Fragrances instead of polymers

New insights on the composition of complex cometary organic matter unveil that dominating representatives are fragrant rather than polymeric, as had been suggested previously, and expose compositional and structural properties compatible with a scenario of inheritance from pre-solar times.

It was in 1986, when a fleet of spacecrafts, sent out by a number of space agencies, would fly past comet 1P/Halley with several mass spectrometers on board. Mass spectrometers were dedicated to study the chemical composition of cometary matter by separating neutral gas and plasma species or impacting material according to their mass. Surprisingly, it turned out that Halley’s coma, i.e., the thin atmosphere due to sublimation of cometary ices close to the Sun, but also impacting dust particles, and the cometary plasma interacting with solar wind was much more complex than expected. The data collected by these instruments contained signatures of cometary species a lot heavier than the molecules commonly studied in comets, which are molecules like water (H2O), carbon dioxide (CO2), carbon monoxide (CO) and sometimes also ammonia (NH3), hydrogen sulfide (H2S), methanol (CH4O), and hydrogen cyanide (HCN). However, their interpretation was controversial and suggested either a fragmenting carbon-oxygen-hydrogen-based polymer (polyoxymethylene) or a set of individual molecules. Now, more than 30 years later, the high-resolution mass spectrometer ROSINA onboard ESA’s Rosetta spacecraft collected data at comet 67P/Churyumov-Gerasimenko (67P) that for the first time allowed to resolve heavy cometary species and, eventually, to differentiate between both hypotheses.

After a 10-year journey, Rosetta rendez-voused with 67P in summer 2014 and spent two years in its close vicinity before it softly crash-landed on its surface. When comet 67P passed its perihelion in summer 2015, at 1.24 au from the Sun, it became very active. Sublimating cometary ices were dragging along dust particles which, on their outbound trajectories, were heated up by solar irradiation (see Fig. 1). The temperatures these particles hence experienced were much higher than those at the cometary surface. Like this, molecules composed of more than just a handful of atoms desorbed from the dust particles and became accessible to the high-resolution mass spectrometer ROSINA-DFMS (Rosetta Orbiter Sensor for Ion and Neutral Analysis-Double Focusing Mass Spectrometer). This instrument was a sector-field mass spectrometer with unprecedented resolution. At a safe distance of a bit more than 200 km above the cometary surface it was possible to run the instrument despite the extraordinary cometary activity and measure under steady conditions. The mass spectra of 3 August 2015 showed a plethora of clear signatures up to 140 u while previously signatures above 100 u were rare.

Fig. 1: Image of the OSIRIS camera picture of the active comet. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA – CC BY-SA 4.0.

 Although these data are a treasure chest for molecule-hunters, the interpretation is extremely challenging. Thanks to Occam’s razor principle, i.e., by explaining as much of these data as possible with as few species as possible, 67P’s pure hydrocarbon budget could be fully resolved up to 140 u. Additional molecules were identified with a high certainty based on their characteristic mass-spectrometric finger prints. This work made clear that 67P’s organic budget was not dominated by polymeric matter like polyoxymethylene, suggested based on heavy ion data from comet Halley, but it was rather composed of a mixture of individual complex organic molecules – many of them fragrant: for instance naphthalene, responsible for the characteristic smell of our grandmothers’ mothballs, benzoic acid, a natural component of incense, benzaldehyde, widely used to confer almond flavor to foods, both naturally and as an additive etc. These heavy organics do not only substantially extend the 2019 cometary zoo of molecules (see Fig. 2), but also make 67P’s scent even more complex, and appealing, as many of the small cometary species like H2S or NH3 are responsible for odors rather unpleasant to the human nose.

Fig. 2: Zoo of cometary molecules based on ROSINA-DFMS data, as of 2019. Credit: ESA.

However, apart from in-situ mass-spectrometric studies and remote rotational spectroscopic observations, not many analytical methods are capable of unambiguously identifying extraterrestrial complex organic molecules individually. For many organic material reservoirs, thus, parameters like the average sum formula or the average bonding geometry of the carbon atoms are important descriptors of the material’s structural properties. Such parameters can easily be inter-compared between different reservoirs of organics in the Solar System or even beyond. On average, 67P’s complex organics budget is identical to the soluble part of meteoritic organic matter and – except from the ratio of H and C atoms – also strongly resembles the material raining down on Saturn from its innermost ring, detected by the INMS mass spectrometer onboard NASA’s Cassini spacecraft. These findings are compatible with and support the scenario of a shared pre-solar origin of the different reservoirs of Solar System organics, confirming that comets indeed are carriers of very pristine material from the times before our Solar System emerged (see Fig. 3). This view is supported, for instance, by a very recent finding that 67P’s small linear alkanes expose extraordinarily high grades of deuteration, which also is in line with a pre-solar origin. In addition, comparative works done for comets C/1995 O1 Hale-Bopp and 67P, reveal similar relative abundances of gaseous species around comets and objects in the Interstellar Medium.

Fig. 3: Material evolving from the Interstellar Medium towards terrestrial carbon-based life. Credit: ESA.

Eventually, this work made clear that data from the high-resolution mass spectrometer ROSINA-DFMS still hide many gems for cometary science, even over 5 years after official conclusion of the Rosetta mission. More work is required in order to better resolve also the heteroatom-bearing budget, knowing that many heteroatom-bearing species – especially also heterocyclic molecules – are relevant to prebiotic chemistry. Delivered to the early Earth by impacts, cometary organics may have contributed substantially to spark carbon-based life as we know it.