Page last updated at 09:27 GMT, Thursday, 1 April 2010 10:27 UK

Rumbling Europa's hidden secret

By Laura Mulholland
Assistant Producer, Wonders of the Solar System

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The wonder of Jupiter's Galilean moons

It's not only one of the most important questions for science; it's one of the great unanswered questions in human history.

We've been peering into the night sky for thousands of years, wondering if we are alone.

As technology has improved, we've been able to look in more and more detail, revealing worlds more bizarre and more beautiful than we could ever have imagined. The harder we search, the closer we get to answering the fundamental question - are we alone in the Universe?

And that's why in the final episode of the BBC's Wonders of the Solar System series, we go in search of alien life.

There is a very strong possibility that the surface ice of Europa may contain viable living microorganisms
Dr Richard Hoover, Nasa astrobiologist

The first challenge is knowing what to look for. What are the key ingredients for life? How does it spring into being and how does it survive?

These are some of the questions we wanted to explore in this episode. To try to find the answers, scientists have turned to the only form of life they know of - life on Earth.

Our planet is teeming with living things. They've spread to every corner of the globe and can survive in the most surprising places.

But no matter how extreme and varied, all the places where living things are found hold one thing in common - liquid water. On Earth, without water there is no life - and there are no exceptions.

This fundamental link between water and life is driving the search for life out in the Solar System. If we can find water on another world, perhaps we'd also find living organisms.

But not just any water will do. Water vapour swirls in Jupiter's atmosphere, Mars has frozen polar caps, and ice was recently discovered on our Moon - but it is thought that none of this is of any use to living things, because life needs liquid water.

Voyager - Artist's impression (Nasa)
The Voyager spacecraft returned fascinating images of Europa

Understandably, for many years the distant planets out in the frozen depths of space were not considered potential homes for life. With surface temperatures hundreds of degrees below zero, water would be frozen hard as rock. But when the twin Voyager spacecraft reached Jupiter in 1979, that all changed.

As they swung past the giant planet they began to take pictures of Jupiter's moons.

The BBC Wonders team travelled to Iceland's cold and barren interior to tell the story of Europa - the moon with a hidden secret.

Series presenter Professor Brian Cox remembers the journey well: "We travelled in these incredible machines, heavily modified 4x4s with wheels as big as people! You need them to get across the ice. It was a proper wilderness and you got a real sense of isolation."

The Voyager images had revealed a world like no other. Europa was covered in a vast network of ridges and cracks criss-crossing its frozen surface.

And the reason for the cracks is intriguing. "Europa orbits Jupiter in what's called an eccentric orbit, so it spends some time close to Jupiter and some time further away," explains Prof Cox.

"And that means that Jupiter's gravitational pull on Europa is constantly changing, so it's stretched and squashed."

Europa surface (Nasa)
The cracked ice shell of Europa shifts about

The regular stretching and squashing causes Europa's icy surface to buckle and crack in a predictable pattern. But more detailed images revealed that the cracks were not where scientists had expected them to be.

As time went by the cracks were shifting across the face of Europa. The only explanation for this was that an icy shell of Europa was rotating independently of the moon's rocky core. And the only way it could do that was if there was a layer of liquid water between the ice and the rock.

Measurements of Europa's fluctuating magnetic field have confirmed that there is a layer of liquid water under the ice of Europa that may be 100km deep. And if that is the case there could be twice as much liquid water on Europa as there is on Earth.

But with a surface temperature of -160 Celsius how can water on Europa possibly be liquid?

The answer lies in its eccentric orbit. As Europa is stretched and squashed by Jupiter's changing gravitational pull, it heats up - just like a squash ball - and that produces enough heat to melt the ice beneath the frozen surface.

The presence of liquid water makes Europa an exciting prospect in the search for alien life, but the technology required to drill through the thick icy shell and reach the liquid ocean could be centuries away.

Yet some scientists believe we won't have to look that far. They're confident that life can be detected on the moon's icy surface.

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Ice cave microbes offer clues to alien life

Nasa astrobiologist Dr Richard Hoover travelled to Iceland with the film team.

"Within the last 20 or 30 years our knowledge and understanding of the possibility of life in ice has been changing tremendously. We have gone from thinking that ice is absolutely sterile to becoming aware of the fact there is an enormous amount of biology in ice," he told us.

EUROPA JUPITER SYSTEM MISSION
Ganymede Orbiter (Nasa/Esa)
Nasa: Jupiter Europa Orbiter could launch on Atlas rocket in 2020
Esa: Jupiter Ganymede Orbiter (above) lofted by an Ariane
Probes use Venus gravity assist to arrive six years later
Orbiters conduct joint observations at other Jupiter moons
Would finally settle into orbits around dedicated targets
Studies will focus on Europa's and Ganymede's interiors
End destructions will allow unique measurement opportunities

Micro-organisms that originate in liquid water have been found to adapt to a new life in ice when their watery home freezes. "Some micro-organisms melt the ice by generating anti-freeze proteins. These antifreeze proteins work very much like the antifreeze in your automobile," Richard explains.

The ice caves beneath the Vatnajökull Glacier in Iceland provided the perfect example of a water-ice interface here on Earth, pristine and untouched for a thousand years. Yet in the heart of this ancient ice, Richard and Brian found micro-organisms not just living but thriving.

As Richard Hoover explains, it's a discovery that can have implications for the hunt for life on Europa.

"You can clearly have bacteria like this living in the frozen ice near the surface crust of Europa," he argues. "There is a very strong possibility that the surface ice of Europa may contain viable living microorganisms."

Europa is such a strong candidate in the search for life that a joint venture between Nasa and Esa is already underway to study the moon in more detail.

The Europa Jupiter Systems Mission (EJSM) will see a spacecraft sent into orbit around Europa to further investigate its potential to support life.

This tiny, distant moon is now the most likely place to find life beyond our planet, making it a true wonder of the Solar System.

Wonders of the Solar System, presented by Professor Brian Cox, continues on BBC Two on Sunday (2100 GMT).



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