Hello? Anybody out there?

On Earth, scientists have found organisms living in some pretty weird places.

Microscopic creatures, often living in communities that form slimy mats, have turned up deep under the ocean, where no light shines. Microbes have been discovered in cold, salty, ice-covered lakes in Antarctica. They're also in very hot, acidic water in Yellowstone National Park. (See Kidspace for May 6.) One type of microbe is so hardy it can survive heavy doses of radiation. (It was found in 1956 in a can of rotten meat that had been zapped to keep it from spoiling.)

Their homes may vary, but all these organisms need one vital ingredient: water.

So if you're looking for life in another part of the solar system, where would you go? Mars? Maybe. It has ice caps - frozen water. And data from robotic spacecraft orbiting Mars suggest that it may have ice beneath its dusty, rusty surface.

But ask Ron Greeley where he'd look, and he'll tell you he'd pick Jupiter's moon Europa any day.

"Europa has more water than all of Earth's oceans combined," says Dr. Greeley. "That's a lot of water!" Especially since Europa is only 1/4th as big as Earth. Greeley studies the geology of other planets at Arizona State University in Tempe.

Greeley and colleague Torrence Johnson at the California Institute of Technology's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., are leading a team of scientists planning a mission to Europa and three other large moons of Jupiter. If all goes well, they will launch a large robotic spacecraft called the Jupiter Icy Moon Orbiter (JIMO) in 2011. Ultimately, scientists want to send special "cryobots" to Europa to try to penetrate its icy cover.

But first they have to get a better idea of how Europa is put together. Three previous spacecraft - two Voyager probes and Galileo - have visited Jupiter and its moons. The Voyagers, launched in 1977, gave scientists their first close-up views of Europa. But the two craft just flew by on their way to other planets.

In 1989, scientists launched Galileo, which spent all its time examining Jupiter and its moons. It ends its trip in September, when it will crash into the planet.

Those missions, and especially Galileo, sent back amazing images of Europa. Its surface is laced with long cracks, ridges, and other evidence that the ice has been shifting. That suggests the ice is riding atop slushy ice or liquid water.

"The crust is extremely disturbed," Greeley says, "with dark stuff concentrated around the cracks and ridges." The "dark stuff" appears to be material brought up from underneath. (On Earth, hot magma rises from deep inside and cools to form new crust along deep-ocean ridges.) Unlike other moons of Jupiter, Europa's surface is nearly crater-free. This suggests that the crust has been jumbled and refreshed by material welling up from underneath during the moon's "recent" past - 30 million years or so.

As if to further tantalize Greeley and other Europa fans, three years ago a team of scientists reported that Europa's magnetic field shifted in ways that could best be explained by the movement of salty slush or water under the ice.

But Greeley cautions that scientists still don't know how thick Europa's icy crust is. Computer simulations suggest it is relatively thin - perhaps 4 miles deep. But it may reach all the way to the planet's rocky surface, 60 to 120 miles down.

JIMO may help solve the puzzle. It could carry special lasers to make precise measurements of how Europa's surface flexes under the influence of Jupiter's strong gravity. If the surface flexes a lot, the icy crust is thin. If it doesn't flex much, it must be thick.

The craft also could carry radar to "see" through the ice and give scientists an idea of whether the ice contains slushy or liquid "bubbles." Such bubbles could be places where life might be found.

In fact, Greeley and some colleagues just got back from studying the Arctic ice cap. One biologist collected some micro-organisms that she's now testing to see if they can survive in conditions like those thought to exist on Europa.

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