The CH₄@Titan project

Scientific background and objectives:

The precise knowledge of the methane absorption in the study of planetary systems, and especially of Titan (Saturn’s largest satellite, whose atmosphere is mainly composed of nitrogen and methane), is very timely and of great importance because it gives access to the determination of the physical properties of these objects. With the advent of the highly-successful Cassini-Huygens mission, in the Saturnian system since July 2004, a large amount of Titan data has been acquired. However, in the absence of reliable absorption coefficients of methane for Titan conditions in the near infrared (especially between 0.8 and 2 microns), the scientific community is faced with the difficulty of analysing the high-quality spectra and images that have been collected at these wavelengths. Even if the Cassini-Huygens mission has been on location for more than 4 years now, Titan is far from having revealed all its secrets. After the landing of the Huygens probe on Titan (14 Jan. 2005), the Cassini spacecraft has been pursuing its trek in the Saturnian system and it will continue to do so at least until 2010, providing new discoveries often enough while, at the same time, raising new questions. Among other, we still lack a precise description of the lower atmosphere and surface of the satellite. That is where the methane coefficients play a very important role: the modeling of the troposphere and surface requires a precise understanding of the methane influence, CH₄ being the main atmospheric absorbing constituent in the near-infrared.

Description of project and methodology:

The collaboration of the four laboratories involved in this project aims at rectifying this critical situation by setting up the means to determine the parameters allowing for the modeling of the methane absorption. Thorough theoretical calculations and experimental data recorded in conditions specific to Titan, and to many other objects in which CH₄ plays an important role, will be produced. The analysis and modeling in frequencies, intensities and lineshapes of the ¹²CH₄, ¹³CH₄ and ¹²CH₃D spectra will be extended towards very excited vibrational and rotational states. Simultaneously, spectra of the very low absorption regions, presently very poorly known, will be recorded using the most up-to-date ultra-sensitivity techniques. Thus, by gathering within a common project worldwide specialists of advanced theoretical calculations on the methane spectrum (Laboratoire Interdisciplinaire Carnot de Bourgogne (ICB) in Dijon and Groupe de Spectroscopie Moleculaire et Atmospherique (GSMA) in Reims) of state-of the-art experimental developments in the field of ultra-sensitive absorption (Laboratoire de Spectrometrie Physique (LSP) in Grenoble and GSMA, Reims) and planetologists strongly implied in the analysis of Cassini-Huygens data (Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA) at Paris-Meudon Observatory and GSMA, Reims), the present project establishes the tools required to determine the parameters allowing for the methane absorption modeling and for a critical comparison with Titan spectra and images. The data mainly concern two instruments of the Cassini-Huygens mission: DISR on the probe and VIMS on the spacecraft. When combined, they cover the spectral region from the UV and up to 5 microns. Once the new methane coefficients become available, we will proceed a) by validating them with the Huygens/DISR spectra that give access to the absorption of the solar radiation by atmospheric methane; b) by deducing the surface composition during the different stages of the Huygens’ descent; c) by analyzing the Cassini/VIMS data to separate and characterize the atmospheric opacity and surface contributions all over the satellite’s disk.

Titan
Image credit:
NASA/JPL/ Space Science Institute

Huygens DISR spectrum
with methane bands