S. Vinatier

Paris Diderot University, Lutetia Parisorum, Île-de-France, France

Are you S. Vinatier?

Claim your profile

Publications (113)192.54 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present an analysis of the VIMS solar occultations dataset, which extracts vertically resolved information on the characteristics of Titan's atmosphere between 100-700 km with a characteristic vertical resolution of 10 km. After a series of data treatment procedures to correct problems in pointing stability and parasitic light, 4 occultations out of 10 are retained. This sample covers different seasons and latitudes of Titan. Through the inversion of the transmission spectra with a line-by-line radiative transfer code we retrieve the vertical distribution of CH$_4$ and CO mixing ratio. The two methane bands at 1.4 and 1.7 {\mu}m are always in good agreement and yield an average stratospheric abundance of $1.28\pm0.06$%. This is significantly less than the value of 1.48% obtained by the GCMS/Huygens instrument. The analysis of the residual spectra after the inversion shows that there are additional absorptions which affect a great part of the VIMS wavelength range. We attribute many of these additional bands to gaseous ethane, whose near-infrared spectrum is not well modeled yet. Ethane contributes significantly to the strong absorption between 3.2-3.5 {\mu}m that was previously attributed only to C-H stretching bands from aerosols. Ethane bands may also influence the surface windows, especially at 2.7 {\mu}m. Other residual bands are generated by stretching modes of C-H, C-C and C-N bonds. In addition to the C-H stretch from aliphatic hydrocarbons at 3.4 {\mu}m, we detect a strong and narrow absorption at 3.28 {\mu}m which we tentatively attribute to the presence of PAHs in the stratosphere. C-C and C-N stretching bands are possibly present between 4.3-4.5 {\mu}m. Finally, we obtained the CO mixing ratio between 70-170 km. The average result of $46\pm16$ ppm is in good agreement with previous studies.
    05/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Cassini/CIRS spectra in the far- and mid-infrared region are used to determine the abundance of methane in Titan’s lower stratosphere and investigate its distribution with latitude. The CIRS spectra include emission from both the CH4 ν4 band at 7.7 μm and pure rotational lines longwards of 50 μm, which show differential sensitivities to thermal profile and methane mole fraction. We analyze nadir and limb data taken over the first part of the Cassini mission (August 2005 to June 2010), including a selection of 12 latitudes that provides a reasonably complete and regular sampling of both hemispheres. Unexpectedly, but in a consistent manner for limb and nadir geometries, large variations of the methane mole fraction near 15 mbar (∼85 km) are found, with values ranging from ∼1.0% (at low latitudes and near ±50–55°) to ∼1.5% (at ±30–35° and polar latitudes). Error bars on the retrieved methane mole fraction are 0.07–0.12% at low latitudes in the Southern hemisphere and 0.14–0.21% northward of 40°N. A 1.0% methane mole fraction at low latitudes permits us to reconcile the HASI-measured temperatures below 147 km altitude (2.7 mbar) with inferences from CIRS. The roughly hemispherically-symmetric distribution of methane gas is reminiscent of that observed or predicted for the tropospheric methane clouds, which on a yearly-averaged basis, show preferential occurrences at tropical and polar latitudes. We speculate that convective events at these latitudes result into local stratospheric methane enrichment, which may persist year-round due to dynamical mixing times in the lower stratosphere only moderately shorter than a Titan year.
    Icarus 01/2014; 231:323–337. · 3.16 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Since the Voyager 1 (V1) flyby in 1980, Titan's exploration from space and the ground has been ongoing for more than a full revolution of Saturn around the Sun (one Titanian year or 29.5 Earth years had elapsed in 2010 May). In this study, we search for temporal variations affecting Titan's atmospheric thermal and chemical structure within that year. We process Cassini/CIRS data taken during the Titan flybys from 2006-2013 and find a rather uneventful equatorial evolution. Conversely, at northern latitudes, we found enhanced abundances around the period of the northern spring equinox in mid-2009, which subsequently decreased (from 2010 to 2012), returning to values similar to those found in the V1 epoch, one Titanian year before. In the southern latitudes, since 2012, we see a trend for an increase of several trace gases (C4H2, C3H4, and HCN), indicative of a seasonal atmospheric reversal setting in. When we compare the CIRS 2010 and the 1980 V1/IRIS spectra (reanalyzed here), we find limited inter-annual variations. A return to the 1980 stratospheric temperatures and abundances is generally achieved from 50°N to 50°S, indicative of the solar radiation being the dominating energy source at 10 AU, as for the Earth, as predicted by general circulation and photochemical models. Exceptions concern the most complex hydrocarbons (C4H2 and C3H4). We also consider data from ground-based and Earth-orbiting observatories (such as from the Infrared Space Observatory, revisited here) and discuss possible atmospheric composition trends during a Titanian year.
    The Astrophysical Journal 12/2013; 779(2):177-. · 6.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The VIMS instrument has acquired 10 solar occultations since the beginning of the Cassini-Huygens mission. This dataset covers different seasons and latitudes and allows to study a large part of the atmosphere (between 50 and 700 km) by the acquisition of spectra in the infrared range (1-5 µm) with a vertical resolution of ~ 10 km on average. We present here the vertical profiles of gases and other atmospheric components, together with their spatial and temporal evolution, for a selection of VIMS occultations. Two main components of Titan's atmosphere, CH4 and CO, are observed in particular. Methane presents strong bands at 1.2, 1.4, 1.7, 2.3 and 3.2 µm. Its vertical profile, computed by the inversion of the 2.3 µm band, shows an almost constant abundance of ~ 1.2-1.3% above 250 km, less than the reference value of 1.41% from the GSMS instrument (Niemann et al. 2010). CO is detectable below ~ 160 km through its band at 4.7 µm. The resulting profiles are in good agreement with CIRS results that indicate a constant mixing ratio of 50 ppm (De Kok et al. 2007). Other spectral signatures have been detected by VIMS solar occultations. The strongest of these signatures is blended with the 3.2 µm CH4 band. It is centered at 3.4 µm and was discovered by a previous analysis of one solar occultation (Bellucci et al. 2009). It has been attributed to the C-H stretch by alkanes and aromatics present in Titan’s aerosols. An additional absorption observed at 2.4 µm, within the 2 µm methane band, can tentatively be attributed to overtone signatures of this C-H stretch. We are discussing this interpretation and its implications on the composition of aerosols and their temporal and spatial variability. Other bands at 2.7 and 4.2 µm have been detected for the first time by the present study and are still unidentified. Their characteristics and possible attribution are analyzed.
    American Astronomical Society, DPS meeting #45, Denver, CO; 10/2013
  • [Show abstract] [Hide abstract]
    ABSTRACT: Above about 400-450 km in Titan's atmosphere, the assumption of local thermodynamic equilibrium (LTE) breaks down for molecular vibrational levels of methane and various trace gases. Above this altitude non-LTE significantly impacts the formation of mid-infrared ro-vibrational band emissions of these species observed in the limb viewing geometry. Due to the limb geometry this impact also propagates down to lower tangent heights. We studied retrievals of temperature and trace gas abundances from the CIRS limb observations of Titan's atmosphere with the help of an iterative retrieval algorithm applied earlier by Vinatier et al, (2007, 2010), which was modified to account for the non-LTE source functions for each gas. We demonstrate that accounting for the non-LTE significantly affects the retrieval results: it leads to higher temperature values (up to 10 K at altitudes of about 550 km). It also leads to substantially higher vertical gradients of volume mixing ratio (VMR) profiles and, as a result, to higher VMR values for some trace gases at high altitudes (e.g., up to about 6 time higher C2H2 mixing ratio at 550 km) compared to those obtained with the LTE assumption. The non-LTE impact on retrievals also propagates down to the altitudes of 350 km.
    10/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Titan is the largest satellite of Saturn and is the only moon in our solar system to have a significant atmosphere. Titan's middle-atmosphere circulation usually comprises a single hemisphere-to-hemisphere meridional circulation cell, with upwelling air in the summer hemisphere and subsiding air at the winter pole with an associated winter polar vortex. Titan has an axial tilt (obliquity) of 26.7degrees, so during its 29.5 Earth year annual cycle pronounced seasonal effects are expected as the relative solar insolation in each hemisphere changes. The most dramatic of these changes is predicted to be the reversal in global meridional circulation as the peak solar heating switches hemispheres after an equinox. Since northern spring equinox in mid-2009, Titan's atmosphere has demonstrated dramatic changes in temperature, composition, and aerosol distribution. These changes indicate major changes to the atmospheric circulation pattern have indeed occurred. Here we use nine years of Cassini/CIRS infrared spectra to determine the temperature and composition evolution of the atmosphere through northern-fall to northern-spring. Particularly dramatic changes are observed at the poles, where a new south polar hot-spot/vortex has been forming. The north polar vortex also appears to be weakening throughout this period. Furthermore, the meridional circulation reversal, predicted by numerical models, occurred a mere six months after equinox, showing that despite Titan's long annual cycle, rapid changes are possible. This gives us new insight into vortex formation processes and atmospheric dynamics.
    10/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: The Voyager 1 flyby of Titan in 1980 gave a first glimpse of the chemical complexity of Titan's atmosphere, detecting many new molecules with the infrared spectrometer (IRIS). These included propane (C3H8) and propyne (CH3C2H), while the intermediate-sized C3Hx hydrocarbon (C3H6) was curiously absent. Using spectra from the Composite Infrared Spectrometer (CIRS) on Cassini, we show the first positive detection of propene (C3H6) in Titan's stratosphere (5-sigma significance), finally filling the three-decade gap in the chemical sequence. We retrieve a vertical abundance profile from 100-250 km, that varies slowly with altitude from ~2 ppbv at 100 km, to ~5 ppbv at 200 km. The abundance of C3H6 is less than both C3H8 and CH3C2H, and we remark on an emerging paradigm in Titan's hydrocarbon abundances whereby: alkanes > alkynes > alkenes within the C2Hx and C3Hx chemical families in the lower stratosphere. More generally, there appears to be much greater ubiquity and relative abundance of triple-bonded species than double-bonded, likely due to the greater resistance of triple bonds to photolysis and chemical attack.
    The Astrophysical Journal 09/2013; 776(1). · 6.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Nine years after Cassini's Saturn orbit insertion, we look at the evolution of the thermal and chemical composition of Titan's atmosphere by combining Cassini CIRS recordings and the related ground- and space- based observations. The fulfillment of one Titanian year of space observations provides us for the first time with the opportunity to evaluate the relative role of different physical processes in the long term evolution of this complex environment. By comparing V1 (1980), ISO (1997) and Cassini (2010) we find that a reversal of composition near the equator from automnal equinox to vernal equinox (1996 min -2009 max), as well as some differences in polar enhancement at the same era as Voyager.
    09/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: A stratospheric ice cloud identified by an emission feature in Titan's far-infrared spectrum has been changing character since Cassini arrived at Saturn. Until recently, the emission at 220 cm-1 had been found only in the polar north, where its spectral intensity had been decreasing gradually during mid-late winter and early spring [1]. Between 2005 and 2012 the emission in the north diminished by about a factor of five. Then in 2012 the emission feature was seen for the first time near the south pole [2]. Although the emergence of the ice cloud in the south was not unexpected, given the approach of winter, the emission appeared rather suddenly in July 2012. The advent of the southern ice cloud coincided with the rapid formation in 2012 of a haze hood and vortex at the south pole seen in Cassini images [3]. The 220 cm-1 emission arises from altitudes of 80-150 km and peaks sharply near 140 km. The material responsible for the spectral feature is not known, but indirect evidence hints at a condensate arising from complex nitriles that build up in the polar shadow at high winter latitudes. As gases migrate downward in the polar regions they encounter a dip in winter stratospheric temperatures near 150 km where some nitriles condense and form a cloud layer. The cloud material eventually precipitates and reaches the surface. By combining our observations from the north and south we can get an idea of the seasonal cycle of the ice cloud over much of a Titan year. From the southern data we see that the cloud begins to form in mid-autumn and from the northern data we infer that a maximum must occur around the beginning of winter, followed by a steady decline by late winter. Little of the ice cloud remains by summer. Our studies of the 220 cm-1 ice cloud, first observed by the Infrared Interferometer Spectrometer (IRIS) on Voyager 1 [4], are performed on Cassini with the Composite Infrared Spectrometer (CIRS) [5-8]. CIRS will track the northern and southern ice clouds through the remainder of the Cassini mission.
    09/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have explored the thermal and chemical composition of Titan's atmosphere by combining Cassini CIRS recordings and the related ground - and space - based observations. The fulfillment of one Titanian year of space observations provides us for the first time with the opportunity to evaluate the relative role of different physical processes in the long term evolution of this complex environment. We find indication for a weakening of the temperature gradient with warming of the stratosphere and cooling of the lower mesosphere. In addition, we infer precise concentrations for the trace gases and their main isotopologues and find that the chemical composition in Titan's stratosphere varies significantly with latitude during the 6 years investigated here, with increased mixing ratios towards the northern latitudes. In particular, we monitor and quantify the amplitu de of a maximum enhancement of several gases observed at northern latitudes up to 50°N around mid-2009, at the time of the NSE. We find that this raise is followed by a rapid decrease in chemical inventory in 2010 probably due to a weakening north polar vortex with reduced lateral mixing across the vortex boundary. By comparing the Cassini/CIRS results from both the limb and the nadir observations with past V1 (1980) and ISO (1997)inferences we find indication for seasonal variations.
    09/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Titan is the largest satellite of Saturn and is the only moon in our solar system with a significant atmo- sphere. Titan's middle-atmosphere (stratosphere and mesosphere) circulation usually comprises a single hemisphere to hemisphere meridional circulation cell, with upwelling air in the summer hemisphere and sub- siding air at the winter pole with an associated winter polar vortex. Titan has an axial tilt (obliquity) of 26.7°, so during its 29.5 Earth year annual cycle pronounced seasonal effects are encountered as the relative solar insolation in each hemisphere changes. The most dramatic of these changes is the reversal in global meridional circulation as the peak solar heating switches hemispheres after an equinox. Titan's northern spring equinox occurred in August 2009, and since then many middle-atmosphere changes have been observed by Cassini that were previously impossible to study (1,2,3,4). Here we present a detailed analysis of the post equinox changes in middle-atmosphere temperature and composition measured with Cassini's Composite InfraRed Spectrometer (CIRS), use these to infer changes in atmospheric circulation, and explore implications for atmospheric photochemical and dynamical processes. Our results show that the meridional circulation has now reversed (1).
    09/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: In 2012 an emission feature at 220 cm-1 in Titan's far-infrared spectrum was seen for the first time in the south [1]. Attributed to a stratosphere ice cloud formed at the winter pole, the 220 cm-1 emission had previously been seen only at high northern latitudes where it had been decreasing since the arrival of Cassini in 2004 [2]. Our far-infrared observations were performed with the Composite Infrared Spectrometer (CIRS) on Cassini [3]. Although it had been expected that the 220 cm-1 emission would eventually appear in the south, the emission appeared rather suddenly, increasing by a factor of at least four between February (when it was not detected) and July 2012. At the time of our observations, one Titan month after equinox, the 220 cm-1feature was present in both the north and south and showed a trend of continued slow decrease in the north and steep increase in the south. As has been the case in the north, the emission in the south was confined to high latitudes associated with winter polar shadowing. Our spectroscopic detection of the southern 220 cm-1 ice cloud coincided with the rapid formation in 2012 of a haze hood and vortex at the south pole as seen in Cassini images [4]. The 220 cm-1 feature was first observed by the Infrared Interferometer Spectrometer (IRIS) on Voyager 1 [5, 6] and has been extensively studied in the north by CIRS [7-10]. Until now the 220 cm-1 emission, like the polar hood, has been associated solely with the north, owing to the fact that Voyager and Cassini have viewed Titan only during winter-spring. In 2012 we witnessed the start of a seasonal shift of this pattern to the south. The 220 cm-1 emission arises from altitudes of 80-150 km and peaks sharply near 140 km. The material responsible for the spectral feature is not known, but indirect evidence hints at a condensate arising from complex nitriles, which also tend to be present only at high winter latitudes. References: [1] Jennings, D. E., et al., ApJ, 761, L15, 2012. [2] Jennings, D. E., et al., ApJ, 754, L3, 2012. [3] Flasar, F. M., et al., Space Sci. Rev., 115, 169, 2004. [4] West, R. A., et al., DPS, paper 300.04, 2012. [5] Kunde et al. 1981, [6] Coustenis et al., 1999. [7] de Kok et al., Icarus, 191, 223, 2007. [8] de Kok et al., Icarus, 197, 572, 2008. [9] Samuelson et al., Icarus, 189, 63, 2007. [10] Anderson, et al., presented at workshop "Titan Through Time 2", 2012.
    04/2013;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In this work [1] we present mid- and far-Infrared absorption spectra of Titan's aerosol analogues produced in the PAMPRE experimental setup. We provide a complete dataset regarding the influence that the concentration of methane vapor in the gas mixture has on the tholins spectra. Among other effects, the intensity of the 2900 cm-1 pattern (also detected in Titan's atmosphere) increases with the methane concentration. On the opposite, tholins produced with low methane concentrations seem to be more amine based polymers. Moreover, we compare tholins spectrum with observation of Titan's atmosphere. It is shown that the position of the bands around 2900 cm-1 depends on the chemical environment of the methyl functional group. We conclude that the presence of these absorption bands in Titan's atmosphere, as measured with the VIMS instrument onboard Cassini [2] is in agreement with an aerosol contribution. In the far-infrared, tholins spectrum presents many similarities with the spectra of Titan's aerosols derived from recent Cassini-CIRS observations [3] and allows identification of bands in the spectrum of Titan's atmosphere REFERENCES 1. T. Gautier N. Carrasco, A. Mahjoub, S. Vinatier, C. Szopa, J.-J. Correia, P. Dumas, A. Giuliani and G. Cernogora. Icarus 221: 320-327(2012). 2. P. Rannou, T. Cours, S. Le Mouelic, S. Rodriguez, C. Sotin, P. Drossart, R. Brown, Icarus 208: 850-867 (2010). 3. C. M. Anderson and R. E. Samuelson, Icarus 212: 762-778 (2011)
    04/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Seasonal variations of Titan's atmospheric temperature/composition from nine years of Cassini-CIRS infrared spectra indicate a general circulation reversal.
    03/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: In this chapter we describe the remote sensing measurement of nitrogen-bearing species in Titan's atmosphere by the Composite Infrared Spectrometer (CIRS) on the Cassini spacecraft. This instrument, which detects the thermal infrared spectrum from 10 to 1,500 cm- 1 (1,000-7 μ m) is sensitive to vibrational and rotational emissions of gases and condensates in Titan's stratosphere and lower mesosphere, permitting the measurement of ambient temperature and the abundances of gases and particulates. Three N-bearing species are firmly detected: HCN, HC3N and C2N2, and their vertical and latitudinal distributions have been mapped. In addition, ices of HC3N and possibly C4N2 are also seen in the far-infrared spectrum at high latitudes during the northern winter. The HC15N isotopologue has been measured, permitting the inference of the 14N/15N in this species, which differs markedly (lower) than in the bulk nitrogen reservoir (N2). We also describe the search in the CIRS spectrum, and inferred upper limits, for NH3 and CH3CN. CIRS is now observing seasonal transition on Titan and the gas abundance distributions are changing accordingly, acting as tracers of the changing atmospheric circulation. The prospects for further CIRS science in the remaining 5 years of the Cassini mission are discussed.
    01/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: An emission feature at 220 cm{sup -1} which has been attributed to a cloud of condensed material in Titan's winter stratosphere has been seen for the first time in the south. This feature had previously been found only at high northern latitudes during northern winter and spring. The material emitting at 220 cm{sup -1}, as yet unidentified, may be volatiles associated with nitrile gases that accumulate in the absence of ultraviolet sunlight. Not detected as recently as 2012 February, the 220 cm{sup -1} feature clearly appeared at the south pole in Cassini spectra recorded on 2012 July 24, indicating a rapid onset of the emission. This is the first indication of the winter buildup of condensation in the southern stratosphere that has been expected as the south pole moves deeper into shadow. In the north the 220 cm{sup -1} feature continued to decrease in intensity with a half-life of 3 years.
    The Astrophysical Journal Letters 12/2012; 761(1). · 6.35 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have developed a line-by-line Atmospheric Radiative Transfer for Titan code that includes the most recent laboratory spectroscopic data and haze descriptions relative to Titan's stratosphere. We use this code to model Cassini Composite Infrared Spectrometer data taken during the numerous Titan flybys from 2006 to 2012 at surface-intercepting geometry in the 600-1500 cm{sup -1} range for latitudes from 50 Degree-Sign S to 50 Degree-Sign N. We report variations in temperature and chemical composition in the stratosphere during the Cassini mission, before and after the Northern Spring Equinox (NSE). We find indication for a weakening of the temperature gradient with warming of the stratosphere and cooling of the lower mesosphere. In addition, we infer precise concentrations for the trace gases and their main isotopologues and find that the chemical composition in Titan's stratosphere varies significantly with latitude during the 6 years investigated here, with increased mixing ratios toward the northern latitudes. In particular, we monitor and quantify the amplitude of a maximum enhancement of several gases observed at northern latitudes up to 50 Degree-Sign N around mid-2009, at the time of the NSE. We find that this rise is followed by a rapid decrease in chemical inventory in 2010 probably due to a weakening north polar vortex with reduced lateral mixing across the vortex boundary.
    The Astrophysical Journal 12/2012; 760(2). · 6.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Saturn's moon Titan has a nitrogen atmosphere comparable to Earth's, with a surface pressure of 1.4 bar. Numerical models reproduce the tropospheric conditions very well but have trouble explaining the observed middle-atmosphere temperatures, composition and winds. The top of the middle-atmosphere circulation has been thought to lie at an altitude of 450 to 500 kilometres, where there is a layer of haze that appears to be separated from the main haze deck. This 'detached' haze was previously explained as being due to the co-location of peak haze production and the limit of dynamical transport by the circulation's upper branch. Here we report a build-up of trace gases over the south pole approximately two years after observing the 2009 post-equinox circulation reversal, from which we conclude that middle-atmosphere circulation must extend to an altitude of at least 600 kilometres. The primary drivers of this circulation are summer-hemisphere heating of haze by absorption of solar radiation and winter-hemisphere cooling due to infrared emission by haze and trace gases; our results therefore imply that these effects are important well into the thermosphere (altitudes higher than 500 kilometres). This requires both active upper-atmosphere chemistry, consistent with the detection of high-complexity molecules and ions at altitudes greater than 950 kilometres, and an alternative explanation for the detached haze, such as a transition in haze particle growth from monomers to fractal structures.
    Nature 11/2012; 491(7426):732-5. · 38.60 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present the vertical distribution of gaseous species and aerosols in Titan’s atmosphere through the analysis of VIMS solar occultations. We employ the infrared channel of VIMS, which covers the 1 - 5 µm wavelength range. VIMS occultations can provide good vertical resolution ( 10 km) and an extended altitude range (from 70 to 700 km), complementing well the information from other Cassini instruments. VIMS has retrieved 8 solar occultations up to now. They are distributed through the whole Cassini mission and they probe different latitudes in both hemispheres. Two main gases can be observed by VIMS occultations: methane, through its bands at 1.2, 1.4, 1.7, 2.3 and 3.3 µm, and CO, at 4.7 µm. We can extract methane’s abundance between 70 and 700 km and CO’s between 70 and 180 km. Regarding aerosols, the VIMS altitude range allows to get information on the properties of both the main haze and the detached layer. Aerosols also affect the transmittance through their spectral signatures. In particular, a spectral signature at 3.4 µm that was attributed to aerosols was recently discovered by the analysis of the first VIMS occultation. We will monitor the latitudinal and temporal variations of the 3.4 µm feature through various occultations. A change in the global circulation regime of Titan sets in with the approaching to the vernal equinox, and a strong decrease of the altitude of the detached layer between the winter solstice and the equinox has indeed been observed. The temporal coverage of VIMS occultations allows the study the effect of these variations in the vertical distribution of aerosol optical and spectral properties.
    American Astronomical Society, DPS meeting #44; 10/2012
  • [Show abstract] [Hide abstract]
    ABSTRACT: Methane is a key species in the Outer Planets. It is the third most abundant molecule in all four Giant Planets, with a deep tropospheric abundance increasing from 0.2-0.4% (of the hydrogen-helium mix) at Jupiter and Saturn and about 2% in Uranus and Neptune. Its abundance is even larger in Titan, reaching 5% (of N2) at the surface. Related to its large abundance, methane plays a dominant role in governing the stratospheric chemistry of all the Outer Planets. In the thermal infrared, the methane 7.7 microns nu4 band has been widely observed. This band is more sensitive to temperature than to the methane abundance, however. On the other hand, weak pure rotational lines of methane at 50-250 microns are much more sensitive to the methane abundance. Therefore, the opening of the submillimeter range with Herschel represented an excellent opportunity for an improved determination of methane abundance and vertical profile in the Outer Planets. Observations of Neptune and Titan were performed in the winter 2011/2012 with the HIFI heterodyne submm instrument on board of the Herschel Space Observatory. The four main components of the CH4(J=6-5) rotational transition at 1882 GHz (i.e. 159.3 micron), were detected in emission. This represents the first times these lines are resolved spectrally (R 10^6). Moreover, observations of Uranus were performed in the same period, with the PACS spectrometer (R 1400) also on board of Herschel. A weak absorption of the CH4(J=6-5) transition was detected, but without resolving the four main component. These lines probe mainly the stratosphere of Neptune and Titan, and the tropopause of Uranus. We will present the methane abundance retrieved from these observations.
    10/2012;

Publication Stats

818 Citations
192.54 Total Impact Points

Institutions

  • 2008–2014
    • Paris Diderot University
      • Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique (LESIA) UMR 8109
      Lutetia Parisorum, Île-de-France, France
    • Laboratoire d'Etudes en Géophysique et Óceanographie Spatiales
      Tolosa de Llenguadoc, Midi-Pyrénées, France
  • 2012
    • University of Bristol
      • School of Earth Sciences
      Bristol, ENG, United Kingdom
  • 2009–2012
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 2007–2012
    • Observatoire de Paris
      Lutetia Parisorum, Île-de-France, France
    • Loyola University Maryland
      Baltimore, Maryland, United States
  • 2010
    • Université de Reims Champagne-Ardenne
      Rheims, Champagne-Ardenne, France
    • University of Maryland, College Park
      • Department of Astronomy
      College Park, MD, United States