Moon holds mystery of life


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 as it closes on its target.

Looming above it is mighty Saturn, the second-largest planet in the solar system, a huge presence, dominating the sky. Its magnificent ring system appears 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 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, measuring the topography of the changing textures passing below. Sometimes flying over flattish, uncratered regions, sometimes over grooved terrain, ridges and trenches, the few craters the probe encounters are fractured and deformed.

Soon the land changes and becomes smoother, signifying that it has reached one of the most significant places in the solar system. The probe is nearing the "tiger stripes" where there is mystery, and possibly life, and on a world where there is almost no atmosphere.

Enceladus began as a tiny point of light. On August 28, 1789, Sir William Herschel 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 - to scout for undiscovered moons. He found one and confirmed its existence a few days later.

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 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.

So it was with much excitement that scientists turned their gaze towards the tiny moon in November 1980 when the Voyager 1 probe skittered 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.

The more sophisticated Cassini spaceprobe arrived at Saturn in 2004, making several close flybys of Enceladus. Icy moons are not unusual, 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 move 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.

Astronomers called them the "tiger stripes" and were excited by them the moment Cassini saw them in 2005. Four almost parallel ridges, each with a central fracture straddling the moon's south pole. They are 30km apart, 130km long, 1.6km wide and 450m deep, with 90m tall flanking regions. Cassini picked them up as being slightly warmer than the surrounding terrain.

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 during strong tides raised inside it, as it is held in a so-called gravitational orbital resonance by the nearby moon, Dione.

Cassini showed the plume was shooting up from Enceladus 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, CO2, as well as more complex hydrocarbon molecules, which 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 super-cold ice as strong as steel. Only at Enceladus does there seem to be a way to get to such an under-ice ocean. If we can, 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. But as Enceladus's reputation grows it is possible that a subsequent mission will be sent there. Until then the mystery will beckon us as it takes a rightful place among the wonders of the solar system. - Sapa-AFP


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