By Paul Rincon
Science reporter, BBC News, Houston
Scientists have produced a global geological map of Jupiter's moon Europa, which has been proposed as a destination for a future space mission.
Interest in Europa has been fuelled by indications that a liquid water ocean lurks beneath its outer shell of ice.
The mapping effort will help build a geological history of the enigmatic moon and target future explorations.
A team at Arizona State University compiled the maps from data sent back by the US-European Galileo probe.
Galileo explored the Jupiter system from 1995 to 2003. The work was presented here at the Lunar and Planetary Science Conference in Houston.
The maps have allowed the scientists to identify several distinct geological units on Europa. Understanding the distribution and age relationships of these units can assist the reconstruction of a geological history for the moon.
Europa's surface is young, active and smooth with few craters. But it is criss-crossed by a network of fractures, thought to be where the icy shell has been prised apart by the tidal forces of Jupiter.
"When we make planetary maps, we're pretty limited by fieldwork, so 'ground truth-ing' is difficult - but not impossible, as we've found with the Mars Rovers," said Professor Ron Greeley, director of planetary geology at Arizona State University (ASU) in Tempe, US.
"We have to rely on remote sensing information. On Earth, this is conventionally done in exploration for oil or mineral deposits. The data is used to make maps to figure out the best places to go and explore.
"That's what we're doing with Europa. We're trying to figure out what's what on the surface so we can go and explore further."
Voyager 2, launched in 1977, was the first spacecraft to fly past Europa, sending back snatched glimpses of the surface at a resolution of about 2km (1.2 miles) per pixel.
Galileo made multiple flybys of the moon, building up a collection of images at different resolutions, ranging from 12.6 to 0.23km (7.8-0.14 miles) per pixel.
Compiling these pictures into a global map required painstaking work by Ron Greeley and Thomas Doggett from ASU and their colleagues at Nasa's Jet Propulsion Laboratory and the US Geological Survey.
"We do not have global coverage, even at the lowest resolution. It is very non-uniform. Also, the illumination of the surface differs widely from one dataset to another," explained Professor Greeley.
Europa, the fourth largest of Jupiter's satellites, is a high priority for future exploration. Study of its magnetic field by the Galileo probe provided strong evidence for a salty ocean beneath the ice.
The moon contains all the ingredients needed for the emergence of life: liquid water, an energy source (provided by the tidal pull exerted by Jupiter's gravity) and organic chemicals.
If life ever emerged on Europa, researchers believe it could exist in an environment similar to terrestrial deep ocean hydrothermal vents or Antarctica's subglacial Lake Vostok.
Nasa has drawn up science definition teams to assess four potential targets for a "flagship" robotic mission to launch after 2015. These targets are the Jupiter system, Europa and Saturn's moons Enceladus and Titan.
The agency has appointed Ron Greeley and Robert Pappalardo from Nasa's Jet Propulsion Laboratory in California to co-chair a team of about a dozen experts who will set out the major science objectives for a Europa mission.
Their work will look at what instruments a spacecraft should take, what orbits would be required for the mission and how the probe might operate in the Jovian moon's harsh radiation environment.
Europa's distinctive icy shell could be 20km thick
After the four teams report back in the summer, officials at Nasa HQ in Washington DC will evaluate the case for a mission and decide which to carry forward.
In addition, an international consortium is preparing a Europa mission proposal to be submitted under the European Space Agency's (Esa) Cosmic Visions programme of space exploration. This consortium is exploring a potential link-up with Japanese partners.
A "dream" concept for the exploration of Europa would be a robotic probe (perhaps powered by radioactive decay) that could melt through the ice and explore the ocean beneath.
But this would pose formidable technical challenges: the ice shell could be about 20km (12 miles) thick, according to one model.
"Is the technology there to do that? Possibly. My own personal feeling is that it is fantastically ambitious," said Professor John Zarnecki, from the UK's Open University and a member of the consortium hoping to mount a European mission to the Jovian moon.
"I think there should be a two-stage approach, beginning with an orbiter, perhaps with ground-penetrating radar and perhaps a way of measuring the gravity field - which is another way to tell what's beneath the surface. Then we go for a lander and a penetrator."
Just getting to Europa is tough - a spacecraft would have to pick up lots of speed and then brake hard to reach it. In addition, the harsh radiation environment gives a spacecraft orbiting the moon a lifetime of just 66 days, according to an Esa feasibility study.