The National Aeronautics and Space Administration (Nasa) is finalising plans to send a small but sophisticated probe to see if there is liquid water - and possibly life - under the frozen surface of the fourth largest Jovian satellite.
In the week that scientists announced there is less water than they had thought on Mars - and what little there is exists as ice - space explorers are turning their sights on Europa as the place best suited for life beyond our planet.
Christopher Chyba, Professor of geological and environmental studies at Stanford University, said the presence of liquid water under the frozen surface of Europa would greatly increase the chances of finding life. "We don't know yet whether there really is a subsurface ocean on Europa, but it's looking more and more likely. If there is an ocean, the exciting question will be whether it contains life," he said.
Nasa is expected to announce next month the details of the instruments it will place on a small, 20kg (44lb) spacecraft that will be launched in 2003 to orbit Europa to look for signs of liquid water.
"Over the past 10 years on Earth we've learnt that there is a deep terrestrial biosphere, a world of micro-organisms that lives beneath our feet in the subsurface," Professor Chyba told a meeting of the American Geophysical Union in San Francisco. "This discovery makes it seem possible that a place like Europa could harbour life of its own."
The Europa Orbiter mission will carry an altimeter, designed to detect any tidal increase in the height of the moon's frozen surface. If the mission is successful, a second probe would be sent to land and melt through the thick ice and send a robot submarine to explore the watery world beneath.
Images of Europa captured by the Galileo spacecraft in 1996 indicated that the moon's surface is like a cracked snooker ball. The ice had broken up and shifted round like jigsaw pieces, suggesting that these icebergs must be lubricated by warm water below.
Professor Chyba said the Orbiter's altimeter could settle the issue of whether there is liquid water under the ice but a second instrument, a ground-penetrating radar, would help to determine whether there is one large subsurface ocean, or a number of discontinuous seas. "If the orbiter confirms that Europa has a liquid ocean, then it will become one of the hottest places in the solar system to search for life," he said.
Europa's surface temperatures hover around minus 160C. Although the cold could freeze the Europan ocean solid,scientists believe friction caused by a tug of gravitational forces between Jupiter and its 12 moons could keep the subsurface water liquid.
There is also the possibility that Europa has hot underwater vents, spweing out mineral-rich deposits that could act as an energy source for life- forms to evolve, in much the same water that some life-forms on Earth thrive around vents on the deep seabed.
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