When the researchers looked at the crystals under a microscope, they saw tiny spaces, some of which contained bubbles, like the glass in a carpenter's level. Further analysis showed that these spaces, known as ''inclusions,'' contained a brine -- water with sodium and potassium as well as other minerals. The amount of water is so small, Dr. Gibson said, that if it was all collected from the meteorite, ''you wouldn't even see a drop on the end of a needle.''

Planetary scientists have long known that the early solar system must have had abundant water, said Adrian Brearley, a professor of geological and planetary sciences at the University of New Mexico. Some meteorites contain water that, rather than being liquid, has been broken down into component ions and bound to minerals. Others show indirect evidence that water once flowed on their parent asteroids.

But this is the first time that liquid water has actually made it to earth. A 1980's report of water-containing crystals in a meteorite was later discounted, as it was shown that the samples had been contaminated by liquid used to prepare them in the laboratory. That error, Dr. Zolensky said, ''put people off trying to even look for these things for years and years.''

The next step for the Monahans samples, Dr. Zolensky said, is to analyze what isotopes of hydrogen and oxygen they contain, ''to see where the water came from -- whether it is interstellar or home-grown solar system water.'' Because the amount of water is so small, that analysis will have to wait at least a year, until more precise instruments are developed.

One remarkable thing about the discovery, Dr. Brearley said, is that the water was found on a type of meteorite that is ordinarily thought of as very dry. Shortly after its asteroid formed billions of years ago, radioactive decay within the rock caused it to heat up, boiling off water and even melting the rock. So the halite crystals probably formed after this, when the rock was cooling.

Dr. Zolensky and his team have suggested two scenarios by which the halite could have formed. In one, water trapped within the asteroid could have been expelled through a crack, perhaps caused by a collision, and evaporated very quickly at the surface, leaving behind the halite crystals.

In the other scenario, the source of the water was an encounter with a comet or other icy body, essentially a cosmic snowball.

''Even if it was hit by a snowball, that snowball is still 4.5 billion years old,'' Dr. Clayton said. ''We're still dealing with very primitive stuff.''