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The composition of the neutral gas comas of most comets is dominated by [H.sub.2]O, CO and C[O.sub.2], typically comprising as much as 95 per cent of the total gas density (1). In addition, cometary comas have been found to contain a rich array of other molecules, including sulfuric compounds and complex hydrocarbons. Molecular oxygen ([O.sub.2]), however, despite its detection on other icy bodies such as the moons of Jupiter and Saturn (2,3), has remained undetected in cometary comas. Here we report in situ measurement of [O.sub.2] in the coma of comet 67P/Churyumov-Gerasimenko, with local abundances ranging from one per cent to ten per cent relative to [H.sub.2]O and with a mean value of 3.80 ± 0.85 per cent. Our observations indicate that the [O.sub.2]/[H.sub.2]O ratio is isotropic in the coma and does not change systematically with heliocentric distance. This suggests that primordial [O.sub.2] was incorporated into the nucleus during the comet's formation, which is unexpected given the low upper limits from remote sensing observations (4). Current Solar System formation models do not predict conditions that would allow this to occur., Measurements of the coma of 67P/Churyumov-Gerasimenko (hereafter 67P) were made between September 2014 and March 2015 with the ROSINA-DFMS mass spectrometer (5) on board the Rosetta spacecraft. For the present [...]

Context. Measurements of isotopic abundances in cometary ices are key to understanding and reconstructing the history and origin of material in the solar system. Comets are considered the most pristine material in the solar system. Isotopic fractionation (enrichment of an isotope in a molecule compared to the initial abundance) is sensitive to environmental conditions at the time of comet formation. Therefore, measurements of cometary isotope ratios can provide information on the composition, density, temperature, and radiation during formation of the molecules, during the chemical evolution from the presolar cloud to the protosolar nebula, and the protoplanetary disk before accretion in solid bodies. Most isotopic abundances of 12C/13C and 16O/18O in comets to date are in agreement with terrestrial abundances. Prior to the Rosetta mission, measurements of 12C/13C in comets were only available for HCN, CN, and C2 and for 16O/18O in H2O. Measurements of 12C/13C in comets were only available from ground based observations and remote sensing, while 16O/18O in H2O had also been measured in-situ. To date, no measurements of the CO2 isotopologues in comets were available. Aims. This paper presents the first measurements of the CO2 isotopologues in the coma of 67P/Churyumov-Gerasimenko (67P). Methods. We analyzed measurements taken by the Double Focusing Mass Spectrometer (DFMS) of the ROSINA experiment on board the ESA spacecraft Rosetta in the coma of 67P. Results. The CO2 isotopologues results for 67P are: 12C/13C = 84 ± 4, 16O/18O = 494 ± 8, and 13C16O2/16C18O16O = 5.87 ± 0.07. The oxygen isotopic ratio is within error bars compatible with terrestrial abundances but not with solar wind measurements. Conclusions. The carbon isotopic ratio and the combined carbon and oxygen isotopic ratio are slightly (14%) enriched in 13C, within 1σ uncertainty, compared to solar wind abundances and solar abundances. The small fractionation of 12C/13C in CO2 is probably compatible with an origin of the material in comets from the native cloud. [ABSTRACT FROM AUTHOR]

Context: The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) was designed to measure the composition of the gas in the coma of comet 67P/Churyumov-Gerasimenko, the target of the European Space Agency's Rosetta mission. In addition to the volatiles, ROSINA measured refractories sputtered off the comet by the interaction of solar wind protons with the surface of the comet. Aims: The origin of different solar system materials is still heavily debated. Isotopic ratios can be used to distinguish between different reservoirs and investigate processes occurring during the formation of the solar system. Methods: ROSINA consisted of two mass spectrometers and a pressure sensor. In the ROSINA Double Focusing Mass Spectrometer (DFMS), the neutral gas of cometary origin was ionized and then deflected in an electric and a magnetic field that separated the ions based on their mass-to-charge ratio. The DFMS had a high mass resolution, dynamic range, and sensitivity that allowed detection of rare species and the known major volatiles. Results: We measured the relative abundance of all three stable silicon isotopes with the ROSINA instrument on board the Rosetta spacecraft. Furthermore, we measured 13C/12C in C2H4, C2H5, and CO. The DFMS in situ measurements indicate that the average silicon isotopic composition shows depletion in the heavy isotopes 29Si and 30Si with respect to 28Si and solar abundances, while 13C to 12C is analytically indistinguishable from bulk planetary and meteorite compositions. Although the origin of the deficiency of the heavy silicon isotopes cannot be explained unambiguously, we discuss mechanisms that could have contributed to the measured depletion of the isotopes 29Si and 30Si. [ABSTRACT FROM AUTHOR]

Context. On 20 January 2015 the Rosetta spacecraft was at a heliocentric distance of 2.5AU, accompanying comet 67P/Churyumov- Gerasimenko on its journey toward the Sun. The Ion Composition Analyser (RPC-ICA), other instruments of the Rosetta Plasma Consortium, and the ROSINA instrument made observations relevant to the generation of plasma waves in the cometary environment. Aims. Observations of plasma waves by the Rosetta Plasma Consortium Langmuir probe (RPC-LAP) can be explained by dispersion relations calculated based on measurements of ions by the Rosetta Plasma Consortium Ion Composition Analyser (RPC-ICA), and this gives insight into the relationship between plasma phenomena and the neutral coma, which is observed by the Comet Pressure Sensor of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis instrument (ROSINA-COPS). Methods. We use the simple pole expansion technique to compute dispersion relations for waves on ion timescales based on the observed ion distribution functions. These dispersion relations are then compared to the waves that are observed. Data from the instruments RPC-LAP, RPC-ICA and the mutual impedance probe (RPC-MIP) are compared to find the best estimate of the plasma density. Results. We find that ion acoustic waves are present in the plasma at comet 67P/Churyumov-Gerasimenko, where the major ion species is H2O+. The bulk of the ion distribution is cold, kBTi = 0:01 eV when the ion acoustic waves are observed. At times when the neutral density is high, ions are heated through acceleration by the solar wind electric field and scattered in collisions with the neutrals. This process heats the ions to about 1 eV, which leads to significant damping of the ion acoustic waves. Conclusions. In conclusion, we show that ion acoustic waves appear in the H2O+ plasmas at comet 67P/Churyumov-Gerasimenko and how the interaction between the neutral and ion populations effects the wave properties. [ABSTRACT FROM AUTHOR]