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The mystery of missing nitrogen and a chance encounter with a salty cloud of dust have provided further evidence that the key ingredients of life were seeded on Earth by comets.

Researchers have discovered why very little nitrogen has been found in the nebulous covering of comets — it is present but in the form of ammonium salts — which have not previously been measured. As nitrogen is a vital building block for life, the finding could be further evidence that the essential ingredients for the development of self-replicating organisms were delivered to the surface of Earth by comets.

The seeds for the team’s research — led by astrophysicist Kathrin Altwegg — were planted 30 years ago when the European comet mission Giotto flew past Halley’s comet. Onboard, an Ion Mass Spectrometer (IMS) — operated by a team led by the University of Bern’s — Professor Hans Balsinger conducted measurements of the chemical composition of the comet. The key finding of this operation was that there appeared to be an unexpected deficiency in the amount of nitrogen in Halley’s nebulous covering, the coma, which develops as the comet passes close to the Sun. Whilst the team operating the IMS did discover nitrogen in the form of ammonia (NH3) and hydrocyanic acid (HCN), the incidence of these molecules was far below other cosmic incidences.

Gas and dust rise from “Chury’s” surface as the comet approaches the point of its orbit closest to the Sun. (© ESA/Rosetta/NAVCAM)

Three decades later Altwegg — a lead researcher on ROSINA — and her team think they have solved the mystery of the missing nitrogen thanks to data collected from the comet 67P/ Churyumov-Gerasimenkov — nicknamed Chury — by the ROSINA mass spectrometer aboard the ESA’s Rosetta space probe.

Shortly before the end of its mission, Rosetta flew through a cloud of dust from Chury. Purely by good fortune rather than design, this dust hit the ion source of mass spectrometer ROSINA-DFMS (Rosetta Orbiter Sensor for Ion and Neutral Analysis-Double Focusing Mass Spectrometer), led by the University of Bern. “This dust almost destroyed our instrument and confused Rosetta’s position control,” comments Altwegg.

The team’s work is documented in a study published in the journal Nature Astronomy.

Ammonium salt and the building blocks of life

Flying through the dust cloud at a distance of just 1.9 km above Chury, allowed the researchers to detect traces of substances that would usually remain in the cold environment of the comet. Amongst the vast amount of the particles detected by the team, many had never been observed on a comet before, something that astounded the team. In particular, they were surprised by the incidence of ammonia, which was many times greater than theorised.

Ammonium chloride is one of five different ammonium salts the ROSINA team has found traces of. (© University of Bern)

“We came up with the idea that the incidence of ammonia in the ROSINA data could potentially be traced back to the occurrence of ammonium salts,” explains Altwegg. “As a salt, ammonia has a much higher evaporation temperature than ice and is therefore mostly present in the form of a solid in the cold environment of a comet. It has not been possible to measure these solids either through remote sensing with telescopes or on the spot until now.”

To prove the presence of these salts locked with the ice of comets the team set about undertaking extensive laboratory testing. “The ROSINA team has found traces of five different ammonium salts: ammonium chloride, ammonium cyanide, ammonium cyanate, ammonium formate and ammonium acetate,” says Dr Nora Hänni, a chemist on the ROSINA team and co-author of the current study. “Until now, the apparent absence of nitrogen on comets was a mystery. Our study now shows that it is very probable that nitrogen is present on comets, namely in the form of ammonium salts.”

Amongst the ammonium salts discovered from Chury were several molecules which can result in the development of urea, amino acids, adenine and nucleotides.

Altwegg concludes: “This is definitely a further indication that comet impacts may be linked with the emergence of life on Earth.”

Original research: K. Altwegg, et al, ‘Evidence of ammonium salts in comet 67P as explanation for the nitrogen depletion in cometary comae.’ Nature Astronomy, (2020), https://doi.org/10.1038/s41550-019-0991-9





















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