By Emma Harding
BBC Radio 4
Europa may harbour an ocean beneath its thick crust of ice
Discovering life on another planet would change our sense of place in the Universe. And the encounter may be closer than we think, according to a BBC Radio 4 documentary - 2010: Space Odyssey to Europa.
Are we alone in the Solar System? In his novel, 2010: Odyssey Two, the sequel to the hugely successful 2001, Arthur C Clarke imagined a manned space mission discovering biological life on one of Jupiter's icy moons, Europa.
And 400 years after Galileo first discovered Europa, scientists believe that more recent data on this icy moon might just prove Clarke right.
In our documentary, astronomer Paul Murdin explores the possibilities for finding life on this moon of Jupiter.
Last year, Nasa and the European Space Agency announced plans for a joint mission to Jupiter's satellites, scheduled to launch around 2020. One of their main objectives is to look for evidence of life on Europa.
Europa was discovered - together with three other satellites of Jupiter - by Italian astronomer Galileo Galilei in January 1610. Four centuries later, this icy moon, 350 million miles from Earth, became a priority in space exploration following the tantalising images sent back from Nasa's Galileo probe in the 1990s.
The Galileo data seemed to confirm an existing scientific theory - that Europa contained a deep, liquid water ocean.
Richard Greenberg was a member of the imaging team for the Galileo mission. He says: "We know from Europa's gravity that a liquid water ocean, roughly 100 miles deep, covers the entire surface - in fact there is twice as much water on Europa as in all of the Earth's oceans combined."
Fuel for life?
This liquid ocean lies beneath a thick layer of ice, but any planetary body that contains liquid water immediately excites the interest of astrobiologists - scientists interested in understanding the origin and evolution of life in the Universe.
Life needs much more than water to grow and evolve, but a principal requirement is some sort of liquid in which biological reactions can occur.
EUROPA JUPITER SYSTEM MISSION
Nasa: Jupiter Europa Orbiter could launch on Atlas rocket in 2020
Esa: Jupiter Ganymede Orbiter (above) lofted by an Ariane
Probes use Venus gravity assist to arrive six years later
Orbiters conduct joint observations at other Jupiter moons
Would finally settle into orbits around dedicated targets
Studies will focus on Europa's and Ganymede's interiors
End destructions will allow unique measurement opportunities
Dr Robert Pappalardo is the study scientist for the Europa orbiter at Nasa's Jet Propulsion Laboratory (JPL) in California, US. He comments: "For life to exist, there have to be chemical reactions that power metabolism. Europa's surface is bathed in radiation.
"There are charged particles that are hitting Europa's surface and making chemical reactions - hitting the H2O and creating oxygen on the surface. If this oxygen can get down into ocean, this could be a fuel for life."
Charles Cockell is a geomicrobiologist and an astrobiologist at the Open University in Milton Keynes, UK: "The temperature is one of the fundamental constraints to life on Europa. There's not a lot of convincing evidence for micro-organisms growing below about -30C. And in Europa's ocean it could be -50C, -60C. It could be just too cold for life."
The Europa Jupiter System Mission (EJSM) could answer some of these questions about Europa's potential to harbour life. Space scientists have already developed a range of imaginative proposals for the sort of technology that could be used, from penetrators that would smash into the surface, to melting probes that would slowly melt through the ice to reach the ocean beneath.
One Nasa study imagines equipping one of these melting probes with a robot submersible that would roam through Europa's ocean.
And if we get that far, it's possible that a submersible might come face-to-face with extra-terrestrial life. But what form is such life likely to take?
The extremely cold temperatures in Europa's ocean would suggest that if life exists there at all, it will have evolved at a glacial pace. It seems we are more likely to find little green microbes than little green men.
Some scientists dream of exploring Europa's ocean with robots
But Richard Greenberg from the University of Arizona, US, has bigger ideas: "More complex organisms, like humans, are less efficient and need a lot of oxygen and a food supply. But I did some calculations of how quickly the oxygen at the surface can work its way down into the ocean, and it looks like the ocean could be saturated with oxygen. Given that, it seems unlimited how complex life could be there."
Four hundred years ago, Galileo changed our sense of place in the Universe, when his discovery of four moons orbiting the planet Jupiter confirmed Copernicus' radical idea - that the planets orbited the Sun.
Galileo's discovery suggested that the Earth - and the humans on it - were no longer the centre of the Solar System. In the 21st century, exploration of those icy moons may once again change our sense of how we fit into the Universe.
According to JPL's Robert Pappalardo, "Icy moons may be the most common habitats for life in the Universe, so studying Europa will help tell us not just whether life exists elsewhere in our Solar System, but how common life may be throughout the Universe."
Paul Murdin presents 2010: Space Odyssey to Europa at 9pm on Thurs 4th February on BBC Radio 4. Or listen afterwards on BBC