Cold Clouds and Water in Space

Astronomers have known for decades that there is a lot of water in space. Hydrogen is the most common element in the Universe, and oxygen is made in stars and dispersed by events such as supernova explosions. The two elements mix in star-forming clouds and form large amounts of water (H2O). But because astronomers couldn"t measure gaseous water in cold clouds in space, they couldn"t be sure of the exact amount of water in those regions.

"We"ve known for a long time that there is a lot of water ice out there," says Louis Allamandola, astrochemist for the NASA Ames Research Center and member of the NASA Astrobiology Institute (NAI). "We also knew some water existed in the form of a gas, but we weren"t sure how much."

Water in space
Image of water in space.
Credit: ESA ISO

Spanish and Italian astronomers have now determined how much of the water in cold regions of space is in gaseous form and how much is frozen. By doing so, they have been able to measure the total amount of water in the cold regions (Water in liquid form does not exist in space because of temperature and pressure conditions).

The team found that 99 percent of the water in cold clouds is ice condensed on cold dust grains, while only 1 percent is in gaseous form.

"This study is interesting because they looked at cold regions in our Galaxy and found how much of that water is a vapor versus ice," says Allamandola. "Before, it was impossible to determine the amount of gaseous water because we could only get a signature for the solid ice. Now we know how much water is out there ‘ in all forms"

The mean temperature of the water in these cold regions is minus 263 degrees Celsius (minus 441 F), just 10 degrees Celsius above absolute zero. This has been a limitation when searching for all the water available in the clouds: The "solid" water, or ice, in cold regions is detected relatively easily from telescopes on Earth, but the signature of water vapor is hidden by the water vapor in our atmosphere. The water vapor in the cold clouds also does not emit radiation detectable by telescopes because of the low temperature and density of the clouds.

To look beyond the Earth"s water vapor, the team used data from the space telescope ISO, the European Space Agency"s Infrared Space Observatory. They knew that if light from a far-away object passed through some water vapor on its way to Earth, the water vapor would leave a chemical "fingerprint" on that light. Astronomers decided to search for this fingerprint in light from two regions in the galactic center that passes through several cold clouds on its way to Earth.

By analyzing data stored in the ISO Archive, Italian astronomer Andrea Moneti and his colleagues found that cold regions have as much total water (ice plus vapor) as warmer regions where stars are actively forming.

cosmic water factory
Oxygen is made in stars and dispersed out into the Universe in events such as supernova explosions. The two elements mix in star-forming clouds and form large amounts of water [H2O]. The molecules of water leave the clouds and end up in many different places – comets, planets, the centers of galaxies.
Credit: ESA ISO

Some hypotheses had suggested that water molecules were best preserved by processes happening exclusively in warm clouds. This ISO finding therefore gives new insights into the question of how water is formed and preserved in space.

As Moneti explains, "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. 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."

These regions are called "quiescent" or "cold" clouds because there are no stars forming within these zones, and they therefore lack strong internal heat sources. Astronomers estimate that there are millions of cold clouds in the Milky Way.

"Another name for these cold regions is dense molecular clouds, says Allamandola. "If you look into the night sky at the Milky Way, these clouds look like dark patches where there doesn"t appear to be anything. Actually, these clouds contain tiny dust particles that block the background starlight. The interstellar medium is generally very cold, but these clouds are even colder because of the blocked starlight."

The clouds form because of elements thrown off by evolving stars, says Allamandola. "The stars give off some of their heavy elements in the form of fine particles ‘ about the size of particles from cigarette smoke ‘ and once these particles are in interstellar space they form the dust. The dust particles probably form into these clouds through a combination of factors, such as gravitational forces and shock waves."

The cold clouds are the future birthplaces of low-mass stars like the Sun, and solar systems like our own. Thus, this research has implications for the study of newborn planetary systems, since the water vapor and ices in the clouds may end up in gaseous planets, planetary atmospheres and solid bodies like comets.

Small, evaporating ice particles in the tail and surrounding the nucleus of Comet Hyakutake are producing most of the water and other gases seen from Earth.
Credit: NASA

Although the role of water in the formation of planets and comets is not yet fully understood, a simplified description is that some of the ice remains unprocessed and ends up in comets, while some of the ice turns into vapor and is used to make planetary atmospheres and gaseous planets.

"When a protoplanetary system is formed around a low-mass star," Moneti says, "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. 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."

"For astrobiology," says Alamandola, "this research could tell us something about the potential distribution of life in our galaxy. Water in these regions mostly freezes onto dust grains, which are the building blocks of comets. It is thought that comets brought a lot of water and other elements to the early Earth and may have helped generate life. And because all of these cold clouds are essentially alike, it underscores the fact that prebiotic conditions seem to be widespread."

What Next?

"An interesting question we need to ask is ‘why is there water vapor out there at all?" says Allamandola. "It’s so cold ‘almost absolute zero ‘ you"d expect all of the water to be frozen."

Astronomers expect that future infrared space telescopes will help answer such questions. NASA"s Space Infrared Telescope Facility (SIRTF) is due to launch in July 2002, and the ESA’s Herschel is due to launch in 2007. These space telescopes will penetrate even more deeply into the cold and dark regions where star and planet formation first begins.

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