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."
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."
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.
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.
Photos are available on the ESA home page on Internet :
European Space Agency
Press Information Note N=B0 14-96
The water that we drink and which fills the world's oceans had its origin among the stars. Astronomers are enthralled by results from the European Space Agency's Infrared Space Observatory, ISO, which reveal the chemistry of our Galaxy in unprecedented detail. Surprisingly conspicuous in the neighbourhood of stars at the end of their lives is water vapour made by the combination of primordial hydrogen with oxygen atoms newly manufactured by the stars themselves. Water then reappears during the formation of new stars and planets from the interstellar medium. This happened at the origin of tbe Solar System, and incidentally supplied the water which accounts for more than half of a human being's body weight.
In retracing this history, ISO also observes water in the form of ice in cooler regions around the stars, and in the dust surrounding young stars, from which planets could evolve. Comets represent an intermediate stage in planet-building, and they contain much water ice. According to one hypothesis the newly formed Earth received some of its water directly from impacting comets.
Water vapour in the Earth's atmosphere has prevented telescopes on the ground from detecting the water vapour among the stars, except in very unusual circumstances. ISO orbiting in space escapes the impediment of the atmosphere. Excellent onboard instruments register the characteristic infrared signatures of water vapour, water ice and many other materials.
When ISO scrutinizes selected objects, it detects emissions or absorptions of infrared rays at particular wavelengths, or "lines" in a spectrum, which reveal the presence of identifiable atoms, molecules and solids. The Short Wavelength Specrometer and the Long Wavelength Spectrometer provide detailed chemical diagnoses, and the photometer ISOPHOT and camera ISOCAM also have important spectroscopic capabilities.
Examples of water detection were among many topics reviewed at the First ISO Science Workshop held at ESA's Research and Technology Centre (ESTEC) in Noordwijk, the Netherlands (29-31 May) when 300 astronomers from Europe, the USA and Japan gathered to assess results from ISO since its launch on 17 November 1995. The Long Wavelength Spectrometer has made remarkable observations of water-vapour lines in the vicinity of dying stars and in star-forming regions. So has the Short Wavelength Spectrometer, which also detects water ice. The photometer ISOPHOT has registered water ice in a large number of objects.
Although fascinated by the natural history of water in the cosmos, astronomers have more technical reasons for welcoming ISO's observations. They can use thc details in a spectrum to reduce the abundance of water and its physical circumstances. In the case of the newly forming star GL 2591 for example, frozen water has vaporized in the warmth of the star and risen to a temperature of about 30 degrees Celsius. The amount of water vapour, roughly 10 parts per million compared with hydrogen, is very high by cosmic standards.
"Its remarkable abundance tells us that water plays an important part in the birth of stars," says Ewine van Dishoeck of Leiden Observatory, whose team of astronomers from the Netherlands and Sweden has used ISO's Short Wavelength Spectrometer in this work. "Stars form by the collapse of a cloud of gas and dust, but a build-up of heat inside the cloud makes the work of gravity harder, when it tries to compress the cloud. By radiating strongly in the infrared, water enables the cloud to shed heat very efficiently. This cooling function of water facilitates star formation. So here ISO gives us a new clue in astrophysics."
The amounts of carbon dioxide and methane detected by ISO are surprising, and ices now account for a larger proportion of the carbon compounds drifting in space. Carbon dioxide ice ranks second to water ice in the vicinity of NGC 7538. Astronomers can start making a complete inventory of the frozen volatile materials in interstellar space and compare them with those found in the Solar System.
"ISO gives us spectra of the kind we dust people used to dream of," says Doug Whittet of the Rensselaer Polytechnic Institute in Troy, New York, who leads a US-Dutch team using the Short Wavelength Spectrometer in this study."Our detection of carbon dioxide and methane in interstellar ices has implications for understanding the behaviour of comets, as well as the origin and evolution of life on Earth."
Silicate minerals, familiar as sand on the seashore. are the principal constituents of the solid Earth. Ground-based infrared telescopes have glimpsed the characteristic signatures of silicate grains in various interslellar settings, but again ISO has a better view. It has observed silicates and other minerals both in the vicinity of dead stars like the planetary nebula NGC 6302, and in disks of dust around young stars where new planets may be forming.
In such protoplanetary disks, astronomers using ISO's Short Wavelength Spectrometer have confirmed the existence of a special form of silicon oxide. It was previously found in comets, and seen in interstellar space only with difficulty and uncertainty by ground-based telescopes. Other silicon oxides are widespread in the Galaxy in non-crystalline (amorphous) form. The special silicon oxide, which may be crystals, is possibly a symptom of planet-making in progress.
Thanks especially to carbon compounds, the Universe is capable of supporting life. A widespread infrared emission at around 12 microns, first noted in 1983 by the IRAS satellite in the Milky Way and in other galaxies, turns out to be due to hydrocarbons gathered in wispy clouds. In interstellar space, complex hydrocarbons make tarry grains similar to the soot from car exhausts or coal fires. ISO's instruments, identifying these hydrocarbons by their characteristic infrared wavelengths, find them almost everywhere they look, except close to stars which tend to decompose the hydrocarbons. Teams are using the ISOPHOT and ISOCAM instruments to survey the hydrocarbons in dozens of locations in the Galaxy. The hydrocarbons appear most conspicuous at the outer surfaces of dense clouds of gas and dust, and should give clues to physical conditions prevailing there.
Shortly before ISO's launch, amateur astronomers reported that the star called R Coronae Borealis was fading from view. This elderly star is normally quite easy to see with binoculars, but intermittently it puffs off clouds of dust that almost hide it from view. Professional astronomers do not have the time to monitor irregularly variable stars, and rely on amateurs to alert them to such events like that in R Coronae Borealis. A few months later when the star could be seen only with powerful telescopes, ISO obtained an infrared spectrum of the star in just one minute, using the high-speed spectroscopic facility of the photometer ISOPHOT.
"We caught this star smoking," says Helen Walker of the Rutherford Appleton Laboratory in England, who was in charge of the observation. "The amateurs saw the star fade from view in visible light in October, but it remained bright in the infrared. The telltale wavelengths revealed sooty carbon compounds newly formed in the star's vicinity. Without ISO we could not hope to analyse such a striking event."
ISO is providing astronomers with more details about the interstellar medium than they can fully understand so far. Not only do chemical mysteries lurk in spectra still being analysed, but some of the spatial features of the Galaxy imaged by ISO leave astronomers scratching their heads. Co-existing cold and hot regions make complicated patterns, which were preeviously thought of only as lukewarm averages.
"The Universe is a very complex place," warns Martin Harwit, a pioneer of infrared astronomy. "But ISO is defining its overall contents, assessing the energy budgets of our Galaxy and others, and teaching us a lot about the demography of old and young stars. For me, the results of ISO so far are inspirational."