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Discovery of ammonium salts in comets


Even three years after the end of the Rosetta mission, comet 67P/Churyumov-Gerasimenko still hasn’t revealed all its secrets. Close to the end of the mission, a comet particle entered the ROSINA/DFMS instrument, which was designed to study only comet gas. In doing so, comet scientists– including researchers from the Royal Belgian Institute for Space Aeronomy (BIRA-IASB) who helped build ROSINA/DFMS – discovered a group of less volatile substances known as "ammonium salts". This discovery explains why comets seem to contain so little nitrogen: the nitrogen is trapped in these substances that are difficult to detect from Earth. Ammonium salts are particularly intriguing because they contribute to the building blocks of life.

A risky adventure for Rosetta

Three years ago, the European Space Agency’s (ESA) Rosetta spacecraft ended its study of comet 67P/Churyumov-Gerasimenko with a soft crash-landing on the comet surface. In the meantime, scientists are still working on figuring out what the measurements mean. Today (Monday January 20, 2020), a remarkable study is published in Nature Astronomy about the chemical composition of the comet.

Rosetta's two-year journey close to comet 67P was a big challenge: Rosetta was not allowed to get too close to the comet, because the navigation system could get confused by the many dust particles in that area. However, towards the end of the mission, ESA was willing to take more risk in order to investigate the comet more closely. This was particularly important for the ROSINA/DFMS mass spectrometer, which would be able to "sniff" the gas coming from the comet's surface much better closer to the comet, and thus determine its composition more precisely.

A dangerous incident leads to an unexpected discovery

But on September 5, 2016 – just below 2 kilometres above the comet surface – a comet particle entered the ROSINA/DFMS instrument. This could have irreparably damaged the instrument. But fortunately this did not happen, and instead the event led to unique measurements. All the volatile and less volatile compounds in the comet particle started to evaporate, and the composition of that gas was measured by ROSINA/DFMS. The presence of ammonium salts was detected, especially ammonium chloride (NH4Cl). This salt is related to kitchen salt (sodium chloride, NaCl, in which sodium has been replaced by ammonium). Ammonium salts can play an important role in forming the building blocks for life on Earth. This is yet another discovery with the ROSINA/DFMS instrument for which Belgium has built the detection system(BIRA-IASB in Uccle, IMEC in Leuven, OIP in Oudenaarde).

Why is this discovery so remarkable?

The researchers at the Royal Belgian Institute for Space Aeronomy (BIRA-IASB) are enthusiastic for several reasons. "The ROSINA/DFMS instrument was designed to measure the volatile gases escaping from the comet, such as water, CO, CO2 and the like. In addition, there were other instruments that investigated the dust of the comet (say, small rock particles, which do not evaporate). But because a comet particle got stuck in the inlet of ROSINA/DFMS and warmed up there, we got to see the less volatile components for the first time," says Johan De Keyser, who leads the comet research at BIRA-IASB. "Like so many comets, Churyumov-Gerasimenko seemed to contain little nitrogen. However, nitrogen is not rare in the solar system. Remember that nitrogen is the main component of the air we breathe. Well, now it turns out that in comets nitrogen is hidden in those ammonium salts. Further quantitative research is needed, but it seems that a pressing question in comet science has been solved."

Earlier, Belgian researchers discovered the presence of chlorine in the comet. Frederik Dhooghe: "It was clear to us that chlorine is contained in hydrogen chloride (HCl), but there seemed to be more chlorine present than one would expect based on the amount of hydrogen chloride alone. Now it appears that ammonium chloride can also contribute to the measured chlorine content".

This research was financially supported by the Belgian Federal Science Policy (BELSPO).

Nature Astronomy article

K. Altwegg, H. Balsiger, J.-J. Berthelier, C. Briois, M. Combi, H. Cottin, J. De Keyser, F. Dhooghe, B. Fiethe, S. A. Fuselier, T. I. Gombosi, N. Hänni, M. Rubin, M. Schuhmann, I. Schroeder, T. Sémon, S. Wampfler. Evidence of ammonium salts in comet 67P as explanation for the nitrogen depletion in cometary comae. Nature Astronomy, DOI:10.1038/s41550-019-0991-9, 2020


Dr. ir. Johan De Keyser, Head of the Space Physics department, 02 37 30 368
Dr. Frederik Dhooghe, Scientific researcher, 02 79 03 917
Dr. Karolien Lefever, Head of the Communication department, 02 37 30 450


News image 1
News image legend 1
Image of particles surrounding the comet. Only the dots indicated by the red lines are actually stars.
News image 2
News image legend 2
ROSINA/DFMS mass spectra at mass 17 and 36. The peaks correspond to substances in the comet gas or their fragments. Orange, purple, black : measurements before the comet particle entered the mass spectrometer; blue : measurements while the particle evaporates; green : measurements afterwards. Not only the quantity of gas, but also its composition appears to be different while the particle is evaporating.