New images taken by the Galileo spacecraft just 96,000 miles away from Europa show dark spots on its surface which could have been created by geysers. "It's really exciting," said Ronald Greeley, a geology professor at the Arizona State University. "We're seeing evidence of a lot of geological activity." He described the surface as resembling "ice floes on polar seas on Earth".
Galileo also sent back pictures of a volcano on another of Jupiter's moons, Io, spewing a glowing blue plume of sulphur dioxide 60 miles into space - far further than any Earth volcano.
This also offers evidence that the moons of Jupiter may be geologically active enough to create the conditions in which life could arise.
The presence of slush on Europa - where the surface temperature is estimated to be -145C - would add weight to the argument that the satellite, which is about the same size as the Earth's moon, is not frozen solid but has water under an icy layer only a few miles thick. Despite being five times further from the Sun than the Earth, it could have been warmed by tidal forces that are created as it orbits Jupiter, the largest planet which orbits the Sun.
The more geologically active Europa turns out to be, the more likely it is to have environmental niches that could harbour life, Professor Greeley said.
Scientists hope that better pictures will emerge after December, when Galileo will pass within 370 miles of Europa's surface.
Daniel Goldin, head of the US space agency, Nasa, called the images "exciting and compelling" but said he greeted the idea of life on the icy moon with "sceptical optimism".
"We're not going to jump the gun," he said. "These pictures do not prove the existence of liquid water on Europa."
But he said "the potential is an intriguing possibility and another step in our quest to explore the solar system, the stars, and the answer to the great mystery of whether life exists anywhere else in the cosmos".
Asked about the possibility of discovering evidence of life on Europa similar to that believed to have existed on Mars, Mr Goldin said: "A few days ago I greeted the possibility of ancient microbial life on Mars with sceptical optimism and invited further scientific examination and debate.
"I greet the new pictures of Europa in the same light."
When asked about the significance of discovering liquid on the moon, Mr Goldin replied: "It raises the possibility of a liquid ocean on Europa - the only other place in our solar system where we suspect such an ocean might exist."
Torrence Johnson, another Nasa scientist, said the volcanic activity on Io is very different from that on Earth.
"Terrestrial eruptions cannot throw materials to such high altitudes. We believe that on Io we are seeing geyser-like eruptions that are driven by sulphur dioxide or sulphur gas that erupts and freezes in Io's extremely tenuous atmosphere," he said.
Europa has long been considered by scientists as one of a handful of places in the solar system, with Mars and one of Saturn's satellites, Titan, that could have an environment where primitive life forms could possibly exist
Last week Nasa announced that scientists had found evidence of possible life on Mars more than 3 billion years ago in remains from a Martian meteorite.Reuse content