Low CO/CO₂ratios of comet 67P measured at the Abydos landing site by thePtolemymass spectrometer

Abstract

Comets are generally considered to contain the best-preserved material from the beginning of our planetary system, although the mechanism of their formation and subsequent evolution are still poorly understood. Here we report the direct in situ measurement of H 2 O, CO, and CO 2 by the Ptolemy mass spectrometer onboard the Philae lander, part of the European Space Agency's Rosetta mission, at the Abydos site of the Jupiter-family comet 67P/Churyumov-Gerasimenko. A CO/CO 2 ratio of around 0.07 ± 0.04 is found at the surface of the comet, a value substantially lower than the one measured by ROSINA in the coma. Such a major difference is a potential indication of heterogeneity of the nucleus and not of changes in the CO/CO 2 ratio of the coma with radial distance.

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

“…the addition or subtraction of one or more protons to the ion species under consideration): m/z 16, 17, 18 and 19 to derive the intensities of H 2 O, m/z 28 and 29 for CO and m/z 44 and 45 to get the intensity of CO 2 . The analysis of the Ptolemy data was described in detail by Morse et al (2015). The peak intensities for H 2 O, CO and CO 2 measured with Ptolemy level off after approximately 20 hours and display a flattening similar to that found in the COSAC measurements.…”

“…We therefore ignore the last two COSAC spectra in our analysis. Figure 2 (right panel) shows Ptolemy data for H 2 O, CO and CO 2 measured between 12 and 14 November 2014 (Morse et al 2015). It should be noted that the ion counts given for these three species are summed ion counts for up to four m/z bins to account for hydrogen abstraction (i.e.…”

“…It should be noted that in our analysis of the COSAC spectra we also consider peaks for ion species with a rather low count (Morse et al 2015).…”

“…The remaining six COSAC spectra obtained at Abydos were not previously presented in detail. The Ptolemy data were discussed by Wright et al (2015) and Morse et al (2015).…”

“…Results were published by Wright et al (2015) and Morse et al (2015). In the latter paper the authors consider the temporal evolution of three abundant cometary species: H 2 O, CO and CO 2 .…”