The organic composition of Comet C/2001 A2 (LINEAR): I. Evidence for an unusual organic chemistry

We used the NIRSPEC instrument on the Keck-2 telescope atop Mauna Kea, HI to observe Comet C/2001 A2 (LINEAR) in a Target of Opportunity campaign on UT 2001 July 9.5, 10.5 August 4.4, 10.5. We measured seven organic parent volatiles (C₂H₆, C₂H₂, HCN, CH₄, CO, CH₃OH, H₂CO) simultaneously with H₂O. We obtained absolute production rates and relative abundances for parent volatiles, and also measured rotational temperatures for several of these species. The chemical composition of C/2001 A2 differs substantially from any comet we have observed to date. The abundances we measure (relative to H₂O) for C₂H₆, C₂H₂, HCN, and CH₃OH are enriched by a factor of ∼2 to 3 in C/2001 A2 compared with most comets in our database. Other molecular species were detected within the typical range of measured abundances. C/2001 A2 presented a unique opportunity to study the chemistry of a fragmenting comet where pristine areas are exposed to the Sun.     

Methane in Oort cloud comets

We detected CH₄ in eight Oort cloud comets using high-dispersion (λ/Δλ∼2×10⁴) infrared spectra acquired with CSHELL at NASA's IRTF and NIRSPEC at the W.M. Keck Observatory. The observed comets were C/1995 O1 (Hale-Bopp), C/1996 B2 (Hyakutake), C/1999 H1 (Lee), C/1999 T1 (McNaught-Hartley), C/1999 S4 (LINEAR), C/2000 WM₁ (LINEAR), C/2001 A2 (LINEAR), and 153/P Ikeya-Zhang (C/2002 C1). We detected the R0 and R1 lines of the ν₃ vibrational band of CH₄ near 3.3 μm in each comet, with the exception of McNaught-Hartley where only the R0 line was measured. In order to obtain production rates, a fluorescence model has been developed for this band of CH₄. We report g-factors for the R0 and R1 transitions at several rotational temperatures typically found in comet comae and relevant to our observations. Using g-factors appropriate to T_rot as determined from HCN, CO and/or H₂O and C₂H₆, CH₄ production rates and mixing ratios are presented. Abundances of CH₄/H₂O are compared among our existing sample of comets, in the context of establishing their place of origin. In addition, CH₄ is compared to native CO, another hypervolatile species, and no correlation is found among the comets observed.     

A search for millimeter-wave emission from HCN,CO, and CH3CN in Comet Bradfield (1978c)

Millimeter wavelength emission from the “parent” molecules HCN, CO, and CH₃CN, the latter in both its ground and ν₈ = 1 excited vibrational states, was sought from Comet Bradfield (1978c) during March 1978 after comet perihelion, when the heliocentric distance was between 0.45 and 0.55 AU. No lines were detected, and upper limits on the molecular column densities (averaged over the antenna beam) and production rates, Q are estimated. The upper limits on Q for HCN, CH₃CN (ν₈ = 0), and CH₃CN (ν₈ = 1) are less than the production rates inferred from the millimeter-wave detection of these species in Comet Kohoutek (1973f). The CO upper limit on Q is comparable to that inferred from a detection of Comet West (1975n) in the rocket uv. It seems likely that the total gas production rate of Comet Bradfield (1978c) was relatively low.     

The organic composition of Comet C/2000 WM1 (LINEAR) revealed through infrared spectroscopy

We investigated the parent volatile composition of the Oort cloud Comet C/2000 WM₁ (LINEAR) on 23-25 November 2001, using the Near Infrared Echelle Spectrograph on the Keck II telescope. Flux-calibrated spectra, absolute production rates, and mixing ratios are presented for H₂O, HCN, CH₄, C₂H₂, C₂H₆, H₂CO, CH₃OH and CO. Compared with “organics-normal” comets, WM₁ is moderately depleted in HCN, CH₄ and CH₃OH, and is even more depleted in C₂H₂ and CO. Its composition is thus intermediate to comets that are severely depleted in their organic volatile composition and those that exhibit “normal” organic volatile abundances. We argue that WM₁ may have formed closer to the young Sun than “organics-normal” comets, but at greater distance than the severely depleted comets, before its ejection to the Oort cloud. The mixing ratios of the above-listed organic volatiles agree day-by-day for 23-25 November 2001. Thus, there is no evidence of macroscopic heterogeneity in chemistry of this comet’s nucleus at the achieved measurement accuracy. As the first comet to show moderate organic depletion in parent volatiles, WM₁ represents an important addition to the emerging taxonomic classification based on chemical composition.     

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.     

Near-infrared spectra of laboratory H2O-CH4 ice mixtures

We present 1.25-19 μm infrared spectra of pure solid CH₄ and H₂O/CH₄=87, 20, and 3 solid mixtures at temperatures from 15 to 150 K. We compare and contrast the absorptions of CH₄ in solid H₂O with those of pure CH₄. Changes in selected peak positions, profiles, and relative strength with temperature are presented, and absolute strengths for absorptions of CH₄ in solid H₂O are estimated. Using the two largest (ν₃+ν₄) and (ν₁+ν₄) near-IR absorptions of CH₄ at 2.324 and 2.377 μm (4303 and 4207 cm⁻¹), respectively, as examples, we show that peaks of CH₄ in solid H₂O are at slightly shorter wavelength (higher frequency) and broader than those of pure solid CH₄. With increasing temperature, these peaks shift to higher frequency and become increasingly broad, but this trend is reversible on re-cooling, even though the phase transitions of H₂O are irreversible. It is to be hoped that these observations of changes in the positions, profiles, and relative intensities of CH₄ absorptions with concentration and temperature will be of use in understanding spectra of icy outer Solar System bodies.     

Mixing ratios of methane,ethane, and acetylene in Neptune's stratosphere

Matching computed spectra for the ν₄ band of methane, the ν₉ band of ethane, and the R branch of the ν₅ band of acetylene to observed spectra for Neptune suggests mixing ratios of CH₄/H₂ ～ 10^?3?10^?2, C₂H₆/H₂ ～ 10^?6, and C₂H₂/H₂ ～ 10^?8 in the stratosphere.     

The 1995–2002 Long-Term Monitoring of Comet C/1995 O1 (HALE–BOPP) at Radio Wavelength

The bright comet Hale–Bopp provided the first opportunity to follow the outgassing rates of a number of molecular species over a large range of heliocentric distances. We present the results of our observing campaign at radio wavelengths which began in August 1995 and ended in January 2002. The observations were carried out with the telescopes of Nançay, IRAM, JCMT, CSO and, since September 1997, SEST. The lines of nine molecules (OH, CO, HCN, CH₃OH, H₂CO, H₂S, CS, CH₃CN and HNC) were monitored. CS, H₂S, H₂CO, CH₃CN were detected up to r_h= 3–4 AU from the Sun, while HCN and CH₃OH were detected up to 6 AU. CO, which is the main driver of cometary activity at heliocentric distances larger than 3–4 AU, was last detected in August 2001, at r_h= 14 AU. The gas production rates obtained from this programme contain important information on the nature of cometary ices, their thermal properties and sublimation mechanisms.Line shapes allow to measure gas expansion velocities, which, at large heliocentric distances, might be directly connected to the temperature of the nucleus surface. Inferred expansion velocity of the gas varied as r_h ^-0.4 within 7 AU from the Sun, but remained close to 0.4 km s^-1 further away. The CO spectra obtained at large r_hare strongly blueshifted and indicative of an important day-to-night asymmetry in outgassing and expansion velocity. The kinetic temperature of the coma, estimated from the relative intensities of the CH₃OH and CO lines, increased with decreasing r_h, from about 10 K at 7 AU to 110 K around perihelion.     

A multi-wavelength study of parent volatile abundances in Comet C/2006 M4 (SWAN)

Volatile organic emissions were detected post-perihelion in the long-period Comet C/2006 M4 (SWAN) in October and November 2006. Our study combines target-of-opportunity infrared observations using the Cryogenic Echelle Spectrometer (CSHELL) at the NASA-IRTF 3-m telescope, and millimeter wavelength observations using the Arizona Radio Observatory (ARO) 12-m telescope. Five parent volatiles were measured with CSHELL (H₂O, CO, CH₃OH, CH₄, and C₂H₆), and two additional species (HCN and CS) were measured with the ARO 12-m. These revealed highly depleted CO and somewhat enriched CH₃OH compared with abundances observed in the dominant group of long-period (Oort cloud) comets in our sample and similar to those observed recently in Comet 8P/Tuttle. This may indicate highly efficient H-atom addition to CO at very low temperature (∼10-20 K) on the surfaces of interstellar (pre-cometary) grains. Comet C/2006 M4 had nearly “normal” C₂H₆ and CH₄, suggesting a processing history similar to that experienced by the dominant group. When compared with estimated water production at the time of the millimeter observations, HCN was slightly depleted compared with the normal abundance in comets based on IR observations but was consistent with the majority of values from the millimeter. The ratio CS/HCN in C/2006 M4 was within the range measured in ten comets at millimeter wavelengths. The higher apparent H-atom conversion efficiency compared with most comets may indicate that the icy grains incorporated into C/2006 M4 were exposed to higher H-atom densities, or alternatively to similar densities but for a longer period of time.     

Spectroscopy of Comet Hale-Bopp in the infrared

High resolution infrared spectra of Comet C/1995 O1 (Hale-Bopp) were obtained during 2-5 March 1997 UT from the NASA Infrared Telescope Facility on Mauna Kea, Hawaii, when the comet was at r≈1.0 AU from the Sun pre-perihelion. Emission lines of CH₄, C₂H₆, HCN, C₂H₂, CH₃OH, H₂O, CO, and OH were detected. The rotational temperature of CH₄ in the inner coma was T_rot=110±20 K. Spatial profiles of CH₄, C₂H₆, and H₂O were consistent with release solely from the nucleus. The centroid of the CO emission was offset from that of the dust continuum and H₂O. Spatial profiles of the CO lines were much broader than those of the other molecules and asymmetric. We estimate the CO production rate using a simplified outflow model: constant, symmetric outflow from the peak position. A model of the excitation of CO that includes optical depth effects using an escape probability method is presented. Optical depth effects are not sufficient to explain the broad spatial extent. Using a parent+extended-source model, the broad extent of the CO lines can be explained by CO being produced mostly (∼90% on 5 March) from an extended source in the coma. The CO rotational temperature was near 100 K. Abundances relative to H₂O (in percent) were 1.1±0.3 (CH₄), 0.39±0.10 (C₂H₆), 0.18±0.04 (HCN), 0.17±0.04 (C₂H₂), 1.7±0.5 (CH₃OH), and 37-41 (CO, parent+extended source). These are roughly comparable to those obtained for other long-period comets also observed in the infrared, though CO appears to vary.     

Meridional variations of C2H2 and C2H6 in Jupiter's atmosphere from Cassini CIRS infrared spectra

Hydrocarbons such as acetylene (C₂H₂) and ethane (C₂H₆) are important tracers in Jupiter's atmosphere, constraining our models of the chemical and dynamical processes. However, our knowledge of the vertical and meridional variations of their abundances has remained sparse. During the flyby of the Cassini spacecraft in December 2000, the Composite Infrared Spectrometer (CIRS) instrument was used to map the spatial variation of emissions from 10 to 1400 cm⁻¹ (1000-7 μm). In this paper we analyze a zonally averaged set of CIRS spectra taken at the highest (0.48 cm⁻¹) resolution, firstly to infer atmospheric temperatures in the stratosphere at 0.5-20 mbar via the ν₄ band of CH₄, and in the troposphere at 150-400 mbar, via the H₂ absorption at 600-800 cm⁻¹. Stratospheric temperatures at 5 mbar are generally warmer in the north than the south by 7-8 K, while tropospheric temperatures show no such asymmetry. Both latitudinal temperature profiles however do show a pattern of maxima and minima which are largely anti-correlated between the two levels. We then use the derived temperature profiles to infer the vertical abundances of C₂H₂ and C₂H₆ by modeling tropospheric absorption (∼200 mbar) and stratospheric emission (∼5 mbar) in the C₂H₂ν₅ and C₂H₆ν₉ bands, and also emission of the acetylene (ν₄+ν₅)−ν₄ hotband (∼0.1 mbar). Acetylene shows a distinct north-south asymmetry in the stratosphere, with 5 mbar abundances greatest close to 20° N and decreasing from there towards both poles by a factor of ∼4. At 200 mbar in contrast, acetylene is nearly flat at a level of ∼3×¹⁰⁻⁹. Additionally, the abundance gradient of C₂H₂ between 10 and 0.1 mbar is derived, based on interpolated temperatures at 0.1 mbar, and is found to be positive and uniform with latitude to within errors. Ethane at both 5 and 200 mbar shows increasing VMR towards polar regions of ∼1.75 towards 70° N and ∼2.0 towards 70° S. An explanation for the meridional trends is proposed in terms of a combination of photochemistry and dynamics. Poleward, the decreasing UV flux is predicted to decrease the abundances of C₂H₂ and C₂H₆ by factors of 2.7 and 3.5, respectively, at latitude 70°. However, the lifetime of C₂H₆ in the stratosphere (3×¹⁰¹⁰ s at 5 mbar) is much longer than the dynamical timescale for meridional mixing inferred from Comet SL-9 debris (5-50×¹⁰⁸ s), and therefore the rising abundance towards high latitudes likely indicates that meridional mixing dominates over photochemical effects. For C₂H₂, the opposite occurs, with the relatively short photochemical lifetime (3×¹⁰⁷ s), compared to meridional mixing times, ensuring that the expected photochemical trends are visible.     

Heliocentric evolution of the degradation of polyoxymethylene: Application to the origin of the formaldehyde (H2CO) extended source in Comet C/1995 O1 (Hale-Bopp)

The H₂CO production rates measured in Comet C/1995 O1 (Hale-Bopp) from radio wavelength observations [Biver, N., and 22 colleagues, 2002a. Earth Moon Planets 90, 5-14] showed a steep increase with decreasing heliocentric distance. We studied the heliocentric evolution of the degradation of polyoxymethylene (formaldehyde polymers: (CH₂O)_n, also called POM) into gaseous H₂CO. POM decomposition can indeed explain the H₂CO density profile measured in situ by Giotto spacecraft in the coma of Comet 1P/Halley, which is not compatible with direct release from the nucleus [Cottin, H., Bénilan, Y., Gazeau, M.-C., Raulin, F., 2004. Icarus 167, 397-416]. We show that the H₂CO production curve measured in Comet C/1995 O1 (Hale-Bopp) can be accurately reproduced by this mechanism with a few percents by mass of solid POM in grains. The steep heliocentric evolution is explained by the thermal degradation of POM at distances less than 3.5 AU. This study demonstrates that refractory organics present in cometary dust can significantly contribute to the composition of the gaseous coma. POM, or POM-like polymers, might be present in cometary grains. Other molecules, like CO and HNC, might also be produced by a similar process.     

The outgassing and composition of Comet 19P/Borrelly from radio observations

Radio spectroscopic observations of Comet 19P/Borrelly were performed during the 1994 apparition and at, and near, the time of the Deep Space 1 flyby in 2001. HCN, CS, CH₃OH, and H₂CO were detected using the 30-m telescope of the Institut de Radioastronomie Millimétrique and the James Clerk Maxwell Telescope, and their production rates relative to water are estimated to be 0.06-0.11, 0.07, 1.7, and 0.4%, respectively. Only upper limits are derived for H₂S and CO. The upper limit for CO/H₂O (<15%) is not very constraining, while the upper limit for the H₂S/H₂O ratio of 0.45% is near the bottom of the range of values measured for other comets. Observations of the OH radical at the Nançay radio telescope provide water production rates a few weeks before the 1994 and 2001 perihelia. Observations of the 1₁₀-1₀₁ water line at 557 GHz with the Odin satellite yield a water production rate of (2.5±0.5)×10²⁸ s⁻¹ on September 22, 2001, at the time of the Deep Space 1 encounter, and (3.3±0.6)×10²⁸ s⁻¹ averaged over the September 22-24, 2001 period. The line shapes are asymmetric and blueshifted by V₀∼−0.18 km s⁻¹ for the best observed HCN lines recorded one week after perihelion. The HCN line shapes, and the similar OH and HCN velocity shifts over the September-November 1994 and August-September 2001 periods, favor anisotropic outgassing towards the Sun. Strong outgassing directed along the primary dust jet seen on visible images is not excluded by the HCN line shapes, but unrealistically high gas expansion velocities are required to explain the line shapes in that case.     

Radio Investigations Of 19p/Borrelly In Support To The Deep Space 1 Flyby

NASA's Deep Space 1 mission flew by Comet 19P/Borrelly on September 22, 2001.We present observations of molecular species obtained with the 30-m telescope of theInstitut de Radioastronomie Millimétrique (IRAM) and the Nançay radio telescopeat and near the time of this flyby. OH, HCN, and CS production rates were measured,while upper limits were deduced for CO, H₂CO and H₂S.     

The molecular composition of Comet C/2007 W1 (Boattini): Evidence of a peculiar outgassing and a rich chemistry

We measured the chemical composition of Comet C/2007 W1 (Boattini) using the long-slit echelle grating spectrograph at Keck-2 (NIRSPEC) on 2008 July 9 and 10. We sampled 11 volatile species (H₂O, OH^∗, C₂H₆, CH₃OH, H₂CO, CH₄, HCN, C₂H₂, NH₃, NH₂, and CO), and retrieved three important cosmogonic indicators: the ortho-para ratios of H₂O and CH₄, and an upper-limit for the D/H ratio in water. The abundance ratios of almost all trace volatiles (relative to water) are among the highest ever observed in a comet. The comet also revealed a complex outgassing pattern, with some volatiles (the polar species H₂O and CH₃OH) presenting very asymmetric spatial profiles (extended in the anti-sunward hemisphere), while others (e.g., C₂H₆ and HCN) showed particularly symmetric profiles. We present emission profiles measured along the Sun-comet line for all observed volatiles, and discuss different production scenarios needed to explain them. We interpret the emission profiles in terms of release from two distinct moieties of ice, the first being clumps of mixed ice and dust released from the nucleus into the sunward hemisphere. The second moiety considered is very small grains of nearly pure polar ice (water and methanol, without dark material or apolar volatiles). Such grains would sublimate only very slowly, and could be swept into the anti-sunward hemisphere by radiation pressure and solar-actuated non-gravitational jet forces, thus providing an extended source in the anti-sunward hemisphere.     

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.     

The 2.9-4.2 micron spectrum of Saturn: Clouds and CH4, PH3, and NH3

We have used synthetic spectra to analyze a medium resolution 2.9-4.2 μm spectrum of Saturn's temperate region observed at UKIRT using CGS4. The synthetic spectra include CH₄, PH₃, and NH₃ lines, for which mixing ratios were adopted from recent Cassini results. The observed absorption features in the spectrum are well accounted for by lines of these molecular species formed 22 +/− 8 km above the 1 bar pressure level at ∼610 mbar. The influence of optically thin haze particles at higher altitudes on the spectrum is not pronounced, with higher spectral resolution probably required to constrain the effects of haze in this wavelength region. Fluorescent line emission by CH₄ in its ν₃ and ν₃+ν₄−ν₄ bands, detected in the 3.2-3.5 μm region, originates between 400 km (∼0.06 mbar) and 800 km (∼0.01 μbar) above the 1 bar level, with peak contributions from the two major contributing bands at 550 km (∼3 μbar) and 700 km (∼0.1 μbar), respectively.     

A theoretical study of Comet Halley's spectrum in the infrared range

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 H₂O, CO, CO₂, CH₄, NH₃, N₂H₄, and H₂CO. The flux emitted by these molecules, mostly due to resonant scattering, should be sufficient for detection by a flyby mission.     

A study of the 3ν3CH4 region in a high-resolution spectrum of Jupiter recorded by Fourier transform spectroscopy

A spectrum of Jupiter between 6000 and 12 000 cm^? at high resolution (0.05 cm^?) was recorded with a Michelson interferometer at Palomar Mountain in October 1974. An analysis of the R branch of the 3ν₃CH₄ band with the reflecting-layer model, taking into account the H₂ absorption which occurs in the same spectral range, leads to a Lorentzian half-width of 0.09 ± 0.02 cm^?1, a rotational temperature of 175 ± 10° K, and a CH₄ abundance of order 52m atm. Five lines of the ¹³CH₄3ν₃ band have been identified; a comparison with new laboratory spectra indicates that the ¹³CH₄/¹²CH₄ ratio in the Jupiter atmosphere is close to the terrestrial ratio.     

The spectra of Uranus and Neptune at 8–14 and 17–23 μm

An array spectrometer was used on the nights of 1985 May 30–June 1 to observe the disks of Uranus and Neptune in the spectral regions 7–14 and 17–23 μm with effective resolution elements ranging from 0.23 to 0.87 μm. In the long-wavelength region, the spectra are relatively smooth with the broad S(1) H₂ collision-induced rotation line showing strong emission for Neptune. In the short-wavelength spectrum of Uranus, an emission feature attributable to C₂H₂ with a maximum stratospheric mixing ratio of 9 × 10⁻⁹ is apparent. An upper limit of 2 × 10⁻⁸ is placed on the maximum stratospheric mixing ratio of C₂H₆. The spectrum of Uranus is otherwise smooth and quantitatively consistent with the opacity provided by H₂ collision-induced absorption and spectrally continuous stratospheric emission, as would be produced by aerosols. Upper limits to detecting the planet near 8 μm indicate a CH₄ stratospheric mixing ratio of 1 × 10⁻⁵ or less, below a value consistent with saturation equilibrium at the temperature minimum. In the short-wavelength spectrum of Neptune, strong emission features of CH₄ and C₂H₆ are evident and are consistent with local saturation equilibrium with maximum stratospheric mixing ratios of 0.02 and 6 × 10⁻⁶, respectively. Emission at 8–10 μm is most consistent with a [CH₃D]/[CH₄] volume abundance ratio of 5 × 10⁻⁵. The spectrum of Neptune near 13.5 μm is consistent with emission by stratospheric C₂H₂ in local saturation equilibrium and a maximum mixing ratio of 9 × 10⁻⁷. Radiance detected near 10.5 μm could be attributed to stratospheric C₂H₄ emission for a maximum mixing ratio of approximately 3 × 10⁻⁹. Quantitative results are considered preliminary, as some absolute radiance differences are noted with respect to earlier observations with discrete filters.