March 20, 2015 – The European Space Agency’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.
Rosetta arrived last August, and has since been collecting extensive data on the comet and its environment with its suite of 11 science instruments.
Scientists have sought detection of molecular nitrogen at a comet, but have only previously detected nitrogen 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 Rosetta’s comet is believed to have formed.
The new results are based on 138 measurements collected by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis instrument, ROSINA, during October 17-23, 2014, when Rosetta was about 10 km from the center of the comet.
“Identifying molecular nitrogen places important constraints on the conditions in which the comet formed, because it requires very low temperatures to become trapped in ice,” says Martin Rubin of the University of Bern, lead author of the paper presenting the results published this week in the journal Science.
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.
One scenario involves temperatures of between roughly –250ºC and perhaps –220ºC, with relatively inefficient trapping of molecular nitrogen in either amorphous water ice or cage-like water ice known as a clathrate, in both cases yielding a low ratio directly.
Rosetta was launched on March 2, 2004 and was reactivated in January 2014 after a record 957 days in hibernation. Rosetta has been in orbit around comet 67P/Churyumov-Gerasimenko since August 6, 2014 and has been studying the comet in unprecedented detail. The orbiter will continue tracking the comet’s changes as it sweeps past the sun. The comet will reach its closest distance to the Sun on 13 August 2015 at about 185 million km. Rosetta will follow it throughout the remainder of 2015, as they head away from the Sun and activity begins to subside.
Rosetta is an ESA mission with contributions from its member states and NASA. Rosetta’s Philae lander is provided by a consortium led by the German Aerospace Center, Cologne; Max Planck Institute for Solar System Research, Gottingen; French National Space Agency, Paris; and the Italian Space Agency, Rome. JPL, a Division of the California Institute of Technology, Pasadena, manages the U.S. contribution of the Rosetta mission for NASA’s Science Mission Directorate in Washington. JPL also built the MIRO and hosts its principal investigator, Samuel Gulkis.
The Southwest Research Institute (San Antonio and Boulder), developed the Rosetta orbiter’s IES and Alice instruments, and hosts their principal investigators, James Burch (IES) and Alan Stern (Alice).