Shedding light on the unexpected chemical complexity of Comet Chury

Comets are fossils from ancient times and from the depths of our solar system, and they are relics of the formation of the sun, planets and moons. A team led by chemist Nora Hänni from the University of Bern’s Institute of Physics, Department of Space Research and Planetary Sciences, has now succeeded for the first time in identifying a whole series of complex organic molecules on a comet then they report in a study published at the end of June in the journal Nature Communication.

More precise analysis thanks to the Bernese mass spectrometer

In the mid-1980s, a fleet of spacecraft was sent by major space agencies to fly past Halley’s Comet. On board were several mass spectrometers which measured the chemical composition of the comet’s coma – the thin atmosphere due to the sublimation of cometary ice close to the Sun – as well as that of the impacting dust particles. However, the data collected by these instruments did not have the necessary resolution to allow unambiguous interpretation.

Now, more than 30 years later, the ROSINA high-resolution mass spectrometer, a Berne-led instrument aboard ESA’s Rosetta spacecraft, has collected data on comet 67P/Churyumov-Gerasimenko, also known as Chury’s name, between 2014 and 2016. These data now allow researchers to shed light for the first time on Chury’s complex organic balance sheet.

The secret was hidden in the dust

When Chury reached perihelion, the closest point to the Sun, she became very active. The sublimation of cometary ices created a flow that carried along dust particles. The expelled particles were heated by solar irradiation to temperatures above those typically encountered on the cometary surface. This allows larger and heavier molecules to desorb, making them available for the high-resolution mass spectrometer ROSINA-DFMS (Rosetta Orbiter Sensor for Ion and Neutral Analysis-Double Focusing Mass Spectrometer). The astrophysicist Prof. em. Dr Kathrin Altwegg, principal investigator of the ROSINA instrument and co-author of the new study, says: “Due to the extremely dusty conditions, the spacecraft had to withdraw to a safe distance of just over 200 km above the cometary surface. so that the instruments can operate under stable conditions.” Thus, it was possible to detect species composed of more than a handful of atoms that had previously remained hidden in the cometary dust.

Interpreting such complex data is difficult. However, the Bernese research team managed to identify a number of complex organic molecules, which had never been found in a comet before. “We found for example naphthalene, which is responsible for the characteristic smell of mothballs. And we also found benzoic acid, a natural component of incense. In addition, we identified benzaldehyde, widely used to impart almond flavor to foods, and many other molecules.” These heavy organics would apparently make Chury’s scent even more complex, but also more appealing, according to Hänni.

In addition to odorous molecules, many species endowed with a so-called prebiotic functionality have been identified in the organic balance of Chury (for example, formamide). Such compounds are important intermediates in the synthesis of biomolecules (eg, sugars or amino acids). “It therefore seems likely that the impact of comets – as essential providers of organic matter – also contributed to the emergence of carbon-based life on Earth,” says Hänni.

Similar organic materials in Saturn and meteorites

In addition to the identification of individual molecules, the researchers also performed a detailed characterization of Comet Chury’s complete set of complex organic molecules, allowing it to be placed in the larger context of the solar system. Parameters such as the average sum formula of this organic material or the average bonding geometry of the carbon atoms it contains are important to a wide scientific community, ranging from astronomers to solar system scientists.

“It turned out that on average, the complex organic balance of Chury is identical to the soluble part of the meteoritic organic matter”, explains Hänni and adds: “Furthermore, besides the relative amount of hydrogen atoms, Chury’s molecular budget also strongly resembles organic matter raining down on Saturn from its innermost ring, as detected by the INMS mass spectrometer aboard NASA’s Cassini spacecraft.”

“We are not only finding similarities between the organic reservoirs of the solar system, but many organic molecules of Chury are also present in the molecular clouds, cradles of new stars”, adds Professor Dr Susanne Wampfler, astrophysicist at the Center for Space. and Habitability (CSH) at the University of Bern and co-author of the publication. “Our findings are consistent with and support the scenario of a shared presolar origin of the Solar System’s various pools of organic matter, confirming that comets are indeed transporting material long before our Solar System emerged.”

Sharon D. Cole