The ESA Rosetta spacecraft (S/C) is tracking comet 67P/Churyumov-Gerasimenko in close vicinity. This prolonged en- counter enables studying the evolution of the volatile coma composition.
Aims. Our work aims at comparing the diversity of the coma of 67P/Churyumov-Gerasimenko at large heliocentric distance to study the evolution of the comet during its passage around the Sun and at trying to classify it relative to other comets.
Methods. We used the Double Focussing Mass Spectrometer (DFMS) of the ROSINA experiment on ESA’s Rosetta mission to determine relative abundances of major and minor volatile species. This study is restricted to species that have previously been detected elsewhere.
Results. We detect almost all species currently known to be present in cometary coma with ROSINA DFMS. As DFMS measured the composition locally, we cannot derive a global abundance, but we compare measurements from the summer and the winter hemisphere with known abundances from other comets. Differences between relative abundances between summer and winter hemispheres are large, which points to a possible evolution of the cometary surface. This comet appears to be very rich in CO2 and ethane. Heavy oxygenated compounds such as ethylene glycol are underabundant at 3 AU, probably due to their high sublimation temperatures, but nevertheless, their presence proves that Kuiper belt comets also contain complex organic molecules.

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Inventory of the volatiles on comet 67P/Churyumov-Gerasimenko from Rosetta/ROSINA

The saturated hydrocarbons ethane (C 2 H 6 ) and methane (CH 4 ) along with carbon monoxide (CO) and water (H 2 O) were detected in comet C/1996 B2 Hyakutake with the use of high-resolution infrared spectroscopy at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. The inferred production rates of molecular gases from the icy, cometary nucleus (in molecules per second) are 6.4 × 10 26 for C 2 H 6 , 1.2 × 10 27 for CH 4 , 9.8 × 10 27 for CO, and 1.7 × 10 29 for H 2 O. An abundance of C 2 H 6 comparable to that of CH 4 implies that ices in C/1996 B2 Hyakutake did not originate in a thermochemically equilibrated region of the solar nebula. The abundances are consistent with a kinetically controlled production process, but production of C 2 H 6 by gas-phase ion molecule reactions in the natal cloud core is energetically forbidden. The high C 2 H 6 /CH 4 ratio is consistent with production of C 2 H 6 in icy grain mantles in the natal cloud, either by photolysis of CH 4 -rich ice or by hydrogen-addition reactions to acetylene condensed from the gas phase.

https://science.sciencemag.org/content/sci/272/5266/1310.full.pdf

Detection of Abundant Ethane and Methane, Along with Carbon Monoxide and Water, in Comet C/1996 B2 Hyakutake: Evidence for Interstellar Origin

The composition of cometary ices provides key information on the chemical and physical properties of the outer solar nebula where comets formed, 4.6 G.y. ago. This chapter summarizes our current knowledge of the volatile composition of cometary nuclei, based on spectroscopic observations and in situ measurements of parent molecules and noble gases in cometary comae. The processes that govern the excitation and emission of parent molecules in the radio, infrared (IR), and ultraviolet (UV) wavelength regions are reviewed. The techniques used to convert line or band fluxes into molecular production rates are described. More than two dozen parent molecules have been identified, and we describe how each is investigated. The spatial distribution of some of these molecules has been studied by in situ measurements, long-slit IR and UV spectroscopy, and millimeter wave mapping, including interferometry. The spatial distributions of CO, H2CO, and OCS differ from that expected during direct sublimation from the nucleus, which suggests that these species are produced, at least partly, from extended sources in the coma. Abundance determinations for parent molecules are reviewed, and the evidence for chemical diversity among comets is discussed.

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The composition of cometary volatiles

Molecular nitrogen (N2) is thought to have been the most abundant form of nitrogen in the protosolar nebula. It is the main N-bearing molecule in the atmospheres of Pluto and Triton and probably the main nitrogen reservoir from which the giant planets formed. Yet in comets, often considered the most primitive bodies in the solar system, N2 has not been detected. Here we report the direct in situ measurement of N2 in the Jupiter family comet 67P/Churyumov-Gerasimenko, made by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis mass spectrometer aboard the Rosetta
spacecraft. A N2/CO ratio of (5.70 +− 0.66) × 10−3 (2s standard deviation of the sampled mean) corresponds to depletion by a factor of ~25.4 +− 8.9 as compared to the protosolar value. This depletion suggests that cometary grains formed at low-temperature conditions below ~30 kelvin.

https://hal.archives-ouvertes.fr/hal-01346031/document

Molecular Nitrogen in Comet 67P/Churyumov-Gerasimenko Indicates a Low Formation Temperature

The branching ratios of the different reaction pathways and the overall rate coefficients of the dissociative recombination reactions of CH3OH2+ and CD3OD2+ have been measured at the CRYRING storage ring located in Stockholm, Sweden. Analysis of the data yielded the result that formation of methanol or deuterated methanol accounted for only 3 and 6% of the total rate in CH3OH2+ and CD3OD2+, respectively. Dissociative recombination of both isotopomeres mainly involves fragmentation of the C–O bond, the major process being the three-body break-up forming CH3, OH and H (CD3, OD and D). The overall cross sections are best fitted by σ = 1.2 ± 0.1 × 10−15E−1.15±0.02 cm2 and σ = 9.6 ± 0.9 × 10−16E−1.20±0.02 cm2 for CH3OH2+ and CD3OD2+, respectively. From these values thermal reaction rate coefficients of k(T) = 8.9 ± 0.9 × 10−7 (T/300)−0.59±0.02 cm3 s−1 (CH3OH2+) and k(T) = 9.1 ± 0.9 × 10−7 (T/300)−0.63±0.02 cm3 s−1(CD3OD2+) can be calculated. A non-negligible formation of interstellar methanol by the previously proposed mechanism via radiative association of CH3+ and H2O and subsequent dissociative recombination of the resulting CH3OH2+ ion to yield methanol and hydrogen atoms is therefore very unlikely.

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Dissociative recombination of protonated methanol

The Giotto space probe's neutral mass spectrometer experiment has determined the abundances and the chemical, elemental and isotopic compositions of gases and low energy ions in the coma of comet Halley. Preliminary results show water predominating, at about 80 percent by volume, with a density of 4.7 x 10 to the 7th molecules/cu cm at 1000 km and a photodestruction scale length of 39,000 km. Limits on the abundances of CO2, NH3 and CH4 relative to H2O are also obtained. An ion temperature change observation indicates a contact surface location at 47,000 + or - 200 km.

In situ gas and ion measurements at comet Halley

A preliminary evaluation of the mass 28 amu/e signal observed in the neutral mode of the Giotto neutral gas mass spectrometer (NMS) is presented. At 1000 km from the nucleus we obtain for the CO number density n (CO)/n (H2O)≦0.07. The production rate of CO as a parent molecule directly from the nucleus is thus less than 7% of the H2O production rate. However, CO is also produced from an extended source in the inner coma (R < 20 000 km) and at 20 000 km from the nucleus we obtain for the total equivalent CO production rate 0.05≦Q (CO)/Q (H2O)≦0.15. For N2 an upper limit Q (N2)/Q (H2O)≦0.1 is derived. No parent molecule for the CO could be identified which would be in agreement with the NMS measurements. It is proposed that CO or a very short-lived parent is relased in the coma from cometary dust grains, such as the “CHON” particles.

The CO and N2 abundance in comet P/Halley

The Neutral Mass Spectrometer on the Giotto spacecraft measured the neutral and ion composition in the coma of comet P/Halley. In the ion mass spectra, the 31 amu/e peak is dominated by the protonated formaldehyde ion H 3 CO + and can be used to calculate the H 2 CO density. The radial profile of the amplitude of the 31 amu/e peak can not be reconciled with H 2 CO being only a parent molecule evaporating from the nucleus. From the radial profile of the H 3 CO + density, we derive the total production of formaldehyde and the strength of its extended source within the contact surface located at 4660 km. The total H 2 CO production inside the contact surface is Q(H 2 CO)/Q(H 2 O)=0.029, essentially independent of model assumptions

The extended formaldehyde source in comet P/Halley

In situ measurements of flow velocity and temperature in the inner coma were obtained from ram energy spectra of molecules and ions observed by the NMS experiment onboard the Giotto probe to comet Halley. Radial flow speed was (800 ± 50) m s−1 at distances from the nucleus between 1000 km and 4000 km. Whereas ions became stagnant just outside the contact surface (located near 4500 km), the outflow of water vapour even accelerated and was above 1 km s −1 at 30000 km. Ion temperatures were around 200 K inside the contact surface where the plasma is collisionally coupled to the gas; at this boundary ion temperature rises by about 1000 K and then further increases to reach about 5000 K at 15 000 km. Gas temperature remains below 300–500 K over this range of distances. A near zero volt spacecraft potential was inferred.

Expansion velocity and temperatures of gas and ions measured in the coma of Comet Halley

The Neutral Mass Spectrometer on the Giotto spacecraft measured the gas and ion composition in the coma of comet P/Halley. A detailed model of the ion chemistry inside the contact surface located at 4660 km is used to interpret the measured ion desnity profiles in the 32 to 35 amu/e mass range. The masses 33 and 35 amu/e are dominated by the protonated methanol and hydrogen sulfide ions CH3OH2(+) and H3S(+). Both profiles are essentially compatible with CH3OH and H2S originating from the nucleus only. The production rates relative to water are Y(CH3OH) = Q(CH3OH)/Q(H2O) = 1.7% and Y(H2S) = 0.41%. Our Y(CH3OH) agrees well with a determination from IR spectra obtained about 6 weeks after the Giotto encounter with P/Halley. In 7 other comets IR and microwave observations give Y(CH3OH) values between about 0.7 and 6%, indicating that the methanol abundance shows a strong variability from comet to comet. In three other comets Y(H2S) values between 0.2 and 0.5% have been reported. In addition to H2S(+), only ions containing minor isotopes of H, C, O and S contribute to mass 34 amu/e (e.g. (34)S(+), (13)CH3OH2(+), CH4DO(+)). These contributions can be calculated from the measured densities of the ions containing the major isotopes and the H2S(+) contribution from the measured H3S(+) density. From mass 34 amu/e we can also derive an upper limit of 1% for the abundance of deuterated methanol. This limit is at most marginally compatible with a direct interstellar origin of the CH3OH in P/Halley as the measured interstellar abundance of deuterated methanol is 1 to 6%.

R. R. Hodges

Papers

In situ gas and ion measurements at comet Halley

The CO and N2 abundance in comet P/Halley

The extended formaldehyde source in comet P/Halley

Expansion velocity and temperatures of gas and ions measured in the coma of Comet Halley

Methanol and hydrogen sulfide in comet P/Halley