[meteorite-list] The Odin Satellite Detects H2-18O In Comet C/2002 C1 (Ikeya-Zhang)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:01:32 2004
Message-ID: <200206082357.QAA28676_at_zagami.jpl.nasa.gov>

Observatoire de Paris
Paris, France

6 June 2002

The Odin satellite detects H2-18O in comet C/2002 C1 (Ikeya-Zhang)

Comet C/2002 C1 (Ikeya-Zhang)

Comet C/2002 C1 (Ikeya-Zhang) displayed its beautiful coma and tails
in the spring sky. Discovered on February 1st, 2002 by Kaoru Ikeya
and Daqing Zhang, the comet could be seen with naked eye in March and
April 2002. Reaching its maximum brightness (visual magnitude of 3)
on the end of March, it was the brightest comet since comet Hale-Bopp
in 1997.

Ikeya-Zhang closest approach to the Sun occurred on March 19, at a
distance of 0.507 AU (1 AU = 150 million kilometres). It came near
the Earth, on April 29, at a distance of 0.405 AU. These close
approaches, as well as abundant productions of dust and gas from
its nucleus, were the reasons for its unusual brightness. With a
revolution period of about 360 years, comet Ikeya-Zhang certainly
visited our environment many times. Comet Ikeya-Zhang is possibly
the great comet that appeared in 1661 and was observed by
J. Hevelius.

Why observe water vapour in comets?

Immediately after its discovery, astronomers carried out a number of
experiments in order to study dust and gases released by comet
Ikeya-Zhang's nucleus. Comets indeed provide valuable information
about the origin and the formation of the Solar System. Mainly made
of water (for 80%), cometary ices contain various other molecules
that are relics of the primitive nebula at the epoch of formation
of planetesimals. Our present knowledge of comets suggests that many
of these molecules were produced in the primordial interstellar
cloud, before its collapse gave birth to the Solar System.

Since comets are variable objects, measurements of the chemical
composition, based on observation of species in the comet atmosphere,
must be referred to the water production rate from the nucleus. Water
in the Universe cannot be easily observed from the ground, because
of the water in the terrestrial atmosphere, which absorbs radiation
from space. Most often, the water production rate in comets is
deduced from observation of the OH radical. The OH radical is
produced by the dissociation of the water molecule by solar UV
radiation. The observation of OH lines at 18 cm in comets is one of
the key programmes of the Nançay radio telescope.

Observations by the Odin spacecraft

The Odin satellite is a small spacecraft, orbiting the Earth,
designed and built by Sweden, in collaboration with Canada, Finland
and France. It was launched in February 2002. Aimed for studying
both the Universe (astronomy) and the terrestrial atmosphere
(aeronomy), it allows for the observation of a number of molecular
lines, at radio (submillimetre range) wavelengths, otherwise not
accessible from the ground: in particular the fundamental water
line at 556.9 GHz. The Odin radiometer includes a 1.1m diameter
antenna, several receivers covering the 480-580 GHz frequency
rangeand corresponding spectrometers (one acousto-optical and two
auto-correlation spectrometers). The acousto-optical spectrometer
was developed under the CNES responsibility by three CNRS
laboratories: the LAS in Marseille, the CESR in Toulouse and the
radio astronomy department (ARPEGES) of the Paris Observatory.

The study of water vapour in comets is one of the main topics
of the Odin observing programme. The sensitivity of the ODIN
radiometer allows us to detect water in weakly active comets,
and its high spectral resolution capability (80 m/s by using the
auto-correlators) provides accurate measurement of line profiles.
The first comet detected by Odin was C/2001 A2 (LINEAR), observed
from end of April until July 2001.

Comet Ikeya-Zhang was observed from April 21 to 29, 2002. First,
a detailed mapping of the water vapour brightness distribution in
the sky was done at 557 GHz. The line intensity at the centre of
the map reached 27 K km/s on April 21 (integrated signal over the
line profile), then decreased slowly as the comet moved further
away from the Sun. The observations provide a first estimate of
the water production rate at about 2 x 10**29 molecules/s (i.e.
about six tons per second). The line profile, very accurately
measured, gives information on the velocity of the water molecules
and on their radiation mechanism. For example, one could observe
that the line is shifted towards positive speed, due to self-
absorption effects in the cometary atmosphere.

The H218O detection

The strong activity of comet Ikeya Zhang and its close approach to
Earth, allowed to carry out more challenging observations. Thus,
the isotopic variety H2-18O [H(subscript)2 (superscript)18 O] was
searched for and finally successfully detected.

Oxygen exists under three isotopic, stable varieties: 16O, the most
abundant isotope, 17O and 18O. In terrestrial water (oceans), the
ratios H216O/H218O and H216O/H217O are 499 and 2681, respectively.
The H216O/H218O ratio could only be measured by the ESA Giotto
spacecraft in comet Halley using mass spectrometers. For the first
time, with Odin, was obtained a spectroscopic measurement of this
ratio.

The H218O line spectrum, at 547.7 GHz, was detected in comet
Ikeya-Zhang after 45 hours of Odin observations and an effective
integration time, on the comet, of 28 hours. The line was observed
in parallel by using two of the four sub-millimetre receivers,
centred at 549 and 555 GHz and tuned at the line frequency. The
receiver outputs were connected to the high resolution spectrometers
and to the acousto-optical spectrometer. The integrated signal was,
as expected, very weak (0.24 K km/s). The successful detection
demonstrates the excellent capability of the Odin instrumentation,
regarding the receiver sensitivity and stability, as well as the
spacecraft pointing  accuracy.

The preliminary measurement of the H216O/H218O ratio in comet
Ikeya-Zhang is in agreement with that measured in terrestrial oceans
(500), and confirms measurements done in comet Halley. This result
is not surprising. With only a very few exceptions, all bodies in
the Solar System show a H216O/H218O ratio similar to the terrestrial
one. Differences by an amount of 5% at maximum can be observed in
some meteorites. Such variations can be explained by slight
differences in nucleo-synthesis sources producing 16O (mainly
supernovae), nearby the Solar System formation region.

Involved French scientists

Alain Lecacheux, LESIA, Observatoire de Paris
alain.lecacheux_at_obspm.fr

Nicolas Biver, ESA ESTEC
nicolas.biver_at_obspm.fr

Jacques Crovisier, LESIA, Observatoire de Paris

Dominique Bockelée-Morvan, LESIA, Observatoire de Paris

References:

A. Lecacheux, "Comet C/2001 A2 (LINEAR)" IAU Circ 7706, 2001
A. Lecacheux, N. Biver, "Comet C/2002 C1 (Ikeya-Zhang)" IAU Circ 7910,
  2002

[NOTE: Images supporting this release are available at
http://www.obspm.fr/actual/nouvelle/jun02/comete-odin.en.shtml ]
Received on Sat 08 Jun 2002 07:57:52 PM PDT