Interstellar chemistry

Molecular nitrogen in space

Astronomers have found evidence of molecular nitrogen in the clouds of gas between the Earth and a distant star. The chemistry involved in the formation of these diffuse clouds might need to be rethought.

On page 636 of this issue, David C. Knauth and colleagues1 claim the first detection of molecular nitrogen (N2) in interstellar space. This simple diatomic molecule, made of one of the most abundant elements in the Universe, is the most common constituent of Earth's modern atmosphere. It is also a major component of the atmosphere of Saturn's moon Titan, and has been detected in trace amounts in the atmospheres of Venus and Mars. But it has proved surprisingly difficult to find N2 in any environment beyond the Solar System.

Chemical models of dark interstellar clouds (whose densities are usually in the range of 103 to 105 particles per cm3) suggest that N2 should be the most abundant form of nitrogen in these regions. This leads to the prediction2,3,4 that the ratio of N2 to hydrogen should be about 10−5. In contrast, models for diffuse interstellar clouds, which are transparent and have densities of about 102 particles per cm3, predict a much lower N2 abundance, in the range between 10−9 and 10−8 that of hydrogen2,5.

Both predictions suggest that N2 might be observable, but searches for this molecule in interstellar space had been fruitless until now. One of the difficulties in detecting interstellar N2 arises from the fact that the symmetric diatomic molecule has no allowed rotational or vibrational (dipole) transitions. Thus, N2 — unlike most of the 120 or more species now detected in dark interstellar clouds — cannot be detected either through millimetre-wavelength obser-vations of rotational emission lines or through infrared spectroscopic detection of vibrational bands (absorption or emission).

The only viable approach to finding interstellar N2 is to search for the spectral lines created by electronic transitions in the molecule. These lines are found exclusively at far-ultraviolet wavelengths (shorter than 100 nm), for which space-based telescopes are required because the Earth's atmosphere blocks such radiation. For technical reasons, however, most ultraviolet telescopes have not covered the far-ultraviolet spectral region where the N2 bands lie. For example, the Hubble Space Telescope cuts off at about 115 nm, well above the wavelength needed for an N2 search. The Copernicus satellite — a small mission that was developed and led by the late Lyman Spitzer and operated from 1972 until 1980 — was the first orbiting spectroscopic observatory capable of far-ultraviolet searches for N2 in interstellar space, but no detection was achieved6.

The best chance for astronomers to search for interstellar N2 has been afforded by the Far Ultraviolet Spectroscopic Explorer (FUSE) mission, now in its fifth year of operation. FUSE was designed specifically to extend ultraviolet spectroscopy to the shorter wavelengths that are not accessible to the Hubble Space Telescope, including the spectral region where the electronic bands of N2 lie. Knauth et al.1 have taken advantage of FUSE's far-ultraviolet sensitivity to search for N2 — and apparently they have found it.

In a classic example of spectroscopic sleuth work, Knauth et al. have detected absorption by N2 in the line of sight towards the star HD 124314 by sorting through and eliminating other features that are blended into the spectrum. These other features arise through the absorption of radiation by the star's own atmosphere, by foreground interstellar gas (mostly molecular hydrogen) and by N2 in the outer vestiges of Earth's atmosphere. The detection of interstellar N2 was aided by the fact that several individual N2 lines are accessible to FUSE and also because FUSE covers the N2 wavelength region with two separate detectors, which means that instrumental artefacts in the data can be eliminated.

The line of sight towards HD 124314 does not intersect a dark molecular cloud; rather, this is a long pathlength, probing one or more diffuse clouds. So, according to model calculations, the ratio of N2 to hydrogen should be closer to the 10−9 to 10−8 level that is predicted for diffuse clouds than to the 10−5 level predicted for dense clouds. Knauth et al. have found an intermediate value, with N2 representing about 10−7 of the total hydrogen abundance in their observed line of sight. This abundance of N2 does not fit either the dense-cloud or diffuse-cloud models.

Among the possible explanations is that the line of sight towards this star contains one or more ‘translucent’ clouds, which are reckoned by astronomers to be intermediate (or possibly transitional) between dense and diffuse clouds7. Alternatively, the models for diffuse clouds might be incorrect, or the detection claimed by Knauth et al. is wrong. The first and third of these options can probably be eliminated, as the line-of-sight dust extinction to this particular star is too small to include a translucent cloud, and the claimed detection of N2 seems secure. So we must surmise that the chemical models for N2 in diffuse clouds are inadequate.

Normally it is assumed that, with the exception of hydrogen, molecules in diffuse clouds form through gas-phase chemical reactions2,5. But in dense clouds an additional process, that of molecule formation on grain surfaces, is probably important8,9. The detection of N2 by Knauth et al.1 suggests that grain-surface reactions might contribute more to diffuse-cloud chemistry than previously thought. This conclusion is consistent with earlier searches in diffuse clouds for NH, another simple diatomic molecule found to be more abundant than expected from gas-phase chemistry alone10,11. If grain-surface reactions are required to explain the measured abundances of N2 and NH, it is possible that other surface reactions should also be incorporated into models of the chemistry inside these diffuse clouds.

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Snow, T. Molecular nitrogen in space. Nature 429, 615–616 (2004). https://doi.org/10.1038/429615a

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