ESA Science & Technology - INFO 12-1997: Europe's space telescope ISO finds water in distant places

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.

The composite picture shows fingerprints of water vapour (labelled H2O) in widely differing infrared wavebands, as registered by two instruments of ESA's space telescope ISO when it was trained on the elderly star W Hydrae. In a high-resolution spectrum from the Short Wavelength Spectrometer (SWS) the main water feature is the strong emission at wavelengths around 38 microns.

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.