By Richard Black
Environment Correspondent, BBC News website
Saturn's tiny moon Enceladus may be the best place to look for life elsewhere in the Solar System.
That is the view of a senior scientist working on the Cassini spacecraft, which has been studying Saturn and its moons for nearly two years.
Dr Bob Brown told a major conference in Vienna, Austria, Enceladus contains simple organic molecules, water and heat, the ingredients for life.
He raised the possibility of future missions to probe inside the moon.
Other research presented at the European Geosciences Union (EGU) annual meeting suggests that Enceladus may have a core of molten rock reaching temperatures of 1,400K (above 1,100C).
Jets and rings
In July 2005 Cassini completed a spectacularly close flyby of Enceladus, passing just 173km above its surface.
From this flyby came confirmation that the moon has an atmosphere, and strong evidence that the gases which make up the atmosphere are coming from cracks in the surface, nick-named "tiger stripes", near the south pole.
It appears that the gases are being forced through the surface, as they emerge in jets which shoot upwards for hundreds of kilometres before dispersing, eventually forming Saturn's E-ring.
This false-colour image shows the extent of the active region (Image: Nasa/JPL/SSI)
Most of the gas is water vapour, suggesting strongly that liquid water lies under the moon's icy surface.
From his base at the University of Arizona, Tucson, Bob Brown leads the scientific team for Cassini's visual and infrared mapping spectrometer (Vims) which analysed the chemical composition of Enceladus's atmosphere and mapped the distribution of various gases.
"We very clearly saw water; there's water everywhere on Enceladus, it's 99.9% water ice in general at the surface, and we've known that for years, so it wasn't a big surprise," he told the BBC News website.
"But when we started looking at our spectra we saw absorption bands from a compound that had to have carbon and hydrogen bonded together.
"And when we mapped the location, it was right in these 'tiger stripes' - right where the jets are coming out, and right where it's hot - and it's pretty hard to imagine it's getting there from anywhere but inside."
The organic molecules appear to be quite simple, he said, probably largely methane.
The jets also contain nitrogen; and putting all this together means, said Dr Brown, that Enceladus contains all the ingredients necessary for the development of life, or of precursors to it.
"What you need to put microbes together of the kind that we're familiar with is carbon, hydrogen, nitrogen and oxygen, and water to act as an intermediary for metabolism," he said.
"You've got a rock core that's hot as hell; you've got all the conditions that we think gave rise to the first self-replicating molecules and eventually to life on this planet.
"So Enceladus in a very real sense becomes a stronger candidate for life than [Jupiter's moon] Europa, for instance."
One of the puzzling facets of Enceladus is how and why it is hot enough that it can generate liquid water and spew vapour into space.
Most of its surface has a temperature of about 80 Kelvin (minus 193C). But in the "tiger stripes" it soars to 140 Kelvin (minus 133C), and the interior must be considerably hotter.
Computer models have been produced which try to explain just how hot the interior needs to be, and examine the processes which could produce and maintain the temperatures observed today.
Dr Dennis Matson from Nasa's Jet Propulsion Laboratory (JPL) took EGU delegates through a model which envisages energy coming from two sources, radioactive decay and tidal heating, where differences in the gravitational forces exerted by a nearby body (in this case the giant planet Saturn) cause churning inside the moon, producing heat through friction.
"Down here [in the centre] we have molten magma," he said. "In this model, in the present day, it's entering a cooling phase which may go on for another billion years or so; but at depth you still have high temperatures."
Temperatures at the centre could reach 1,400 Kelvin, he said.
But there are still puzzles. Radioactive decay would have produced the vast majority of its heat shortly after the solar system's formation; somehow, Enceladus has retained some of that energy.
"We think there's a thermostatic mechanism going on in the magma," observed Dr Matson.
If the magma were to cool, he said, it would become more viscous, increasing friction from tidal churning and so producing more heat. But if temperatures veered higher, the magma would flow more easily, and tidal heat production would reduce accordingly.
Along with all the other Cassini mission findings, the research presented here emphasises what an unexpected treasure trove of scientific novelty researchers have discovered on Enceladus.
Its tiger stripes amount to a "water volcano", the only one seen in the solar system other than on Earth.
Among our neighbours, it is the only known geophysically active world other than Jupiter's moon Io.
But as always with space missions, one set of answers leads to another set of questions.
The way to answer some is a further flyby in about two years' time, shortly before the end of Cassini's scheduled mission, which could take the $3.2bn craft just 25km above the tiger stripes and through their jets.
"There's a little bit of a danger, because observations suggest that the particles get larger as you get in closer," said Dr Brown.
"If they're only 20 or 30 microns [in diameter] they won't hurt the spacecraft; but if they're a millimetre or two, and hit the spacecraft in the wrong place, we're dead."
If there is enough fuel left on board Cassini and enough money in the coffers of its masters, the US, European and Italian space agencies (Nasa, Esa and Asi), the mission may gain an extension to its scheduled life, which could yield further flybys of the tiny moon.
But investigations aimed at looking for self-replicating molecules or even primitive forms of life would have to wait for a further mission.
For Europa, landers have been proposed which would burrow down through the top layer of ice into liquid water below, perhaps using heat from radioactive decay to penetrate the surface.
The same approach could potentially work on Enceladus; but Bob Brown believes there may be another, simpler way in.
"You could target the cracks; they clearly give you a way to get down inside, into the reservoir," he said.
"Now whether we can make something smart enough to do that robotically I don't know. But if there are bugs, they don't have to be in the ocean; they could live inside the vents, they just have to be somewhere where it's hot enough and they have enough energy to conduct metabolism.
"My guess is that if stuff has evolved in this ocean, it's figured out a way to work itself up into these vents; and maybe it's not completely crazy to think some of this stuff is sitting there near the surface."
ENCELADUS 'COLD GEYSER'