The millimeter spectrum of Titan: Detectability of HCN,HC3N,and CH3CN and the CO abundance

The flux emitted by Titan's disk in millimeter lines of HCN, HC₃N, CH₃CN, and CO is calculated by means of a radiative transfer formulation which takes into account the sphericity of the atmosphere. It is demonstrated that the plane-parallel approximation for radiative transfer is no longer valid, especially in the core of emission lines, when Titan is not spatially resolved. The antenna temperatures which would be measured by large radiotelescopes observing Titan at frequencies of (1?0) and (2?1) transitions of CO, of (1?0), (2?1), and (3?2) transitions of HCN, and of selected transitions of HC₃N and CH₃CN in the range 80–300 GHz are calculated. The observability of these transitions is investigated. It is concluded that there is the possibility of inferring the vertical stratospheric distribution of these species from line shape measurements to be achieved with existing or forthcoming radioastronomical instrumentation. The determination of the CO abundance by D. O. Muhleman, G. L. Berge, and R. T. Clancy (1984, (Science (Washington, D.C.), 223, 393–396) from measurements at 115.3 GHz in two 200 MHz bands, is reinterpreted by means of this radiative transfer formulation. A CO mixing ratio between 3 × 10^?5 and 18 × 10^?5, with a most plausible value of 7.5 × 10^?5, is found.     

Vertical profiles of HCN, HC3N, and C2H2 in Titan's atmosphere derived from Cassini/CIRS data

Mid-infrared limb spectra in the range 600-1400 cm⁻¹ taken with the Composite InfraRed Spectrometer (CIRS) on-board the Cassini spacecraft were used to determine vertical profiles of HCN, HC₃N, C₂H₂, and temperature in Titan's atmosphere. Both high (0.5 cm⁻¹) and low (13.5 cm⁻¹) spectral resolution data were used. The 0.5 cm⁻¹ data gave profiles at four latitudes and the 13.5 cm⁻¹ data gave almost complete latitudinal coverage of the atmosphere. Both datasets were found to be consistent with each other. High temperatures in the upper stratosphere and mesosphere were observed at Titan's northern winter pole and were attributed to adiabatic heating in the subsiding branch of a meridional circulation cell. On the other hand, the lower stratosphere was much colder in the north than at the equator, which can be explained by the lack of solar radiation and increased IR emission from volatile enriched air. HC₃N had a vertical profile consistent with previous ground based observations at southern and equatorial latitudes, but was massively enriched near the north pole. This can also be explained in terms of subsidence at the winter pole. A boundary observed at 60° N between enriched and un-enriched air is consistent with a confining polar vortex at 60° N and HC₃N's short lifetime. In the far north, layers were observed in the HC₃N profile that were reminiscent of haze layers observed by Cassini's imaging cameras. HCN was also enriched over the north pole, which gives further evidence for subsidence. However, the atmospheric cross section obtained from 13.5 cm⁻¹ data indicated a HCN enriched layer at 200-250 km, extending into the southern hemisphere. This could be interpreted as advection of polar enriched air towards the south by a meridional circulation cell. This is observed for HCN but not for HC₃N due to HCN's longer photochemical lifetime. C₂H₂ appears to have a uniform abundance with altitude and is not significantly enriched in the north. This is consistent with observations from previous CIRS analysis that show increased abundances of nitriles and hydrocarbons but not C₂H₂ towards the north pole.     

Latitudinal variations of HCN, HC3N, and C2N2 in Titan's stratosphere derived from Cassini CIRS data

Mid- and far-infrared spectra from the Composite InfraRed Spectrometer (CIRS) have been used to determine volume mixing ratios of nitriles in Titan's atmosphere. HCN, HC₃N, C₂H₂, and temperature were derived from 2.5 cm⁻¹ spectral resolution mid-IR mapping sequences taken during three flybys, which provide almost complete global coverage of Titan for latitudes south of 60° N. Three 0.5 cm⁻¹ spectral resolution far-IR observations were used to retrieve C₂N₂ and act as a check on the mid-IR results for HCN. Contribution functions peak at around 0.5-5 mbar for temperature and 0.1-10 mbar for the chemical species, well into the stratosphere. The retrieved mixing ratios of HCN, HC₃N, and C₂N₂ show a marked increase in abundance towards the north, whereas C₂H₂ remains relatively constant. Variations with longitude were much smaller and are consistent with high zonal wind speeds. For 90°-20° S the retrieved HCN abundance is fairly constant with a volume mixing ratio of around 1 × 10⁻⁷ at 3 mbar. More northerly latitudes indicate a steady increase, reaching around 4 × 10⁻⁷ at 60° N, where the data coverage stops. This variation is consistent with previous measurements and suggests subsidence over the northern (winter) pole at approximately 2 × 10⁻⁴ m s⁻¹. HC₃N displays a very sharp increase towards the north pole, where it has a mixing ratio of around 4 × 10⁻⁸ at 60° N at the 0.1-mbar level. The difference in gradient for the HCN and HC₃N latitude variations can be explained by HC₃N's much shorter photochemical lifetime, which prevents it from mixing with air at lower latitude. It is also consistent with a polar vortex which inhibits mixing of volatile rich air inside the vortex with that at lower latitudes. Only one observation was far enough north to detect significant amounts of C₂N₂, giving a value of around 9 × 10⁻¹⁰ at 50° N at the 3-mbar level.     

Clouds, haze, and CH4, CH3D, HCN, and C2H2 in the atmosphere of Titan probed via 3 μm spectroscopy

Using synthetic spectra derived from an updated model atmosphere together with a continuum model that includes contributions from haze, cloud and ground, we have re-analyzed the recently published (Geballe et al., 2003, Astrophys. J. 583, L39-L42) high-resolution 3 μm spectrum of Titan which contains newly-detected bands of HCN (in emission) and C₂H₂ and CH₃D (in absorption), in addition to previously detected bands of CH₄. In the 3.10-3.54 μm interval the analysis yields strong evidence for the existence of a cloud deck or optically thick haze layer at about the 10 mbar (∼ 100 km) level. The haze must extend well above this altitude in order to mask the strong CH₄ lines at 3.20-3.50 μm. These cloud and haze components must be transparent at 2.87-2.92 μm, where analysis of the CH₃D spectrum demonstrates that Titan's surface is glimpsed through a second cloud deck at about the 100 mbar (∼ 50 km) level. Through a combination of areal distribution and optical depth this cloud deck has an effective transmittance of ∼ 20%. The spectral shape of Titan's continuum indicates that the higher altitude cloud and haze particles responsible for suppressing the CH₄ absorptions have a largely organic make-up. The rotational temperature of the HCN ranges from 140 to 180 K, indicating that the HCN emission occurs over a wide range of altitudes. This emission, remodeled using an improved collisional deactivation rate, implies mesospheric mixing ratio curves that are consistent with previously predictions. The stratospheric and mesospheric C₂H₂ mixing ratios are ∼¹⁰⁻⁵, considerably less than previous model predictions (Yung et al., 1984), but approximately consistent with recent observational results. Upper limits to mixing ratios of HC₃N and C₄H₂ are derived from non-detections of those species near 3.0 μm.     

Titan's stratospheric C2N2, C3H4, and C4H2 abundances from Cassini/CIRS far-infrared spectra

Far-IR (25-50 μm, 200-400 cm⁻¹) nadir and limb spectra measured during Cassini's four year prime mission by the Composite InfraRed Spectrometer (CIRS) instrument have been used to determine the abundances of cyanogen (C₂N₂), methylacetylene (C₃H₄), and diacetylene (C₄H₂) in Titan's stratosphere as a function of latitude. All three gases are enriched at northern latitudes, consistent with north polar subsidence. C₄H₂ abundances agree with those derived previously from mid-IR data, but C₃H₄ abundances are about 2 times lower, suggesting a vertical gradient or incorrect band intensities in the C₃H₄ spectroscopic data. For the first time C₂N₂ was detected at southern and equatorial latitudes with an average volume mixing ratio of 5.5±1.4×¹⁰−11 derived from limb data (>3-σ significance). This limb result is also corroborated by nadir data, which give a C₂N₂ volume mixing ratio of 6±3×¹⁰−11 (2-σ significance) or alternatively a 3-σ upper limit of 17×¹⁰−11. Comparing these figures with photochemical models suggests that galactic cosmic rays may be an important source of N₂ dissociation in Titan's stratosphere. Like other nitriles (HCN, HC₃N), C₂N₂ displays greater north polar relative enrichment than hydrocarbons with similar photochemical lifetimes, suggesting an additional loss mechanism for all three of Titan's main nitrile species. Previous studies have suggested that HCN requires an additional sink process such as incorporation into hazes. This study suggests that such a sink may also be required for Titan's other nitrile species.     

Radioastronomical observations of comets IRAS-Araki-Alcock (1983d) and Sugano-Saigusa-Fujikawa (1983e)

Detections and upper limits to the continuum emission (1 ≤ λ ≤6 cm) and spectral line emission (OH, CO, CS, HCN, HCO⁺, CN, CH₃CN, CH₃C₂H, NH₃, H₂O, HC₃N, CH₃CH₂CN) are reported from radio observations of Comets 1983d and 1983e. Comparison is made with observations of CN at optical wavelengths. These results may be useful in planning future cometary observations.     

Vertical abundance profiles of hydrocarbons in Titan's atmosphere at 15° S and 80° N retrieved from Cassini/CIRS spectra

Limb spectra recorded by the Composite InfraRed Spectrometer (CIRS) on Cassini provide information on abundance vertical profiles of C₂H₂, C₂H₄, C₂H₆, CH₃C₂H, C₃H₈, C₄H₂, C₆H₆ and HCN, along with the temperature profiles in Titan's atmosphere. We analyzed two sets of spectra, one at 15° S (Tb flyby) and the other one at 80° N (T3 flyby). The spectral range 600-1400 cm⁻¹, recorded at a resolution of 0.5 cm⁻¹, was used to determine molecular abundances and temperatures in the stratosphere in the altitude range 100-460 km for Tb and 170-495 km for T3. Both temperature profiles show a well defined stratopause, at around 310 km (0.07 mbar) and 183 K at 13° S, and 380 km (0.01 mbar) with 207 K at 80° N. Near the north pole, stratospheric temperatures are colder and mesospheric temperatures are warmer than near the equator. C₂H₂, C₂H₆, C₃H₈ and HCN display vertical mixing ratio profiles that increase with height at 15° S and 80° N, consistent with their formation in the upper atmosphere, diffusion downwards and condensation in the lower stratosphere, as expected from photochemical models. The CH₃C₂H and C₄H₂ mixing ratios also increase with height at 15° S. But near the north pole, their profiles present an unexpected minimum around 300 km, observed for the first time thanks to the high vertical resolution of the CIRS limb data. C₂H₄ is the only molecule having a vertical abundance profile that decreases with height at 15° S. At 80° N, it also displays a minimum of its mixing ratio around the 0.1-mbar level. For C₆H₆, an upper limit of 1.1 ppb (in the 0.3-10 mbar range) is derived at 15° S, whereas a constant mixing ratio profile of is inferred near the north pole. At 15° S, the vertical profile of HCN exhibits a steeper gradient than other molecules, which suggests that a sink for this molecule exists in the stratosphere, possibly due to haze formation. All molecules display a more or less pronounced enrichment towards the north pole, probably due, in part, to subsidence of air at the north (winter) pole that brings air enriched in photochemical compounds from the upper atmosphere to lower levels.     

The origin of the CN radical in comets: A review from observations and models

The origin of CN radicals in comets is not completely understood so far. We present a study of CN and HCN production rates and CN Haser scale lengths showing that: (1) at heliocentric distances larger than 3 AU, CN radicals could be entirely produced by HCN photolysis; (2) closer to the Sun, for a fraction of comets CN production rates are higher than HCN ones whereas (3) in the others, CN distribution cannot be explained by the HCN photolysis although CN and HCN production rates seem to be similar. Thus, when the comets are closer than 3 AU to the Sun, an additional process to the HCN photolysis seems to be required to explain the CN density in some comets.The photolysis of HC₃N or C₂N₂ could explain the CN origin. But the HC₃N production rate is probably too low to reproduce CN density profile, even if uncertainties on its photolysis leave the place for all possible conclusions. The presence of C₂N₂ in comets is a reliable hypothesis to explain the CN origin; thus, its detection is a challenging issue. Since C₂N₂ is very difficult to detect from ground-based observations, only in situ measurements or space observations could determine the contribution of this compound in the CN origin.Another hypothesis is a direct production of CN radicals by the photo- or thermal degradation of complex refractory organic compounds present on cometary grains. This process could explain the spatial profile of CN inside jets and the discrepancy noted in the isotopic ratio ¹⁴N/¹⁵N between CN and HCN. Laboratory studies of the thermal and UV-induced degradation of solid nitrogenated compounds are required to model and validate this hypothesis.     

Particle size and abundance of HC3N ice in Titan’s lower stratosphere at high northern latitudes

Up to now, there has been no corroboration from Cassini CIRS of the Voyager IRIS-discovery of cyanoacetylene (HC₃N) ice in Titan’s thermal infrared spectrum. We report the first compelling spectral evidence from CIRS for the ν₆ HC₃N ice feature at 506 cm⁻¹ at latitudes 62°N and 70°N, from which we derive particle sizes and column abundances in Titan’s lower stratosphere. We find mean particle radii of 3.0 μm and 2.3 μm for condensed HC₃N at 62°N and 70°N, respectively, and corresponding ice phase molecular column abundances in the range 1-10 × 10¹⁶ mol cm⁻². Only upper limits for cloud abundances can be established at latitudes of 85°N, 55°N, 30°N, 10°N, and 15°S. Under the assumption that cloud tops coincide with the uppermost levels at which HC₃N vapor saturates, we infer geometric thicknesses for the clouds equivalent to 10-20 km or so, with tops at 165 km and 150 km at 70°N and 62°N, respectively.     

The Titan N/N and C/C isotopic ratios in HCN from Cassini/CIRS

We report the detection of H¹³CN and HC¹⁵N in mid-infrared spectra recorded by the Composite Infrared Spectrometer (CIRS) aboard Cassini, along with the determination of the ¹²C/¹³C and ¹⁴N/¹⁵N isotopic ratios. We analyzed two sets of limb spectra recorded near 13-15° S (Tb flyby) and 83° N (T4 flyby) at 0.5 cm⁻¹ resolution. The spectral range 1210-1310 cm⁻¹ was used to retrieve the temperature profile in the range 145-490 km at 13° S and 165-300 km at 83° N. These two temperature profiles were then incorporated in the atmospheric model to retrieve the abundance profile of H¹²C¹⁴N, H¹³CN and HC¹⁵N from their bands at 713, 706 and 711 cm⁻¹, respectively. The HCN abundance profile was retrieved in the range 90-460 km at 15° S and 165-305 km at 83° N. There is no evidence for vertical variations of the isotopic ratios. Constraining the isotopic abundance profiles to be proportional to the HCN one, we find at 15° S, and at 83° N, two values that are statistically consistent. A combination of these results yields a ¹²C/¹³C value equal to 75±12. This global result, as well as the 15° S one, envelop the value in Titan's methane (82.3±1) [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779-784] measured at 10° S and is slightly lower than the terrestrial inorganic standard value (89). The ¹⁴N/¹⁵N isotopic ratio is found equal to at 15° S and at 83° N. Combining the two values yields ¹⁴N/¹⁵N = 56 ± 8, which corresponds to an enrichment in ¹⁵N of about 4.9 compared with the terrestrial ratio. These results agree with the values obtained from previous ground-based millimeter observations [Hidayat, T., Marten, A., Bézard, B., Gautier, D., Owen, T., Matthews, H.E., Paubert, G., 1997. Icarus 126, 170-182; Marten, A., Hidayat, T., Biraud, Y., Moreno, R., 2002. Icarus 158, 532-544]. The ¹⁵N/¹⁴N ratio found in HCN is ∼3 times higher than in N₂ [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779-784], which implies a large fractionation process in the HCN photochemistry.     

High-resolution spectroscopy of Saturn at 3 microns: CH4, CH3D, C2H2, C2H6, PH3, clouds, and haze

We report observation and analysis of a high-resolution 2.87-3.54 μm spectrum of the southern temperate region of Saturn obtained with NIRSPEC at Keck II. The spectrum reveals absorption and emission lines of five molecular species as well as spectral features of haze particles. The ν₂+ν₃ band of CH₃D is detected in absorption between 2.87 and 2.92 μm; and we derived from it a mixing ratio approximately consistent with the Infrared Space Observatory result. The ν₃ band of C₂H₂ also is detected in absorption between 2.95 and 3.05 μm; analysis indicates a sudden drop in the C₂H₂ mixing ratio at 15 mbar (130 km above the 1 bar level), probably due to condensation in the low stratosphere. The presence of the ν₃+ν₉+ν₁₁ band of C₂H₆ near 3.07 μm, first reported by Bjoraker et al. [Bjoraker, G.L., Larson, H.P., Fink, U., 1981. Astrophys. J. 248, 856-862], is confirmed, and a C₂H₆ condensation altitude of 10 mbar (140 km) in the low stratosphere is determined. We assign weak emission lines within the 3.3 μm band of CH₄ to the ν₇ band of C₂H₆, and derive a mixing ratio of 9±4×¹⁰⁻⁶ for this species. Most of the C₂H₆ 3.3 μm line emission arises in the altitude range 460-620 km (at ∼μbar pressure levels), much higher than the 160-370 km range where the 12 μm thermal molecular line emission of this species arises. At 2.87-2.90 μm the major absorber is tropospheric PH₃. The cloud level determined here and at 3.22-3.54 is 390-460 mbar (∼30 km), somewhat higher than found by Kim and Geballe [Kim, S.J., Geballe, T.R., 2005. Icarus 179, 449-458] from analysis of a low resolution spectrum. A broad absorption feature at 2.96 μm, which might be due to NH₃ ice particles in saturnian clouds, is also present. The effect of a haze layer at about 125 km (∼12 mbar level) on the 3.20-3.54 μm spectrum, which was not apparent in the low resolution spectrum, is clearly evident in the high resolution data, and the spectral properties of the haze particles suggest that they are composed of hydrocarbons.     

The mesosphere and lower thermosphere of Titan revealed by Cassini/UVIS stellar occultations

Stellar occultations observed by the Cassini/UVIS instrument provide unique data that probe the mesosphere and lower thermosphere of Titan at altitudes between 400 and 1400 km. This region is a site of complex photochemistry that forms hydrocarbon and nitrile species, and plays a crucial role in the formation of the organic hazes observed in the stratosphere, but has yet to be adequately characterized. We analyzed publicly available data obtained between flybys Tb in December 2004 and T58 in July 2009, with an emphasis on two stable occultations obtained during flybys T41 and T53. We derived detailed density profiles for CH₄, C₂H₂, C₂H₄, C₄H₂, HCN, HC₃N and C₆H₆ between ∼400 and 1200 km and extinction coefficients for aerosols between 400 and 900 km. Our analysis reveals the presence of extinction layers in the occultation data that are associated with large perturbations in the density profiles of the gaseous species and extinction profiles of the aerosols. These relatively stable features vary in appearance with location and change slowly over time. In particular, we identify a sharp extinction layer between 450 and 550 km that coincides with the detached haze layer. In line with recent images obtained by Cassini/ISS, the altitude of this layer changes rapidly around the equinox in 2009. Our results point to unexpected complexity that may have significant consequences for the dynamics and physical processes taking place in the upper atmosphere of Titan.     

Cassini CIRS update on stratospheric ices at Titan's winter pole

A strong, broad spectral emission feature at 85° N latitude centered at 221 cm⁻¹ remains unidentified after candidate ices of H₂O and pure crystalline CH₃CH₂CN are unambiguously ruled out. A much shallower weak emission feature starts at 160 cm⁻¹ and blends into the strong feature at ∼190 cm⁻¹. This feature is consistent with one formed by an HCN ice cloud composed of ?5 μm radius particles that resides in the lower stratosphere somewhere below an altitude of 160 km. Titan's stratospheric aerosol appears to have a spectral emission feature at about 148 cm⁻¹. The aerosol abundance at 85° N is about a factor 2.2 greater than at 55° S.     

IR band intensities of DC3N and HC3N: Implication for observations of Titan's atmosphere

We have obtained the infrared spectra and the corresponding absolute band intensities for two HC₃N isotopomers: DC₃N and HC₃¹⁵N. Our results for DC₃N are in good agreement with previous measurements except for the ν₂ and ν₃ stretching modes. For HC₃¹⁵N, this study is the first including intensity measurements.We have also studied the possible detection of these isotopomers in Titan's atmosphere using the CIRS spectrograph onboard the Cassini spacecraft. Our simulation of the expected spectra shows that for a signal-to-noise ratio better than 100, the ¹⁵N isotopomer of HC₃N could be detected. But, further study of HC₃N hot bands are needed since some of them overlap the HC₃¹⁵N Q-branch.     

Sounding of Titan’s atmosphere at submillimeter wavelengths from an orbiting spacecraft

An investigation of the capabilities and science goals of a submillimeter-wave heterodyne sounder onboard a Titan orbiter is presented. Based on a model of Titan’s submillimeter spectrum, and including realistic instrumental performances, we show that passive limb observations of Titan’s submillimeter radiation would bring novel and unique information on the dynamical and chemical state of Titan’s atmosphere, particularly in the so far poorly probed 500-900 km region. The 300-360, 540-660 and 1080-1280 GHz spectral ranges appear especially promising, and could be explored with an instrument equipped with a tunable local oscillator system. Vertical temperature profiles can be determined up to ∼1200 km using rotational lines of CH₄, CO, and HCN. Winds can be measured over the 200-1200 km altitude range with an accuracy of 3-5 m/s from Doppler shift measurements of any strong optically thin line. Numerous molecular species are accessible, including H₂O, NH₃, CH₃C₂H, CH₂NH, and several nitriles (HC₃N, HC₅N, CH₃CN, and C₂H₃CN). Many of them are expected to be detectable in a large fraction of the atmosphere and in some cases at all levels, providing an observational link between stratospheric and thermospheric chemistry. Isotopic variants of some of these species can also be measured, providing new measurements of H, C, N, and O isotopic ratios. Mapping of the thermal, wind, and composition fields, best achieved from a polar orbit and with an articulated antenna, would provide a new view of the couplings between chemistry and dynamics over an extended altitude range of Titan’s atmosphere. Additional science goals at Saturn and Enceladus are briefly discussed.     

Spectroscopic Observations of Comet C/1996 B2 (Hyakutake) with the Caltech Submillimeter Observatory

The apparition of Comet C/1996 B2 (Hyakutake) offered an unexpected and rare opportunity to probe the inner atmosphere of a comet with high spatial resolution and to investigate with unprecedented sensitivity its chemical composition. We present observations of over 30 submillimeter transitions of HCN, H¹³CN, HNC, HNCO, CO, CH₃OH, and H₂CO in Comet Hyakutake carried out between 1996 March 18 and April 9 at the Caltech Submillimeter Observatory. Detections of the H¹³CN (4–3) and HNCO (16_0,16–15_0,15) transitions represent the first observations of these species in a comet. In addition, several other transitions, including HCN (8–7), CO (4–3), and CO (6–5) are detected for the first time in a comet as is the hyperfine structure of the HCN (4–3) line. The observed intensities of the HCN (4–3) hyperfine components indicate a line center optical depth of 0.9 ± 0.2 on March 22.5 UT. The HCN/HNC abundance ratio in Comet Hyakutake at a heliocentric distance of 1 AU is similar to that measured in the Orion extended ridge— a warm, quiescent molecular cloud. The HCN/H¹³CN abundance ratio implied by our observations is 34 ± 12, similar to that measured in giant molecular clouds in the galactic disk but significantly lower than the Solar System¹²C/¹³C ratio. The low HCN/H¹³CN abundance ratio may be in part due to contamination by an SO₂line blended with the H¹³CN (4–3) line. In addition, chemical models suggest that the HCN/H¹³CN ratio can be affected by fractionation during the collapse phase of the protosolar nebula; hence a low HCN/H¹³CN ratio observed in a comet is not inconsistent with the solar system¹²C/¹³C isotopic ratio. The abundance of HNCO relative to water derived from our observations is (7 ± 3) × 10⁻⁴. The HCN/HNCO abundance ratio is similar to that measured in the core of Sagittarius B2 molecular cloud. Although a photo-dissociative channel of HNCO leads to CO, the CO produced by HNCO is a negligible component of cometary atmospheres. Production rates of HCN, CO, H₂CO, and CH₃OH are presented. Inferred molecular abundances relative to water are typical of those measured in comets at 1 AU from the Sun. The exception is CO, for which we derive a large relative abundance of 30%. The evolution of the HCN production rate between March 20 and March 30 suggests that the increased activity of the comet was the cause of the fragmentation of the nucleus. The time evolution of the H₂CO emission suggests production of this species from dust grains.     

A Multi-Wavelength Simultaneous Study of the Composition of the Halley Family Comet 8P/Tuttle

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.     

New Molecular Species in Comet C/1995 O1 (Hale-Bopp) Observed with the Caltech Ssubmillimeter Observatory

We present millimeter-wave observations of HNCO, HC₃N, SO, NH₂CHO, H¹³CN, and H₃O⁺ in comet C/1995 O1 (Hale-Bopp)obtained in February–April, 1997 with the Caltech Submillimeter Observatory (CSO). HNCO, first detected at the CSO in comet C/1996B2 (Hyakutake), is securely confirmed in comet Hale-Bopp via observations of three rotational transitions. The derived abundance with respect to H₂O is (4-13) × 10^-4. HC₃N, SO, and NH₂CHO are detected for the first time in a comet. The fractional abundance of HC₃N based on observations of three rotational lines is (1.9 ± 0.2) × 10^-4. Four transitions of SO are detected and the derived fractional abundance, (2-8) ×10^-3, is higher than the upper limits derived from UV observations of previous comets. Observations of NH₂CHO imply a fractional abundance of (1-8) × 10^-4. H₃O is detected for the first time from the ground. The H¹³CN (3-2)transition is also detected and the derived HCN/H¹³CN abundance ratio is 90 ± 15, consistent with the terrestrial¹²C/¹³C ratio. In addition, a number of other molecular species are detected, including HNC, OCS, HCO⁺, CO⁺, and CN(the last two are first detections in a comet at radio wavelengths). This revised version was published online in July 2006 with corrections to the Cover Date.     

A Molecular Survey of Comet C/1995 O1 (Hale-Bopp) at the IRAM Telescopes

Spectroscopic observations of comet Hale-Boppwere undertaken near perihelion at millimetre wavelengths with the Institut de Radioastronomie Millimétrique (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. This revised version was published online in July 2006 with corrections to the Cover Date.     

Infrared study of ion-irradiated N2-dominated ices relevant to Triton and Pluto: formation of HCN and HNC

Infrared spectra and radiation chemical behavior of N₂-dominated ices relevant to the surfaces of Triton and Pluto are presented. This is the first systematic IR study of proton-irradiated N₂-rich ices containing CH₄ and CO. Experiments at 12 K show that HCN, HNC, and diazomethane (CH₂N₂) form in the solid phase, along with several radicals. NH₃ is also identified in irradiated N₂ + CH₄ and N₂ + CH₄ + CO. We show that HCN and HNC are made in irradiated binary ice mixtures having initial N₂/CH₄ ratios from 100 to 4, and in three-component mixtures have an initial N₂/(CH₄ + CO) ratio of 50. HCN and HNC are not detected in N₂-dominated ices when CH₄ is replaced with C₂H₆, C₂H₂, or CH₃OH.The intrinsic band strengths of HCN and HNC are measured and used to calculate G(HCN) and G(HNC) in irradiated N₂ + CH₄ and N₂ + CH₄ + CO ices. In addition, the HNC/HCN ratio is calculated to be ∼1 in both icy mixtures. These radiolysis results reveal, for the first time, solid-phase synthesis of both HCN and HNC in N₂-rich ices containing CH₄.We examine the evolution of spectral features due to acid-base reactions (acids such as HCN, HNC, and HNCO and a base, NH₃) triggered by warming irradiated ices from 12 K to 30-35 K. We identify anions (OCN⁻, CN⁻, and N3−) in ices warmed to 35 K. These ions are expected to form and survive on the surfaces of Triton and Pluto. Our results have astrobiological implications since many of these products (HCN, HNC, HNCO, NH₃, NH₄OCN, and NH₄CN) are involved in the syntheses of biomolecules such as amino acids and polypeptides.