Fishing for aliens

There could be life in Europa's ocean - if we can get to it, writes Matthew Genge

Water, simple H20, is the fluid of choice for life. Just about everything that goes on in living organisms goes on in water - billions of complex chemical reactions all happening submerged. Not surprising then that on Earth the oceans hide a swarming proliferation of life from the simplest of bacteria to the aquatic mammals with their complex social existence.

It is in another deep blue sea, on Jupiter's moon Europa, that scientists, therefore, hope to find life beyond the Earth, and last week that search took its first important step when the Nasa Europa Orbiter project was endorsed as one of the agency's flagship missions. The most recent results, however, suggest that even if Europa is the most likely haven for extraterrestrial life, it may also be the most difficult place to find it.

The very existence of its ocean came as a complete surprise. Europa is too small to keep its heat and should, therefore, be cold and geologically dead. However, images of Jupiter's satellite sent back by spacecraft lead us to suppose that Europa is anything but a ball of ice. The rarity of craters on Europa's surface, compared with the pockmarked moons Ganymede and Callisto, suggests that these scars have been removed by the eruption of water from below the surface. More telling, however, are areas with crenulated rafts of ice, looking like planetary crazy paving, which strongly resemble images of sea ice on Earth and so betray the ocean that lies beneath.

The presence of an ocean on Europa, it turns out, is due to its stressful relationship with the planet Jupiter. Europa stretches in and out like a rubber ball as it circles the giant planet because one side of the moon, closest to Jupiter, is pulled harder by the planet's gravity than the other.

Since the path followed by Europa takes it sometimes nearer and sometimes further away from Jupiter, the moon's shape wobbles as it circles the planet. It is this constant stretch and release exercise that heats up Europa's interior and prevents its ocean from freezing. Calculations suggest, in fact, that enough heat is generated to maintain an ocean around 60km deep, 20 times the average depth of those on Earth.

Simply the presence of an ocean on Europa, however, does not mean that life evolved beneath its surface. Without the right chemical building blocks for life, such as carbon, sulphur and phosphorous, and an appropriate source of energy, Europa's ocean would be more barren than any desert on Earth.

Recent calculations, therefore, present a real problem for life on the icy satellite since they suggest that shortly after it formed, it was highly deficient in the chemicals needed to build living things. The same, nevertheless, was also true of our own planet and yet life is here in abundance.

In the case of the Earth, it is thought that components important to life were added by the collision of comets. Last month, new research showed that despite its small size, which would allow much of the debris from a colliding comet to be blasted back into space, enough should be retained to ensure that Europa has the right stuff for life.

So what about an energy source? On Earth, sunlight ultimately provides the energy for most living things. On Europa, however, the sunlight, which is already weak, would only penetrate several metres into the ice and will not illuminate the deep oceans. In places on the bottom of our planet's oceans, however, life flourishes in the absence of light and gets its energy and nourishment from mineral-rich hot water vents. Such vent communities might also exist in Europa's oceans if geothermal vents, produced by Jupiter's heating of the moon's stony interior, issue from the base of its ocean.

The results of the latest research on Europa, published in the journal Science, have, however, delivered the cruellest blow, if not to life on the satellite, certainly to our chances of finding it. Dr Paul Schenk, of the Lunar and Planetary Institute, Houston, has discovered that what craters are present on Europa are completely the wrong shape. Those larger than 25km in diameter should have lofty central peaks but instead, they are disappointingly pancake-flat. The reason, Schenk believes, is that bigger craters have punched all the way through the icy crust to the ocean below which, not surprisingly, has flowed out to fill up the holes. The craters thus tell us how thick the icy crust of Europa is - 19km.

Nasa had planned to explore Europa using robotic probes that would melt their way down through the icy crust to search for life in the ocean below. This Eskimo "hole in the ice" trick, however, was enough of a technical challenge when the icy crust was thought to be 1km thick; at 19km, it's probably entirely unfeasible. Evidence for life might be found, however, when the Nasa Europa Orbiter takes a close look at the surface in 2010, since the water from the ocean that has filled craters and erupted elsewhere on the surface may have carried tell-tale signs of life with it. The orbiter may even locate some thin ice, for example close to the huge fractures caused by Europa's stretching, where a probe could reach the ocean.

For now, however, it seems that Europan cod and chips will be off the menu for some time to come.