Then, in the Sixties, the sci-fi dream started to crumble, when the first space probes began sending back data from their cameras and measuring instruments. Slowly and surely, each planet was written off as a likely home for life. The Moon is an airless, sterile land. The Mariner spacecraft showed the famous canals of Mars to be no more than an optical illusion. Venus, far from being a watery paradise, is Hell. Its surface a 400C furnace, crushed under a pressure of 90 atmospheres, under a dull yellow sky of sulphuric acid clouds. Jupiter, Saturn, Uranus and Neptune turn out not even to have a solid surface to walk on. Pluto is a deep-frozen dead loss. Everywhere, it turns out, is too hot, or too cold, or smothered in poisonous gases.
But it is water, in liquid form, that is the key to extraterrestrial life. Without water, organic chemistry is impossible, and it is hard to imagine self-replicating molecules evolving in any other solvent, such as liquid ammonia or methane. Without water, building colonies even on the Moon - our nearest neighbour - would be difficult and expensive. A small probe is now searching for ice near the lunar south pole. Ice is one thing, but liquid water, extraterrestrial blue gold, appears to be in short supply in the solar system.
But in the imagination of one science fiction writer, there may just be one other place in the solar system with majestic oceans to rival those of the Earth. Arthur C Clarke dreamt up a fantastic universe in his 1968 novel 2001: A Space Odyssey, with omnipotent aliens, mysterious monoliths and a small moon of Jupiter covered with a huge ocean teeming with life.
Since the Seventies, we have known that the surface of Europa, Jupiter's second satellite, is almost entirely covered by water ice. Clarke imagined a fictional Europa covered by a vast watery abyss, hidden beneath a permanently frozen ice layer. The ocean would be kept liquid under the ice by the heat of volcanoes deep on the seafloor, tens of miles down. Living off this geothermal energy, the pan-Europan ocean would be populated by strange fish-like creatures, as well as microbes and seaweeds. And although Clarke's monoliths and God-like aliens are clearly fantasies, his picture of a watery Europa teeming with life may well be uncannily accurate.
Jupiter and its moons form a mini-solar system in their own right. A "gas giant", Jupiter is a huge ball of hydrogen and methane, 10,000 times the size of the Earth, swirling with titanic hurricanes and brightly-coloured gaseous maelstroms. Orbiting this huge planet is a whole suite of moons, more than two dozen in all. Most are tiny, insignificant chunks of rock, but four, the Galilean satellites (named after their Italian discoverer Galileo) are big enough to be seen with binoculars from Earth. Callisto is a cratered world, rather like our own moon. Ganymede, the largest moon in the solar system, is covered with strange and unexplained grooved geological formations. Io, the innermost satellite of the giant planet, is a seething cauldron of sulphur volcanoes. The volcanic activity is driven by the immense tidal forces experienced by Io as it swings round its parent. The second moon, Europa, looks, superficially, the least interesting of the four. Slightly smaller than our own moon, billiard-ball smooth, no mountains, few craters, dusted with yellow sulphur squirted across space by nearby Io. But in the Seventies, when spectroscopic analysis by Earth-based telescopes showed that the surface of Europa was made of almost pure ice, astronomers - and science fiction writers - got very excited indeed.
In 1979, the Voyager space probes took spectacular photographs of the whole Jupiter system, including Europa. Close up, Europa looked very odd. No mountains, but instead strange dark ridges hundreds of kilometres long and only a few hundred metres wide. Areas that looked like crazy paving, and, significantly, very few craters, showing that the surface of Europa was very young. Then, in December 1995, Nasa's Galileo probe arrived at Jupiter after a six-year journey. In a series of orbits around the giant planet, the Galileo probe has swung in close around each of the four main satellites, taking extremely detailed and close-up pictures. Two weeks ago Galileo started on its closest-yet pass of Europa. The pictures taken by its cameras will take many weeks to be converted into digital form, transmitted across space to Earth and decoded. Meanwhile, scientists are analysing pictures taken in February 1997, and in an article in last month's Nature magazine, four groups of researchers in the US argued that theses images show that under the ice layer is a huge, Europa-wide liquid water ocean, tens of kilometres deep. Europa is too far from the Sun for liquid water to exist normally - the temperature at the surface rarely struggles above a perishing -150C - so something must be melting the ice layer from underneath.
Michael Carr, a scientist with the US Geological Survey in California, has studied the latest pictures from Galileo, which, to the untrained eye, look like aerial photos of the Arctic Ocean. Parts of the surface seem to be covered with "icebergs", huge blocks of ice several kilometres across, standing hundreds of metres proud of a liquid or slushy layer underneath. These icebergs - if that is what they are - seem to have moved relative to one another. It is quite easy in places to reconstruct the surface, putting the blocks back together like a huge, icy jigsaw puzzle. Carr thinks it is likely that under an ice layer, maybe only a couple of kilometres thick, there is an ocean of liquid water, and convection currents in this ocean are driving the ice-rafts along.
"I think the chances are pretty good that there's liquid water down there ... you could be looking at an 80-mile deep ocean," he said. Sceptics have argued that the movement in the ice could be explained by "solid-state convection". Even solid ice will flow, given enough pressure and enough time, and heat rising from below could fuel this movement, just as the solid crust of the Earth is able to slide and flow over millions of years. Some scientists argue that the Europan "ocean" is frozen solid to the rocky surface below. But Carr is not convinced by this explanation. "I am very, very sceptical about whether this could be explained by solid- state convection. These blocks drifted around the surface and appear to be plunging into the surface. Water appears to have crept up on top of the blocks and splashed around." Some of the most intriguing pictures from Galileo show what Carr describes as a "puddle" - a small, very dark, very smooth area. Could this be an area of open water on the surface, a tiny "window" to the vast ocean below? Any liquid water would freeze instantly, but it is possible that warm currents from below could melt through the surface, causing water and vacuum to come into an uneasy coexistence.
Further evidence for a liquid ocean comes from a team at the University of Arizona which has discovered that Europa, or at least its surface, spins slowly in its orbit around Jupiter. Normally, a moon that small orbiting such a large planet would be "locked" by its gravity, so the same side always faced its giant neighbour. This is why we always see the same side of our own Moon in the night sky. But Paul Geissler and his colleagues have found that the surface of Europa is not locked to face Jupiter. Instead, it slowly turns, once every 10,000 years or so. This can only be explained if the surface is able to flow, en masse, over the rocky interior. "We have found evidence that the crust is mechanically decoupled from the interior, just as it should be if it is underlain by an ocean," Geissler said.
Water is essential for life on Earth, and it is hard, though not impossible, to imagine life evolving in its absence. If there are oceans of the stuff on Europa, this little moon might be the best place to start looking for alien life.
John Delaney, an oceanographer based in Seattle, is an enthusiast of the idea that Europa is covered by a vast, hidden ocean kept warm by immense undersea volcanoes - an ocean very probably host to living beings. "The bottom line is there's growing evidence that there is liquid water beneath the ice. If one interprets the data in a straightforward way, then the ice crust is only 2 to 4km thick. If the ice is only that thick, then there has to be a great deal of water below it - maybe 100km deep. I do not believe that it is far-fetched that there is water on this planet, and that the combination of vulcanism and water could produce life within half a million years. I believe that is a very viable proposition for why life came into being on this planet. In the Earth's early years, the dominant show in town was basaltic volcanoes under water. There was tremendous energy, new compounds being formed when 1,200-degree lava came into contact with water ... If there is anyplace else in the solar system likely to have life, it is Europa."
But what sort of life? This is not, Delaney says, going to be something that we could wave at - or at least it would be unlikely to wave back. On Earth's deep-sea floors, whole ecosystems flourish around the volcanic vents that mark the edges of the crust's tectonic plates. Until recently, these places were as mysterious as the moons of Jupiter. Now, in parallel with the exploration of outer space, submarines are exploring Earth's inner space, and the discoveries have astounded scientists. Deep on the Pacific floor, colonies of life exist clustered around the volcanic vents known as black smokers. The lava and dissolved gases brought to the ocean floor are rich in nutrients, and sulphur-gobbling bacteria live and die in huge numbers. These form the basis of a geothermal food chain of plants and animals, quite separate from their solar-powered cousins on the land and in the ocean shallows. There are giant worms, metres long. Strange, bioluminescent eyeless fishes. On Europa, there may be something similar.
We will not know if there is water on Europa for sure until we send an orbiting probe capable of taking radar soundings through the ice, one is due to be launched in 2003. If the suspicions of scientists are confirmed, and there is water there in huge quantities, Nasa is likely to push for funding for an unmanned lander mission.
Getting to the Europan surface is only half the battle, though. Unless places can be found where the frozen crust is very thin indeed, or even non-existent, getting to the ocean will involve drilling through hundreds of metres of solid ice. Any mission to land on Europa will be expensive - far more expensive than the recent Pathfinder mission to Mars, and supporters of a Europa mission are competing for federal funds with a team designing a mission to Pluto.
Jim Cutts, a special projects engineer at Nasa, thinks Europa should be a priority. He is working on a probe that would be able to carry out this fantastic task. Nicknamed "Cryobot", the probe will land on Europa's surface and slowly melt its way down through the ice, using a radioactive heat source. As the probe descends, the hole will refreeze behind it. Eventually, all being well, the Cryobot will plop through the bottom of the ice layer and cruise around the ocean depths below, analysing the chemistry of the water, looking for life and sending back no-doubt bizarre underwater photographs.
"It's actually very simple, in principle," Cutts says. "The whole probe will sink through the ice and keep in contact with the surface - and Earth - by radio. There will be no cable connecting it to the surface, it will be completely self-contained." Adding that because of Europa's low gravity - only one seventh that of Earth - a probe could drill much deeper than expected. "The gravity is low, so the pressure is low. Maybe we could drill through 25km or more."
Fortunately, it is possible to test the technology for an ice-drilling probe here on Earth. Twenty years ago, a huge lake was discovered deep below the Antarctic ice cap. Lake Vostock, as it was named, is completely inaccessible. Nasa is involved with a project that will, hopefully next year, see a small probe burying its way through thousands of metres of Antarctic ice to reach the lake. If this is successful, it will pave the way for a Europa mission to be launched around 2015.
As far as finding life is concerned, Cutts says that task is actually quite simple. "We don't need complex analysis of DNA or anything, just something capable of detecting the simple chemicals produced by bacteria." The hardest thing, he says, will be to ensure that the probe is properly sterilised before it leaves Earth. Many scientists fear that some of the Mars probes sent by the USSR in the Sixties, with their cotton flags and Brezhnev autographs, may have introduced terrestrial micro-organisms to the red planet. In a liquid environment, contamination would be much easier. It would be a tragedy if the human race were to find life on an alien world, only to wipe it out with an accidental plague.Reuse content