y Tai Viinikka

NASA's Opportunity and Spirit robots, now probing the geology of Mars, have a motto for their mission: "Follow the water." Biologists (and astrobiologists -- scientists who search for unearthly life) are sure that if life as we know it turns up, it will require liquid water.

Geological evidence from the Mars rovers and orbiter photos indicate that the planet was moist in the distant past. It looks very dry now, with any remaining water locked under the polar caps. But Europa, one of Jupiter's giant moons, seems almost certain to have liquid water, in a 60-mile-deep ocean sealed under a 10-mile-thick global ice sheet. By comparison, oceans here on earth are at most about six miles deep.

“Europa's oceans should have more than twice the volume of the Earth's,” says Torrence Johnson, the chief scientist of the Galileo mission to Jupiter and its moons. There's plenty of room for life, but can life flourish in a high pressure waterworld?

Deborah Kelley was looking at just such a world, at the bottom of the Atlantic ocean, in 2000. Her focus then was the geology of the Atlantis Massif, a sizeable mountain on the bottom of the ocean near the Mid-Atlantic Ridge. Late one evening, some other scientists on the mission were watching from a remote camera being "flown" over the sea-bottom valleys. Kelley remembers an excited Swiss oceanographer barging into her cabin to say: “I think we've seen something. I don't know what it is, but we haven't seen anything like it before." When they gathered around the monitors, the scientists saw white towers of rock, 200 feet tall, venting hot water, covered in bacterial slime. What the expedition had found at the Mid-Atlantic Ridge was a new kind of ...