In the future, swooping low over a lonely ice-moon of distant Saturn, an unmanned spacecraft will manoeuvre carefully, for it will be in a fragile orbit around a small world of feeble gravity.
Under the command of an onboard computer that received its last orders from Earth days ago, it races low over a shining landscape of ice and rock, picking out landmarks and waypoints as it closes on its target. Looming above it is mighty Saturn, the second largest planet in the solar system, a huge presence, 30 degrees across, dominating the sky. Its magnificent ring system is edge-on from this viewpoint, appearing as a faint, thin band of light. Its true extent is revealed by the huge shadows it casts over Saturn's southern hemisphere. Another of Saturn's moons – Mimas – is beginning a passage across the face of the planet, and not far from it is the fuzzy, orange blob that is Saturn's major moon Titan, 10 times larger than the tiny world the probe is exploring which astronomers call Enceladus.
Small it may be, only about the size of England, but for some it is Enceladus, and not Mars, that offers the most promising place to look for life beyond the Earth, and some conceive, as yet just in their imaginations, of missions that would reach this icy world and search for its secrets.
The probe's cameras would record high-resolution images as it fires laser pulses to bounce off the surface, its travel time measuring the topography of the changing textures passing below. Sometimes flying over flattish, uncratered regions like the Sarandib and Diyar Planitia, sometimes over grooved terrain like Harran Sulci, ridges like Cufa Dorsa and trenches like the Daryabar Fossa, the few craters the probe encounters are fractured and deformed. Soon the land changes and becomes noticeably smoother signifying that it has reached one of the most significant places in the solar system. The probe is nearing the so-called "tiger stripes" where there is mystery, and possibly life, and on a world where there is almost no atmosphere a strange mist looms on the horizon.
Like most objects in astronomy, Enceladus began as a tiny point of light. On the evening of 28 August 1789, Sir William Herschel, a 51-year old musician from Hannover who had lived in England since he was 19, saw a tiny speck along the ringed planet Saturn. Herschel was dubbed "the King's Astronomer" by George III after his discovery of the planet Uranus in 1781.
He was taking advantage of Saturn's ring crossing – when its rings appear edge-on and sometime disappear completely. With no glare from the rings it was a good time to scout for undiscovered moons. He found one and confirmed its existence a few days later with the first observations from his new 40ft telescope in Slough.
Herschel's son John, also to become a famous scientist, called it Enceladus after one of the Titans and for almost two centuries it remained an enigmatic point of light circling Saturn. Few things were known about it, its orbit took it around Saturn every 33 hours, so that it could be easily seen to move around its parent planet during an evening's observations. It also seemed to have water ice on its surface. It circled Saturn in the so-called e-ring, the outermost and faintest of Saturn's rings. Beyond that, there was nothing.
So it was with much excitement that scientists turned their gaze towards the tiny moon in November 1980 when the Voyager 1 probe skitted through the Saturnian system, followed the following year by Voyager 2. Enceladus was a surprise. Much smaller than our moon, it had a surface of rock and ice with few impact craters. It was not battered and scarred like many other worlds of the same size Instead it had a youthful appearance. Clearly, something was going on at Enceladus.
The more sophisticated Cassini spaceprobe arrived at Saturn in 2004 and took orbit of the planet making several close flybys of Enceladus, and with each one it became more interesting.
Its sheen of ice made it the most reflective world in the solar system. Icy moons are not unusual out there in the solar system's cold, outer reaches, but a closer inspection of Enceladus revealed something unique. Across its northern ranges are layers of ice through which are exposed rocky mountain folds. In the southern regions the surface is younger and deformed by crustal movements as if the ice and rock moves like a viscous liquid taking millions of years to flow. The surface is young, in places just 100 million years old, in other places perhaps less than a million. For the surface of a world in our solar system this is young indeed. Astronomers called them the "tiger stripes" and were excited by them the moment Cassini saw them in May 2005. Four almost parallel low ridges each with a central fracture straddling the moon's south pole winding over a heavily deformed region. They are 20 miles apart, 80 miles long, one mile wide, and 1,500ft deep with 300ft tall flanking regions. Cassini's infra-red spectrometer picked them up as being slightly warmer than the surrounding terrain, giving off an estimated three to seven gigawatts of power.
But it was when Cassini looked at the limb of the moon that they got the greatest shock. Silhouetted against the darkness of space were jets of what turned out to be water vapour and ice, and they were coming from the tiger stripes which were obviously cracks in the surface of Enceladus reaching down to an ocean beneath the ice. It seems that the ice beneath the moon's icy skin has melted because Enceladus is made to flex due to strong tides raised inside it as it is held in a so-called gravitational orbital resonance by the nearby moon Dione.
In July 2005, Cassini flew through the plume with its spectrometer and dust analyser working overtime. The plume was shooting up from Enceladus at more than a thousand miles per hour from pressurised underground water caverns. The ice and dust escaped into space and went into orbit around Saturn. The plumes of Enceladus are the source of the e-ring. Cassini also found a unique chemistry. There was salt water, carbon dioxide as well as more complex hydrocarbon molecules such propane, ethane, and acetylene. Such molecules could be the building blocks of life. There are also indications from the plume that it originated from very hot water.
In recent years scientists have become intrigued by the possibility of life in oceans trapped beneath the icy crust of distant worlds. Europa and Ganymede, moons of Jupiter, have such oceans where there is water, nutrients, energy and a long-term stable environment, all thought to be conducive to life. But the creatures of those worlds, if they exist, lie trapped beneath many miles of supercold ice as strong as steel. Only at Enceladus does there seem to be a way to get to such an under-ice ocean. Nowhere else do we know of a such a cracked world. If we can really get into them we could be witnesses to the first aliens we have ever encountered.
Despite its scientific importance and the mystery of its leaking ocean there are no definite plans to return to Enceladus. There are just so many other things to do, and money is tight. For the next round of missions Nasa prefers Titan, a very different world with mysteries of its own. But as Enceladus' reputation grows it is possible that a subsequent mission will be sent there. A probe to return to Earth a sample of the plume material would be scientifically invaluable, as well as technically highly challenging.
Until then the mystery of Enceladus will beckon us as it takes a rightful place among the wonders of the solar system. Someday men will stand on Mars and watch the shifting sand dunes of the Elysium desert, touch down amid the strange formations of the dwarf world Ceres; witness first hand the sulphur spewing alien volcanoes of Io, a moon of Jupiter; and, hopefully, drive a tractor to the rim of the Tiger Stripes of Enceladus and lower a probe into dark, misty ice caverns, and watch pools of water flow over the floor paying particular attention to the green slime at its margins.
Dr David Whitehouse is a space scientist and author.