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

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Science  10 Apr 2015:
Vol. 348, Issue 6231, pp. 232-235
DOI: 10.1126/science.aaa6100

Making comets in the cold

The speciation of nitrogen compounds in comets can tell us about their history. Comets are some of the most ancient bodies in the solar system and should contain the nitrogen compounds that were abundant when they formed. Using the ROSINA mass spectrometer aboard the Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko, Rubin et al. found molecular nitrogen at levels that are depleted compared to those in the primordial solar system. Depletion of such a magnitude suggests that the comet formed either from the low-temperature agglomeration of pristine amorphous water ice grains or from clathrates.

Science, this issue p. 232

Abstract

Molecular nitrogen (N2) is thought to have been the most abundant form of nitrogen in the protosolar nebula. It is the main N-bearing molecule in the atmospheres of Pluto and Triton and probably the main nitrogen reservoir from which the giant planets formed. Yet in comets, often considered the most primitive bodies in the solar system, N2 has not been detected. Here we report the direct in situ measurement of N2 in the Jupiter family comet 67P/Churyumov-Gerasimenko, made by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis mass spectrometer aboard the Rosetta spacecraft. A N2/CO ratio of Embedded Image (2σ standard deviation of the sampled mean) corresponds to depletion by a factor of ~25.4 ± 8.9 as compared to the protosolar value. This depletion suggests that cometary grains formed at low-temperature conditions below ~30 kelvin.

Thermochemical models of the protosolar nebula (PSN) suggest that molecular nitrogen (N2) was the principal nitrogen species during the disk phase (1) and that the nitrogen present in the giant planets was accreted in this form (2). Moreover, Pluto and Triton, which are both expected to have formed in the same region of the PSN as Jupiter family comets (JFCs), have N2-dominated atmospheres and surface deposits of N2 ice (3, 4). This molecule has never been firmly detected in comets; however, CN, HCN, NH, NH2, and NH3 among others have been observed spectroscopically (5, 6). The abundance of N2 in comets is therefore a key to understanding the conditions in which they formed.

Condensation or trapping of N2 in ice occurs at similar thermodynamic conditions as those needed for CO in the PSN (7, 8). This requires very low PSN temperatures and implies that the detection of N2 in comets and its abundance ratio with respect to CO would put strong constraints on comet formation conditions (7, 8). Ground-based spectroscopic observations of the N2+ band in the near ultraviolet are very difficult because of the presence of telluric N2+ and other cometary emission lines. Searches conducted with high-resolution spectra of comets 122P/de Vico, C/1995 O1 (Hale-Bopp), and 153P/2002 C1 (Ikeya-Zhang) have been unsuccessful and yielded upper limits of 10−5 to 10−4 for the N2+/CO+ ratio (9, 10). Only one N2+ detection in C/2002 VQ94 (LINEAR) from ground-based observations is convincing, because the comet was at sufficient distance from the Sun to prevent terrestrial twilight N2+ contamination (11). The in situ measurements made by Giotto in 1P/Halley were inconclusive, because the resolution of the mass spectrometers aboard the spacecraft (12) was insufficient to separate the nearly identical masses of N2 and CO during the 1P/Halley encounter, and only an upper limit could be derived for the relative production rates [Q(N2)/Q(CO) ≤ 0.1] (13).

Here we report the direct in situ measurement of the N2/CO ratio by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) in the JFC 67P/Churyumov-Gerasimenko (hereafter 67P). ROSINA is the mass spectrometer suite on the European Space Agency's Rosetta spacecraft (14) and measures the gas density and composition at the location of the spacecraft (15). The Double Focusing Mass Spectrometer (DFMS) has a high mass resolution of mm about 3000 at the 1% level (corresponding to ~9000 half peak width at the 50% level) at atomic mass per unit of charge 28 u/e, allowing the separation of N2 from CO (Δm = 0.011 u) by numerical peak fitting. Neutral gas is ionized by electron impact and then deflected through an electrostatic, then magnetic, filter onto a position-sensitive microchannel plate (MCP) detector. The peak shape of a single species on the MCP is well known, and therefore numerical fitting can distinguish overlapping contributions from different atoms and molecules (see the supplementary materials).

Starting on 5 August 2014, ROSINA observed the cometary gas flux rise above the spacecraft background signal for the major species, including H2O, CO, and CO2. For N2, which has a higher relative spacecraft background, the cometary signal became apparent a few days later. The spacecraft background signal (16) for both species, CO and N2, was derived at different times before detecting the coma and shown to be temporally quite stable. N2 and CO were both observed in the Rosetta spacecraft background mass spectra, e.g., on 11 May 2014, while the spacecraft was still at a distance of 1.65 × 106 km from the comet (Fig. 1A). A comparable N2 background was measured on 1 August 2014, at almost 800 km from the nucleus before the cometary signal became apparent. Another mass spectrum, representative of the measurements within a distance of 10 km from the nucleus, was obtained on 18 October 2014 (Fig. 1B) and includes both cometary and spacecraft background signal. The indicated background was subsequently removed, leaving only cometary CO and N2. Furthermore, CO from dissociative electron-impact ionization of cometary CO2 inside DFMS’ ion source was removed (a 7 to 36% reduction), and the signal was corrected for the instrument alignment with respect to the comet (supplementary materials).

Fig. 1 Mass per charge 28 u/e spectra before (A) and after (B) entering the coma of 67P, including statistical and 10% pixel gain error.

(A) was obtained in May 2014 and (B) is a representative spectrum from October containing the sum of the cometary parents and fragments and the spacecraft background signals.

This procedure was carried out for 138 spectra over two terminator orbits of the Rosetta spacecraft from 17 to 23 October 2014. Clear diurnal variations in the cometary signal of both species associated with the 12.4-hour rotation period of the comet have been observed (Fig. 2A). The signal is to first order correlated to the comet’s cross-section exposed to the Sun and the relative position of Rosetta (Fig. 2, B to D). The resulting mean N2/CO ratio of Embedded Image in the observed time period corresponds to the mean ratio of each individual measurement and includes the 2σ SD of the sampled mean. Higher outgassing is found at positive latitudes corresponding to the summer hemisphere. Over the sunlit hemisphere, the CO/H2O ratio varies between 0.1 to 0.3 (17), which is in agreement with variations observed at other comets (6). Because these measurements were achieved at a heliocentric distance of 3.1 astronomical units (AU), the water production rate may increase relative to both CO and N2 as the comet approaches the Sun. We therefore expect the N2/CO ratio to be more representative of the N2 content in the coma than the N2/H2O ratio. The N2/CO ratio exhibits a strong variation depending on the position of Rosetta above the surface of the comet nucleus between 0.17 to 1.6% (Fig. 3). There are also hints of a nonlinear relationship between N2 and CO, further indicating that thermal processes in the upper layer of the nucleus and/or surface inhomogeneities might influence the measured N2/CO ratio in the coma.

Fig. 2 Cometary parent CO and N2 signal during 17 to 23 October 2014.

(A) The error bars are associated with the accuracy of the fit, background subtraction, detector gain, and statistical error. Gaps in the data indicate times when ROSINA was off due to thruster operations. The sections below show phase angle and local time (B), latitude and longitude of the subspacecraft point (C) in the Cheops coordinate system, and the distances of Rosetta to the comet (r67P) and the comet to the Sun (rSun) (D). The summer hemisphere is at positive latitudes.

Fig. 3 Cometary parent N2 versus CO signal.

The min and max lines bracket most measurements. To derive the N2/CO ratio, the detector signal ratio in the plot has to be divided by the differential sensitivity of 1.175. The average N2/CO ratio of Embedded Image is given by the solid black line; the min and the max lines show the observed variation and correspond to ratios of Embedded Image and Embedded Image.

With a protosolar ratio N/C of 0.29 ± 0.10 (18) and assuming to first order that all of N and C were in the form of N2 and CO in the PSN (1), we derived an N2/CO ratio of 0.145 ± 0.048 in the PSN gas phase. The comparison with the N2/CO measurement performed in the near coma of 67P shows that the cometary N2/CO ratio is depleted by a factor of about 25.4 ± 8.9 as compared to the value derived from protosolar N and C abundances. This depletion of N2 relative to CO in comet 67P may be a consequence of how cometary ice formed. According to one model, comets agglomerated from pristine amorphous water ice grains originating from the interstellar medium (ISM) (19). In this case, the low N2/CO ratio in 67P is the result of inefficient trapping of N2 in amorphous water ice as compared to CO. This possibility is supported by laboratory experiments in which a mixture of water vapor with N2 and CO was directed onto a cold plate in the 24 to 30 K temperature range (7). In these experiments, gases initially trapped in growing amorphous ice were later released when ice warmed up, and the evolved gases were measured by mass spectrometry. At 24 K, the depletion factor for the N2/CO ratio was found to be ~19, a value within the range of the one observed in 67P of 25.4 ± 8.9. This yields a lower limit for the temperature experienced by the grains agglomerated by 67P, because the N2/CO ratio in amorphous ice would increase at temperatures lower than 24 K because of increasing efficiency of N2 trapping.

An alternative interpretation of the low N2 abundance is that 67P agglomerated from grains consisting of clathrates, which are icelike crystalline solids formed by cages of water molecules that contain small nonpolar molecules (20). This hypothesis is based on models showing that the vaporization distance of ISM ices could have been as high as about 30 AU from the Sun when they entered the PSN (21). With time, the decrease of the gas temperature and pressure allowed water to condense at ~140 to 150 K in the form of crystalline ice, leaving negligible water in the gas phase to condense at low temperatures where amorphous ice is expected to form (22). Depending on the nature of the entrapped species, clathrates formed from preexisting crystalline water ice when the PSN temperature was lower than about 80 K, provided that the slow kinetics of the process was balanced by sufficient formation time (8). As in the case of trapping in amorphous ice, experiments and models suggest that N2 is poorly trapped in clathrate cages, because of its small size (8, 2325). In particular, statistical thermodynamics models (26) used to compute the composition of clathrates formed from a protosolar-composition gas in the PSN show that an N2/CO ratio in the comet’s nucleus is consistent with the measured value in the coma if the nucleus agglomerated from grains formed in the 26 to 56 K temperature range (8).

Both interpretations are consistent with the idea that 67P agglomerated from grains formed at about 30 K or below. However, the measured N2/CO ratio may reflect in whole or in part the comet’s post-formation evolution. A possibility is that 67P agglomerated from grains formed at a lower temperature (around 20 K) in the PSN, favoring the trapping of much more N2 in its building blocks, in a way consistent with the known compositions of the atmospheres and surfaces of Pluto and Triton (3, 4). This possibility would be consistent with an inferred Kuiper Belt origin for 67P and its high D/H ratio (27). In these conditions, 67P could have been initially N2-rich but subsequent post-accretion heating due to the radiogenic decay of nuclides and/or thermal cycles during its transit from the Kuiper Belt and its subsequent history in a short period orbit could have been sufficient to trigger the outgassing of N2 (8). A scenario such as this may explain how initial nitrogen-rich cometesimals similar to Triton and Pluto evolved into nitrogen-depleted comets.

Because N2 trapped in 67P is presumably PSN gas, its 14N/15N ratio should be about 441, the value found in Jupiter and the solar wind (28). This is much higher than values measured in other cometary N-bearing species such as NH3 and HCN (~130) (5). Thus, depending on the proportions of N2 relative to other N-bearing species, the terrestrial 14N/15N ratio of 272 could possibly be cometary in origin, given an appropriate mix of the different nitrogen species in the comets that contributed to terrestrial volatiles (e.g., ~50% N2 and ~50% NH3 or HCN). Our initial ROSINA measurement for N2/CO of 0.57% may be compared with NH3/CO of 6% and HCN/CO of ~2% in the Oort cloud comet Hale-Bopp (6). The production rates of volatiles relative to water vary from one comet to another, but their values normalized to CO remain close to those measured in Hale-Bopp (6). If 67P is a typical JFC, then the ROSINA value for N2/CO implies that the amount of N2 reaching the surface of a solid body in the inner solar system from a JFC impact was almost 15 times less than the amounts of NH3, HCN, and certain organic compounds (6). This comparison suggests that JFC comets were not the main source of Earth’s nitrogen.

Supplementary Materials

References and Notes

  1. Acknowledgments: The authors thank the following institutions and agencies, which supported this work: Work at the University of Bern was funded by the State of Bern, the Swiss National Science Foundation, and the European Space Agency PRODEX Program. Work at the Max Planck Institute for Solar System Research was funded by the Max-Planck Society and Bundesministerium für Wirtschaft und Energie under contract 50QP1302. Work at the Southwest Research Institute was supported by subcontract no. 1496541 from the Jet Propulsion Laboratory (JPL). Work at BIRA-IASB was supported by the Belgian Science Policy Office via PRODEX/ROSINA PEA 90020. This work has been carried out thanks to the support of the A*MIDEX project (no. ANR-11-IDEX-0001-02) funded by the “Investissements d’Avenir” French government program, managed by the French National Research Agency (ANR). This work was supported by CNES grants at IRAP; LATMOS; LPC2E; Univers, Transport, Interfaces, Nanostructures, Atmosphère et Environnement, Molécules (UTINAM); and CRPG and by the European Research Council (grant no. 267255 to B.M.). A.B.-N. thanks the Ministry of Science and the Israel Space agency. Work at the University of Michigan was funded by NASA under contract JPL-1266313. Work by J.H.W. at the Southwest Research Institute was funded by NASA JPL subcontract NAS703001TONMO710889. The results from ROSINA would not be possible without the work of the many engineers, technicians, and scientists involved in the mission, in the Rosetta spacecraft, and in the ROSINA instrument team over the past 20 years, whose contributions are gratefully acknowledged. We thank herewith the work of the whole European Space Agency (ESA) Rosetta team. Rosetta is an ESA mission with contributions from its member states and NASA. All ROSINA data are available on request until they are released to the Planetary Science Archive of ESA and the Planetary Data System archive of NASA.
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