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Etude préparatoire à l'interprétation des données micro-ondes de l'instrument radar de la sonde Cassini-Huygens : impact de l'atmosphère de Titan

Abstract : The Cassini mission has been designed to send a spacecraft to the planet Saturn, and deploy an instrument probe, Huygens, that will descend to the surface of Saturn's moon Titan. The Cassini Radar instrument (f=13.78 GHz) onboard the orbiter is expected to characterize the surface of Titan. Correct interpretation of the measurements made with the radar requires simulations and laboratory measurements to anticipate possible atmospheric and surface effects and then better invert and interpret radar data. Due to the lack of permittivity values for Titan's haze particles in the microwave range, we performed dielectric constant measurements around 13.78 GHz on titanian aerosols synthesized in laboratory, also called tholins. We obtained a real part of in the range of 2-2.5 and a loss tangent between 10^(-3) and 5.10^(-2). We also gathered dielectric constants of others materials suspected to compose Titan's atmosphere and surface and for which experimental values in the range of Cassini radar frequency already existed: liquid and solid light hydrocarbons, water ice and silicates. We have then developed simple simulations to study the behaviour of a 13.78 GHz plane wave going through Titan's atmosphere and then backscattered by its surface. Titan's atmosphere (hydrocarbons clouds and/or aerosol haze) was modeled by a diluted medium filled with spherical particles of size, density and permittivity estimated from what could be found in the literature and laboratory experiments. Surface was characterized by its composition and roughness properties. The results about atmospheric radar attenuation are of two kinds: (1) The haze scenarios do not cause any attenuation and the radar wave will reach the surface of Titan without any losses. The particle radius never exceeds 1 um and the corresponding attenuation never exceeds 0.01 dB. (2) When we add in our simulation a rain layer in the last few kilometers, the attenuation reaches and rapidly exceeds the instrument sensitivity limit. A wave emerging from such a cloud layer could be so attenuated that the radar antenna would not be able to detect a returned echo from Titan's surface. Simulations of the backscattered signal by Titan's surface covered by tholins, various ices or silicates show also that rather low return could be expected, even for rough surfaces. This could dramatically increase the possible attenuation role of Titan's clouds. Finally, we show that the altimetry pulse shape could also be analyzed in order to detect an atmospheric effect (hypothetic clouds) and get unexpected informations about Titan's lower atmosphere (rain occurrence and extension of cloud systems, size, density and velocity of the particles) thanks to the backscattering of the signal on cloud layer.
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https://tel.archives-ouvertes.fr/tel-00005551
Contributor : Sébastien Rodriguez <>
Submitted on : Monday, April 5, 2004 - 6:51:27 PM
Last modification on : Wednesday, December 9, 2020 - 3:11:15 PM
Long-term archiving on: : Friday, April 2, 2010 - 8:24:10 PM

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  • HAL Id : tel-00005551, version 1

Citation

Sébastien Rodriguez. Etude préparatoire à l'interprétation des données micro-ondes de l'instrument radar de la sonde Cassini-Huygens : impact de l'atmosphère de Titan. Astrophysique [astro-ph]. Université Pierre et Marie Curie - Paris VI, 2003. Français. ⟨tel-00005551⟩

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