Analyse d'occultations solaires et stellaires par Titan observées par l'instrument Cassini/VIMS

Abstract : The observation of solar and stellar occultations by Titan allows us to study the thick atmosphere of this Saturnian satellite, regarding its gas and haze composition. The principle of these observations, done by the visible and infrared mapping spectrometer (VIMS) onboard the Cassini spacecraft, is to measure the transmission of the stellar or solar flux through Titan?s atmosphere. Data consist in a set of lightcurves at different wavelengths and a set of spectra for different altitudes of the line of sight. The study of those lightcurves reveals that we observe occultations by absorption and not differential refraction, as it is the case for Earth based observations of occultation events. The observed drop of signal is thus caused by the absorption of light by atmospheric gases and haze.
Absorption bands are visible in the transmission spectrum at 1.2, 1.4, 1.7, 2.3 and 3.3 µm for methane and 4.7 µm for carbon monoxide. In order to model those bands, a radiative transfer code was developed taking into account spherical geometry and based on a line by line calculation. The methane study is focused on the 2.3 µm band. Above 200 km, our data are in good agreement with models with a 1.4-1.6 % CH4 mixing ratio as measured by other instruments.
Below 200 km, a systematic effect, yet misunderstood, prevents a reliable measurement. The CO molecule is detected below the 180 km altitude level. A mixing ratio of 33±10 ppm is measured between the altitudes of 70 and 130 km. An extra absorption, centred at 3.4 µm, appears below about 500 km and is mixed with the 3.3 µm methane band. This band characterizes the stretching modes of C - H bonds in long aliphatic chains attached to large organic molecules that compose the haze particles.
Haze absorption defines the continuum level in the studied spectrum. This absorption is stronger at short wavelengths and increases as altitude decreases. An inversion code of the continuum was developed in order to retrieve the density profiles of the haze and to model its transmission. The main assumption is that haze particles are fractal aggregates made of spherical monomers of 0,05 µm radius with the optical properties of the tholins produced by Khare et al. (1984). The transmission models reveal that only aggregates with more than 1 000 monomers can reproduce the observations. Furthermore, the characteristic absorptions at 3 and 4.6 µm are not observed in our data, which underlines the significant differences between the tholins and the actual Titan haze. Haze density profiles indicate a exponential growth below 450 km, characterized by a scale height of about 60 km for the solar occultation (71°S) and 50 km for the Gamma Crucis occultation (24°N). This difference may be explained by the difference of latitude of those observations. Finally, the data set from the grazing occultation of Antares indicates the presence of numerous spikes that are attributed to gravity waves propagating in Titan?s atmosphere.
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Contributor : Aurélie Bellucci <>
Submitted on : Monday, December 15, 2008 - 11:15:40 AM
Last modification on : Wednesday, May 15, 2019 - 3:56:21 AM
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  • HAL Id : tel-00347235, version 1

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Aurélie Bellucci. Analyse d'occultations solaires et stellaires par Titan observées par l'instrument Cassini/VIMS. Astrophysique [astro-ph]. Université Pierre et Marie Curie - Paris VI, 2008. Français. ⟨tel-00347235⟩

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