we stress that laboratory data are missing on spectroscopy, photodissociation and collisions with H_{2}O.

Publisher Summary This chapter discusses the infrared spectroscopy of comets with Infrared Space Observatory (ISO). The ISO offers the opportunity to observe the spectrum of celestial bodies from 2.4 to 196 μm. This spectral range is of interest for the study of cometary dust. The chapter describes the IS0 observations of comets C/1995 O1 (Hale–Bopp) and 103P/Hartley 2, and discusses the nature of cometary dust. The apparition of the long-period, exceptional comet C/1995 01 (Hale–Bopp) coincided with the IS0 operation. The spectrum of this comet was observed as part of a target-of-opportunity program by an international team. The short-period comet 103P/Hartley 2 was observed in December 1997 to January 1998, close to its perihelion at 1 AU, as part of a guaranteed time program. 103P/Hartley 2 is a Jupiter-family comet that supposedly originated from the Edgeworth–Kuiper belt.

The volatile species released in the coma are an important clue to the composition of the cometary nucleus ices. Their identification and the measurement of their abundances is possible by remote sensing. Considerable progress has been made recently using radio and infrared spectroscopy, especially with the observations of the two exceptional comets C/1996 B2 (Hyakutake) and C/1995 01 (Hale-Bopp).) 24 molecules likely to be parent molecules outgassed from the nucleus have now been identified. Significant upper limits exist for many other species, and the presence of unidentified lines suggests that further species are to be identified. In addition, isotopic varieties have been observed for hydrogen, carbon, nitrogen and sulphur. We will review these results with a special emphasis on the reliability of the identifications and of the molecular production rate determinations. A critical point is to assess whether a given species is a genuine parent molecule outgassed from nuclear ices, or is a secondary product coming from grains or from gas-phase photochemistry. Ground-based spectral imaging, such as radio interferometry, may help resolving this problem.

Comets occupy a key position in the cycles of ices and dust in the Universe. Their nuclei have mostly kept intact the composition and morphology they had at their formation. The return of a comet close to the Earth and Sun is thus an appreciated opportunity to investigate the original state of the material which built up our Solar System. The physical processes occurring in cometary nuclei and cometary atmospheres, as well as the interface between the two media, are reviewed. The status of our present knowledge of the chemical composition and physical state of comets is presented. The recent observations of comets C/1996 B2 (Hyakutake) and C/1995 01 (Hale-Bopp), which provided a bonanza of new cometary molecules, are outlined. There are strong analogies between interstellar and cometary ices. In contrast, the silicates of cometary dust (for comet Hale-Bopp) are akin to circumstellar, rather than interstellar dust. Different families of comets may have different composition and history, however. These clues will have to be taken into consideration to achieve a consistent scenario of cometary formation.

The D/H ratio in cometary water is believed to be an important indicator of the conditions under which icy planetesimals formed and can provide clues to the contribution of comets to the delivery of water and other volatiles to Earth. Available measurements suggest that there is isotopic diversity in the comet population. The Herschel Space Observatory revealed an ocean-like ratio in the Jupiter-family comet 103P/Hartley 2, whereas most values measured in Oort-cloud comets are twice as high as the ocean D/H ratio. We present here a new measurement of the D/H ratio in the water of an Oort-cloud comet. HDO, H_2O, and H_2^18O lines were observed with high signal-to-noise ratio in comet C/2009 P1 (Garradd) using the Herschel HIFI instrument. Spectral maps of two water lines were obtained to constrain the water excitation. The D/H ratio derived from the measured H_2^16O and HDO production rates is 2.06+/-0.22 X 10**-4. This result shows that the D/H in the water of Oort-cloud comets is not as high as previously thought, at least for a fraction of the population, hence the paradigm of a single, archetypal D/H ratio for all Oort-cloud comets is no longer tenable. Nevertheless, the value measured in C/2009 P1 (Garradd) is significantly higher than the Earth's ocean value of 1.558 X 10**-4. The measured H_2^16O/H_2^18O ratio of 523+/-32 is, however, consistent with the terrestrial value., Comment: 6 pages with 4 figures and 1 table. Accepted for publication as a Letter in Astronomy & Astrophysics

High-resolution far-infrared and sub-millimetre spectroscopy of water lines is an important tool to understand the physical and chemical properties of cometary atmospheres. We present observations of several rotational ortho- and para-water transitions in comet C/2008 Q3 (Garradd) performed with HIFI on Herschel. These observations have provided the first detection of the 2_{12}-1_{01} (1669 GHz) ortho and 1_{11}-0_{00} (1113 GHz) para transitions of water in a cometary spectrum. In addition, the ground-state transition 1_{10}-1_{01} at 557 GHz is detected and mapped. By detecting several water lines quasi-simultaneously and mapping their emission we can constrain the excitation parameters in the coma. Synthetic line profiles are computed using excitation models which include excitation by collisions, solar infrared radiation, and radiation trapping. We obtain the gas kinetic temperature, constrain the electron density profile, and estimate the coma expansion velocity by analyzing the map and line shapes. We derive water production rates of 1.7-2.8 x 10^{28} s^{-1} over the range r_h = 1.83-1.85 AU., Comment: 7 figures, v2: minor changes, updated version of arXiv:1005.2969v1. Accepted for publication in Astronomy and Astrophysics, Herschel special issue on Herschel First Science Results

Spectroscopic observations of comet Hale-Boppwere undertaken near perihelion at millimetre wavelengths with the Institut de Radioastronomie Millimetrique (IRAM) telescopes at Plateau de Bure (France)and Pico Veleta (Spain). They resulted in the first detections of HCOOH,SO2, NH2CHO and HCOOCH3 in a comet. HDO was detected through its312–221 line at 225.897 GHz,complementing the observation of the 101–000 line at 464.925 GHz at the James Clerk Maxwell Telescope (Meier et al., 1998,Science 279, 842).Several unidentified lines are present in the spectra. Observations of HC3N, HNCO, OCS, SO, CN, CO+, HCO+, in addition to more ‘classical’species CO, HCN, HNC, CH3CN, CH3OH, H2CO, CS and H2S (Biveret al., this issue) permit us to make out an extensive inventory of the composition of the coma of comet Hale-Bopp at its perihelion. It presents strong analogies with gas-phase abundances measured in interstellar hot cores and bipolar flows, which are believed to reflect the composition of interstellar grains.

Comet C/1995 O1 (Hale-Bopp) has been observed on October 5 and 25, 1996 and from March 6 to March 22, 1997 with the Institut de Radioastronomie Millimetrique (IRAM) interferometer at Plateau de Bure (France). Millimetre lines of HCN,HNC, CO, H2CO, CH3OH, H2S, CS and SO were mapped with spatial resolutions of 1.5–3.5 arc sec. These observations allow us to investigate whether these species are released by the nucleus or produced in the coma by extended sources or photo-processes. The brightness distribution of the HCN J (1-0) line is consistent with release from the nucleus. The HNC J (1-0) distribution deviates from that of HCN in the innermost coma, and indicates production of HNC in the coma. This is in agreement with the heliocentric variation of the HNC/HCN ratio (Biver et al., 1997, Science 275, 1915; Irvine et al., 1998, this issue) and formation by chemical reactions (Rodgers and Charnley, 1998, Ap. J. 501, L227; Irvine et al., 1998, Nature 393, 547). There is clear evidence that SO is a photo dissociation product. The observations also confirm that H2CO is mainly produced by an extended source, as first evidenced in comet P/Halley. The contribution of the nucleus to the total H2CO production rate does not exceed 6%. The molecular lines have also been monitored hourly with the five antennas of the interferometer in single-dish mode. The line velocity shifts show aperiodic modulation linked to the nucleus rotation. The amplitude of the modulation differs from one species to another. The periodic modulation seen for the CO J (2-1) line on March 11 suggests that a significant fraction of CO is released continuously night and day by an active source situated at equatorial latitudes on the nucleus surface.

Synthetic spectra of Comet Halley between 2.5 and 15 μm are calculated on the basis of current cometary models. This study shows that molecules which are most likely detectable in the infrared range include H 2 O, CO, CO 2 , CH 4 , NH 3 , N 2 H 4 , and H 2 CO. The flux emitted by these molecules, mostly due to resonant scattering, should be sufficient for detection by a flyby mission.

The radio observations of the 18 cm wavelength OH main lines in comets Kobayashi-Berger-Milon (1975 h) and West (1975 n) confirm the model of ultraviolet pumping by the sun, that we proposed to explain our previous observations of comet Kohoutek (1973 f). The OH scale length in (1975 n) is at least 106 km at a heliocentric distance of 1 AU, in agreement with that for (1973 f).

The IKS infrared spectro-photometer will fly on board the VEGA platforms. It is designed to characterize the size, temperature and emissivity of the Comet Halley nucleus, to identify the major gaseous components of the inner coma and to detect the emission of the cometary grains. This paper presents the “calibration” experiments required to reduce the raw data: (i) absolute wavelength calibration of the filter wheels; (ii) modeling of the internal signal, as a function of the temperature of the different sub-systems; (iii) absolute and spectral responsivities of each of the spectrometric and photometric channels, as a function of the wavelength and position of the source in the field of view. Finally, we shall indicate the expected S/N ratios.

In order to prepare infrared sounding of comet Halley from the flyby VEGA probes, we have computed the synthetic spectrum between 2.5 and 15 μ of a typical comet at a heliocentric distance of ∼ 0.8 AU. The present paper is particularly devoted to the contribution from the cometary gases. For a selection of 20 possible parent molecules, the most efficient excitation process is resonant fluorescence by the solar radiation field. The H 2 O, CO, CO 2 , CH 4 , NH 3 and H 2 CO molecules are the best candidates for detection by the IKS infrared spectrometers aboard the VEGA probes. For the water molecule, collisions are too rare to ensure thermal equilibrium in the whole coma ; therefore a limited number of fluorescence lines are expected to be present in the H 2 O vibrational bands.

The IKS instrument carried by the Vega 1 spacecraft performed infrared sounding of the inner coma and nuclear region of comet Halley. An emissive centre, a few kilometres in size, was detected with a temperature significantly in excess of 300 K. In the surrounding central coma, hydrocarbon species or other carbonaceous material seem to be present. The fluorescence signatures of H2O and CO2 were identified.

The Infrared thermal emission of the nucleus has been observed by the imaging channel of the infrared spectrometer IKS during the fly-by of Comet Halley by the VEGA 1 probe. An emissive region with a temperature in excess of 300 K has been detected. The results are compatible with a simplified model assuming a spherical nucleus covered by an insulating black material.

The infrared thermal emission of the nucleus of Halley’s comet was measured by the imaging channel of the IKS spectrometer in two wavelengths bands (7–10 µm and 9–14 µm) and in two perpendicular directions of analysis. The effective dimensions of the infrared emissive region in these directions are estimated from a simulation of the flyby configuration, and compared to observations made in the visible range. A colour temperature larger than 360 K was deduced using on-ground preflight calibrations. A temperature of the same order is obtained independently, comparing the absolute value of the measured fluxes with model simulations. Such temperatures, significantly higher than the sublimating temperature of ices, and near the blackbody temperatures at 0.8 au, suggest the existence of an essentially inactive dark crust covering a significant part of the surface of the nucleus.

High resolution Fourier-transform spectra of comet P/Halley have been recorded at the CFH 3.60m telescope (Hawaii) on 1985 December 20–23, and 1986 April 28–30. The spectral interval ranges from 3800 to 12000cm–1 (0.83 to 2.6 μm) and the spectral resolution is equal to 10cm-1. Dust temperatures of 300 ± 40 K and 240 ± 40 K are derived for 1985 December 20−23, and 1986 April 28–30, respectively, from the slope of the thermal component. Bands of the CN red system are observed at 1.46 μm (1–2), 1.10 μm (0–0), 0.94 μm (2–1), and 0.92 μm (1–0). In addition, an emission feature centered at 2.44 μm (4100 cm−1) is detected. We tentatively identify this feature as the v 1 + v 3 - 2v2 band of H2O. Ground-based monitoring of this feature on future bright comets might be a direct method for measuring their H2O production rate.

In order to interpret infrared observations of parent molecules in comets, we have developed an excitation model which takes into account excitation by collisions, pumping of the fundamental bands of vibration by the infrared solar radiation field, and radiation trapping in the rotational and rovibrational lines. This model allows us to derive infrared synthetic spectra for various species. It is applied here to interpret recent infrared observations of water in comet P/Halley. The saturation of the 2.7 μm water band observed by the IKS instrument on Vega is evaluated. The relative water line intensities observed by the KAO in March 1986 are well fitted by the model, when a coma kinetic temperature of ~ 60 K is assumed in the r ≳ 1000 km region.

Results of the 2.5–5 micron spectroscopic channel of the IKS instrument on Vega are reported and the data reduction process is described. H2O and CO2 molecules have been detected with production rates of 1030 s−1 and 1.5 1028 s−1 respectively. Emission features between 3.3 and 3.7 microns are tentatively attributed to CH - bearing compounds - CO is marginally detected with a mixing ratio CO/H2O ⩽ 0.2. OH emission and H2O - ice absorption might also be present in the spectra.

The 1667 MHz and 1665 MHz transitions of the OH radical were detected and monitored nearly daily in comet Halley 1982i with the Nancay radio telescope. The observations cover the period July 1985 to July 1986 and provide a unique test for the excitation mechanisms of the OH molecule and the kinematic models of the OH coma. The heliocentric velocity range was fully observed between −26.8 and 26.8 kms−1 and 6 line reversals were covered. The OH-parent production rate of P/Halley was measured between 3.4 AU (pre-perihelion) and 2.8 AU from the Sun (post-perihelion). We find new effects like significant short-term variations of the OH-parent production rate in a few days, large variations of the half power profile width and anomalous line shapes.

A detailed analysis is presented of observations of the CO (2-1) line at 230 GHz in Jupiter performed with the IRAM 30 m telescope after the collision of comet Shoemaker-Levy 9. Observations of site K, 33 h after impact, are focused on. Using independent information on the perturbed jovian stratospheric thermal profile, the CO column density, vertical distribution and total mass deposited at the K impact site are derived. It is found that CO is restricted to upper stratospheric levels, at pressures less than 50–200 μbar, with an average mixing ratio of about 1.5 × 10 −4 . The total CO mass is (1.5 ± 0.6) × 10 14 g. It is shown that these results are generally consistent with observations of CO at other wavelengths. The K fragment mass is inferred to be in the range (1–10) × 10 14 g, where the largest source of uncertainty comes from the unknown behavior of the cometary dust in the shock chemistry. Comparison with observations performed at IRAM over a 1 yr period following the impacts indicates that the CO mass brought into Jupiter's atmosphere by the SL9 fragments has remained approximately constant over this period.

Following the selection of comet 67P/Churyumov-Gerasimenko as the new target of the Rosetta/ESA mission, observations of this comet were performed at the 30-m telescope of the Institut de radioastronomie millimetrique in May–June 2003. Observations focussed on the CO J(2–1) and J(1–0) lines and dust thermal continuum at 1.2 mm in order to constrain the CO and dust productions at 3 AU from the Sun. Neither the CO lines, nor the continuum emission were detected. The derived 3–σ upper limit on the CO production rate is QCO < 9.2 × 1026 molecules s−1. That derived for the dust production rate is strongly model dependent, and at best is Qdust < 36 kg s−1. The first synthetic CO line profiles computed with a 3-D gas-dynamical simulation of a gas coma are presented.