The Fountains of Enceladus

Scientists are turning their attention to a moon of Saturn for possible signs of life. Martin Hickes reports.

IN the book and film 2010, Arthur C Clarke’s sequel to his seminal 2001: A Space Odyssey, the great author predicted life would flourish on a distant moon of Jupiter.

In it, he imagined Jupiter turning into a second sun, powering the start of life on the nearby moon of Europa.

And while such dreams remain firmly within the realm of science fiction, scientists are beginning to question if the first seeds of extra-terrestrial  life might lie not on Mars – from a scientific and literary point of view ,the favourite candidate for years -  but almost a billion miles further away.

Enceladus, a moon of Saturn, just 310 miles in diameter, and 1.5bn km from the Sun until recently has looked like any other frozen satellite.

And while many scientists are looking to the so-called newly discovered exo-solar planets for signs of life, many believe icy Enceladus offers the best hope we have of discovering life within our own solar system.

First observed by William Herschel in 1789 and named after one of the children of the Earth goddess Gaia – the tiny moon spectacularly caught the attention of the robot spacecraft Cassini, which has been in orbit around Saturn for the past eight years.

The $3bn probe has shown that the little moon not only has an atmosphere, but has immense jets of water  - the so-called Fountains of Enceladus - erupting from its surface into space.

Even more astonishing has been a more recent discovery, which has shown that these geysers contain complex organic compounds, which is turn could form the building blocks of life.

The fractured moon also has an array of ‘tiger stripes’  - deep fissures cut into its surface which may hint at powerful geothermal activity.

In May 2011 NASA scientists at an Enceladus Focus Group Conference reported that Enceladus "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it".

"It just about ticks every box you have when it comes to looking for life on another world," says NASA astrobiologist Chris McKay.

"It has got liquid water, organic material and a source of heat. It is hard to think of anything more enticing short of receiving a radio signal from aliens on Enceladus telling us to come and get them."

For years, ironically, Europa – a moon of Jupiter and the subject of Arthur C Clarke’s imaginings, has been a prime candidate in the search for life.

Europa is thought to have a hot inner core which is kept alive by the immense gravitation push-pull effects of Jupiter which it undergoes.

It is thought this space concertina effect is enough to generate sufficient heat to create a liquid water ocean beneath the smooth icy surface of the small moon – and one which might harbour basic life.

Enceladus also seems to have liquid water under its icy surface – but unlike Europa, this surmised ocean is much closer to the moon’s icy surface – and not buried kilometres beneath impenetrable ice.

Coupled with the ‘Fountains’ seemingly jetting organic laden water to the surface, it’s perhaps understandable how the tiny moon has become the focus of much astro-biological attention.

Some of this water falls back onto the moon as "snow", some of it adds to Saturn's rings, and some of it reaches Saturn itself.

Because of this apparent water at or near the surface, space scientists say Enceladus may be one of the best places for humans to look for life.

"We are not sure where that energy is coming from," McKay admits. "The source is producing around 16 gigawatts of power and looks very like the geothermal energy sources we have on Earth – like the deep vents we see in our ocean beds and which bubble up hot gases."

At the moon's south pole, Enceladus's underground ocean even appears to rise close to the surface.

Fissures are bubbling to the surface before being vented into space, along with complex organic chemicals that also appear to have built up in its subterranean sea.

"Those plumes do not represent a torrent," says McKay. "This is not the Mississippi pouring into space. The output is roughly equivalent to that of the Old Faithful geyser in Yellowstone national park. On the other hand, it would be enough to create a river that you could kayak down.

"The fact that this water is being vented into space and is mixed with organic material is truly remarkable, however. It is an open invitation to go there. The place may as well have a big sign hanging over it saying: 'Free sample: take one now'."

Other scientists have surmised that the ocean beneath Enceladus might be a salt water one.

In late 2008, Candice Hansen, a scientist with NASA's Jet Propulsion Lab, headed up a research team on the plumes after they were found to be moving at well over 1,000 mph.

Since that speed is difficult to attain unless liquids are involved, they decided to investigate the compositions of the plumes.

Eventually it was discovered that in one of Saturn’s  rings, ‘fed’  by the Fountains of Enceladus, about 6% of particles contained a significant degree of sodium salt.

In other parts of the plume close to Enceladus, the fraction of "salty" particles increases to 70% by number and >99% by mass.

Scientists have surmised the particles could be frozen spray from a ‘salty’ underground ocean.

The exciting discoveries of the Cassini mission have prompted a return shot to the moons of Saturn – though we may have to wait until 2020 before further discoveries are made.

Meanwhile many scientists argue that water could exist deep below the Martian surface, supporting bacteria-like lifeforms.  

However, these reservoirs could be many metres, if not kilometres, below Mars's surface and it could take decades to find them.

Similarly, the oceans under the thick ice that covers Europa – and two other moons of Jupiter, Ganymede and Callisto – could also support life.

But again, it will be tough to drill through the deep ice fields that cover the presumed oceans of these worlds.

For the moment, its looks like Enceladus, rather than Europa or Mars, might come closest to fulfilling Arthur C Clarke’s visionary dream.

·         Titan Saturn System Mission (TSSM) is a joint NASA/ESA proposal for an exploration of Saturn and its moons Titan and Enceladus.

·         With an estimated NASA cost of $2.5 Billion (FY07), TSSM could launch in 2020, get gravity assists from Earth and Venus, and arrive at the Saturn system in 2029.

·         TSSM was competing against a rival Europa Jupiter System Mission (EJSM) proposal for funding; in February 2009 it was announced that NASA/ESA had given EJSM priority ahead of TSSM, although TSSM will continue to be studied for a launch date, around 2020.