Science: What life lurks in the lost lake?

An ocean twice the size of Yorkshire could give us clues about life on other planets, says David Whitehouse. There's one problem - it's under the Antarctic ice cap

In 1974, a survey aircraft was flying over central Antarctica. Protruding out of its underside was a microwave antenna. As the pilot set course for the desolate and little-known central polar wastes, the scientists on board began beaming radio waves down at the ice to measure its profile and properties. A few hundred kilometres further on, the survey team detected a very peculiar signal - a kind of double echo where there should have been none. What they had seen was the radar signal of water trapped four kilometres under the ice.

But it was only last year, when scientists went back to take a closer look, that they realised what it was they had found. It was a vast subterranean ocean 19,000 square kilometres in size - twice the size of Yorkshire - a cold under-ice ocean which had been cut off from the rest of the world for at least two million years. Lake Vostok - named after the nearby Russian research station - was formed when geothermal heat from the ground below melted the ice above.

Despite being cut off, it may be inhabited by microbes and other forms of life. A possible source of nutrients would be the sediments deposited at the bottom of the lake by the glacier as it grinds over the bedrock. In this closed ecosystem, something may have evolved which is totally alien to what has been found elsewhere on Earth. Consequently, scientists are using it to search for alien life on other worlds.

They call Lake Vostok "the Europa simulator" because something similar may exist elsewhere in the solar system - on the smallest but brightest of Jupiter's four major moons.

Europa is 3,130 kilometres in diameter, only slightly smaller than our own Moon; like ours, it is a world that begs for exploration. First seen in close-up by Voyager 2 in 1979, its icy surface has tantalised astronomers for almost 20 years. It is an ice world covered with cracks and ridges, looking rather like the veins that spread out over an eyeball. Unlike our own Moon, the photographs of its landscape show no craters from meteorite impact - perhaps because the ice melted and reformed, smoothing out any trace of their impact. Indeed, Europa is the smoothest world in the solar system. Named after the old world, Europa is one of the major foci of the future exploration of the solar system.

The Galileo spacecraft, in orbit around Jupiter since December 1995, has sent back remarkable images of Europa's ice-fractured surface. When the most detailed Galileo pictures of Europa were shown to the public for the first time recently, one scientist paused a while and said to the assembled reporters, "You know, the last time we discovered an ocean, it was the Pacific." Five hundred years is a long time between oceans.

The ice floes and ridges of Europa are indeed familiar. The same thing is seen every polar winter on earth when the sea freezes. The polar patterns, caused when huge blocks of ice fuse and become solid, are re-enacted on Europa.

But it is what may be underneath the ice that is so exciting. Some scientists suggest - and are increasingly confident - that there may be liquid oceans beneath the crust of ice. Perhaps the ocean of water is heated by underwater- venting volcanoes. On the Earth, deep ocean vents have been found to be home to strange kinds of bacteria, tube worms and shrimps.

To many scientists, it is Europa and not Mars that may be the most likely abode of life living somewhere other than on the Earth. This is why they plan to practice on Lake Vostok before they get to Europa.

To plan the search for life in our solar system, the US National Science Foundation (NSF) has just started a research program called Life in Extreme Environments. Its aim is to consider how life lives in such places as volcanic vents on earth, and what that may tell us about the possibility of life on other worlds.

"Clearly, it's an exciting prospect, as life exists on Earth in some incredibly extreme environments," said Mike Purdy, director of NSF's division of Ocean Sciences. "By studying them, methods can be designed to detect life elsewhere in the solar system." In particular, this means the ice world of Europa.

Some scientists want to first place a Europa Orbiter around the moon. Using microwave sounding technology, such as that which has recently been used to uncover some of the tectonic secrets beneath the Antarctic ice cap, it would look for the region where the ice seems the thinnest. Most scientists, however, see this as an expensive, timewasting luxury. They want to go directly to Europa's icy surface and dig.

Preliminary work has begun at Nasa's jet propulsion laboratory in California on a probe to Lake Vostok that would imitate the exploration of Europa. A key technology for under-ice exploration at Europa would involve a "Cryobot" melter probe, based on a probe used for decades by polar scientists. After penetrating Europa's ice crust using jets of hot water, it would emerge into the sunless sea beneath the ice and deploy a robot mini-submarine called a Hydrobot, which would be released to make measurements and retrieve samples.

"The Lake Vostok expedition would validate the technology," said Jim Cutts of JPL. "We can get real science from Lake Vostok and practice for a mission to Europa." Nasa officials have begun to talk about a Europa probe for acceptance as a core Nasa project in four years.

If, in a decade, a tiny submersible ventures beneath the ice crust of Europa, it will owe a lot to the experience gained in exploring those other caverns measureless to man beneath the ice of the South Pole. It will all be the result of some scientists happening to think that it might be useful - in some abstract way - to know how thick the ice sheet of Antarctica was.

Dr David Whitehouse is the BBC's science correspondent