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Rosetta spacecraft made the first detection of molecular nitrogen at a comet

on 23 March 2015

ESA’s Rosetta spacecraft has made the first measurement of molecular nitrogen at a comet, providing clues about the temperature environment in which Comet 67P/Churyumov–Gerasimenko formed. The new results are based on 138 measurements collected by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis instrument, ROSINA, during 17–23 October 2014, when Rosetta was about 10 km from the centre of the comet.

The in situ detection of molecular nitrogen has long been sought at a comet, but nitrogen had only previously been detected bound up in other compounds, including hydrogen cyanide and ammonia, for example.

Its detection is particularly important since molecular nitrogen is thought to have been the most common type of nitrogen available when the Solar System was forming. In the colder outer regions, it likely provided the main source of nitrogen that was incorporated into the gas planets. It also dominates the dense atmosphere of Saturn’s moon, Titan, and is present in the atmospheres and surface ices on Pluto and Neptune’s moon Triton.

It is in these cold outer reaches of our Solar System in which the family of comets that includes 67P/Churyumov–Gerasimenko comet is believed to have formed too.

The trapping of molecular nitrogen in ice in the protosolar nebula is thought to take place at temperatures similar to those required to trap carbon monoxide. So in order to put constraints on comet formation models, the scientists compared the ratio of molecular nitrogen to carbon monoxide measured at the comet to that of the protosolar nebula, as calculated from the measured nitrogen to carbon ratio in Jupiter and the solar wind.

That ratio for Comet 67P/Churyumov–Gerasimenko turns out to be about 25 times less than that of the expected protosolar value. The scientists think that this depletion may be a consequence of the ice forming at very low temperatures in the protosolar nebula.

First detection of molecular nitrogen at a comet

Image credit: Spacecraft: ESA/ATG medialab; comet: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0; Data: Rubin et al (2015)