[meteorite-list] First Estimate of the Formation Temperature of Ammonia Ice in a Comet

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:47:05 2004
Message-ID: <200111040305.TAA27810_at_zagami.jpl.nasa.gov>


Subaru Approaches Origin of Comets ---
First Estimate of the Formation Temperature of Ammonia Ice in a Comet

National Astronomical Observatory Of Japan
November 1, 2001

Observations made with the High-Dispersion Spectrograph (HDS) of
Subaru Telescope have, for the first time, allowed astronomers to
measure the formation temperature of ammonia ice in a comet. The
temperature of 28 +/- 2 Kelvin (about -245oC or -410oF) suggests
that this comet, Comet LINEAR (C/1999 S4), was formed between the
orbits of Saturn and Uranus. These observations provide us with
not only direct evidence of the environment in which the comet was
born, but also establish brand new methods for probing the origin
of comets.

Comet LINEAR was discovered in 1999 by the Lincoln Near Earth
Asteroid Research project (LINEAR), operated by the MIT Lincoln
Laboratory. Figure 1 shows two images of Comet LINEAR obtained by
Subaru Telescope in 2000 (see Latest News on July 24th, 2000). A
team of researchers from the National Astronomical Observatory of
Japan, the HDS group, and the Gunma Astronomical Observatory made
spectroscopic observations of Comet LINEAR on July 5th, 2000,
during the commissioning phase of HDS, when the comet was bright.

The team concentrated on the emission lines produced when NH2
molecules which have been previously excited, lose some energy and
emit light at a series of characteristic wavelengths (Figure 2).
Previous studies indicate that NH2, which consists of one nitrogen
and two hydrogen atoms, is produced when the powerful Solar UV
rays free a hydrogen atom from the ammonia (NH3) gas which is
constantly boiling off the comet. The emission lines of the NH2
molecules should therefore contain information on their parent
ammonia molecules.

Molecules like NH2 and NH3 which contain two or three hydrogen
atoms are classified as either "ortho" or "para", depending on
whether the quantum mechanical spins of the hydrogen atoms are
aligned or not. The ortho-to-para ratio strongly depends on the
physical environment, and would have been preserved when the
molecules were confined into the icy cometary nuclei. The observed
ratio can therefore reveal the temperature at the time the ice was

Molecules in the ortho and para states emit radiation at
wavelengths which are very close together, but subtly different
due to the differences in alignment between the spins of the
hydrogen atoms. The resolving power of HDS is high enough to
separate these lines and determine how much light is being emitted
by molecules in the ortho and para states. Using code written by
Mr. Hideyo Kawakita of the Gunma Astronomical Observatory, the
strengths of the emission lines from NH2 could be modeled and
compared with the observations to determine the ratio of ortho to
para molecules in Comet LINEAR. Furthermore, the team investigated
the ortho-to-para ratio of the parent NH3 molecules and estimated
that the formation temperature of the ammonia ice to be 28 +/- 2
Kelvin, which suggests that Comet LINEAR was formed between the
orbits of Saturn and Uranus in the primordial Solar System nebula.

Until now, the formation temperature had only been determined for
water ice in comets, and this is the first time that it has been
measured for another molecule. Dr. Jun-ichi Watanabe of the
National Astronomical Observatory of Japan, and a member of the
team who performed this research, says "The brand new methods
using NH2 molecules have great potential for studying the origin
of comets. I have a high expectation for future results obtained
by these methods, especially for short-period comets which are
thought to have a different origin from long-period comets such as
Comet LINEAR."

This result has been published in Science, November 2nd, 2001

   * Figure 1: Two images of Comet LINEAR observed in 2000 with
Subaru Telescope (Latest News on July 24th, 2000)

   * Figure 2: A Comparison between the spectrum observed with HDS
and the spectrum simulated with the model calculations.

Note: Comet LINEAR (C/1999S4) was an unusual comet because it
disintegrated about one month after the HDS observations. A 50-cm
telescope at the Mitaka campus of the National Astronomical
Observatory of Japan monitored this disintegration.

   o http://www.nao.ac.jp/pio/Comets/99S4/ (in Japanese)
Received on Sat 03 Nov 2001 10:05:57 PM PST

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