Behavior of Titan's atmosphere during a total eclipse

The edge-on presentation of Saturn's rings and satellites system has provided a rare opportunity to observe total eclipses of Titan. During its emersion from the Saturnian shadow (1980, June 28), Titan has been observed simultaneously in the visible and the infrared ranges (6000–9000 Å, 11.8 μm and 20 μm). No change has been recorded in these three spectral ranges. Our observations tend to support the thick-atmosphere model, which has been shown to be valid by Voyager a few months later.     

On stellar occultations by comets

Stellar occultations by comets can be used for studying the dust opacity of the coma in the vicinity of the cometary nucleus. This method provides high spatial resolution. If faint stars (V ≤ 15) are used in the densest regions of the sky, the event probability is typically one every 2 hr. This probability can be enhanced by a factor of 40 if the Space Telescope is used.     

On the possibility of nucleosynthesis in any matter-antimatter symmetric cosmology

In any matter-antimatter symmetric cosmology the primeval nucleosynthesis imposes a typical size of a region of matter (antimatter) much larger than the diffusion length of neutrons. Observational results on the abundances of cosmic deuterium and helium are used to evaluate how large it has to be. As a result we find that a minimal sizeL ₁～1.5×10⁸ cm at the temperatureT=1 MeV is compatible with the standard abundances of primeval helium and deuterium.     

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 determination of the composition of the Saturnian stratosphere using the IUE

We reduced ultraviolet spectra of Saturn from the IUE satellite to produce a geometric albedo of the planet from 1500 to 3000 Å. By matching computer models to the albedo we determined a chemical composition consistent with the data. This model includes C₂H₂ and C₂H₆ with mixing ratios and distributions of (9 ± 3) × 10^?8 in the top 20 mbar of the atmosphere with none below for C₂H₂ and (6 ± 1) × 10^?6 also in the top 20 mbar with none below for C₂H₆. The C₂H₂ and C₂H₆ distributions and the C₂H₆ mixing ratio are taken directly from the Voyager IRIS model [R. Courtin et al., Bull. Amer. Astron. Soc.13, 722 (1981), and private communication]. The Voyager IRIS model also includes PH₃, which is not consistent with the uv albedo from 1800 to 2400 Å. Our model requires a previously unidentified absorber to explain the albedo near 1600 Å. After considering several candidates, we find that the best fit to the data is obtained with H₂O, having a column density of (6 ± 1) × 10^?3 cm-am.     

Unveiling the chemistry of hot protostellar cores with ALMA

High angular resolution mm-wave observations of the Orion-KL region, made with the IRAM Plateau de Bure interferometer (PdBI), reveal the presence of several cores of size 10³ AU, which have distinct spectral signatures. Complex molecules such as ethanol, vinyl cyanide and dimethyl ether show different distributions and their relative abundance varies from core to core by orders of magnitude. The molecular column densities derived in the cores also differ widely from the beam-averaged column densities observed with large single-dish telescopes. Obviously, the predictions of hot core chemistry models must be checked against high resolution observations. ALMA, which allies sensitivity and high angular resolution, will be a key instrument for this type of studies. The PdBI observations were part of a search for interstellar glycine, also carried out with the IRAM 30-m telescope and the Green Bank Telescope. We derive a 3σ upper limit on the column density of glycine of 1×10¹⁵ cm⁻² per 2″×3″ beam in the Orion Hot Core and Compact Ridge. Based on observations made with the IRAM PdB Interferometer, the IRAM 30-m telescope and the NRAO Green-Bank telescope. IRAM is supported by CNRS, MPG and IGN.     

Minor constituents in the Martian atmosphere from the ISM/Phobos experiment

Global Martian atmospheric results derived from the infrared imaging spectrometer ISM flown aboard the Phobos 2 Soviet spacecraft are presented. Over low altitude regions the expected CO mixing ratio of (8 +/- 3) x 10(-4) is measured. Variations of the 2.35-micrometers feature are inconsistent with this value over the Great Martian Volcanoes. If the 2.35-micrometers band is entirely attributable to carbon monoxide, the CO mixing ratio is typically depleted by a factor of 3 over these high altitude areas. Orography should play a major role in the existence of this CO "hole." If, however, these spectral variations at 2.35 micrometers are due to the surface composition, the fraction of the surface covered by the responsible mineral must smoothly decrease as the surface elevation decreases. This phenomenon implies a strong interaction between the surface and the atmosphere for the Great Martian Volcanoes. Diurnal behavior and latitudinal variations of water vapor are globally consistent with Viking measurements. During the Phobos observations, the water vapor amounts over the bright equatorial regions range around 11 pr-micrometers during the day. These amounts are slightly larger than those inferred from 1976 to 1979. The lack of global dust storms during 1988-1989 could explain the enhancement of H2O in the atmosphere.     

Saturn's icy satellites investigated by Cassini-VIMS: I. Full-disk properties: 350-5100 nm reflectance spectra and phase curves

Saturn's icy satellites are among the main scientific objectives of the Cassini-VIMS (Visual and Infrared Mapping Spectrometer) experiment. This paper contains a first systematic and comparative analysis of the full-disk spectral properties of Dione, Enceladus, Epimetheus, Hyperion, Iapetus, Mimas, Phoebe, Rhea and Tethys as observed by VIMS from July 2004 to June 2005. The disk integrated properties (350-5100 nm reflectance spectra and phase curves at 550-2232 nm) and images of satellites are reported and discussed in detail together with the observed geometry. In general, the spectra in the visible spectral range are almost featureless and can be classified according to the spectral slopes: from the bluish Enceladus and Phoebe to the redder Iapetus, Hyperion and Epimetheus. In the 1000-1300 nm range the spectra of Enceladus, Tethys, Mimas and Rhea are characterized by a negative slope, consistent with a surface largely dominated by water ice, while the spectra of Iapetus, Hyperion and Phoebe show a considerable reddening pointing out the relevant role played by darkening materials present on the surface. In between these two classes are Dione and Epimetheus, which have a flat spectrum in this range. The main absorption bands identified in the infrared are the 1520, 2020, 3000 nm H₂O/OH bands (for all satellites), although Iapetus dark terrains show mostly a deep 3000 nm band while the 1520 and 2020 nm bands are very faint. In this spectral range, the Iapetus spectrum is characterized by a strong reddening. The CO₂ band at 4260 nm and the Fresnel ice peak around 3100 nm are evident only on Hyperion, Phoebe and Iapetus. The phase curves at 550 and at 2232 nm are reported for all the available observations in the 0°-144° range; Rhea shows an opposition surge at visible wavelengths in the 0.5°-1.17° interval. The improvement on the retrieval of the full-disk reflectance spectra can be appreciated by a direct comparison with ground-based telescopic data available from literature. Finally, data processing strategies and recent upgrades introduced in the VIMS-V calibration pipeline (flat-field and destriping-despiking algorithm) are discussed in appendices.