Titan, the largest moon of Saturn, is a fascinating object, with its dense atmosphere, mostly constituted of nitrogen and methane. Solar radiation and energetic particles from Saturn’s magnetosphere induce a complex chemistry in this atmosphere, leading to the formation of hydrocarbons and nitriles. Very recently, the Cassini-Huygens space mission has revealed the presence in the ionosphere of negatively charged ions of the type CxHyNz-, with a mass-to-charge ratio up to 10,000. This is an exceptional discovery, as negative ions had not been predicted. Negative ion production and chemistry is poorly known. In order to understand the ionospheric processes leading to the ions, complex kinetic models of Titan’s ionospheric chemistry have to be developed. The models require ion molecule reaction rates and product branching ratios that are determined by laboratory experiments. However, kinetic data about negative ions are often outdated or even lacking. I propose to measure reaction rate constants, and to obtain structural information concerning ionic species of the type CxHyNz-. A commercial high-resolution mass spectrometer of a new type (LTQ-Orbitrap) has recently been acquired at the Laboratoire de Planétologie de Grenoble in order to study positive ion chemistry. I will take advantage of the fact that the Orbitrap is perfectly adapted for the study of negative ions. This experimental work will be guided by kinetic modeling of Titan’s ionosphere conducted in close collaboration with planetary scientists. Another part of the project will be to gain some insight on the haze that surrounds Titan. Laboratory synthesized analog compounds, the so-called « tholins », are produced in plasmas of nitrogen/methane mixtures. We plan to use the high-resolution mass spectrometer to perform structural analysis of their volatile fraction, and also to investigate the processes underlying their nucleation and growth.
Field of science
- /natural sciences/physical sciences/astronomy/planetary science/natural satellites
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