Comets, traditionally viewed as aggregates of refractories and ices formed in the outer Solar System, have been shown through the Rosetta mission's study of comet 67P/Churyumov-Gerasimenko (67P) to possess a more complex composition. Rosetta revealed a surprisingly diverse range of molecules in the comet's coma, spanning a wide volatility range from super-volatiles (e.g., CO) to refractories with significant organic components. While smaller volatiles have been extensively studied, heavier and more complex species are rarer, typically associated with dust activity. Prior studies of comet 1P/Halley yielded ambiguous results regarding the nature of complex organics: were they fragmented polymers (like polyoxymethylene, POM) or an ensemble of individual molecules? The limited resolution of the mass spectrometers employed prevented a definitive conclusion. The Stardust mission's analysis of comet 81P/Wild 2 grains indicated high oxygen and nitrogen abundance and the presence of aromatic moieties but lacked the resolution to identify polymers or complex molecules. The Rosetta mission's Double Focusing Mass Spectrometer (DFMS), with its high-resolution capabilities, offers the opportunity to resolve this long-standing question by directly characterizing the complex organic molecules in comet 67P's coma.

Previous analyses of cometary organic molecules, particularly from the Halley and Wild 2 missions, hinted at the presence of complex organic structures but lacked the resolution to fully characterize their nature. The debate centered around two main possibilities: a mixture of individual complex organic molecules or fragments of larger polymeric structures such as polyoxymethylene (POM). While some studies suggested the presence of POM, based on the observed distribution of certain molecules in the coma, this interpretation remained contentious. The lack of high-resolution mass spectrometry data in previous missions hindered definitive conclusions about the exact composition and structure of the complex organic molecules present in comets. This study leverages the high-resolution data obtained by the Rosetta mission's DFMS to address this gap in knowledge and resolve the longstanding ambiguity.

The research utilized high-resolution mass-spectrometric data obtained by the Rosetta mission's Double Focusing Mass Spectrometer (DFMS) onboard the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument. Data were collected on August 3, 2015, when comet 67P was near perihelion, exhibiting high dust activity. This heightened activity was expected to enhance the sublimation of larger organic molecules from dust particles. The DFMS measured outgassing from both the nucleus and detached particles at a cometocentric distance of ~215 km. Coma species were ionized via electron impact (EI) in the DFMS ion source. The mass resolution (m/Δm = 3000 for m/z = 28) allowed distinction between pure hydrocarbons and heteroatom-bearing species. Signal intensities, corrected for mass-dependent sensitivity, are presented in arbitrary units. The analysis involved deconvolving the complex mass spectra to identify individual molecules and their fragments. The deconvolution process prioritized identifying pure hydrocarbon species first due to their abundance and simpler fragmentation patterns. This facilitated the identification of other heteroatom-bearing molecules through analysis of characteristic fragmentation patterns. The average composition of the organic molecules was determined by multiplying species' stoichiometric coefficients by their signal intensities and normalizing to carbon. The study used descriptive parameters such as the average sum formula, sp²:sp³ and CH₂/CH₃ ratios, and the hydrogen deficiency index (HDI) to compare the cometary organics with other Solar System reservoirs and the Interstellar Medium (ISM). Specific attention was paid to the identification of potential polymeric structures like polyoxymethylene (POM) and hexamethylenetetramine (HMT). Calibration measurements with POMs were referenced to rule out their substantial presence.

The study identified a new ensemble of complex organic molecules in comet 67P's coma, with masses up to 140 Da. Several molecules were identified for the first time in a comet, including nonane (C₉H₂₀), naphthalene (C₁₀H₈), and benzylamine (C₇H₉N). The analysis revealed that the ensemble consists predominantly of chain-based, cyclic, and aromatic hydrocarbons in an approximate ratio of 6:3:1. The absence of significant signatures of polyoxymethylene (POM) and hexamethylenetetramine (HMT) polymers indicates that polymeric matter, if present, is of extremely low abundance. The average composition of the ensemble is C₁H₁.₅₆O₀.₁₃₄N₀.₀₄₆S₀.₀₁₇, remarkably similar to meteoritic soluble organic matter (SOM). The sp²:sp³ ratio of ~0.3 (relative to C atoms) and ~0.1 (relative to H atoms) shows similarities to interstellar carbonaceous dust. The CH₂/CH₃ ratio of ~1.9 suggests that cometary dust contains more extended or less branched aliphatic moieties than meteoritic matter or the diffuse ISM. The heteroatomic abundances relative to carbon (O/C, N/C, S/C) are remarkably consistent across various Solar System organic reservoirs (comet 67P, comet 1P/Halley, meteorites, and Saturn's ring rain), suggesting a common prestellar origin. The presence of various aromatic molecules and their hydrogenated derivatives points towards potential hydrogenation/dehydrogenation processes occurring before comet formation. A comparison of the cometary organic molecules with those detected in the Interstellar Medium (ISM) reveals both overlaps (e.g., benzene, potentially some heterocycles) and differences (e.g., polyynes, radicals), highlighting the evolution of organic molecules from the ISM to comets. The analysis suggests that the observed complex organic molecules were inherited from earlier stages of Solar System history, rather than being formed in-situ on the comet.

The findings directly address the long-standing debate regarding the nature of complex organic molecules in comets, conclusively demonstrating that the observed organic matter in comet 67P consists primarily of an ensemble of individual molecules rather than fragmented polymers. The remarkable similarity in the average composition and structural properties of cometary organics with those found in meteoritic SOM and Saturn's ring rain strongly supports a shared prestellar origin, potentially originating in the ISM. The observed differences, particularly in the hydrogenation levels, likely result from subsequent processing events during Solar System formation and cometary evolution. The presence of both aromatic molecules and their hydrogenated counterparts suggests a dynamic interplay of hydrogenation and dehydrogenation reactions in the pre-cometary environment. While the ISM and cometary censuses of organic molecules show some overlap, significant differences also exist, underscoring the complex evolution of organic molecules from the early ISM to comets. Future comparative analyses of ISM and cometary data, combined with laboratory experiments, can further refine our understanding of the formation and evolution of these molecules.

This study provides the first comprehensive characterization of a new ensemble of complex organic molecules in comet 67P, resolving the long-standing ambiguity regarding their polymeric versus individual molecule nature. The findings strongly suggest a prestellar origin for these molecules, with their elemental composition and structural features resembling those of meteoritic SOM and Saturn's ring rain. The data highlight the complexity of organic chemistry from the ISM to comets, prompting further investigation into the evolution of these molecules and the processes shaping their distribution and composition. Future research should focus on expanding the census of both ISM and cometary molecules, refining our understanding of the processes leading to hydrogenation/dehydrogenation, and examining additional Solar System reservoirs to further strengthen the case for a common origin of Solar System organics.

While the high-resolution mass spectrometry data provided unprecedented detail, limitations exist. The deconvolution process involved assumptions regarding fragmentation patterns and the availability of reference spectra, introducing uncertainties in the identification and quantification of some molecules. The analysis focused primarily on pure hydrocarbons, potentially underrepresenting the abundance of certain heteroatom-bearing molecules. The influence of in-situ processes on the observed composition is also a potential limitation. Moreover, the study was confined to a single observation point and time, limiting the generalizability of the findings to other regions and times in the comet's coma.