Rare form of nitrogen

USING the Subaru Telescope’s High Dispersion Spectrograph, a team of astronomers from Kyoto Sangyo University detected two forms of nitrogen, ¹⁴ NH ₂ and ¹⁵ NH ₂ , in Comet ISON during its bright outburst in the middle of November 2013. This is the first time that astronomers have reported a clear detection of the relatively rare isotope ¹⁵ N in a single comet and also measured the relative abundance of two different forms of nitrogen of cometary ammonia (NH ₃ ). Their results support the hypothesis that there are two distinct reservoirs of nitrogen in the massive, dense cloud, or solar nebula, from which the solar system may have formed and evolved.

Comets are relatively small solar system objects composed of ice and dust that formed 4.6 billion years ago in the solar nebula when the solar system was in its infancy. Because they usually reside in the cold regions far from the sun, they probably preserve information about the physical and chemical conditions in the early solar system. Different forms and abundances of the same molecule provide information about their source and evolution. Scientists do not yet understand very well how cometary molecules separate into isotopes with different abundances. Ammonia is a particularly important molecule because it is the most abundant nitrogen-bearing volatile in cometary ice and one of the simplest molecules in an amino group (–NH ₂ ) closely related to life. This means that these different forms of nitrogen in NH could link the components of interstellar space to life on the earth as we know it.

However, the direct detection of cometary NH ₃ is difficult. Therefore, the team concentrated on studying NH ₂ , a derivative of NH ₃ that forms when NH ₃ is broken down by the light (photodissociation) in the cometary coma. The team was fortunate to observe the comet as it neared the sun, when its icy composition was evaporating. They were also fortunate that NH ₂ is easy to observe in the optical wavelength and that the relative abundance of nitrogen isotopes of cometary ammonia is probably close to that of NH ₂ . The observation clearly detected ¹⁵ NH ₂ , and astronomers were able to infer from it that the ratio of ¹⁴ N/ ¹⁵ N (139±38) was consistent with the average ( ¹⁴ N/ ¹⁵ N~130) of that from the spectra of 12 other comets. In other words, Comet ISON is typical in its relative abundance of ¹⁴ N/ ¹⁵ N in cometary NH ₃ . In the case of a dense molecular cloud core, the isotopic ratio of hydrogen cyanide (HCN) is similar to that of comets, while its ratio in ammonia is different from its cometary value. This may mean that the NH ₃ formed in an environment of a low-temperature dust surface, not in the gas of the molecular cloud, and the cometary nucleus could, in addition to ammonia, contain a complex molecule that relates to the origin of life. If this is so, it raises the possibility that the comet brought these materials to the earth.