Decay of COSAC and Ptolemy mass spectra at comet 67P/Churyumov-Gerasimenko

摘要

Context. The Rosetta lander Philae successfully landed on the nucleus of comet 67P/Churyumov-Gerasimenko on 12 November 2014. Philae is equipped with two gas analysers: The Cometary Sampling and Composition experiment (COSAC) and the gas chromatograph and mass spectrometer Ptolemy. Aims. COSAC is designed for in situ analysis of organic molecules on 67P while Ptolemy is optimised to measure ratios of stable isotopes. Methods. On 12 to 14 November 2014, both instruments measured the organic composition of the comet nucleus material through seven measurements in sniffing mode during Philae’s hopping and at its final landing site Abydos. We compare the temporal evolution of intensities of several ion species identified by both mass spectrometers. For COSAC, this is the first analysis of the temporal behaviour of the measured ion species. Results. All ion species showed the highest intensities in the first spectra measured by both instruments approximately 20 to 30 min after Philae’s first touchdown at Agilkia, and a decay during the six consecutive measurements at Abydos. Both instruments measured an almost identical decay of the water peak ( m / z 18), and CO ( m / z 28) behaved similarly. In the COSAC measurements, the peak at m / z 44 decays much slower than all the other ion species, including the water peak. In particular, the m / z 44 peak decays much slower in the COSAC measurements than in the Ptolemy data. This supports our earlier interpretation that COSAC analysed, for the first time, a regolith sample from a cometary nucleus in situ, while Ptolemy measured cometary gas from the ambient coma. The m / z 44 peak measured by COSAC was likely dominated by organic species, whereas the peak measured by Ptolemy was interpreted to be mostly due to CO_(2) . Ion species heavier than m / z 30 tend to decay somewhat slower in the COSAC measurements than in the Ptolemy data, which may be related to differences in the exhaust designs between both instruments.