Molecular nitrogen in comet 67P/Churyumov-Gerasimenko indicates a low formation temperature

Abstract

One sentence summary: The first direct detection of dinitrogen in a cometary coma by Rosetta/ROSINA indicates a low formation temperature of comet 67P/Churyumov-Gerasimenko.3 Abstract: Molecular nitrogen (N 2 ) is thought to have been the most abundant form of nitrogen in the protosolar nebula. N 2 is also the main N-bearing molecule in the atmospheres of Pluto and Triton, and was probably the main nitrogen reservoir from which the giant planets formed. Yet in comets, often considered as the most primitive bodies in the solar system, N 2 has not been detected. Here we report the direct in situ measurement of N 2 in the Jupiter family comet 67P/Churyumov-Gerasimenko made by the ROSINA mass spectrometer aboard the Rosetta spacecraft. A N 2 /CO ratio of 5.70 ± 0.66"3 was measured, corresponding to depletion by a factor of ~25.4 ± 8.9 compared to the protosolar value. This depletion suggests that cometary grains formed at low temperature conditions below ~30 K, and that the amount of N 2 delivered by comets to the terrestrial planets was a small fraction of that contributed by the other N-bearing species.Main text: Thermochemical models of the protosolar nebula (PSN) suggest that molecular nitrogen N 2 was the principal nitrogen species during the disk's phase (1) and that the nitrogen present in the giant planets was accreted in this form (2).

The Photodissociation of Formaldehyde in Comets

“…Using the CO production rate relative to that of H 2 O of 0.072 derived in Paper I, we deduce Q(N 2 )/Q(H 2 O) < 1.9 × 10 −3 . Our N 2 /CO limit of 0.027 is consistent with the recently reported mass spectrometer measurement of N 2 /CO = 5.7 × 10 −3 made from the Rosetta spacecraft at comet 67P/ Churyumov-Gerasimenko (Rubin et al 2015). However, we note that 67P is a Jupiter family comet that is highly evolved, while C/2001 Q4 is a long period comet, so that comparison may not be valid.…”

The Main Belt Comets and ice in the Solar System

“…The full implications are likely to be debated for some time as further analysis of the Rosetta data is performed, but first attempts to model comet formation based on Rosetta results (e.g., Davidsson et al 2016) point to the low density/high porosity of the comet and the presence of hyper-volatile species such as O 2 and N 2 Rubin et al 2015) to show that it must have formed far from the Sun and avoided significant heating during its formation and subsequent evolution. This must be significantly different to the MBC case discussed in Sect.…”

Constraints from Comets on the Formation and Volatile Acquisition of the Planets and Satellites

“…In contrast to inner solar system bodies, all comets analyzed so far present strong enrichments (factors of about 2) in 15 N in HCN (Bockelée-Morvan et al 2008;Manfroid et al 2009) and NH 3 (Rousselot et al 2014;Shinnaka et al 2014). Although N 2 has been detected by ROSINA DFMS in 67P/CG, its abundance was found to be depleted by a factor of ∼ 25.4 ± 8.9 relative to CO (Rubin et al 2015) and no isotope ratio has been measured. It is unclear if the depletion of N 2 in comets is primordial or due to loss after accretion (Mousis et al 2012), so it is difficult to conclude based on this early measurement if comets contributed significantly to the Earth's nitrogen.…”

“…This means that temperature conditions in the region where comets (and Pluto) formed could have been too warm for N 2 ice to form, that they did not retain N 2 beyond their first pass through the solar system (Owen et al 1993), or that detection methods for N 2 are too limited to determine its abundance in comets. The recent detection of N 2 in 67P/CG has been interpreted to mean that this comet formed at temperatures less than 30 K (Rubin et al 2015), although the N 2 /CO ratio is compatible with formation temperatures as high as 56 K if the nucleus agglomerated from ice grains made of clathrates (Mousis et al 2012;Rubin et al 2015). As with Titan (Mandt et al 2014), if formation temperatures were low enough the source of Pluto's nitrogen was N 2 in the PSN, but if temperatures were higher than 38 K then the source of nitrogen for Pluto's surface and atmosphere would have been NH 3 in the PSN that was somehow converted to N 2 .…”