[meteorite-list] Subaru Telescope Detects Rare Form of Nitrogen in Comet ISON

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 20 Feb 2014 12:41:08 -0800 (PST)
Message-ID: <201402202041.s1KKf86S020262_at_zagami.jpl.nasa.gov>


Subaru Telescope Detects Rare Form of Nitrogen in Comet ISON
Subaru Telescope Press Release
Feburary 19, 2014

A team of astronomers, led by Ph.D. candidate Yoshiharu Shinnaka and Professor
Hideyo Kawakita, both from Kyoto Sangyo University, successfully observed
the Comet ISON during its bright outburst in the middle of November 2013.
Subaru Telescope's High Dispersion Spectrograph (HDS) detected two forms
of nitrogen--14NH2 and 15NH2--in the comet. This is the first time that
astronomers have reported a clear detection of the relatively rare isotope
15NH2 in a single comet and also measured the relative abundance of two
different forms of nitrogen ("nitrogen isotopic ratio") of cometary ammonia
(NH3) (Figure 1). Their results support the hypothesis that there were
two distinct reservoirs of nitrogen the massive, dense cloud ("solar nebula")
from which our Solar System may have formed and evolved.

[Figure 1]
Figure 1: Close-up of spectra of NH2 emission lines (of the same transitions
for both 14NH2 and 15NH2) in Comet ISON, showing the difference in wavelengths
and relative intensity between the isotopes. The red and green-dashed
lines indicate the observed spectrum. The blue line indicates 15NH2, clearly
detected for the first time. (Credit: NAOJ)

Why did the team focus on studying these different forms of nitrogen in
the comet? Comets are relatively small Solar System objects composed of
ice and dust, which formed 4.6 billion years ago in the solar nebula when
our Solar System was in its infancy. Because they usually reside in cold
regions far from the Sun, e.g., the Kuiper belt and Oort cloud, 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. Were they from a
stellar nursery (a primordial interstellar cloud) or from a distinctive
cloud (solar nebula) that may have formed our Solar System's star, the
Sun? Scientists do not yet understand very well how cometary molecules
separate into isotopes with different abundances. Isotopes of nitrogen
from ammonia (NH3) may hold the key.

Ammonia (NH3) is a particularly important molecule, because it is the
most abundant nitrogen-bearing volatile (a substance that vaporizes) in
cometary ice and one of the simplest molecules in an amino group (?NH2)
closely related to life. This means that these different forms of nitrogen
could link the components of interstellar space to life on Earth as we
know it.

Since ammonia is the major carrier of nitrogen in a comet, it is necessary
to clear it from the relative abundance of its isotopes to understand
how 15NH2 separates in cometary molecules. However, the direct detection
of cometary ammonia is difficult, and there are only a few reports of
its clear detection. Therefore, the team concentrated on studying the
form of NH2 developed after the ammonia was 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 NH2, a derivative of ammonia
(NH3), is easy to observe in the optical wavelength, and the relative
abundance of nitrogen isotopes of cometary ammonia is probably close to
that of NH2.

The team used Subaru Telescope's HDS to successfully observe Comet ISON
on November 15th and 16th (UT) ("Spectrum of Outburst from Comet ISON
Obtained by Subaru Telescope's High-Dispersion Spectrograph", December
2, 2013 Subaru Telescope press release), when the comet had its bright
outburst that began on November 14th. The observation clearly detected
15NH2 from Comet ISON, and the team inferred that the ratio of cometary
ammonia of 14N/15N (139?38) is consistent with the average (14N/15N~130)
of that from the spectra of 12 other comets. In other words, Comet ISON
is typical in its relative abundance of 14N/15N in cometary ammonia.

These findings support the hypothesis that there were two distinct reservoirs
of nitrogen in the solar nebula: 1) primordial N2 gas having a protosolar
value of 14N/15N, and 2) less volatile and probably solid molecules having
a ratio of about 14N/15N~150 in the solar nebula. 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 (Figure 2).

[Figure 2]
Figure 2: Comparison of nitrogen isotopic ratios obtained from comets
(left) and molecular cloud core (right). The blue line indicates the ratio
of nitrogen isotopes in the Earth's atmosphere while the wider, yellow
line indicates that of the protosolar nebula. The figure shows that the
nitrogen isotopic ratios obtained from cometary molecules are similar
to each other while those of HCN (hydrogen cyanide) and HN3 (ammonia)
in the molecular cloud core are different. (Credit: NAOJ)

This may mean that the ammonia formed in an environment of a low temperature
dust surface, not in the gas of the molecular cloud. Laboratory experiments
show that various complex molecules can form on the surface of low temperature
dust. If the ammonia molecule formed on the low temperature dust surface,
the cometary nucleus could contain a complex molecule that relates to
the origin of life, in addition to the ammonia. If this is so, it raises
the possibility that the comet brought these materials to Earth.

In the future, the team would like to increase the sample of comets for
which nitrogen isotopic ratios of cometary ammonia have been determined.
They would also like to carry out laboratory measurements of 15NH2 to
obtain more precise isotopic ratios. On a larger scale, the team hopes
to investigate the origin of Comet ISON and the mechanisms that triggered
its outburst so that we can better understand the evolution of the Solar


These results will be published on February 20, 2014 as:
Shinnaka, Y., Kawakita, H., Kobayashi, H., Nagashima, M., & Boice, D.C.
2014 "14NH2/15NH2 ratio in Comet C/2010 S1 (ISON) observed during its
Outburst in November 2013)" Astrophysical Journal Letters, V 782, L106

* "Spectrum of Outburst from Comet ISON Obtained by Subaru Telescope's
High-Dispersion Spectrograph", December 2, 2013 Subaru Telescope press
Received on Thu 20 Feb 2014 03:41:08 PM PST

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