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Material of low geometric albedo (pV⩽0.1pV⩽0.1) is found on many objects in the outer Solar System, but its distribution in the saturnian satellite system is of special interest because of its juxtaposition with high-albedo ice. In the absence of clear, diagnostic spectral features, the composition of this low-albedo (or “dark”) material is generally inferred to be carbon-rich, but the form(s) of the carbon is unknown. Near-infrared spectra of the low-albedo hemisphere of Saturn's satellite Iapetus were obtained with the Visible–Infrared Mapping Spectrometer (VIMS) on the Cassini spacecraft at the fly-by of that satellite of 31 December 2004, yielding a maximum spatial resolution on the satellite's surface of ∼65 km. The spectral region 3–3.6 μm reveals a broad absorption band, centered at 3.29 μm, and concentrated in a region comprising about 15% of the low-albedo surface area. This is identified as the CH stretching mode vibration in polycyclic aromatic hydrocarbon (PAH) molecules. Two weaker bands attributed to CH2 stretching modes in aliphatic hydrocarbons are found in association with the aromatic band. The bands most likely arise from aromatic and aliphatic units in complex macromolecular carbonaceous material with a kerogen- or coal-like structure, similar to that in carbonaceous meteorites. VIMS spectra of Phoebe, encountered by Cassini on 11 June 2004, also show the aromatic hydrocarbon band, although somewhat weaker than on Iapetus. The origin of the PAH molecular material on these two satellites is unknown, but PAHs are found in carbonaceous meteorites, cometary dust particles, circumstellar dust, and interstellar dust.

Recent attempts using high resolution spectra to detect N+2 in several comets were unsuccessful [Cochran, A.L., Cochran, W.D., Baker, E.S., 2000. Icarus 146, 583–593; Cochran, A.L., 2002. Astrophys. J. 576, L165–L168]. The upper limits on N+2 in comparison with the positively detected CO+ for Comets C/1995 O1 Hale–Bopp, 122P/1995 S1 de Vico and 153P/2002 C1 Ikeya–Zhang range between N 2 + /CO + ( 0.65 – 5.4 ) × 10 −4 . Ar was not detected in three recent comets [Weaver, H.A., Feldman, P.D., Combi, M.R., Krasnopolsky, V., Lisse, C.M., Shemansky, D.E., 2002. Astrophys. J. 576, L95–L98], with upper limits of Ar/CO ( 3.4 – 7.8 ) × 10 −2 for Comets C/1999 T1 McNaught–Hartley, C/2001 A2 LINEAR and C/2000 WM1 LINEAR. The Ar detected by Stern et al. [Stern, S.A., Slater, D.C., Festou, M.C., Parker, J.Wm., Gladstone, G.R., A'Hearn, M.F., Wilkinson, E., 2000. Astrophys. J. 544, L169–L172] for Comet C/1995 O1 Hale–Bopp, gives a ratio Ar/CO = 7.25 × 10 −2 , which was not confirmed by Cosmovici et al. [Cosmovici, C.B., Bratina, V., Schwarz, G., Tozzi, G., Mumma, M.J., Stalio, R., 2006. Astrophys. Space Sci. 301, 135–143]. Trying to solve the two problems, we studied experimentally the trapping of N 2 + CO + Ar in amorphous water ice, at 24–30 K. CO was found to be trapped in the ice 20–70 times more efficiently than N2 and with the same efficiency as Ar. The resulting Ar/CO ratio of 1.2 × 10 −2 is consistent with Weaver et al.'s [Weaver, H.A., Feldman, P.D., Combi, M.R., Krasnopolsky, V., Lisse, C.M., Shemansky, D.E., 2002. Astrophys. J. 576, L95–L98] non-detection of Ar. However, with an extreme starting value for N2/CO = 0.22 in the region where the ice grains which agglomerated to produce comet nuclei were formed, the expected N2/CO ratio in the cometary ice should be 6.6 × 10 −3 , much higher than its non-detection limit.

During the last two minutes before closest approach of Deep Space 1 to Comet 19P/Borrelly, a long exposure was made with the short-wavelength infrared (SWIR) imaging spectrometer. The observation yielded 46 spectra covering 1.3–2.6 μm; the footprint of each spectrum was ∼160 m × width of the nucleus. Borrelly's highly variegated and extremely dark 8-km-long nucleus exhibits a strong red slope in its short-wavelength infrared reflection spectrum. This slope is equivalent to J–K and H–K colors of ∼0.82 and ∼0.43, respectively. Between 2.3–2.6 μm thermal emission is clearly detectable in most of the spectra. These data show the nucleus surface to be hot and dry; no trace of H2O ice was detected. The surface temperature ranged continuously across the nucleus from ⩽300 K near the terminator to a maximum of ∼340 K, the expected sub-solar equilibrium temperature for a slowly rotating body. A single absorption band at ∼2.39 μm is quite evident in all of the spectra and resembles features seen in nitrogen-bearing organic molecules that are reasonable candidates for compositional components of cometary nuclei. However as of yet the source of this band is unknown.

The effect of water ice formation temperature and rate of ice deposition on a cold plate on the amount of trapped argon (equivalent to CO), and the ratios of Ar/Kr/Xe trapped in the water ice were studied at 50, 27 and 22 K and at ice formation rates ranging over four orders of magnitude, from 10−1 to 10−5 μm min−1. Contrary to our previous conclusions that cometary ices were formed at 50–60 K, we now conclude that these ices were formed at about 25 K. At 25 K the enrichment ratios for Ar, Kr, and Xe remained the same as those at 50 K, reinforcing our suggestion of cometary contribution of these noble gases to the atmospheres of Earth and Mars.

Observations of emissivity features of 10 Hygiea have been made for the first time in the relatively unexplored thermal-infrared wavelength region with the ISO (Infrared Space Observatory) satellite. Spectrophotomer (PHT-S) and short wavelength spectrometer (SWS) spectra of 10 Hygiea, obtained at 5.8–11.6 and 7–45 μm, respectively, are presented. In order to remove the thermal emission continuum, an advanced thermo–physical model has been applied to the observational data. To better interpret the spectral features above the thermal emission continuum, we compared the ISO observations with laboratory spectra available in the literature. Several laboratory experiments on minerals and meteorites have been performed to complete the analysis and to study the spectral behavior at various grain sizes. A possible spectral similarity with CO carbonaceous chondrites at small grain size is demonstrated.

Adaptive optics (0.15″ resolution) infrared images of the rings and satellites of Saturn were obtained in August 1995 as the Earth was crossing the ring plane. Twelve clumps were detected in the F ring, including HST S5 and S7 objects. For the first time H magnitudes were obtained for Prometheus, Pandora, Telesto, and Calypso, and J magnitudes for Epimetheus, Janus, Mimas, Telesto, and Helene.

The isotopic enrichment of HDO over H 2 O when water vapor freezes into ice at 60–170 K was studied experimentally. No such enrichment was detected (1.003–1.007 in the 95% confidence interval). Thus HDO cannot be enriched when ice is formed by freezing of water vapor. The very similar D/H ratio in the water of Comets Halley, Hyakutake, and Hale–Bopp (∼3 × 10 −4 ) is 10–20 times larger then the D/H ratio in the solar nebula. Therefore the cometary water had to originate in a giant molecular cloud, where the HDO is enriched by ion–molecule reactions. We cannot determine whether the ice grains which agglomerated into these comets were formed in a ∼50 K warm clump in the giant molecular cloud and settled intact to the solar nebula or sublimated and refroze in the ∼50 K Uranus–Neptune region. The HDO/H 2 O ratio in Earth's oceans suggests that the water was delivered by both comets and rocky material formed in Earth's region of the solar nebula.

The near-infrared reflectance spectrum of Saturn's satellite Phoebe shows a broad absorption band at 2.0 micrometers and absorption at lambda > 2.2 micrometers, both characteristic of H2O ice. We have successfully modeled the surface of Phoebe with an intimate (granular) mix of H2O ice (3% by weight, grain size 500 micrometers) mixed with fine grains of H2O ice (0.25%) with amorphous carbon (grain size 900 micrometers) as the dominant component. This model reproduces the shape of the measured spectrum and the observed albedo of 0.10 for Phoebe, but it is not unique. The presence of ice establishes Phoebe as an original member of the outer Solar System rather than a renegade asteroid.

Since August 1995, near-infrared images of Neptune have regularly been obtained with the University-of-Hawaii telescope. These images reveal Neptune's cloud structure with an angular resolution reaching 0.12″ in the H band.

We present a new spectrum of the Centaur object 5145 Pholus between 1.15 and 2.4 micro meters. We model this, and the previously published (0.4- to 1.0- micrometer) spectrum, using Hapke scattering theory. Seen in absorption are the 2.04- micrometer band of H2O ice and a strong band at 2.27 micrometer, interpreted as frozen methanol and/or a photolytic product of methanol having small molecular weight. The presence of small molecules is indicative of a chemically primitive surface, since heating and other processes remove the light hydrocarbons in favor of macromolecular carbon of the kind found in carbonaceous meteorites. The unusually red slope of Pholus' spectrum is matched by fine grains of a refractory organic solid (tholin). Olivine (which we model with Fo 82) also appears to be present on Pholus. We present a five-component model for the composite spectrum of all spectroscopic and photometric data available for 5145 Pholus and conclude that this is a primitive object which has not yet been substantially processed by solar heat. The properties of Pholus are those of the nucleus of a large comet that has never been active.

We obtained I–J–H–K band spectroscopy of 18 low-albedo objects of the Solar System using the University of Hawaii 88-in. telescope equipped with the near-infrared spectrograph KSPEC. Most of our targets were selected from among the population of the Cybele, Hilda, and Trojan groups of asteroids, based on their low albedos and the relatively steep slopes of their reflection spectra at visible wavelengths. The outer jovian satellite Himalia and the Apollo object 3200 Phaethon were also observed. None of the spectra show absorptions similar to those found in the spectrum of 5145 Pholus, which remains the only asteroid-like object whose H–K band reflection spectrum contains a few broad absorptions due to organics. The near-infrared slope and the (1.0- to 2.2-μm) infrared color-index have been derived for each surface. This sample of primitive objects of the Solar System can be interpreted in terms of cometary-like objects that have undergone different degrees of aging.

Millimeter and submillimeter heterodyne observations performed with the IRAM 30-m telescope (Pico Veleta, Spain) and the JCMT (Mauna Kea, Hawaii) have been used to derive the stratospheric distribution of carbon monoxide on Titan. Rotational transition lines from 12 CO J (0 → 1), J (1 → 2), J (2 → 3) at 115.271, 230.538, 345.796 GHz, respectively, as well as the J (1 → 2) and J (2 → 3) lines of the 13 CO isotope at 220.399 and 330.588 GHz, respectively, were recorded with a spectral resolution of 1 MHz. Flux calibration uncertainties were estimated to 10% for all the data. A terrestrial value of the 12 C/ 13 C ratio has been assumed in the analysis as suggested by T. Hidayat et al . (1997, Icarus 126, 170–182) from recent observations of the H 12 CN(1–0) and H 13 CN(4–3) lines. The 13 CO lines sound the 60- to 180-km altitude range, while the 12 CO lines permit us to probe the atmosphere up to an altitude of about 350 km. Below 180 km, the 13 CO data impose a constant-with-height CO mixing ratio of ∼2.5 × 10 −5 . Extending this uniform mixing ratio profile throughout the stratosphere, all the 12 CO observations could be matched only if the systematic calibration errors were greater than our estimated value by at least a factor of 2. Uncertainties related to the temperature profile adopted in the stratosphere have been also investigated. Taking into account random and systematic uncertainties, the entire set of data indicates a CO mixing ratio equal to 2.9 +0.9 −0.5 × 10 −5 at 60 km, decreasing to 2.4 ± 0.5 × 10 −5 at 175 km, and reaching a value of 4.8 +3.8 −1.5 × 10 −6 at 350 km.

The close approach to the Earth of comet C/1996 B2 (Hyakutake) in March 1996 allowed searches for minor volatile species outgassing from the nucleus. We report the detection of deuterated water (HDO) through its 1(sub 01)-0(sub 00) rotational transition at 464.925 GHz using the Caltech Submillimeter Observatory. We also present negative results of a sensitive research for the J(5-4) line of deuterated hydrogen cyanide (DCN) at 362.046 GHz. Simultaneous observations of two rotational lines of methanol together with HDO in the same spectrum allow us to determine the average gas temperature within the telescope beam to be 69 +/- 10 K. We are thus able to constrain the excitation conditions in the inner coma and determine reliably the HDO production rate as (1.20 +/- 0.28) x 10(exp 26)/s on March 23-24, 1996. Available IR, UV and radio measurements lead to a water production rate of (2.1 +/- 0.5) x 10(exp 29)/s at the time of our HDO observations. The resulting D/H ratio in cometary water is thus (29 +/- 10) x 10(exp -5) in good agreement with the values of (30.8(sub - 5.3, sup +3.8) (Balsiger et al. 1995) and (31.6 +/- 3.4) x 10(exp -5) (Eberhardt et al. 1995) determined in comet P/Halley from in situ ion mass spectra. The inferred 3 a upper limit for the D/H ratio in HCN is 1%. Deuterium abundance is a key parameter for studying the origin and the early evolution of the Solar System and of its individual bodies. Our HDO measurement confirms that, in cometary water, deuterium is enriched by a factor of at least 10 relative to the protosolar ratio, namely the D/H ratio in H2 in the primitive Solar Nebula which formed from the collapse of the protosolar cloud. This indicates that cometary water has preserved a major part of the high D/H ratio acquired in this protosolar cloud through ion-molecule isotopic exchanges or grain-surface reactions and was not re-equilibrated with H2 in the Solar Nebula. Scenarios of formation of comets consistent with these results are discussed.

We report new observations of the (1–0) line of HCN at 88.63 GHz, emitted by Titan, made in May 1995 with the IRAM 30-m radiotelescope located at Pico Veleta (Spain). Compared to previous observations of Tanguy et al. (1990, Icarus 85, 43–57), the signal-to-noise ratio has been improved by a factor of 5, permitting us to retrieve the vertical distribution of HCN in the 80–350 km altitude range. Using the production rate of HCN predicted from photochemical models, we derive a profile of the eddy diffusion coefficient up to 350 km altitude consistent with the observed HCN vertical distribution. This coefficient increases with height more rapidly than the standard n −0.5 dependence in the stratosphere. The (4–3) line of H 13 CN at 345.34 GHz was subsequently observed with the JCMT on Mauna Kea (Hawaii) in June 1995. The analysis of the two spectra yields a 12 C/ 13 C ratio between 70 and 120, consistent with the terrestrial value. Taking into account possible fractionation processes, we show that the C-bearing species which formed Titan should have had a carbon isotopic ratio in the same range.

Near-infrared (J, H and K bands) spectra of nine dark asteroids (chosen among a sample of supposed primitive objects between C and D classes) have been obtained at the Mauna Kea Observatory (Hawaii) with the 2.2-m telescope using KSPEC as spectrograph. The aim of this work was to search for evidence of the presence of organic materials in these objects as found in other planetary bodies as 5145 Pholus, and in some cometary nuclei. A careful analysis of the data has revealed flat or slightly redder spectra than the solar one for all observed asteroids. No evidence of distinct absorption features was found.

The reflectance spectrum of Jupiter's sixth satellite, Himalia, is featureless in the wavelength region 1.95–2.50 μm as seen at a spectral resolution of 0.005 μm, with no absorptions deeper than a few percent. From model calculations we establish an upper limit of 10% by weight of H 2 O (30-μm grains) mixed intimately in the soil of Himalia, or alternatively 0.3% of the surface covered by exposures of H 2 O ice spatially segregated from the darker soil. For CH 4 and CO 2 ices the upper limits in spatially segregated models are both 0.3%.

On 15 August 1994 we launched the EUVS sounding rocket payload to observe the 825-1110 angstrom region of Venus's far ultraviolet airglow spectrum. The EUVS telescope/spectrograph obtained good data at five times higher spectral resolution than was previously available in the far ultraviolet. We present these data and compare our results to those obtained by the Galileo UVS and Venera 11/12 UV spectrophotometers. We identify several new spectral emission features, including both singly ionized nitrogen and molecular nitrogen in Venus's spectrum. We also see evidence for electron-impact-induced emission from CO. Finally, the EUVS data indicate that the "Ar" emissions detected in Venus's far ultraviolet spectrum by Venera 11/12 spectrophotometers are in fact not due to argon, thus eliminating the discrepancy between in situ and remote sensing measurements.