A few years ago the European Space Agency’s Rosetta spacecraft visited comet 67P Churyumov-Gerasimenko (2014-2016). You may remember the spectacular images of the Philae lander as it settled down on the comet surface. The scientific instruments onboard Rosetta collected detailed measurements of the comet. In particular, the Double Focusing Mass Spectrometer (Rosina-DFMS) – built by the University of Bern with important contributions from the Royal Belgian Institute for Space Aeronomy (BIRA-IASB) in Uccle – has turned out to provide fascinating information about the comet’s composition.
As a comet approaches the Sun, its surface heats up. This surface consists of ice, which starts to sublimate: it turns into gas. The DFMS instrument “sniffs” this gas and determines its constituents – most of it is water, carbon dioxide, carbon monoxide, and molecular oxygen. Embedded in the ice are also solid particles or ‘dust grains’, coated with a layer of less volatile material. As these dust grains come off the comet surface, sunlight heats them up to higher temperatures, causing the less volatile material to turn into gas as well. This contributes to the gas that has been sampled by DFMS at the time the comet was closest to the Sun.
A detailed study of the composition of this gas by a team led by Dr. Nora Hänni of the University of Bern has now been published in the prestigious journal Nature Communications. ‘Less volatile gas’ tends to mean ‘larger molecules’ and thus a more complex chemical composition. Indeed, the team found a number of complex organic (carbon-based) molecules, which have never been identified in a comet before.
Dr. Nora Hänni:
We found for instance 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 confer almond flavour to foods, and many other molecules.
Other complex molecules are believed to be “pre-biotic”, that is, they are intermediates in the synthesis of biomolecules (e.g., sugars or amino acids). This strengthens the hypothesis that comets bombarded Earth in the young solar system and contributed to the emergence of carbon-based life.
The researchers compared their results with other materials that are found in our Solar System. These complex organics appear to be very similar to the soluble part of organic matter found in meteorites. There is also a strong resemblance to the composition of organic material found in Saturn’s rings. And astronomers are observing similar chemicals in interstellar space.
Dr. Johan De Keyser of BIRA-IASB, co-author on the study:
All of this points to a common origin of this material: we suspect it was already present in the interstellar gas and dust clouds out of which our Solar System was formed. Actually, DFMS has brought us different lines of evidence that point to this same conclusion: comets indeed contain pristine material dating back from before the birth of the solar system.
The fact that such important scientific results appear more than five years after Rosetta was set down on the comet and switched off, has to do with the complexity of the measurements.
Johan De Keyser :
To obtain reliable data, we do our utmost to correct the measurements for all sorts of instrumental effects. Temperature changes in the instrument, exposure to the space environment during so many years (Rosetta was launched in 2004), aging of the detector … In the past years we have tried to understand and compensate for all of these. Our colleagues in Bern have also spent considerable time in the lab with a copy of DFMS in order to characterize its sensitivity for some of these complex organics. This represents years of painstaking work. But I am sure: there are more scientific findings to come!
Nature Communications publication
N. Hänni, K. Altwegg, M. Combi, S. A. Fuselier, J. De Keyser, M. Rubin, and S. F. Wampfler: Identification and characterization of a new ensemble of cometary organic molecules, Nature Communications, 13, 3639 (2022).
DOI : 10.1038/s41467-022-31346-9
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