Titan and the Cassini-Huygens mission
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Titan and the Cassini-Huygens mission Athena Coustenis LESIA, Paris-Meudon Observatory 5, place Jules Janssen, 92195 Meudon Cedex, FRANCE Abstract. The Cassini-Huygens mission has initiated its long exploration of the Saturnian system in July 2004, after a 7,5 year trek through our solar system. Since the Saturn Orbit Insertion, we have witnessed the great success of the Huygens mission, the probe descent through Titan’s atmosphere, on January 14, 2005. One of the main targets of the Cassini- Huygens mission was Titan. The combined orbiter and probe data have been a precious tool in the description of Titan’s atmosphere and surface returning wonderful new data whose analysis have revealed an amazing new world, 10 time further from our Sun and yet so close to our own planet. Indeed, Titan is currently the only exobiological system that we can study in reference to conditions which may have prevailed on the primitive Earth. INTRODUCTION Titan, Saturn’s largest satellite, has attracted the interest of the scientific community ever since its discovery by C. Huygens in 1655, and in particular since the realization in the early 20 th century that it has an extended atmosphere. This atmosphere is compose d essentially of molecular nit rogen (1.5 bar), like the Earth, and is the host of an active organic chemistry due to the presence of methane. CH 4 and N 2 combine to produce a large variety of hydrocarbons and nitriles. Some of the latter (such as HCN, called “prebiotic”) are considered to be the precursors of life on our own planet. An exterior source brings traces of oxygen in Titan, in the form of H 2 O, CO and CO 2 . The temperature on Titan is very low (varies from 70 to180 K in the atmosphere) due to the distance to the Sun (10 AU). At the surface, the temperature reaches 94 K thanks to a greenhouse effect, as on our planet. Titan was mainly explored in the 80s by the Voyager 1 mission which during one flyby returned its atmospheric composition and other characteristics. However, such things as the nature of the surface or the aerosol distribution, or the structure in the troposphere remained unknown. The Cassini-Huygens mission to the Saturnian system was launched in 1997 and arrived in the vicinity of the Saturnian system in 2004. The instruments aboard the mission have since then been returning wonderful new information on the satellite during each flyby. 44 of them are expected until the end of the mission in 2008. In what follows I discuss some of this information that has been revealed to us by the Cassini-Huygens mission and how our perception of Titan physics (Figure 1) has evolved over the past year or so.
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