The COSAC experiment of the Rosetta mission: Performance under representative conditions and expected scientific return

The Cometary Sampling and Composition (COSAC) experiment is a combined gas chromatograph–mass spectrometer onboard the Philae cometary surface probe of the Rosetta mission. Its goal is to analyse, directly at the surface of a comet, the molecular and elemental compositions of the cometary nucleus samples collected by the probe. The present article describes the results of the first calibration campaign done with the whole instrumental suite, in order to evaluate and to improve its analytical performance. This calibration campaign enabled us: (i) to optimize the operations of the gas chromatograph and thus to improve the chromatographic analytical power; (ii) to confirm the analytical performance of the COSAC chromatograph, by comparison with tests performed at the instrumental component level, excepted a little loss of the chromatographic resolution; (iii) to validate and to evaluate the performance of the different chromatographic analytical modes, i.e. mainly single and multi columns separation modes, and the gas chromatograph–mass spectrometer coupling mode. This series of tests therefore confirms the potential for the COSAC experiment to provide the first insight of the main molecular composition of a cometary nucleus, and thus to improve our knowledge of these bodies and their history.