Scientists are relatively certain that whatever extraterrestrial life might look like, it would require liquid water to develop and survive. The laws of chemistry are such that chemists and biologists have a difficult time hypothesizing conditions in which any kind of biochemistry could occur without water.

That is why scientists believe that the search for signs of primitive extraterrestrial life can be confined to sites where liquid water once existed. Several missions are planned or have already explored the places in the solar system where scientists believe evidence of water past or present might next be found:

-- The Moon. In August scientists closed the 18-month Lunar Prospector mission by directing the spacecraft to crash into a lunar crater on the off chance that the orbiter might rustle up a cloud of dust that would betray signs of hidden water ice. In 1998, an instrument aboard the Prospector verified that ice exists in the soil at the lunar north pole. A proposed NASA mission called Icebreaker would send a lander to the moon in 2003 to drill for ice and perform other scientific experiments.

-- Mars. Vast floods once covered Mars. The martian atmosphere likely held a good deal of water vapor. Several Mars missions are involved in finding where the missing water is now. The Mars Surveyor 2001 Orbiter is flying a spectrometer similar to the one aboard the Lunar Prospector which found ice on the moon. The instrument will map the distribution of hydrogen in the top few feet (one meter) of the Martian surface. That will show where on the planet water may be hidden, if it exists near the surface.

NASA will also team up with the European Space Agency to put a water-searching instrument called MARSIS aboard the European Mars Express Spacecraft, scheduled for launch in 2003.

-- Europa. The surface of Jupiter's moon Europa is composed of water ice, and many scientists think that layer may cover a global ocean of liquid water. If there is an ocean on Europa, some biologists believe that life could develop at undersea hotspots, just as life is found at undersea thermal vents at the bottom of oceans on Earth.

-- Ganymede and Callisto. Data returned by the Galileo spacecraft orbiting Jupiter has given preliminary indications that water may exist beneath the surface of Jupiter's moons Callisto and Ganymede.

-- Titan. Saturn's moon Titan could hold some amount of liquid water, but scientists have not been able to penetrate the moon's thick, hazy atmosphere to glimpse any clues. The Cassini spacecraft en route to Saturn is carrying a probe called Huygens, which will be dropped to the surface of Titan. Scientists hope the probe will help answer many of the questions that have been blocked by Titan's optically-impenetrable air.

-- Save the dough, wait for water to fall from space. Although NASA has big plans for fetching geologic samples from other planets, asteroids and comets, pieces of extraterrestrial material are dropping to Earth all the time. These samples arrive free of charge when they fall to the ground as meteorites. Scientists may have even found samples of extraterrestrial water locked inside two recently-fallen meteorites.