Comets are considered to be some of the most pristine and unprocessed solar system objects accessible to in-situ exploration. Investigating their molecular and elemental composition takes us on a journey back to the early period of our solar system and possibly even further. In this work, we deduce the bulk abundances of the major volatile species in comet 67P/Churyumov-Gerasimenko, the target of the European Space Agency's Rosetta mission. The basis are measurements obtained with the ROSINA instrument suite on board the Rosetta orbiter during a suitable period of high outgassing near perihelion. The results are combined with both gas and dust composition measurements published in the literature. This provides an integrated inventory of the major elements present in the nucleus of 67P/Churyumov-Gerasimenko. Similar to comet 1P/Halley, which was visited by ESA's Giotto spacecraft in 1986, comet 67P/Churyumov-Gerasimenko also shows near-solar abundances of oxygen and carbon, whereas hydrogen and nitrogen are depleted compared to solar. Still, the degree of devolatilization is lower than that of inner solar system objects, * E-mail: martin.rubin@space.unibe.ch 2 including meteorites and the Earth. This supports the idea that comets are among the most pristine objects in our solar system.

Cometary comae are generally depleted in nitrogen. The main carriers for volatile nitrogen in comets are NH3 and HCN. It is known that ammonia readily combines with many acids like e.g. HCN, HNCO, HCOOH, etc. encountered in the interstellar medium as well as in cometary ice to form ammonium salts (NH4 + X -) at low temperatures. Ammonium salts, which can play a significant role in prebiotic chemistry, are hard to detect in space as they are unstable in the gas phase and their infrared signature is often hidden by thermal radiation or by e.g. OH in minerals. Here we report the presence of all possible sublimation products of five different ammonium salts at comet 67P/Churyumov-Gerasimenko measured by the ROSINA instrument on Rosetta. The relatively high sublimation temperatures of the salts leads to an apparent lack of volatile nitrogen in the coma. This then also explains the observed trend of higher NH3/H2O ratios with decreasing perihelion distances in comets. MainNitrogen in the volatile part of a comet nucleus is predominantly in the form of NH3 and HCN, which are on average (0.80 ± 0.20) % and (0.21 ± 0.02) %, respectively relative to water, e.g. (1). The numbers for HCN are somewhat uncertain as IR observations generally differ from radio observations (2). Apart from these two molecules, nitrogen bearing species have rather low abundances in comets (2). Especially, neutral N2 escaped detection before the Rosetta mission. Already in 1988, after the Giotto flyby at comet 1P/Halley, Geiss (3) recognized that, while carbon and oxygen relative to silicon are close to solar abundance, comet Halley was clearly lacking nitrogen. One explanation for this depletion at that time was the high volatility of N2, which may not have been condensed in the cometary ice or may have been lost in the last 4.6 Gy. For comet 67P / Churyumov-Gerasimenko (67P hereafter), neutral N2 has now been found on the level of (8.9 ± 2.4) × 10 -4 relative to water (~3 % relative to CO) (4). Recently, a high N2/CO ratio of 6% has been reported for comet C/2016 R2 (Pan-STARRS) (5). This shows that N2 is condensed and stored in cometary ice, but the reported abundances are by far not enough to explain the deficiency in nitrogen. N/C atomic ratio in the solar photosphere is about 0.3 ± 0.1 (6). In the refractory phase, comets are also depleted in nitrogen with N/C = 0.05 ± 0.03 in comet 1P (7) and N/C= 0.035 ± 0.011 in comet 67P (8).While the spread in relative abundances of HCN is quite small among comets, the variation for NH3 seems to be much larger (1). What is quite remarkable is the fact that comets with small perihelion distances seem to have much higher NH3/H2O values (1). This suggests that ammonia has a higher sublimation temperature in comets than water, although for pure ice sublimation temperatures are 90 K and 140 K, respectively. In comet D/2012 S1 (ISON) between 1.2 and 0.34 AU, Di Santi et al. (9) found an increase in NH3/H2O from < 0.78 % up to (3.5 ± 0.3) % and in addition a distribution of NH3, inconsistent with release from ...

The European Space Agency spacecraft Rosetta accompanied the Jupiter-family comet 67P/Churyumov-Gerasimenko for over two years along its trajectory through the inner solar system. Between 2014 and 2016, it performed almost continuous in situ measurements of the comet's gaseous atmosphere in close proximity to its nucleus. In this study, the 16 O/ 18 O ratio of H 2 O in the coma of 67P/Churyumov-Gerasimenko, as measured by the ROSINA DFMS mass spectrometer on board Rosetta, was determined from the ratio of H 2 16 O / H 2 18 O and 16 OH / 18 OH. The value of 445 ± 35 represents an ∼11% enrichment of 18 O compared with the terrestrial ratio of 498.7 ± 0.1. This cometary value is consistent with the comet containing primordial water, in accordance with leading self-shielding models. These models predict primordial water to be between 5% to 20% enriched in heavier oxygen isotopes compared to terrestrial water. ABSTRACTRefer to main paper.

Deuterated methanol is one of the most robust windows astrochemists have on the individual chemical reactions forming deuterium-bearing molecules and the physicochemical history of the regions where they reside. The first-time detection of mono- and di-deuterated methanol in a cometary coma is presented for comet 67P/Churyumov–Gerasimenko using Rosetta–ROSINA data. D-methanol (CH3OD and CH2DOH combined) and D2-methanol (CH2DOD and CHD2OH combined) have an abundance of 5.5 ± 0.46 and 0.00069 ± 0.00014 per cent relative to normal methanol. The data span a methanol deuteration fraction (D/H ratio) in the 0.71 − 6.6 per cent range, accounting for statistical corrections for the location of D in the molecule and including statistical error propagation in the ROSINA measurements. It is argued that cometary CH2DOH forms from CO hydrogenation to CH3OH and subsequent H-D substitution reactions in CH3-R. CHD2OH is likely produced from deuterated formaldehyde. Meanwhile, CH3OD and CH2DOD, could form via H-D exchange reactions in OH-R in the presence of deuterated water ice. Methanol formation and deuteration is argued to occur at the same epoch as D2O formation from HDO, with formation of mono-deuterated water, hydrogen sulfide, and ammonia occurring prior to that. The cometary D-methanol/methanol ratio is demonstrated to agree most closely with that in prestellar cores and low-mass protostellar regions. The results suggest that cometary methanol stems from the innate cold (10 − 20 K) prestellar core that birthed our Solar System. Cometary volatiles individually reflect the evolutionary phases of star formation from cloud to core to protostar.

Although the debate regarding the origin of the cyano (CN) radical in comets has been ongoing for many decades, it has yielded no definitive answer to date. CN could previously only be studied remotely, strongly hampering efforts to constrain its origin because of very limited spatial information. Thanks to the European Space Agency's Rosetta spacecraft, which orbited comet 67P/Churyumov-Gerasimenko for two years, we can investigate, for the first time, CN around a comet at high spatial and temporal resolution. On board Rosetta's orbiter module, the high-resolution double-focusing mass spectrometer DFMS, part of the ROSINA instrument suite, analyzed the neutral volatiles (including HCN and the CN radical) in the inner coma of the comet throughout that whole two-year phase and at variable cometocentric distances. From a thorough analysis of the full-mission data, the abundance of CN radicals in the cometary coma has been derived. Data from a close flyby event in February 2015 indicate a distributed origin for the CN radical in comet 67P/Churyumov-Gerasimenko.