Chemical reactions producing water are common in space. The existence of huge amounts of water in many different regions was established four years ago.

Other researchers in Spain have been studying regions of space where low-mass stars like our Sun are born. These regions are called 'quiescent' or 'cold' clouds, because they don't form massive stars and hence lack strong internal heat sources. They are at a mean temperature of minus 441 degrees F, just 10 degrees above absolute zero. Astronomers estimate that there are millions of cold clouds in the Milky Way.

But researchers say the frigid temperatures make it difficult to detect water vapor, which does not emit radiation detectable by current telescopes.

On the other hand, water in liquid form does not exist in space because the temperature and pressure conditions are not suitable.

Consequently, only ices had been detected so far in cold clouds. But astronomers knew that water vapor should also be there, even if in small amounts. It was impossible to estimate the global amount of water in cold clouds, and its relative abundance as compared with other molecules, unless water vapor could be found.

"In cold regions you expect to find most of the water forming ices because water vapor condenses on cold dust grains, much as it does on car roofs and windows in the winter," explained Italian astronomer Andrea Moneti. "In warmer regions, on the contrary, the stars heat the environment and the ice on the dust grains evaporates -- as when the Sun makes the frost evaporate off your car. So the rule is: the colder the cloud, the less water vapor. But we expected that there had to be at least some water vapor in quiescent clouds, only it had not been detected."

To search for the cold water vapor the team tried a different strategy than that used for the ices. They knew that if the light from a far-away object goes through some water vapor on its way to Earth, the vapor will leave a chemical fingerprint on that light.

Astronomers decided to search for the fingerprint in the light coming from two regions in the galactic center. These regions were selected because light emitted from there passes through several cold clouds on its way to Earth.

Moneti and his colleagues looked through archived data and detected the presence of both water vapor and solid ices in the cold regions, leading to the first estimates of the global amount of water available in these places and its relative abundance as compared with other molecules.

"We find that in cold regions there is as much water (ice and vapor) as in the very active star-forming regions," said Moneti. "The important result is that after molecular hydrogen and carbon monoxide, water is the most abundant molecule."

According to this result, in a cold cloud with a thousand times the mass of the Sun, the amount of water (ice and vapor) is equivalent to about a hundred Jupiter masses.

The fact that cold and warm clouds have the same amount of water is surprising, the researchers say, because some hypotheses suggested that the molecule was best preserved by processes happening exclusively in warm clouds. The finding therefore gives new insights into the question of how water is formed and preserved in space.

The team has also found that 99 percent of the water is ice, condensed on cold dust grains, while only 1 percent is in gaseous form. This provides new data to help understand the role of water in the formation of planets and comets. Although the process is not yet fully understood, a simplified description is that part of the ice remains unprocessed and ends up in comets, while the rest turns into vapor and is used, together with the original water vapor, to make planetary atmospheres and gaseous planets.

"When a protoplanetary system is formed around a low-mass star, most of the gas and dust that does not form the central star will eventually find its way to the planets, while part of it will form comets," Moneti explained. "It is not yet clear how the water contained in the gas and in the grains contributes to form planetary atmospheres, but it is clear that a lot of processing occurs in the planets. On the other hand, we are fairly certain that comets are made of relatively unprocessed material; comets are essentially large, dirty snowballs, and we believe that the 'snow' that makes the comets shares many characteristics with the water ice in cold molecular clouds."

Astronomers expect that future infrared space telescopes, like the European Space Agency's Herschel, due to be launched in 2007, will penetrate even more deeply into the cold and dark regions where star, and eventually planet formation, first begins.