[meteorite-list] Optical Detection of Anomalous Nitrogen in Comets

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:29:54 2004
Message-ID: <200309121525.IAA11743_at_zagami.jpl.nasa.gov>

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

ESO Press Release 25/03
12 September 2003
For immediate release

Optical Detection of Anomalous Nitrogen in Comets

VLT Opens New Window towards Our Origins

Summary

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 rôle of comets in bringing to the young Earth materials
essential for the subsequent formation of life on our planet.

 PR Photo 28a/03: Comet LINEAR (C/2000 WM1) - direct image and UVES slit
 position.
 PR Photo 28b/03: Part of the UVES spectrum of Comet LINEAR (C/2000 WM1)
 with CN-band.
 PR Photo 28c/03: Identification of nitrogen-15 in the spectrum.
----------------------------------------------------------------------------

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

  [ESO PR Photo 28a/03] ESO PR Photo [ESO PR Photo 28b/03] ESO PR Photo
                         28a/03 28b/03

 [Preview - JPEG: 495 x 400 pix - [Preview - JPEG: 502 x 400 pix -
 183k 115k
 [Normal - JPEG: 990 x 800 pix - [Normal - JPEG: 1004 x 800 pix -
 450k] 290K]



 Captions: PR Photo 28a/03 displays an image of Comet LINEAR (C/2000 WM1)
 with the UVES slit viewer image. The colour composite in the large frame
 (sky field: 16 x 16 arcmin2) was obtained by Gordon Garradd (Loomberah,
 NSW, Australia). [Image Copyright (c) 2002 Gordon Garradd
 (loomberah_at_ozemail.com.au]. The UVES slit viewer photo (small frame; 40 x
 40 arcsec2) is a false-colour image taken in the (red) R-band with
 UVES+KUEYEN on March 22, 2002; it shows the position of the narrow
 spectrograph slit (0.45 arcsec wide and 8 arcsec long) crossing the inner
 coma and through which the comet's light was captured to produce the
 high-resolution spectra. The slit has been offset from the center of light
 to reduce contamination from solar light reflected off dust particles in
 the comet's coma - there is most dust near the nucleus. PR Photo 28b/03
 shows a small part of the UVES spectrum with an emission band (ultraviolet
 light at wavelength 390 nm) from CN molecules [3] in the comet's
 atmosphere. The emission lines are produced by absorption of the solar
 light by these molecules, followed by re-emission of lines of specific
 wavelengths. This physical process is known as "resonance-fluorescence" -
 it is the same process that causes glowing teeth and shirts in a Disco.
 The upper panel displays the "raw" spectrum; the lower is the "extracted"
 spectrum, now clearly displaying the individual emission lines.

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

  [ESO PR Photo 28c/03] ESO PR Photo Captions: PR Photo 28c/03 is an
                         28c/03 enlarged view of a small section of
                                       the high-resolution UVES spectrum of
                                       Comet LINEAR (PR Photo 28b/03) with
 [Preview - JPEG: 400 x 524 pix - emission lines from CN-molecules
 109k (blue line), compared to the
 [Normal - JPEG: 800 x 1047 pix - "synthetic" spectrum based on
 285k] theoretical calculations and
                                       laboratory measurements (black line
                                       ; some of the lines are labeled with
                                       quantum numbers). In the upper
                                       panel, the synthetic spectrum has
                                       been produced on the basis of the
                                       most abundant isotopic species
                                       (12C14N) . The lower panel shows
                                       that the observed spectrum is in
                                       nearly perfect agreement with a
                                       synthetic spectrum which includes
                                       contributions from two other
                                       isotopic species, 13C14N (emission
                                       lines at wavelengths indicated by
                                       red ticks) and 12C15N (blue ticks);
                                       they are added in proportions of
                                       1/115 and 1/140, respectively. The
                                       isotopic abundances of carbon-13 and
                                       nitrogen-15 are measured
                                       accordingly. Introducing instead the
                                       terrestrial ratio for nitrogen-15
                                       (1/272) significantly degrades the
                                       fit and thus that ratio can clearly
                                       be ruled out in Comet LINEAR.

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 Liège University is
also issuing a press release (in French) on this occasion.

Notes

[1]: The team consists of Claude Arpigny, Jean Manfroid and Damien
Hutsemékers (Institut d'Astrophysique et de Géophysique de l'Université de
Liège (IAGL), Belgium), Emmanuël Jehin (ESO-Chile), Rita Schulz (ESA/RSSD,
Noordwijk, The Netherlands), Joachim A. Stüwe (Leiden Observatory, The
Netherlands), Jean-Marc Zucconi (Observatoire de Besançon, 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 Géophysique
Université de Liège
Belgium
Phone : +32 (0)4 366 97 12
E-mail : arpigny_at_astro.ulg.ac.be

Emmanuël Jehin
ESO
Santiago de Chile
Phone : +56 2 463 30 65
E-mail : ejehin_at_eso.org
Received on Fri 12 Sep 2003 11:25:22 AM PDT