Regis Courtin

UV spectroscopy of Titan's atmosphere, planetary organic chemistry and prebiological synthesis: II. Interpretation of new IUE observations in the 220–335 nm Range

We report on new observations of Titan with the International Ultraviolet Explorer in the mid-UV range (lambda approximately 220-335 nm). We use these data to determine upper limits for the abundances of simple organic compounds in the gas phase and to further constrain the properties of the high altitude haze on Titan. As a baseline, we adopted the parametrized microphysical model of McKay et al. (1989) which is successful at reproducing Titans thermal structure while satisfying several other observational constraints in the visible and IR regions. However, we find that such a model--in which all particles at a given altitude are assumed to have the same size--cannot match simultaneously the IUE observations and the visible/IR data, even when allowance is made for a wide range of values in the adjustable parameters. On the other hand, a good overall agreement is obtained when considering a biomodal size distribution, with small haze particles or polymers (r < 0.02 micrometer) acting as strong Rayleigh absorbers below 300 nm and larger haze particles (r approximately 0.1-0.5 micrometer) being responsible for the characteristics of the albedo spectrum in the near-UV, visible, and IR regions. This approach is consistent with the results of several previous investigations of the properties of Titans haze, although our preferred vertical structure for the haze + polymer material somewhat differs from earlier solutions. On the basis of simple dynamical considerations, we adopted a uniformly mixed layer between 150 and 600 km. The IUE data allow us to place fairly stringent constraints on the abundance of the Rayleigh absorbers, if we assume that their optical properties are similar to those of laboratory-synthesized tholins: The column-mass density of this material--the essential observable that can be determined from our study--is of the order of 5 micrograms cm-2. This would correspond to number-densities between 10(3) and 10(7) cm-3 in the 150-600 km altitude range, if the average particle radius is between 0.001 and 0.02 micrometer. Such high number densities are a priori at odds with the estimated coagulation lifetime for particles of that size. Thus, our proposed bimodal size distribution is plausible only if inhibiting processes act to slow down considerably the coagulation of polymers in Titans stratosphere.

Ammonia vertical density profiles in Jupiter and Saturn from their radioelectric and infrared emissivities

Abstract The distribution of ammonia with height in the atmospheres of Jupiter and Saturn has been determined by inversion of microwave thermal spectra and from spectroscopic measurements of the Jovian flux in the v 2 vibration-rotation band of NH 3 . The uncertainty in both the thermal profiles and brightness temperatures used in the inversion leads to upper and lower estimates of the retrieved NH 3 distributions. In the case of Jupiter, undersaturation of ammonia is very likely in the stratosphere and in the upper troposphere. In the lower troposphere, we find two characteristic regions with the following parameters: 1. (I) 150–175 ⩽ T ⩽ 230–250° K , 0.7 × 10 −4 ⩽ NH 3 H 2 2.0 × 10 −4 , 2. (II) T > 330–340° K , 1.2 × 10 −4 ⩽ NH 3 H 2 ⩽ 1.5 × 10 −3 . These results give further evidence for an altitude-dependent distribution of NH 3 in the deep atmosphere and for the existence of a cloud of nitrogen compounds at the bottom of region I. In the case of Saturn, a large uncertainty still exists, since the possible values of the tropospheric ammonia mixing ratio lie between 1.3 and 7.8 × 10 −4 .

Mapping of Titan's tropopause and surface temperatures from Voyager IRIS spectra

Abstract We have derived temperatures at the tropopause and near the surface of Titan from a set of radiance measurements made by the Voyager IRIS instrument at 300 and 510 cm −1 (33.3 and 19.6 μm ). At 510 cm −1 , Titans atmosphere is relatively transparent and the surface contribution to the measured radiance is approximately 60%. On the other hand, the 300 cm −1 radiance is mostly sensitive to the tropopause temperature, as well as to the degree of methane supersaturation in the upper troposphere. The retrievals are based on a simple inversion scheme of the radiative transfer equation and rely on a model of the atmospheric opacity derived by Courtin et al. (Icarus 114 (1995) 144). Although the longitude–latitude coverage afforded by the IRIS dataset is severely limited, as is also the surface area covered by the IRIS footprints (∼22%), we were able to extract longitudinal and latitudinal profiles of the tropopause and near-surface (brightness) temperatures. We find that the zonal distributions of the tropopause and near-surface temperatures are fairly homogenous. The meridional distribution of the tropopause temperature shows only a slight increase ( ∼1 K ) from −60° to 60°. On the other hand, the surface brightness temperature varies significantly and symmetrically from equator to pole (2.5– 3.2 K ). Less then one-third of this decrease can potentially be attributed to meridional variations of the stratospheric thermal structure. This confirms earlier results obtained from more limited selections of the IRIS data, both in terms of symmetry and amplitude. The role that could be played by meridional variations of the supersaturated distribution of methane is explored, although no definite answer can be derived in this respect. A significant variation of the bulk temperature gradient between the surface and the tropopause is suggested, the troposphere at high latitudes appearing more stable with respect to moist convection than in the equatorial regions. Finally, a very tentative result concerns an area where the near-surface (brightness) temperature shows a significant differential with respect to the extrapolation of the latitudinal distribution of surface temperatures. Possible interpretations of this differential include: a low emissivity surface feature with 25% contrast, an actual surface cold spot (with a temperature contrast of ∼3 K ), a transient tropospheric cloud of optical thickness τ∼1, or an elevated area of height ∼5 km . Further high spatial resolution data, especially those that will be collected by the Cassini orbital mission, will be necessary to discriminate between the various possibilities.

Absolute absorption coefficient of C6H2 in the mid-UV range at low temperature; implications for the interpretation of Titan atmospheric spectra

The interpretation of mid-UV albedo spectra of planetary atmospheres, especially that of Titan, is the main goal of the SIPAT (Spectroscopie uv dInteret Prebiologique dans lAtmosphere de Titan) research program. This laboratory experiment has been developed in order to systematically determine the absorption coefficients of molecular compounds which are potential absorbers of scattered sunlight in planetary atmospheres, with high spectral resolution, and at various temperatures below room temperature. From photochemical modelling and experimental simulations, we may expect triacetylene (C6H2) to be present in the atmosphere of Titan, even though it has not yet been detected. We present here the first determination of the absolute absorption coefficient of that compound in the 200-300 nm range and at two temperatures (296 K and 233 K). The temperature dependence of the C6H2 absorption coefficient in that wavelength range is compared to that previously observed in the case of cyanoacetylene (HC3N). We then discuss the implications of the present results for the interpretation of Titan UV spectra, where it appears that large uncertainities can be introduced either by the presence of trace impurities in laboratory samples or by the variations of absorption coefficients with temperature.

Indications of supersaturated stratospheric methane in Neptune from its atmospheric thermal profile

Abstract An iterative inversion method is used to retrieve the thermal structure of Neptune from available infrared measurements. The most plausible thermal profile leads to an effective temperature of 58.4°K, implying the presence of an internal heat source equal to about 1.6 times the solar-absorbed power. The interpretation of Neptunian infrared measurements in the 8-μm range requires a very strong supersaturation of CH 4 in the upper atmosphere of the planet.

The Raman signature of H2 in the uv spectra of Uranus and Neptune: Constraints on the haze optical properties and on the para-H2 fraction

Abstract The geometric albedos of Uranus and Neptune, inferred from archived Hubble Space Telescope observations and from the ground-based measurements of Karkoschka, 1994 , are modeled in the wavelength range 2200–4200 A. The radiative transfer model, which includes Rayleigh–Raman scattering and Mie scattering by haze particles, aims at reproducing the fine structure of the geometric albedos at a resolution of 2–10 A. The steep variation of the total optical depth allows to investigate the influences of both the stratospheric and tropospheric haze layers and that of the deep tropospheric cloud, although their relative importance is difficult to estimate accurately. Using the haze models of Baines et al., 1995 , the optical properties of the Mie scatterers are inferred. The haze material on Uranus is characterized by a slowly decreasing imaginary index of refraction: n i varies from about 0.10 to 0.01–0.02 between 2200 and 4200 A. Below 3000 A, the absorptivity of Neptuness haze material is comparable to that on Uranus or slightly lower ( n i ∼ 0.03–0.10). Above 3000 A, it exhibits a steeper decrease (from 0.30 to 0.003). The main source of uncertainty at longer wavelengths is the reflectivity of the underlying (H 2 S ?) cloud. At shorter wavelengths, molecular scattering strongly dominates Mie scattering and the determination of the absorptivities is estimated to be accurate within a factor of 2. For Neptune, there is an additional uncertainty due to the inability of the initial haze model to provide a fit to the observed albedo. The Baines et al. model was modified by multiplying the number-densities of the hydrocarbons haze layers by a factor of 2.5–4.8, making it more consistent with the results of Pryor et al., 1992 . For Uranus, these results suggest a darkening of the southern hemisphere since the Voyager epoch, in agreement with recent HST imaging. As a whole, the Neptunian haze seems to be more transparent than that of Uranus, possibly owing to the more turbulent dynamical state of the troposphere. Longwards of 3000 A, the inferred absorptivities are consistent with laboratory measurements on tholins produced from CH 4 –H 2 gas mixtures ( Khare et al., 1987 ). The para -H 2 mole fraction on both planets is constrained from the strength of a prominent H 2 Raman feature at 2853 A. On Uranus, at latitudes between 45 and 75°S and in the 50–500 mbar pressure range, the best agreement is obtained with an equilibrium para -H 2 distribution. On Neptune, there is an indication of a slight departure from equilibrium in the same pressure range at mid-southern latitudes. Although this new method is significantly less accurate, its results are consistent with those of previous investigations based on the analysis of H 2 quadrupole lines ( Baines et al., 1995 ) and of the Voyager IRIS spectra ( Conrath et al., 1998 ).

Divisions i and III / Working Group: Cartographic Coordinates and Rotational Elements

As in the past, the primary activity of the IAU Working Group on Cartographic Coordinates and Rotational Elements has been to prepare and publish a triennial (``2009) report containing current recommendations for models for Solar System bodies (Archinal et al. (2011a)). The authors are B. A. Archinal, M. F. AHearn, E. Bowell, A. Conrad, G. J. Consolmagno, R. Courtin, T. Fukushima, D. Hestroffer, J. L. Hilton, G. A. Krasinsky, G. Neumann, J. Oberst, P. K. Seidelmann, P. Stooke, D. J. Tholen, P. C. Thomas, and I. P. Williams. An erratum to the ``2006 and ``2009 reports has also been published (Archinal et al. (2011b)). Below we briefly summarize the contents of the 2009 report, a plan to consider requests for new recommendations more often than every three years, three general recommendations by the WG to the planetary community, other WG activities, and plans for our next report.

Comparative performances of an objective grating concept in U.V. spectroscopic observations of solar system objects. IV: Molecular absorption structures in planetary atmospheres

Prospects for an Earth-orbiting planetary observatory are fairly high for the next decade. Therefore, scientific priorities, subsequent requirements and their instrumental consequences have to be carefully analyzed.Detailed studies of spatio-temporal variations in the composition and chemistry of planetary atmospheres are of prime importance for the understanding of their evolution. Ultraviolet observations with an imaging spectrograph would be a means of partially fulfilling this objective. The performances of such an imaging spectrograph are studied in the case of observations of molecular absorption features in planetary atmospheres. A simple model of the source is used to simulate three-dimensional (spectral, spatial and temporal) data sets. We propose a method of data reduction which consists in focusing the images corresponding to different positions of the absorbing areas on the disk back onto a common frame of reference. The influence of the various parameters defining the absorption structure in the source on the contrast and width of the observed absorption dip is investigated as a function of the spectral dispersion of the instrument, as well as the effect due to spurious assumptions on the longitudinal position of the absorption area. A comparison with the performances of a long slit spectrograph capable of performing similar measurements shows that the objective grating concept, when it is optimized to the particular absorption bands of interest, has a significant advantage in terms of sensitivity, simultaneous spatial coverage and data reduction flexibility.RésuméIl est probable quun observatoire planétaire orbital verra le jour dans les dix ans qui viennent. Par conséquent, il est nécessaire danalyser avec soin les priorités scientifiques dun tel observatoire, les contraintes qui en découleraient et leur traduction sur le plan instrumental.Létude détaillée des variations spatio-temporelles dans la composition et la chimie des atmosphères planétaires est de premiére importance pour la compréhension de leur évolution. La possibilité dobserver dans lultraviolet moyen avec un spectrographe imageur serait un moyen de répondre au moins partiellement à cet objectif. Les performances dun tel instrument appliqué à lobservation de structures dabsorption moléculaire dans les atmosphères planétaires sont le sujet de la présente étude. Un modèle simple de la source est utilisé pour simuler les données tri-dimensionnelles (spectrales, spatiales et temporelles). Nous proposons une méthode de réduction des données qui consiste à ajouter dans un système de référence commun les images correspondant aux positions successives dune région dabsorption sur le disque. Linfluence des divers paramètres qui définissent la structure dabsorption sur le contraste et la largeur de la région dabsorption telle quelle est observée dans les données réduites, est étudiée en fonction de la dispersion spectrale de linstrument, de même que les effets produits par des hypothèses erronées sur la position longitudinale de cette structure. Comparé un spectrographe à fente de caractéristiques identiques, le concept à réseau objectif, dans la mesure où il est optimisé pour les bandes dabsorption intéressantes, apparait présenter un avantage significatif en termes de sensibilité, de couverture spatiale simultanée et de souplesse dans le traitement des données.