Nr 12-97 - Paris, 29 April 1997
[ Version française ]
Europe's space telescope ISO finds water in distant places
Water is the medium of life, and ESA's cosmic water diviner continues
to detect it in a wide variety of sources in the cosmos where it was previously
unknown. Astronomers using ESA's Infrared Space Observatory, ISO, have
found water vapour in dark clouds lying towards the centre of the Milky
Way. They calculate that water is abundant in our Galaxy.
Equally striking is ISO's discovery of water vapour in the outer planets,
Saturn, Uranus and Neptune. As those chilly planets cannot release water
from within, they probably have a supply of water coming from elsewhere
in the Solar System.
Since ISO went into orbit at the end of 1995, it has used its unique power
of analysing infrared rays coming from the Universe to identify water vapour
and water ice near dying stars and newborn stars. It has also measured
the water vapour steaming from Comet Hale-Bopp.
"Before ISO no instrument was capable of detecting water in so many places,"
comments ESA's director of science, Roger Bonnet. "To start revealing the
cosmic history of the Earth's water is a big success for ESA and for the
astronomers who use our unique infrared observatory. And ISO's discovery
that water is commonplace in the Galaxy will encourage renewed speculation
about life that may exist in the vicinity of other stars."
Water amid the stars
Primaeval hydrogen atoms make water by joining with oxygen atoms that are
manufactured within stars, in nuclear reactions occurring towards the end
of a star's life. Oxygen from defunct stars enriches the Galaxy, and abundant
hydrogen is available to react with it. Although the existence of water
in interstellar space is not surprising, the Earth's moist atmosphere makes
life difficult for any astronomer who wishes to spot water vapour in the
Universe with ground-based instruments.
Observations from aircraft and balloons gave early hints of cosmic water,
but thorough investigations had to wait for ISO's unhampered view from
space. Three of the satellite's instruments, the Short Wavelength Spectrometer
(SWS), the Long Wavelength Spectrometer (LWS) and the photometer ISOPHOT
operating in spectroscopic mode, take part in the hunt for water.
Last year, for example, users of both SWS and LWS reported water vapour
in the vicinity of the aged star, W Hydrae, from which oxygen-rich winds
blow into space. The bright infrared source GL 2591, surrounding a newly
formed massive star, revealed to SWS hot and abundant water vapour. Jets
of gas from very young stars can create luminous shock waves at great distances,
and LWS made the first detection of water vapour in such an object, HH-54.
Among the objects subsequently examined by LWS, IRAS 16293-2422 is a cosmic
egg in the process of creating a star of about the same size as the Sun.
Characteristic emissions from water vapour at 108, 113, 174 and 179 microns
show up clearly. The water plays a practical part in starmaking. It helps
to radiate away excess heat which could otherwise prevent the parent gas
from condensing under gravity to make the star.
When ISO looks towards the centre of the Galaxy, which lies about 28,000
light-years away in the constellation of Sagittarius, it sees, not emissions
of the the characteristic wavelengths of water, but absorptions. These
appear as dips in the infrared spectrum and tell of the presence of dark,
cool clouds, called molecular clouds, which are the primary source of new
stars. Very close to the true Galactic Centre is the bright infrared source
Sagittarius B2, and it too shows the presence of water vapour.
In a programme of observations which began in the autumn of 1996 and is
still continuing, ISO's Long Wavelength Spectrometer has made observations
of such high precision that it distinguishes different molecular clouds
on the way towards the Galactic Centre. The clouds are moving at different
speeds relative to the Earth. They alter each water wavelength by the Doppler
effect, to produce a broad absorption line representing water vapour in
the various clouds intervening between the Earth and the bright source
Sagittarius B2. The detection by LWS of water molecules containing the
rare, heavy form of oxygen, oxygen-18, helps the astronomers to estimate
the abundance of water.
Other watery clouds show up when ISO aims towards other dense regions of
the Galaxy somewhat away from the Galactic Centre. There really is, in
the words of an English poet, "Water, water everywhere".
A Spanish astronomer, Jose Cernicharo of the Instituto de Estructura de
la Materia in Madrid, has played a prominent part in this work. He is delighted
by the results.
"For the first time, we have a clear impression of the abundance of water
in the Galaxy," Cernicharo says. "In relatively dense clouds as many as
ten per cent of all oxygen atoms are incorporated into molecules of water
vapour. Even more may be in the form of water ice. Water vapour is, after
molecular hydrogen and carbon monoxide, one of the most important molecules
in space. It plays an important role in the dynamical evolution of the
gas inside the molecular clouds of our Galaxy, and hence in the formation
of new stars."
The water supply of the outer planets
The water vapour in Saturn, Uranus and Neptune showed up in analyses of
very accurate observations made with ISO's Short Wavelength Spectrometer
during October and November 1996. A report to the world's astronomical
community tells of a particularly clear water signature from Uranus, in
distinctive infrared emissions at eight wavelengths between 28.43 and 44.19
microns. A preliminary analysis indicated that the water vapour exists
in the giant planet's outer atmosphere, at a temperature around 0 degrees
C. ISO detected six of the same water "lines" in the infrared spectrum
of distant Neptune, and three in Saturn, which is closer than Uranus. The
puzzle for planetary astronomers is now to figure out where the water comes
from. These giant planets are a long way from the Sun. Uranus, for example,
is twenty times farther out than the Earth is, and sunlight is feebler
by a factor of 400. The planets have their own internal sources of heat,
and they are thought to contain plenty of water incorporated when the planets
formed. But it would be difficult for water vapour to escape into the outer
atmosphere. On the other hand, water in the form of ice is a major constituent
of comets, which sometimes collide with the planets, as seen in the spectacular
impacts of Comet Shoemaker-Levy 9 on Jupiter in 1994.
The leader of the ISO team that found the water vapour in the outer planets
is Helmut Feuchtgruber of the Max-Planck Institut fur Extraterrestrische
Physik at Garching, Germany. He works at the ISO operations centre at Villafranca,
Spain. For him, the theoretical puzzle of the water vapour is full of significance
for planetary science.
"The upper atmosphere of the Earth is very dry because water vapour rising
from the oceans or the land freezes into clouds," Feuchtgruber comments.
"We would expect the same kind of lid to seal in the water vapour of the
outer planets. What we see in Saturn, Uranus and Neptune probably comes
from an outside source. This has important implications for our theories
of the origin and evolution of all planetary atmospheres, including the
Earth's."
Helmut Feuchtgruber, Emmanuel Lellouch and their colleagues are preparing
a theoretical analysis of the likely origin of the water vapour in the
outer planets, which they hope to publish in the next few months.
European success story
Rated by a panel of American astronomers as "the major infrared mission
of the decade", ISO is a special achievement for ESA -- and for Europe's
astronomers and engineers. Advanced technology created ISO's extremely
cold telescope capable of observing cool regions of the Universe.
Multinational teams, with leaders in France, Germany, the Netherlands and
the United Kingdom, developed the special scientific instruments.
A European Ariane 44P launcher put ISO into orbit on 17 November 1995.
ISO's supply of superfluid helium, which keeps the telescope and instruments
cold, is expected to run out at about the end of 1997, giving it a life
several months longer than required in the specification. Requests from
the world's astronomers for observations with ISO have always far exceeded
the available operating time, even though the spacecraft's controllers
at ESA Villafranca supervise an average of 45 astronomical observations
every day.
For further information please contact :
Simon Vermeer
ESA Public Relations Division
+ 33(0)1.5369.7155
Dr Martin Kessler
ESA Project Scientist for ISO
+34.1.813.1253
Prof. Peter Clegg
Principal Investigator, Long Wavelength Spectrometer
+ 44.171.975.5038
Dr Thijs de Graauw
Principal Investigator, Short Wavelength Spectrometer
+ 31.50.363.4074
Prof. Dietrich Lemke
Principal Investigator, Photometer
+49.6221.528.259
Dr Catherine Cesarsky
Principal Investigator, Camera
+33(0)1.6908.7515
Water amid the Stars :
Dr Jean-Paul Baluteau, +33.(0)4.9105.5900
Dr Pierre Cox, +33(0)1.6985.8737
Dr Jose Cernicharo, +34.1.590.1611
Water in the Outer Planets :
Dr Helmut Feuchtgruber, +34.1.813.1373
Dr Threse Encrenaz, +33(0)1.4507.7691
Dr Emmanuel Lellouch, +33(0)1.4507.7672
Photos are available from internet: here.