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The puzzling complexity of terrestrial biomolecules is driving the search for complex organic molecules in the Interstellar Medium (ISM) and serves as a motivation for many in situ studies of reservoirs of extraterrestrial organics from meteorites and interplanetary dust particles (IDPs) to comets and asteroids. Comet 67P/Churyumov-Gerasimenko (67P) -- the best-studied comet to date -- has been visited and accompanied for two years by the European Space Agency's Rosetta spacecraft. Around 67P's perihelion and under dusty conditions, the high-resolution mass spectrometer on board provided a spectacular glimpse into this comet's chemical complexity. For this work, we analyzed in unprecedented detail the O-bearing organic volatiles. In a comparison of 67P's inventory to molecules detected in the ISM, in other comets, and in Soluble Organic Matter (SOM) extracted from the Murchison meteorite, we also highlight the (pre)biotic relevance of different chemical groups of species. We report first evidence for abundant extraterrestrial O-bearing heterocycles (with abundances relative to methanol often on the order of 10% with a relative error margin of 30-50%) and various representatives of other molecule classes such as carboxylic acids and esters, aldehydes, ketones, and alcohols. Like with the pure hydrocarbons, some hydrogenated forms seem to be dominant over their dehydrogenated counterparts. An interesting example is tetrahydrofuran (THF) as it might be a more promising candidate for searches in the ISM than the long-sought furan itself. Our findings not only support and guide future efforts to investigate the origins of chemical complexity in space, but also they strongly encourage studies of, e.g., the ratios of unbranched vs. branched and hydrogenated vs. dehydrogenated species in astrophysical ice analogs in the laboratory as well as by modeling., Comment: Reproduced with permission from Astronomy & Astrophysics, copyright ESO

Ammonium hydrosulphide has long since been postulated to exist at least in certain layers of the giant planets. Its radiation products may be the reason for the red colour seen on Jupiter. Several ammonium salts, the products of NH3 and an acid, have previously been detected at comet 67P/Churyumov-Gerasimenko. The acid H2S is the fifth most abundant molecule in the coma of 67P followed by NH3. In order to look for the salt NH4+SH-, we analysed in situ measurements from the Rosetta/ROSINA Double Focusing Mass Spectrometer during the Rosetta mission. NH3 and H2S appear to be independent of each other when sublimating directly from the nucleus. However, we observe a strong correlation between the two species during dust impacts, clearly pointing to the salt. We find that NH4+SH- is by far the most abundant salt, more abundant in the dust impacts than even water. We also find all previously detected ammonium salts and for the first time ammonium fluoride. The amount of ammonia and acids balance each other, confirming that ammonia is mostly in the form of salt embedded into dust grains. Allotropes S2 and S3 are strongly enhanced in the impacts, while H2S2 and its fragment HS2 are not detected, which is most probably the result of radiolysis of NH4+SH-. This makes a prestellar origin of the salt likely. Our findings may explain the apparent depletion of nitrogen in comets and maybe help to solve the riddle of the missing sulphur in star forming regions., Comment: accepted by MNRAS

The cyano radical (CN) is one of the most frequently remotely observed species in space, also in comets. Data from the high-resolution Double Focusing Mass Spectrometer (DFMS) on board the Rosetta orbiter, collected in the inner coma of comet 67P/Churyumov-Gerasimenko, revealed an unexpected chemical complexity, and, recently, also more CN than expected from photodissociation of its most likely parent hydrogen cyanide (HCN). This work is dedicated to the derivation of abundances relative to HCN of three cometary nitriles (including structural isomers) from DFMS data. Mass spectrometry of complex mixtures does not always allow distinction of structural isomers. We assumed the most stable and most abundant (in similar environments) structure in our analysis, that is HCN for CHN, CH3CN for C2H3N, HC3N for C3HN, and NCCN for C2N2. For cyanoacetylene (HC3N) and acetonitrile (CH3CN) the complete mission timeline was evaluated, while cyanogen (NCCN) often was below detection limit. By carefully selecting periods where cyanogen was above detection limit, we were able to follow the abundance ratio between NCCN and HCN from 3.16 au inbound to 3.42 au outbound. These are the first measurements of NCCN in a comet.We find that neither NCCN, nor any of the other two nitriles, is sufficiently abundant to be a relevant alternative parent to CN.

The ratios of the three stable oxygen isotopes 16O, 17O and 18O on Earth and, as far as we know in the solar system, show variations on the order of a few percent at most, with a few outliers in meteorites. However, in the interstellar medium there are some highly fractionated oxygen isotopic ratios in some specific molecules. The goal of this work is to investigate the oxygen isotopic ratios in different volatile molecules found in the coma of comet 67P/Churyumov-Gerasimenko and compare them with findings from interstellar clouds in order to assess commonalities and differences. To accomplish this goal, we analyzed data from the ROSINA instrument on Rosetta during its mission around the comet. 16O/18O ratios could be determined for O2, methanol, formaldehyde, carbonyl sulfide and sulfur monoxide/dioxide. For O2 the 16O/17O ratio is also available. Some ratios are strongly enriched in the heavy isotopes, especially for sulfur bearing molecules and formaldehyde, whereas for methanol the ratios are compatible with the ones in the solar system. O2 falls in-between, but its oxygen isotopic ratios clearly differ from water, which likely rules out an origin of O2 from water, be it by radiolysis, dismutation during sublimation or the Eley-Rideal process from water ions hitting the nucleus as postulated in the literature.

The Rosetta spacecraft escorted Comet 67P/Churyumov-Gerasimenko for 2 years along its journey through the Solar System between 3.8 and 1.24~au. Thanks to the high resolution mass spectrometer on board Rosetta, the detailed ion composition within a coma has been accurately assessed in situ for the very first time. Previous cometary missions, such as $\text{Giotto}$, did not have the instrumental capabilities to identify the exact nature of the plasma in a coma because the mass resolution of the spectrometers onboard was too low to separate ion species with similar masses. In contrast, the Double Focusing Mass Spectrometer (DFMS), part of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis on board Rosetta (ROSINA), with its high mass resolution mode, outperformed all of them, revealing the diversity of cometary ions. We calibrated and analysed the set of spectra acquired by DFMS in ion mode from October 2014 to April 2016. In particular, we focused on the range from 13-39 u$\cdot$q$^{-1}$. The high mass resolution of DFMS allows for accurate identifications of ions with quasi-similar masses, separating $^{13}$C$^+$ from CH$^+$, for instance. We confirm the presence in situ of predicted cations at comets, such as CH$_m^+$ ($m=1-4$), H$_n$O$^+$ ($n=1-3$), O$^+$, Na$^+$, and several ionised and protonated molecules. Prior to Rosetta, only a fraction of them had been confirmed from Earth-based observations. In addition, we report for the first time the unambiguous presence of a molecular dication in the gas envelope of a Solar System body, namely CO$_2^{++}$., Comment: Accepted for publication in A&A

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.

To understand how Phosphorus-bearing molecules are formed in star-forming regions, we have analysed ALMA observations of PN and PO towards the massive star-forming region AFGL 5142, combined with a new analysis of the data of the comet 67P/Churyumov-Gerasimenko taken with the ROSINA instrument onboard Rosetta. The ALMA maps show that the emission of PN and PO arises from several spots associated with low-velocity gas with narrow linewidths in the cavity walls of a bipolar outflow. PO is more abundant than PN in most of the spots, with the PO/PN ratio increasing as a function of the distance to the protostar. Our data favor a formation scenario in which shocks sputter phosphorus from the surface of dust grains, and gas-phase photochemistry induced by UV photons from the protostar allows efficient formation of the two species in the cavity walls. Our analysis of the ROSINA data has revealed that PO is the main carrier of P in the comet, with PO/PN>10. Since comets may have delivered a significant amount of prebiotic material to the early Earth, this finding suggests that PO could contribute significantly to the phosphorus reservoir during the dawn of our planet. There is evidence that PO was already in the cometary ices prior to the birth of the Sun, so the chemical budget of the comet might be inherited from the natal environment of the Solar System, which is thought to be a stellar cluster including also massive stars., Comment: Accepted in MNRAS (20 pages, 12 figures)

This white paper proposes that AMBITION, a Comet Nucleus Sample Return mission, be a cornerstone of ESA's Voyage 2050 programme. We summarise some of the most important questions still open in cometary science after the successes of the Rosetta mission, many of which require sample analysis using techniques that are only possible in laboratories on Earth. We then summarise measurements, instrumentation and mission scenarios that can address these questions, with a recommendation that ESA select an ambitious cryogenic sample return mission. Rendezvous missions to Main Belt comets and Centaurs are compelling cases for M-class missions, expanding our knowledge by exploring new classes of comets. AMBITION would engage a wide community, drawing expertise from a vast range of disciplines within planetary science and astrophysics. With AMBITION, Europe will continue its leadership in the exploration of the most primitive Solar System bodies., Comment: 8 figures, 26 pages

Magnetohydrodynamics simulations have been carried out in studying the solar wind and cometary plasma interactions for decades. Various plasma boundaries have been simulated and compared well with observations for comet 1P/Halley. The Rosetta mission, which studies comet 67P/Churyumov-Gerasimenko, challenges our understanding of the solar wind and comet interactions. The Rosetta Plasma Consortium observed regions of very weak magnetic field outside the predicted diamagnetic cavity. In this paper, we simulate the inner coma with the Hall magnetohydrodynamics equations and show that the Hall effect is important in the inner coma environment. The magnetic field topology becomes complex and magnetic reconnection occurs on the dayside when the Hall effect is taken into account. The magnetic reconnection on the dayside can generate weak magnetic filed regions outside the global diamagnetic cavity, which may explain the Rosetta Plasma Consortium observations. We conclude that the substantial change in the inner coma environment is due to the fact that the ion inertial length (or gyro radius) is not much smaller than the size of the diamagnetic cavity., Comment: 23 pages, 6 figurs

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 $^{13}$C/$^{12}$C in C$_2$H$_4$, C$_2$H$_5$, and CO. The DFMS in situ measurements indicate that the average silicon isotopic composition shows depletion in the heavy isotopes $^{29}$Si and $^{30}$Si with respect to $^{28}$Si and solar abundances, while $^{13}$C to $^{12}$C 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 $^{29}$Si and $^{30}$Si., Comment: Accepted for publication in Astronomy & Astrophysics

Because of the high fraction of refractory material present in comets, the heat produced by the radiogenic decay of elements such as aluminium and iron can be high enough to induce the loss of ultravolatile species such as nitrogen, argon or carbon monoxide during their accretion phase in the protosolar nebula. Here, we investigate how heat generated by the radioactive decay of 26Al and 60Fe influences the formation of comet 67P/Churyumov-Gerasimenko, as a function of its accretion time and size of parent body. We use an existing thermal evolution model that includes various phase transitions, heat transfer in the ice-dust matrix, and gas diffusion throughout the porous material, based on thermodynamic parameters derived from Rosetta observations. Two possibilities are considered: either, to account for its bilobate shape, 67P/Churyumov-Gerasimenko was assembled from two primordial ~2 kilometer-sized planetesimals, or it resulted from the disruption of a larger parent body with a size corresponding to that of comet Hale-Bopp (~70 km). To fully preserve its volatile content, we find that either 67P/Churyumov-Gerasimenko's formation was delayed between ~2.2 and 7.7 Myr after that of Ca-Al-rich Inclusions in the protosolar nebula or the comet's accretion phase took place over the entire time interval, depending on the primordial size of its parent body and the composition of the icy material considered. Our calculations suggest that the formation of 67P/Churyumov-Gerasimenko is consistent with both its accretion from primordial building blocks formed in the nebula or from debris issued from the disruption of a Hale-Bopp-like body., Comment: Accepted for publication in The Astrophysical Journal Letters

The nature of the icy material accreted by comets during their formation in the outer regions of the protosolar nebula is a major open question in planetary science. Some scenarios of comet formation predict that these bodies agglomerated from crystalline ices condensed in the protosolar nebula. Concurrently, alternative scenarios suggest that comets accreted amorphous ice originating from the interstellar cloud or from the very distant regions of the protosolar nebula. On the basis of existing laboratory and modeling data, we find that the N$_2$/CO and Ar/CO ratios measured in the coma of the Jupiter family comet 67P/Churyumov-Gerasimenko by the ROSINA instrument aboard the European Space Agency's Rosetta spacecraft match those predicted for gases trapped in clathrates. If these measurements are representative of the bulk N$_2$/CO and Ar/CO ratios in 67P/Churyumov-Gerasimenko, it implies that the ices accreted by the comet formed in the nebula and do not originate from the interstellar medium, supporting the idea that the building blocks of outer solar system bodies have been formed from clathrates and possibly from pure crystalline ices. Moreover, because 67P/Churyumov-Gerasimenko is impoverished in Ar and N$_2$, the volatile enrichments observed in Jupiter's atmosphere cannot be explained solely via the accretion of building blocks with similar compositions and require an additional delivery source. A potential source may be the accretion of gas from the nebula that has been progressively enriched in heavy elements due to photoevaporation., Comment: The Astrophysical Journal Letters, in press

Molecular oxygen has been detected in the coma of comet 67P/Churyumov-Gerasimenko with abundances in the 1-10% range by the ROSINA-DFMS instrument on board the Rosetta spacecraft. Here we find that the radiolysis of icy grains in low-density environments such as the presolar cloud may induce the production of large amounts of molecular oxygen. We also show that molecular oxygen can be efficiently trapped in clathrates formed in the protosolar nebula, and that its incorporation as crystalline ice is highly implausible because this would imply much larger abundances of Ar and N2 than those observed in the coma. Assuming that radiolysis has been the only O2 production mechanism at work, we conclude that the formation of comet 67P/Churyumov-Gerasimenko is possible in a dense and early protosolar nebula in the framework of two extreme scenarios: (1) agglomeration from pristine amorphous icy grains/particles formed in ISM and (2) agglomeration from clathrates that formed during the disk's cooling. The former scenario is found consistent with the strong correlation between O2 and H2O observed in 67P/C-G's coma while the latter scenario requires that clathrates formed from ISM icy grains that crystallized when entering the protosolar nebula., Comment: The Astrophysical Journal Letters, in press

We report on magnetic field measurements made in the innermost coma of 67P/Churyumov-Gerasimenko in its low activity state. Quasi-coherent, large-amplitude ($\delta B/B \sim 1$), compressional magnetic field oscillations at $\sim$ 40 mHz dominate the immediate plasma environment of the nucleus. This differs from previously studied comet-interaction regions where waves at the cometary ion gyro-frequencies are the main feature. Thus classical pick-up ion driven instabilities are unable to explain the observations. We propose a cross-field current instability associated with newborn cometary ion currents as a possible source mechanism., Comment: 6 pages, 3 Figures

Grand Canonical Monte Carlo simulations are used to reproduce the N$_2$/CO ratio ranging between 1.7 $\times$ 10$^{-3}$ and 1.6 $\times$ 10$^{-2}$ observed {\it in situ} in the Jupiter family comet 67P/Churyumov-Gerasimenko by the ROSINA mass spectrometer aboard the Rosetta spacecraft, assuming that this body has been agglomerated from clathrates in the protosolar nebula. Simulations are done using an elaborated interatomic potentials for investigating the temperature dependence of the trapping within a multiple guest clathrate formed from a gas mixture of CO and N$_2$ in proportions corresponding to those expected for the protosolar nebula. By assuming that 67P/Churyumov-Gerasimenko agglomerated from clathrates, our calculations suggest the cometary grains must have been formed at temperatures ranging between $\sim$31.8 and 69.9 K in the protosolar nebula to match the N$_2$/CO ratio measured by the ROSINA mass spectrometer. The presence of clathrates in Jupiter family comets could then explain the potential N$_2$ depletion (factor up to $\sim$87 compared to the protosolar value) measured in 67P/Churyumov-Gerasimenko., Comment: Accepted for publication in The Astrophysical Journal Letters

Remote sensing observations meet some limitations when used to study the bulk atmospheric composition of the giant planets of our solar system. A remarkable example of the superiority of in situ probe measurements is illustrated by the exploration of Jupiter, where key measurements such as the determination of the noble gases abundances and the precise measurement of the helium mixing ratio have only been made available through in situ measurements by the Galileo probe. This paper describes the main scientific goals to be addressed by the future in situ exploration of Saturn placing the Galileo probe exploration of Jupiter in a broader context and before the future probe exploration of the more remote ice giants. In situ exploration of Saturn's atmosphere addresses two broad themes that are discussed throughout this paper: first, the formation history of our solar system and second, the processes at play in planetary atmospheres. In this context, we detail the reasons why measurements of Saturn's bulk elemental and isotopic composition would place important constraints on the volatile reservoirs in the protosolar nebula. We also show that the in situ measurement of CO (or any other disequilibrium species that is depleted by reaction with water) in Saturn's upper troposphere would constrain its bulk O/H ratio. We highlight the key measurements required to distinguish competing theories to shed light on giant planet formation as a common process in planetary systems with potential applications to most extrasolar systems. In situ measurements of Saturn's stratospheric and tropospheric dynamics, chemistry and cloud-forming processes will provide access to phenomena unreachable to remote sensing studies. Different mission architectures are envisaged, which would benefit from strong international collaborations., Comment: Submitted to Planetary and Space Science