Optical Detection of Anomalous Nitrogen in Comets

VLT Opens New Window towards Our Origins

A team of European astronomers [1] has used the UVES spectrograph on
the 8.2-m VLT KUEYEN telescope to perform a uniquely detailed study of
Comet LINEAR (C/2000 WM1). This is the first time that this powerful
instrument has been employed to obtain high-resolution spectra of a
comet. At the time of the observations in mid-March 2002, Comet LINEAR
was about 180 million km from the Sun, moving outwards after its
perihelion passage in January.

As comets are believed to carry “pristine” material – left-overs from
the formation of the solar system, about 4,600 million years ago –
studies of these objects are important to obtain clues about the
origins of the solar system and the Earth in particular.

The high quality of the data obtained of this moving 9th-magnitude
object has permitted a determination of the cometary abundance of
various elements and their isotopes [2]. Of particular interest is the
unambiguous detection and measurement of the nitrogen-15 isotope. The
only other comet in which this isotope has been observed is famous
Comet Hale-Bopp – this was during the passage in 1997, when it was
much brighter than Comet LINEAR.

Most interestingly, Comet LINEAR and Comet Hale-Bopp display the same
isotopic abundance ratio, about 1 nitrogen-15 atom for each 140
nitrogen-14 atoms (14N/15N = 140 +- 30). That is about half of the
terrestrial value (272). It is also very different from the result
obtained by means of radio measurements of Comet Hale-Bopp (14N/15N =
330 +- 75). Optical and radio measurements concern different molecules
(CN and HCN, respectively), and this isotopic anomaly must be
explained by some differentiation mechanism.

The astronomers conclude that part of the cometary nitrogen is trapped
in macromolecules attached to dust particles.

The successful entry of UVES into cometary research now opens eagerly
awaited opportunities for similiar observations in other,
comparatively faint comets. These studies will provide crucial
information about the detailed composition of a much larger number of
comets than hitherto possible and hence, more information about the
primordial matter from which the solar system formed.

A better understanding of the origins of the cometary material (in
particular the HCN and CN molecules [3]) and the connection with
heavier organic molecules is highly desirable. This is especially so
in view of the probable role of comets in bringing to the young Earth
materials essential for the subsequent formation of life on our
planet.

The full text of this Press Release, with three photos and all related
links, is available at:

http://www.eso.org/outreach/press-rel/pr-2003/pr-25-03.html

Cometary material

Knowledge of the abundance of the stable isotopes [2] of the light
elements in different solar system objects provides critical clues to
the origin and early evolution of these objects and of the system as a
whole.

In order to gain the best possible insight into the origins and
formation of the niche in which we live, it is therefore important to
determine such isotopic abundance ratios in as many members of the
solar family as possible. This is particularly true for comets,
believed to be carriers of well-preserved specimens of the pristine
material from which the solar system was made, some 4,600 million
years ago.

However, the detailed study of cometary material is a difficult task.
Measurements of isotopic ratios is an especially daunting undertaking,
mainly because of the extreme weakness of the spectral signatures
(emissions) of the less abundant species like carbon-13, nitrogen-15,
etc..

Measurements of microwave emission from those atoms suffer from
additional, inherent uncertainties connected to the much stronger
emission of the more abundant species. Measurements in the optical
spectral region thus take on particular importance in this
context. However, it is exceedingly difficult to procure the
high-quality, high-resolution spectra needed to show the very faint
emissions of the rare species.

So far, they were only possible when a very bright comet happened to
pass by, perhaps once a decade, thereby significantly limiting such
studies. And there has always been some doubt whether the brightest
comets are also truly representative of this class of objects.

Observations of fainter, more typical comets had to await the advent
of powerful instruments and telescopes.

First UVES spectrum of a comet

Observations of Comet LINEAR (C/2000 WM1) were carried out with the
UV-Visual Echelle Spectrograph (UVES) mounted on the 8.2-m VLT KUEYEN
telescope at the ESO Paranal Observatory (Chile) on four occasions
during March 2002. At that time, the comet had moved past its
perihelion and was by far the faintest comet for which such a detailed
spectral analysis had ever been attempted.

A number of 25-min exposures were secured, resulting in a total
observing time of about 4 hours. The final spectrum covers the entire
visual region (330 – 670 nm) and is one of the most detailed and
information-rich cometary spectra ever obtained. PR Photo 28b/03
displays a small part of this spectrum.

These observations are the first high resolution spectra of a comet
taken with the VLT.

Identification of nitrogen-15

At the time of the VLT observations, the comet was of 9th magnitude,
i.e. about 15 times fainter than what can be perceived with the
unaided eye. The distance from the Sun was about 180 million km; the
distance from the Earth was 186 million km. The observations included
calibration spectra of sunlight reflected from the lunar surface; they
were used to “subtract” the solar signatures in the comet’s spectrum,
caused by reflection of sunlight from the dust particles around the
comet.

As expected, in addition to emission from “normal” CN-molecules
(12C14N), the UVES data also show emission lines of the
13C14N-molecule that contains the rare isotope carbon-13. The derived
12C/13C isotopic ratio is 115 +- 20, quite similar to the “standard”
solar system value of 89.

However, there is also a series of weak features that are positioned
exactly at the theoretical wavelengths of emission lines from
12C15N-molecules, cf. PR Photo 28c/03. The excellent fit that is
evident in this diagram proves beyond any doubt the presence of
nitrogen-15 in Comet LINEAR and allows a quite accurate determination
of the isotopic ratio.

The “anomalous” nitrogen isotope ratio in comets

In 1997, the same group of astronomers obtained spectra of the (at
that time) much brighter Comet Hale-Bopp with the 2.6-m NOT telescope
(Roque de los Muchachos Observatory, La Palma, Canary Islands, Spain)
in order to investigate the isotopic ratio of carbon-12 to
carbon-13. Claude Arpigny remembers: “Interestingly, our spectra of
Hale-Bopp showed a number of weak and unidentified emission lines. We
later realised that they were positioned close to the theoretical
wavelengths of some lines from the 12C15N-molecule. This was a
pleasant surprise, as lines from that molecular species were
previously believed to be so faint that they would not be observable.”

He continues: “This identification is now fully confirmed with the
UVES observations of Comet LINEAR. Our detections in these two comets
are the first ever of those emission lines in comets”.

The estimates of the 14N/15N isotopic ratios are very similar, 140 +-
35 for Hale-Bopp and 140 +- 30 for LINEAR. These ratios are remarkably
low and different from the terrestrial value of 272. This means that
these comets have comparatively more nitrogen-15 than has the
Earth. No measurement has yet been made of the abundance of
nitrogen-15 in the Sun.

So which of the values corresponds to the composition of the material
from which the solar system was made?

Different origins?

To date, only four cometary values of the 14N/15N isotopic ratio have
been reported: two in the radio wavelength range and the two now
measured by means of optical spectra.

The radio measurements concern the HCN-molecule (hydrocyanic acid) in
Comet Hale-Bopp, a “parent” molecule for the CN-molecules present in
comets. Contrary to the optical measurements, the radio values (about
330 +- 75) are compatible with the terrestrial value (272). But radio
measurements of carbon and nitrogen isotopic ratios are only possible
on extraordinarily bright comets like Hale-Bopp, and even then, the
achievable accuracy is very limited. This emphasizes the importance of
performing this kind of research by means of optical observations.

The origin of the isotopic discrepancy between different CN parents is
likely due to fractionation mechanisms in the forming presolar nebula,
e.g. when oxygen- and carbon-bearing molecules in high-density
nebulae stick to cold (10K) dust grains.

Macromolecules in space

The astronomers think that the new results indicate that the
HCN-molecule cannot be the only “parent” of the CN-molecule; the
latter must also be produced by some as yet unknown parent(s) in which
the nitrogen-15 isotope is even more abundant.

In this connection, it is very interesting that an “excess” of
nitrogen-15 is also known to exist in interplanetary dust particles
(IDPs), captured by high-flying aircraft in the Earth’s
atmosphere. They represent the oldest material in the solar system
that can be subjected to detailed laboratory analysis. Many of these
particles are thought to originate from passing comets – this
possibility is obviously supported by the new measurements.

The nitrogen-15 carriers in IDPs have not been securely identified but
are possibly organic macromolecules or polycyclic aromatic
hydrocarbons (PAHs). It is thus possible that the additional
parent(s) of cometary CN may belong to this ensemble of organic
substances.

Whatever the case, the longstanding question of nitrogen and its
isotopic ratio(s) in the solar system, whether present and primordial,
is notoriously enigmatic in several respects. However, the present
results demonstrate that a detailed study of comets may deliver very
useful clues.

The team has now been granted more observing time with UVES and KUEYEN
in order to pursue this important study by observing more comets.

More information

The results described in this ESO press release are presented in a
research report published today in the “Science” journal (“Anomalous
Nitrogen Isotope Ratio in Comets”, by Claude Arpigny and
co-authors). The Liege University is also issuing a press release (in
French) on this occasion.

Notes

[1]: The team consists of Claude Arpigny, Jean Manfroid and Damien
Hutsemekers (Institut d’Astrophysique et de Geophysique de
l’Universite de Liege (IAGL), Belgium), Emmanuel Jehin (ESO-Chile),
Rita Schulz (ESA/RSSD, Noordwijk, The Netherlands), Joachim A. Stuewe
(Leiden Observatory, The Netherlands), Jean-Marc Zucconi (Observatoire
de Besancon, France) and Ilya Ilyin (University of Oulu, Finland).

[2]: Different isotopes of the same elements have different numbers of
neutrons in their nuclei. For instance, carbon-12 nuclei contain six
protons and six neutrons (i.e., 12 particles in all) – this is the
most abundant carbon isotope; carbon-13 contains six protons and seven
neutrons. Nitrogen-14 – the most abundant isotope of this element –
has seven protons and seven neutrons; nitrogen-15 has seven protons
and eight neutrons.

[3]: In chemical terms, CN is referred to as a “radical”.

Contacts

Claude Arpigny

Institut d’Astrophysique et de Geophysique

Universite de Liege

Belgium

Phone : +32 (0)4 366 97 12

E-mail : arpigny@astro.ulg.ac.be

Emmanuel Jehin

ESO

Santiago de Chile

Phone : +56 2 463 30 65

E-mail : ejehin@eso.org