The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument on ESA’s Rosetta spacecraft has made the first measurement of molecular nitrogen at a comet, providing clues about the temperature environment in which this comet formed.
Rosetta NavCam image taken on 14 March 2015 at 85.7 km from the center of 67P/Churyumov-Gerasimenko; it has been cleaned to remove the more obvious bad pixels and cosmic ray artefacts, and intensities have been scaled. Image credit: ESA / Rosetta / NAVCAM / CC BY-SA IGO 3.0.
The results, published online in journal Science, are based on more than 100 measurements made by the ROSINA in October 2014.
“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,” explained lead author Dr Martin Rubin of the University of Bern.
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, Dr Rubin and his colleagues 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 67P/Churyumov-Gerasimenko turns out to be about 25 times less than that of the expected protosolar value.
The researchers think that this depletion may be a consequence of the ice forming at very low temperatures (minus 253 to minus 220 degrees Celsius) in the protosolar nebula. Subsequent heating of the comet through the decay of radioactive nuclides, or as Rosetta’s comet moved into orbits closer to the Sun, could have been sufficient to trigger outgassing of the nitrogen and thus a reduction of the ratio over time.
“Like the origin of our water, the missing molecular nitrogen in comets was another open question raised during the Giotto mission to comet 1P/Halley almost 30 years ago. It is very satisfying that it can be finally answered now,” said co-author Dr Kathrin Altwegg of the University of Bern.
The only other body in the Solar System with a nitrogen-dominated atmosphere is our planet. The current best guess at its origin is via plate tectonics, with volcanoes releasing nitrogen locked in silicate rocks in the mantle. However, the question remains as to the role played by comets in delivering this important ingredient.
“Just as we wanted to learn more about the role of comets in bringing water to Earth, we would also like to place constraints on the delivery of other ingredients, especially those that are needed for the building blocks of life, like nitrogen,” Dr Altwegg said.
Ths graph shows the variation in the signals measured for molecular nitrogen and carbon monoxide by Rosetta’s ROSINA instrument. The signals vary as a function of time, comet rotation and position of the spacecraft above the comet. By comparing the ratio of nitrogen to carbon monoxide at the comet with that of the protosolar nebula, it was determined that the comet must have formed at low temperatures, consistent with a Kuiper Belt origin. The study also finds that Jupiter-family comets like 67P/Churyumov-Gerasimenko were unlikely the source of Earth’s nitrogen. Image credit: ESA / ATG medialab / Rosetta / NavCam / CC BY-SA IGO 3.0 / Rubin et al.
To assess the possible contribution of comets like 67P/Churyumov-Gerasimenko to the nitrogen in the atmosphere of our planet, the team assumed that the isotopic ratio of 14N to 15N in the comet is the same as that measured for Jupiter and solar wind, which reflects the composition of the protosolar nebula. However, this isotopic ratio is much higher than measured for other nitrogen-bearing species present in comets, such as hydrogen cyanide and ammonia.
Earth’s 14N/15N ratio lies roughly between these two values, and thus if there was an equal mix of the molecular form on the one hand, and in hydrogen cyanide and ammonia on the other in comets, it would be at least conceivable that Earth’s nitrogen could have come from comets.
“However, the amount of nitrogen found in 67P/Churyumov-Gerasimenko is not an equal mix between molecular nitrogen and the other nitrogen-bearing molecules. Rather, there is 15 times too little molecular nitrogen, and therefore Earth’s 14N/15N ratio cannot be reproduced through delivery of Jupiter family comets like Rosetta’s,” Dr Rubin said.
“ROSINA has the required resolution to distinguish on site molecules with almost identical molecular weights, which is the case for carbon monoxide and molecular nitrogen. It is great to see that an instrument, designed and built almost 20 years ago, finally delivers the data so long sought-after. This is one of the key measurements of ROSINA.”
“It’s another piece of the puzzle in terms of the role of Jupiter-family comets in the evolution of the Solar System, but the puzzle is by no means finished yet,” added Dr Matt Taylor of ESA, who was not involved in the study.
“Rosetta is about 5 months away from perihelion now, and we’ll be watching how the composition of the gases changes over this period, and trying to decipher what that tells us about the past life of this comet.”
_____
M. Rubin et al. Molecular nitrogen in comet 67P/Churyumov-Gerasimenko indicates a low formation temperature. Science, published online March 19, 2015; doi: 10.1126/science.aaa6100
