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We live on a world of wonders.

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A place of astonishing beauty and complexity.

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There are vast oceans and incredible weather.

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Giant mountains

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and stunning landscapes.

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I'm a physicist, and I'm fascinated by the way that the universal laws of nature

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that made all this, also created such diverse and different worlds

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out there in the solar system.

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I think we're living through the greatest age of discovery our civilisation has known.

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We've voyaged to the farthest reaches of the solar system.

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We've photographed strange new worlds,

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stood in unfamiliar landscapes, tasted alien air.

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But the one thing we haven't found on those worlds

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is the thing that makes our planet unique.

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Life.

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But is that really true?

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Is the Earth the only place in the solar system that could support life?

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In this film we will search the solar system for worlds that harbour the conditions to support life.

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What we find on these worlds may help us answer the question, are we alone in the universe?

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That's not only one of the great fundamental questions for science,

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but one of the great unanswered questions in human history.

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Floating in the Sea of Cortez off the cost of Mexico is the research vessel Atlantis,

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the mother ship for the exploration of one of the most alien worlds we know.

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But it's an alien world on our planet.

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The Atlantis is the launch vessel for Alvin,

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one of the world's most rugged submarines.

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Built like a spacecraft,

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it's designed to explore the deepest depths of the ocean.

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And I'm lucky enough to have hitched a ride down to the sea floor,

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two kilometres beneath the surface.

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That has got to be the closest thing to going into space that you can do.

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And, given that I'm not going to go into space any time soon, I think it's the next best thing.

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See you in eight hours.

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Roger, Alvin. Your checks are good. Permission to dive.

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Roger. Alvin diving.

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The parallels to spaceflight are obvious.

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As the tiny capsule descends, we are leaving the familiar world

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of the surface of our planet, and entering a strange, hostile world.

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If anything goes wrong, we will be completely on our own.

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MACHINE BEEPS

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Beeping is never good.

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'Fortunately, Alvin is one of only a handful of submarines

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'that can withstand the colossal pressure of the deep ocean.'

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At the Earth's surface, we're used to one atmosphere of pressure.

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As we descend, the pressure increases by another atmosphere every ten metres.

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And it soon adds up.

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We're approaching a kilometre deep. The pressure outside there is now

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100 atmospheres, that's higher than the atmospheric pressure on the surface of Venus.

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Without knowing, if you were asked a question, could life exist down here, 100 atmospheres,

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cold, dark no sign of sunlight at all,

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it's pitch black there, you would say no.

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Well, I would say no.

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But the depths of the ocean are not lifeless.

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Illuminated by Alvin's lights,

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we find oases of life in the deserts of the ocean floor.

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So we have landed, after about an hour of descent.

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We've just stopped in the most incredible place.

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Look at those.

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We've landed on top of the tube worms.

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Amazing things.

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This underwater city is one of the most bizarre environments on our planet.

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It's built around a hydrothermal vent, a volcanic opening in the Earth's crust

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that pumps out clouds of sulphurous chemicals and water heated to nearly 300 Celsius.

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And somehow, life has found a way to thrive in these most extreme conditions.

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This is a genuinely remarkable place.

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There are mats,

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carpets of yellow bacteria.

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Look at that. It's not only just bacterial blobs,

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there is real complex organisms.

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Alien. I want to say that word, alien environment.

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It really is alien to us.

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For me, the fascinating thing about finding life down here

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is that the conditions on the

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deep ocean floor are more similar in many ways to the conditions on

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worlds hundreds of millions of kilometres away out there

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in the solar system than they are to the conditions just two kilometres from my head on the Earth's surface.

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It's incredibly dark, there is no sunlight,

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there's a brutal mixture of hot and cold water,

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and just rock and minerals.

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So, if life can not only survive but even flourish in these conditions,

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then you've got to feel that it's much more likely that life can

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also survive and flourish out there in the solar system.

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Ever since the invention of the telescope 400 years ago,

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we have looked to our neighbouring worlds for signs of life.

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As technology has improved, we've been able to search the planets in more and more detail,

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and we have found nothing.

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But that doesn't mean the rest of the solar system is dead,

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because we're only beginning to scratch the surface of what's out there.

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There are literally hundreds of other worlds.

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Planets and their moons which we have barely explored.

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Among them may be worlds that hold the conditions to support life.

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And the best way to find out what those conditions are

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is to look at the one place we know life flourishes.

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The Earth.

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Life is pretty much only chemistry.

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It's just the reactions between atoms and molecules.

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And so for life to exist, you only really need three things.

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First of all, you need the right chemistry set.

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Now, I'm made of something like 40 elements,

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almost half of the known elements, which is pretty complicated.

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But actually 96% of me is only made of four of them, carbon, nitrogen, oxygen and hydrogen.

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Secondly, you need a power source.

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You need a battery, something to make a flow of electrons

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that powers the processes of life.

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Now here on Earth, most life uses the power of the sun.

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And thirdly, you need some kind of medium for life to play itself out in,

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for processes to happen.

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And here on Earth,

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you don't have to look very far at all to find that medium, that solvent.

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Because it's this, water.

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If you want to see how important water is to life,

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there's no better place to come than the Atacama desert in Chile.

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The soil here is more sterile than a hospital operating theatre.

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In fact, scientists have looked for the most basic form of life, bacteria,

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in some parts of the Atacama, and they found absolutely nothing.

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All deserts are characterised by a lack of moisture.

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But the Atacama takes that to the extremes.

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The Sahara is 50 times wetter than the Atacama.

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There are weather stations here that have measured 1mm of rainfall in 10 years.

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There are river valleys that have been dry for 120,000 years.

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There are rocks that haven't seen rainfall for 20 million years.

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It's this dryness that explains why nothing can survive here.

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Even the most primitive form of life on Earth, the bacteria,

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need water for their survival.

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And there are no exceptions.

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And this seemingly fundamental link between water and life

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is driving the search for life out there in the solar system.

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Because, wherever we find water,

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that will be the best place to look for life beyond the Earth.

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The Earth is the only planet that currently has liquid water on its surface.

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The other planets are either too close to the sun,

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like Mercury, and baked dry.

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Or they are too far away.

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Saturn's rings are made of water, but in the depths of space, it's frozen into lumps of solid ice.

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But that doesn't mean that liquid water has never existed elsewhere in the solar system.

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And if it has, we should be able to find the evidence,

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because wherever water goes, it leaves its footprints.

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These are the Scablands, a remote part of the North Western United States.

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It's one of the most spectacular places to come to see how water

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carves its signature into the landscape.

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The largest flood on Earth went through this area here.

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Jim Rice is an astro-geologist. He believes that understanding the events that created this landscape

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can help in the search for water on other planets.

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We are kind of like CSI arriving at the scene of a crime, this is the evidence left here.

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We've come to piece it together. I can see this is not a normal river system.

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You can see, because it is so straight.

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There is no meandering of a river here, it's just a big hole.

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This entire landscape was created at the end of the last Ice Age.

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200 miles to the east lay a huge lake, held in place by a wall of glacial ice.

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When that wall ruptured, over 2,000 cubic kilometres of water swept out in a single catastrophic event.

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The flood waters were at least 400 feet deep here.

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But actually they were another 200 feet stacked on top of that, coming across here.

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So we would be under 200 feet of water standing right here.

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So am I to imagine a wave?

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Yeah, a massive wave rolling, rumbling, this water would

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be charged full of big chunks of ice from that ice dam.

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It would be loaded with big chunks of the salt bed rock being gouged, ripped out of here.

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It would be an impressive sight.

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As the floodwaters tore across the landscape, they carved out this 20 mile long canyon.

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And at its head, it left these giant horseshoes.

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At over 400 feet high and five miles across,

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this was the largest waterfall the world has ever known.

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The easiest way of thinking about it is if you took every river in the world, put them in

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the same location, had them flowing at the same time, these floods are 10 times larger than that.

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And how long do we think it took to sculpt this landscape?

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48 hours to a week.

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It's instantaneous, geologically.

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The Scablands reveal the characteristic signature

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that water carves into the landscape.

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It's a signature that can be seen from space, and not just on the Earth.

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When we turn our telescopes on our next door neighbour and prime candidate for finding

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alien life, the planet Mars,

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we find almost identical features cut into its surface.

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The Red Planet is covered in outflow channels.

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Straight, wide canyons, exactly like the Scablands.

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And they are filled with identical geological features.

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It all suggests that similar huge floods once tore across the surface of Mars.

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This is a picture of here from the air.

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I am sat somewhere around here.

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And here are the horseshoe shapes of the dry folds which are just over there.

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This is a picture taken of the surface of Mars,

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and you see those typical horseshoe shapes of the folds.

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Also, you see the structures upstream of the folds, these grooves cut into the landscape.

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And you see that here, grooves cut into the landscape as the water

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cascades down and then flows over the folds

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and cuts the gigantic valleys out as it moves downstream.

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So, all this adds up, I think, to an overwhelming smoking gun

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that there were vast amounts of water that flowed very quickly

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over the surface of Mars at some point in the past.

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But although the outflow channels are proof that liquid water once flowed across Mars,

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it may not point to the existence of life.

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Because if the Martian landscapes were formed by the same processes that formed the Scablands on Earth,

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the floods that created them may only have lasted a matter of days.

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For life to get a foothold, you need more than that.

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You need areas of standing water.

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Lakes and rivers that persist for millions of years.

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In order to look for evidence of that standing water, we've done the only thing we can,

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we have sent an army of robotic explorers to the surface of the planet.

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We have touch down, we have touch down.

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Over the last 35 years,

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we've landed six robot probes on Mars.

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And one of them, Opportunity, is still rolling across the surface, investigating the Martian geology.

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The Mars rovers has really captured our imaginations.

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I suppose, because they genuinely are explorers in the old-fashioned sense.

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They are the extension of our senses to the surface of another world.

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But they have also been very important scientifically, because

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you can't really get to know another planet from orbit.

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You have got to get down to the surface, you've got to touch it,

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you've got to dig down and examine it microscopically.

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And the Rovers really have, by doing that,

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made some extremely important scientific discoveries.

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One of the most significant of those discoveries was made in November 2004.

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The Opportunity rover was examining an impact feature called the Endurance crater,

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when it detected deposits of a remarkable mineral.

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This is the world's largest salt works on the Baha peninsula in Mexico.

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And what they do here is pump sea water into these lagoons and let it evaporate.

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What they're after is this stuff, which is sodium chloride, table salt.

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But, at different stages, different salts, different minerals, crystallise out.

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So all the things really that are in sea water emerge, crystallise out

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at different stages of the process.

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In one of the lagoons, pond number nine, the sea water is at exactly the right concentration

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to precipitate out these beautiful crystals that cover the entire floor of the lagoon.

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This is gypsum,

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and it's exactly the same stuff that Opportunity found on the surface of Mars.

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Now, what's interesting about that discovery is how you make gypsum.

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You see, its chemical formula is CaSO4.

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So it's calcium sulphate.

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Dihydrate, 2H2O.

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That's water.

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So, the only way we know of, the only way to make gypsum here on Earth, is to have calcium

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and sulphate ions in the presence of liquid water.

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So, large deposits of gypsum on the surface of Mars tells you

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that there must have been big areas of water present for a very long time.

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The discovery of gypsum has helped to build a picture

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of an ancient Mars that was much warmer and wetter.

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Subsequent discoveries of gypsum in networks of sand dunes

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suggest that large areas of Mars were once covered in standing water.

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And where there is standing water, there is the chance of life.

239
00:24:37,720 --> 00:24:40,320
This area of the salt flats is, we think,

240
00:24:40,360 --> 00:24:44,520
very similar to areas that have been seen on Mars.

241
00:24:44,560 --> 00:24:48,080
And it certainly looks extremely inhospitable.

242
00:24:48,120 --> 00:24:51,640
It's hard at first sight to see how anything could live here.

243
00:24:51,680 --> 00:24:54,640
But, if you just dig

244
00:24:54,680 --> 00:24:56,880
a tiny bit below the surface,

245
00:24:56,920 --> 00:25:02,840
then you see that this layer of gypsum is only a few millimetres thick,

246
00:25:02,880 --> 00:25:07,520
and then immediately the ground beneath it turns this greeny colour.

247
00:25:07,560 --> 00:25:14,320
It's green because that is bacteria that thrive in these seemingly inhospitable conditions.

248
00:25:14,360 --> 00:25:17,160
Now if these bacteria can survive here,

249
00:25:17,200 --> 00:25:24,160
then there seems to be no good reason why they couldn't also have survived and even flourished on Mars

250
00:25:24,200 --> 00:25:28,880
when there was water present at some point in the very distant past.

251
00:25:33,440 --> 00:25:36,800
But although it may once have been more hospitable,

252
00:25:36,840 --> 00:25:41,760
any liquid water has long since disappeared from the surface of Mars.

253
00:25:44,640 --> 00:25:50,200
About three billion years ago, it died as a planet.

254
00:25:50,240 --> 00:25:56,120
Its core froze and the volcanoes that had produced its atmosphere seized up.

255
00:25:59,080 --> 00:26:03,760
The solar wind stripped away the remains of that atmosphere.

256
00:26:03,800 --> 00:26:06,760
Any liquid water would have evaporated

257
00:26:06,800 --> 00:26:10,000
or soaked into the soil where it froze.

258
00:26:11,120 --> 00:26:15,320
It left the surface of Mars too cold, too exposed

259
00:26:15,360 --> 00:26:18,200
and too dry to support life.

260
00:26:24,520 --> 00:26:29,320
It's highly unlikely that there will be life on the surface of Mars today.

261
00:26:29,360 --> 00:26:33,760
But that's not to say that life couldn't exist somewhere on the Red Planet,

262
00:26:33,800 --> 00:26:36,680
maybe we're just looking in the wrong place.

263
00:26:39,040 --> 00:26:42,680
There are other potential habitats for life on Mars.

264
00:26:44,520 --> 00:26:49,360
Detailed pictures of the surface show the entrances to caves,

265
00:26:49,400 --> 00:26:53,800
revealing the existence of a world beneath the Martian surface.

266
00:26:57,840 --> 00:26:59,880
We know there may be water down there.

267
00:26:59,920 --> 00:27:06,000
Satellite data shows permafrost, ice frozen in the soil.

268
00:27:06,040 --> 00:27:11,320
Deep below the surface, that ice may melt to form liquid water.

269
00:27:16,840 --> 00:27:21,000
It all hints at an undiscovered subterranean world

270
00:27:21,040 --> 00:27:25,480
that may be a more likely place to find life.

271
00:27:46,680 --> 00:27:50,520
If you were to imagine the perfect habitat for life,

272
00:27:50,560 --> 00:27:53,840
then it would surely be somewhere like this.

273
00:27:53,880 --> 00:27:55,920
A warm climate, lots of liquid water,

274
00:27:55,960 --> 00:28:00,200
a beautiful, dense atmosphere.

275
00:28:00,240 --> 00:28:04,920
You see the results everywhere, just life everywhere you look.

276
00:28:07,360 --> 00:28:11,400
All the life we're familiar with thrives in pretty much the same

277
00:28:11,440 --> 00:28:17,120
conditions that we do, driven by the heat and light of the sun.

278
00:28:17,160 --> 00:28:20,280
But this is by no means the only life on Earth.

279
00:28:24,800 --> 00:28:29,520
There's another living planet hidden beneath the surface

280
00:28:29,560 --> 00:28:32,480
that exists in completely different conditions.

281
00:28:32,520 --> 00:28:38,000
It raises fascinating possibilities for the caves on Mars.

282
00:28:46,800 --> 00:28:49,760
This is the Cueva de Villa Luz in Tabasco, Mexico,

283
00:28:49,800 --> 00:28:51,600
the Cave of the House of Light.

284
00:28:51,640 --> 00:28:56,320
And it is the definition of a hostile environment to me.

285
00:28:56,360 --> 00:29:01,000
Because (HE SNIFFS) it's full of hydrogen sulphide gas, hence

286
00:29:01,040 --> 00:29:07,600
the gas monitor which says at the moment one part per million hydrogen sulphide, very toxic for me,

287
00:29:07,640 --> 00:29:12,480
which is why I have got this gas mask in case it all gets too much.

288
00:29:12,520 --> 00:29:15,960
So, it's a place where you, at first sight,

289
00:29:16,000 --> 00:29:21,280
would not expect a great many life forms to survive and flourish.

290
00:29:49,520 --> 00:29:55,440
Although the cave is a death-trap for us, that doesn't mean that nothing lives here.

291
00:29:55,480 --> 00:29:57,720
In fact, it's teeming with life.

292
00:30:00,760 --> 00:30:05,560
Look at these fish, just everywhere in the cave water. And they're

293
00:30:05,600 --> 00:30:08,560
adapted to live in these conditions.

294
00:30:08,600 --> 00:30:10,680
In fact, if you look at them closely,

295
00:30:10,720 --> 00:30:12,480
they're quite pink.

296
00:30:12,520 --> 00:30:16,040
That's thought to be because they've got lots of haemoglobin

297
00:30:16,080 --> 00:30:18,560
because there's not much oxygen down here,

298
00:30:18,600 --> 00:30:23,680
so they need to have an efficient way of moving oxygen around their bodies.

299
00:30:23,720 --> 00:30:25,200
Beautiful.

300
00:30:31,120 --> 00:30:35,520
But the really interesting life is found in the depths of the caves,

301
00:30:35,560 --> 00:30:41,360
where the concentration of poisonous gas is high enough to set off my alarm.

302
00:30:42,920 --> 00:30:45,640
Down here, far from the light of the sun,

303
00:30:45,680 --> 00:30:50,160
are organisms whose energy source comes from the air around them.

304
00:30:52,480 --> 00:30:58,280
They use the hydrogen sulphide gas bubbling up through these springs.

305
00:30:58,320 --> 00:31:02,520
The same gas that could be fatally poisonous to me

306
00:31:02,560 --> 00:31:04,640
is their source of life.

307
00:31:15,120 --> 00:31:18,480
These things are what I came deep underground to see.

308
00:31:18,520 --> 00:31:24,000
These are snottites. And you can see why they're called that.

309
00:31:24,040 --> 00:31:29,920
They're really one of the most alien life forms that I can conceive of

310
00:31:29,960 --> 00:31:31,080
on the Earth

311
00:31:31,120 --> 00:31:36,440
Because they metabolise hydrogen sulphide, so they metabolise this

312
00:31:36,480 --> 00:31:40,680
faintly acidic and nasty gas that I'm just breathing in now.

313
00:31:40,720 --> 00:31:45,440
You can almost feel it on your tongue, actually, the acidity of it.

314
00:31:45,480 --> 00:31:50,880
They metabolise it, they react it with oxygen, and they produce sulphuric acid.

315
00:31:50,920 --> 00:31:55,800
So their breathing process, if you like, their version of what I do,

316
00:31:55,840 --> 00:31:59,760
I breathe in oxygen, react that with sugars and breathe out CO2 and get energy

317
00:31:59,800 --> 00:32:05,560
these guys breathe in hydrogen sulphide and oxygen and produce sulphuric acid.

318
00:32:05,600 --> 00:32:07,840
In fact, I can test it here with this.

319
00:32:16,200 --> 00:32:18,800
Yes, you see, look at that.

320
00:32:18,840 --> 00:32:24,840
That, well, what looks like water, that secretion of dripping off the snottites, has actually got a pH...

321
00:32:24,880 --> 00:32:29,080
well, it's now about between 0.5 and 0.

322
00:32:29,120 --> 00:32:30,840
That's strong acid.

323
00:32:30,880 --> 00:32:33,520
That's as strong as battery acid.

324
00:32:33,560 --> 00:32:37,880
It's actually highly concentrated sulphuric acid.

325
00:32:37,920 --> 00:32:41,240
So, what a strange organism.

326
00:32:41,280 --> 00:32:43,840
Alien in every sense of the word.

327
00:32:43,880 --> 00:32:48,480
Except that it's present on, well, just below the surface, of our planet.

328
00:32:52,360 --> 00:32:55,080
And the snottites are not alone.

329
00:32:55,120 --> 00:32:59,240
Organisms that can extract energy from the minerals around them

330
00:32:59,280 --> 00:33:01,680
are found under the ground all over the world.

331
00:33:03,960 --> 00:33:07,960
In fact, this way of life is so successful that it's thought there

332
00:33:08,000 --> 00:33:13,880
may be more life living beneath the Earth's surface than there is on it.

333
00:33:15,240 --> 00:33:18,680
And that raises an intriguing possibility.

334
00:33:18,720 --> 00:33:22,760
If life can thrive below the Earth's surface,

335
00:33:22,800 --> 00:33:27,200
why couldn't organisms like snottites survive and flourish

336
00:33:27,240 --> 00:33:29,400
beneath the surface of Mars?

337
00:33:33,880 --> 00:33:37,080
If you think about it, living below the surface of Mars might actually

338
00:33:37,120 --> 00:33:41,320
be quite a good idea, because the surface is incredibly hostile.

339
00:33:41,360 --> 00:33:45,200
It's subjected to intense ultraviolet radiation from the sun.

340
00:33:45,240 --> 00:33:48,920
It's a very cold place, and the atmospheric pressure doesn't

341
00:33:48,960 --> 00:33:51,960
allow liquid water to exist on the surface.

342
00:33:52,000 --> 00:33:57,840
But, if there is life below the surface of Mars, then obviously we have a problem.

343
00:33:57,880 --> 00:33:59,800
How could you possibly detect it?

344
00:33:59,840 --> 00:34:03,640
Well, actually, there is a perhaps tantalising clue that

345
00:34:03,680 --> 00:34:10,480
there might be something interesting going on below the Martian surface.

346
00:34:21,040 --> 00:34:25,320
These are termites, or white ants.

347
00:34:25,360 --> 00:34:31,760
And they're very unusual animals because they eat wood.

348
00:34:31,800 --> 00:34:33,400
This is their food.

349
00:34:33,440 --> 00:34:39,960
There are many, many species of these, billions of individuals across the planet.

350
00:34:40,000 --> 00:34:45,520
And, in the process of digesting wood, they produce the gas methane.

351
00:34:45,560 --> 00:34:49,200
Because there are so many of them, they actually produce an estimated

352
00:34:49,240 --> 00:34:51,720
50 million tonnes of methane

353
00:34:51,760 --> 00:34:56,440
and pump it into the Earth's atmosphere every year.

354
00:34:56,760 --> 00:34:59,280
And it's not just termites.

355
00:34:59,320 --> 00:35:02,960
There's lots of methane naturally in our atmosphere.

356
00:35:03,000 --> 00:35:06,520
It's all produced either biologically...

357
00:35:08,160 --> 00:35:13,760
or by active geological processes like mud volcanoes.

358
00:35:19,440 --> 00:35:22,600
And that makes it all the more surprising that methane

359
00:35:22,640 --> 00:35:28,240
has been detected in the atmosphere of the supposedly dead planet Mars.

360
00:35:32,680 --> 00:35:37,040
It was telescopes on Earth, using infrared spectroscopy,

361
00:35:37,080 --> 00:35:41,720
that first identified methane in Mars's tenuous atmosphere.

362
00:35:46,200 --> 00:35:51,920
Those first measurements appeared to show only tiny amounts.

363
00:35:51,960 --> 00:35:55,000
But closer observations have revealed that the gas

364
00:35:55,040 --> 00:36:01,520
is concentrated in a handful of plumes that vary with the seasons.

365
00:36:02,480 --> 00:36:04,840
In the warmer summer months,

366
00:36:04,880 --> 00:36:10,720
thousands of tonnes of the gas is released from vents in the surface.

367
00:36:10,760 --> 00:36:15,560
Something under the surface of Mars must be producing it.

368
00:36:15,600 --> 00:36:20,520
It may be coming from previously unknown geological processes.

369
00:36:23,120 --> 00:36:27,200
But it could be that it's coming from a biological source.

370
00:36:28,800 --> 00:36:32,080
Now no-one, I don't think, is seriously suggesting that there

371
00:36:32,120 --> 00:36:36,360
are termites running around beneath the surface of Mars.

372
00:36:36,400 --> 00:36:41,680
But it's not actually the termites that are particularly interesting about this story.

373
00:36:41,720 --> 00:36:43,960
It's the way they digest the wood.

374
00:36:44,000 --> 00:36:50,400
You see, they use symbiotic bacteria, bacteria that live in their guts, called Archaea.

375
00:36:50,440 --> 00:36:56,040
And Archaea, these bacteria that can digest wood and produce methane,

376
00:36:56,080 --> 00:37:01,240
are the most common organisms beneath the surface of the Earth.

377
00:37:03,840 --> 00:37:08,480
The snottites are members of the Archaea,

378
00:37:08,520 --> 00:37:16,000
as are many of the microorganisms found living around deep-sea hydrothermal vents.

379
00:37:16,320 --> 00:37:23,880
In fact, it's Archaea that we find thriving in many of the Earth's most extreme environments.

380
00:37:26,800 --> 00:37:32,600
So I think it's quite a fascinating prospect that the methane we see

381
00:37:32,640 --> 00:37:39,120
in Mars's atmosphere might just be produced by organisms like Archaea,

382
00:37:39,160 --> 00:37:42,320
living below the Martian surface.

383
00:37:44,200 --> 00:37:48,280
But while Mars remains a tantalising possibility,

384
00:37:48,320 --> 00:37:52,000
it's no longer the only place in the solar system

385
00:37:52,040 --> 00:37:54,800
we think could harbour alien life.

386
00:38:00,920 --> 00:38:04,240
Far out, a billion kilometres from the sun,

387
00:38:04,280 --> 00:38:08,120
the solar system becomes a very different place.

388
00:38:10,080 --> 00:38:14,400
The planets, like Saturn, are made of gas, not rock.

389
00:38:18,120 --> 00:38:21,600
There's plenty of water out here, but it's frozen solid.

390
00:38:24,480 --> 00:38:30,360
The planets are surrounded by networks of moons, carved from ice.

391
00:38:30,400 --> 00:38:33,560
They're cold and desolate.

392
00:38:35,080 --> 00:38:40,560
They don't seem likely places to find life.

393
00:38:40,600 --> 00:38:45,040
Any places on Earth remotely similar are completely barren.

394
00:39:04,320 --> 00:39:06,360
This is central Iceland.

395
00:39:06,400 --> 00:39:12,520
And, at this time of year, in mid-November, it's an increasingly inhospitable place.

396
00:39:12,560 --> 00:39:16,520
It's about 3 o'clock in the afternoon, it's already well below freezing.

397
00:39:16,560 --> 00:39:19,240
The sun is dipping below the horizon.

398
00:39:19,280 --> 00:39:22,800
And it will stay this way for another six months.

399
00:39:22,840 --> 00:39:28,960
And there's pretty much no visible life here at all.

400
00:39:29,000 --> 00:39:34,040
There are no trees, no grass, and just listen.

401
00:39:34,080 --> 00:39:36,560
SILENCE

402
00:39:38,560 --> 00:39:41,600
No insects, no birds.

403
00:39:41,640 --> 00:39:43,120
Nothing.

404
00:39:47,360 --> 00:39:51,440
But it's because these places are so cold and inhospitable

405
00:39:51,480 --> 00:39:55,320
that they're of increasing interest to astro-biologists.

406
00:39:57,240 --> 00:40:01,920
Because discoveries in these frozen places of Earth have raised new hope

407
00:40:01,960 --> 00:40:07,360
of finding life among the icy worlds of the outer solar system.

408
00:40:09,080 --> 00:40:10,960
And in those frozen wastes

409
00:40:11,000 --> 00:40:15,040
we have found one world that is of particular interest.

410
00:40:16,040 --> 00:40:18,200
It's one of Jupiter's moons.

411
00:40:25,160 --> 00:40:29,320
Jupiter has a vast network of moons.

412
00:40:29,360 --> 00:40:32,400
The four largest have been known

413
00:40:32,440 --> 00:40:36,760
since they were discovered by Galileo in 1610.

414
00:40:36,800 --> 00:40:38,760
And they're a varied bunch.

415
00:40:41,880 --> 00:40:45,720
Closest to the planet is the tortured moon Io.

416
00:40:45,760 --> 00:40:52,400
It's torn apart by volcanoes that carpet its surface with bright yellow sulphur.

417
00:40:58,800 --> 00:41:03,280
In total contrast to the heat of Io comes its neighbour,

418
00:41:03,320 --> 00:41:06,040
the ice moon Europa.

419
00:41:09,520 --> 00:41:12,960
It's about the same size as our moon.

420
00:41:13,000 --> 00:41:16,400
And it's the smoothest body in the solar system.

421
00:41:19,920 --> 00:41:25,080
Its surface is made of an unbroken shell of ice.

422
00:41:25,120 --> 00:41:30,080
Though it's etched with a network of mysterious red markings.

423
00:41:34,200 --> 00:41:39,800
It exists at a chilly minus 160 Celsius.

424
00:41:39,840 --> 00:41:43,560
It seems an incredibly unlikely home for life.

425
00:41:50,480 --> 00:41:54,320
The photographs of Europa from space

426
00:41:54,360 --> 00:41:59,160
reveal a vast, icy wilderness.

427
00:42:00,800 --> 00:42:06,400
But, if you look more closely, then you start to see surface features.

428
00:42:06,440 --> 00:42:13,200
And those features tell you a lot about what's going on deep beneath the ice.

429
00:42:21,400 --> 00:42:26,760
Close-up, we can see deep cracks that criss-cross the surface of Europa.

430
00:42:30,520 --> 00:42:32,440
At higher magnification

431
00:42:32,480 --> 00:42:36,680
we see areas where the ice has been broken into icebergs

432
00:42:36,720 --> 00:42:39,440
and jumbled up before refreezing.

433
00:42:42,680 --> 00:42:46,720
We see the same formations in sea ice on Earth,

434
00:42:46,760 --> 00:42:51,640
where the movements of the ocean have caused the ice to bend and crack.

435
00:42:53,800 --> 00:42:57,720
It suggests something similar may be happening on Europa.

436
00:42:59,720 --> 00:43:03,920
But it's the way the cracks are broken and fractured that provide

437
00:43:03,960 --> 00:43:08,480
the compelling evidence that there is liquid water on Europa.

438
00:43:12,320 --> 00:43:15,560
You see, as Europa orbits around Jupiter,

439
00:43:15,600 --> 00:43:20,320
Jupiter's intense gravity stretches and squashes the moon.

440
00:43:20,360 --> 00:43:25,720
And that stresses the ice and causes it to fracture and crack.

441
00:43:25,760 --> 00:43:30,760
But the position of those cracks is not quite where you would expect it to be.

442
00:43:30,800 --> 00:43:35,560
And the explanation for that is that the icy surface of Europa

443
00:43:35,600 --> 00:43:39,760
has shifted, it's moved relative to the rocky core.

444
00:43:39,800 --> 00:43:46,520
And the only way that could happen is if there's a layer, or an ocean of liquid water,

445
00:43:46,560 --> 00:43:52,600
surrounding the rocky core that allows the outer ice surface to slip around.

446
00:43:56,240 --> 00:44:01,680
Measurements of Europa's magnetic field have confirmed that its icy shell

447
00:44:01,720 --> 00:44:09,200
is sitting on top of a salty ocean that may be a staggering 100km deep.

448
00:44:24,120 --> 00:44:28,480
That would mean that there is more than twice as much life-giving

449
00:44:28,520 --> 00:44:34,000
liquid water on this tiny moon than there is on planet Earth.

450
00:44:38,320 --> 00:44:41,280
But it's not just the discovery of the hidden ocean

451
00:44:41,320 --> 00:44:47,160
that makes us believe that Europa may be the most likely home to alien life.

452
00:44:48,160 --> 00:44:55,000
And that's why I've come to this spectacular ice cave in the Vatnajokull glacier.

453
00:44:55,480 --> 00:44:58,080
You see, the laws of nature are universal.

454
00:44:58,120 --> 00:45:03,680
That may not only apply to laws of physics, but also to the laws of biology as well.

455
00:45:03,720 --> 00:45:05,600
And if that's the case,

456
00:45:05,640 --> 00:45:11,320
then what we find in these ice caves of Iceland may tell us something

457
00:45:11,360 --> 00:45:16,720
about what we could expect to find below the frozen surface of Europa.

458
00:45:42,000 --> 00:45:45,320
It's hard to describe this place.

459
00:45:47,280 --> 00:45:50,600
It's absolutely magnificent.

460
00:45:50,640 --> 00:45:55,320
Visually, the quality of the ice, it's just completely

461
00:45:55,360 --> 00:45:58,640
transparent and clear.

462
00:45:58,680 --> 00:46:01,200
You can see straight through it.

463
00:46:03,520 --> 00:46:07,480
The cave tunnels into the heart of the glacier,

464
00:46:07,520 --> 00:46:12,240
where the ice has been frozen for a thousand years.

465
00:46:12,720 --> 00:46:16,800
It's what astro-biologists find in this ice

466
00:46:16,840 --> 00:46:21,960
that makes us think that Europa could be teeming with life.

467
00:46:30,680 --> 00:46:33,000
NASA scientist Richard Hoover

468
00:46:33,040 --> 00:46:37,120
has spent his career looking for life in unlikely places.

469
00:46:40,120 --> 00:46:42,080
Well, that went very well.

470
00:46:44,880 --> 00:46:50,840
OK.So, will any organisms that you find in that ice be living in a sense that I would understand it?

471
00:46:50,880 --> 00:46:53,920
They're actually alive now, and metabolising?

472
00:46:53,960 --> 00:46:59,320
For a long time it was thought that ice microorganisms

473
00:46:59,360 --> 00:47:03,840
were present only in a state of what is called deep anabiosis.

474
00:47:03,880 --> 00:47:08,200
Suspended animation. It's now becoming quite clear that that isn't necessarily

475
00:47:08,240 --> 00:47:15,760
the case for all the microorganisms, there may be others that are actually actively living in the ice.

476
00:47:15,800 --> 00:47:19,600
So in this glacier, the whole place, this whole cave

477
00:47:19,640 --> 00:47:24,880
may be populated by living things, not frozen things?

478
00:47:24,920 --> 00:47:29,240
Things existing, living, cell dividing, reproducing, all the things you do?

479
00:47:29,280 --> 00:47:30,760
All of this.

480
00:47:36,200 --> 00:47:40,240
It's this prospect of finding things living in solid ice

481
00:47:40,280 --> 00:47:42,920
that has had the greatest impact

482
00:47:42,960 --> 00:47:47,520
on our ideas of where life could survive in the solar system.

483
00:47:49,120 --> 00:47:51,320
OK, we're at lowest magnification.

484
00:47:51,360 --> 00:47:53,880
So, that is 100,000 millionths of a metre?

485
00:47:53,920 --> 00:47:57,040
Yes. We have bacteria.

486
00:47:59,640 --> 00:48:04,600
So, these are organisms that have been trapped in that glacier for thousands of years?

487
00:48:04,640 --> 00:48:06,400
Yes, look at this.

488
00:48:06,440 --> 00:48:09,400
Beautiful. You're seeing life in ice.

489
00:48:09,440 --> 00:48:13,560
We now know that some microorganisms

490
00:48:13,600 --> 00:48:18,920
are capable of actually causing the ice to melt,

491
00:48:18,960 --> 00:48:23,040
because they generate, essentially, anti-freeze proteins.

492
00:48:23,080 --> 00:48:29,560
They change the temperature at which ice goes from a solid state to a liquid state.

493
00:48:29,600 --> 00:48:31,960
And they could have been forming little tiny pockets,

494
00:48:32,000 --> 00:48:34,920
maybe only a few microns in diameter,

495
00:48:34,960 --> 00:48:39,600
but if he can make a two or three micron diameter ball of liquid water,

496
00:48:39,640 --> 00:48:41,920
and he has the ability to move,

497
00:48:41,960 --> 00:48:46,960
then that bacterium is now not in a glacier, but he's in an ocean.

498
00:48:47,000 --> 00:48:51,880
What are the implications of these discoveries?

499
00:48:51,920 --> 00:48:58,200
The fact that you've got living bacteria inside ice on Earth, what are the implications for Europa?

500
00:48:58,240 --> 00:49:04,520
You can clearly have bacteria like this in the frozen ice near the surface crust.

501
00:49:04,560 --> 00:49:07,320
And the thing that is most exciting to me,

502
00:49:07,360 --> 00:49:12,320
is that surface crust of Europa has a wide variety of colours

503
00:49:12,360 --> 00:49:16,280
that are highly suggestive of microbial life.

504
00:49:16,320 --> 00:49:20,960
And so there is a very, very strong possibility

505
00:49:21,000 --> 00:49:26,640
that the ice of Europa may contain viable, living microorganisms.

506
00:49:30,480 --> 00:49:35,640
It's a controversial idea, but it is a dizzying thought

507
00:49:35,680 --> 00:49:39,600
that the mysterious red stains on the surface of Europa

508
00:49:39,640 --> 00:49:44,400
could be the visible signs of alien life.

509
00:49:49,280 --> 00:49:53,280
The discovery of the huge ocean of liquid water

510
00:49:53,320 --> 00:49:59,680
under the surface of this tiny moon, combined with the potential for life in ice,

511
00:49:59,720 --> 00:50:04,160
and the intriguing red markings that criss-cross its surface,

512
00:50:04,200 --> 00:50:10,800
have made Europa the most fascinating and important alien world we know.

513
00:50:20,200 --> 00:50:23,200
A true wonder of the solar system,

514
00:50:23,240 --> 00:50:29,080
because it's our best hope of finding extraterrestrial life.

515
00:50:47,680 --> 00:50:50,600
That question, are we alone in the universe?

516
00:50:50,640 --> 00:50:57,280
Is this the only planet amongst the billions of planets in our galaxy,

517
00:50:57,320 --> 00:51:02,560
amongst the billions of galaxies in the universe, that harbours life?

518
00:51:02,600 --> 00:51:07,000
Is, I think, one of the most important questions,

519
00:51:07,040 --> 00:51:10,520
perhaps THE most important question that we can ask.

520
00:51:10,560 --> 00:51:13,360
Think about what it would mean for us

521
00:51:13,400 --> 00:51:18,200
if the answer was that there was no other life in the solar system,

522
00:51:18,240 --> 00:51:21,720
in our galaxy, perhaps even in the universe.

523
00:51:21,760 --> 00:51:25,080
How valuable would that make planet Earth?

524
00:51:25,120 --> 00:51:27,640
How valuable would that make us?

525
00:51:27,680 --> 00:51:33,720
But then imagine that the answer is that, on every moon of every planet

526
00:51:33,760 --> 00:51:38,360
where the conditions are right, then life survives and flourishes.

527
00:51:38,400 --> 00:51:43,160
That makes us part of a wider cosmic community,

528
00:51:43,200 --> 00:51:47,040
if the universe is teeming with life.

529
00:51:48,600 --> 00:51:53,520
If knowing the answer to the question is so profoundly important,

530
00:51:53,560 --> 00:51:59,680
then surely striving to find the answer should be of overwhelming importance.

531
00:51:59,720 --> 00:52:03,040
I believe it's the most important question you can possibly ask.

532
00:52:03,080 --> 00:52:06,360
Because we have a chance of answering it.

533
00:52:21,000 --> 00:52:24,880
What we've learned from the extreme places on Earth

534
00:52:24,920 --> 00:52:30,120
is that, if there is life out there in the solar system, it will almost certainly be simple.

535
00:52:30,160 --> 00:52:38,240
Single-celled organisms like bacteria eking out an existence in the most hostile of environments.

536
00:52:50,360 --> 00:52:52,400
One thing seems certain.

537
00:52:52,440 --> 00:52:57,200
The only place in the solar system where there is complex life,

538
00:52:57,240 --> 00:53:00,000
life that can build a civilisation,

539
00:53:00,040 --> 00:53:03,280
is here on planet Earth.

540
00:53:04,840 --> 00:53:11,080
But how did that happen? What is it that makes our world so special?

541
00:53:11,120 --> 00:53:18,040
Because, after all, everything in the solar system shares the same genesis.

542
00:53:20,320 --> 00:53:29,040
It was all created out of nothing more than a spinning cloud of gas and dust 4.5 billion years ago.

543
00:53:38,240 --> 00:53:42,800
Solid worlds condensed out of the swirling mists.

544
00:53:42,840 --> 00:53:46,160
But those worlds were radically different.

545
00:53:50,160 --> 00:53:56,080
Around the solar system, there are worlds that erupt with volcanoes of sulphur.

546
00:53:58,040 --> 00:54:00,920
And others with geysers of ice.

547
00:54:03,200 --> 00:54:08,880
There are worlds with rich atmospheres and swirling storms.

548
00:54:11,160 --> 00:54:13,960
And there are moons carved from ice

549
00:54:14,000 --> 00:54:17,320
that hide huge oceans of liquid water.

550
00:54:22,240 --> 00:54:27,120
But there's only one world where the laws of physics have conspired

551
00:54:27,160 --> 00:54:30,360
to combine all these features in one place.

552
00:54:33,960 --> 00:54:38,520
On Earth, the temperature and atmospheric pressure are just right

553
00:54:38,560 --> 00:54:43,360
to allow oceans of liquid water to exist on the surface of the planet.

554
00:54:46,840 --> 00:54:51,080
And it's big enough to have retained its molten core

555
00:54:51,120 --> 00:54:54,600
that not only powers geysers and volcanoes,

556
00:54:54,640 --> 00:54:58,160
but also produces our magnetic field

557
00:54:58,200 --> 00:55:03,720
that fends off the solar wind and protects our thick, nurturing atmosphere.

558
00:55:12,400 --> 00:55:16,400
It's the combination of all those wonders in one place

559
00:55:16,440 --> 00:55:21,720
that allowed life to begin and to get a foothold here on Earth.

560
00:55:21,760 --> 00:55:27,160
But, to allow that life to evolve into such complex creatures as ourselves

561
00:55:27,200 --> 00:55:30,160
requires one more ingredient.

562
00:55:30,200 --> 00:55:33,000
And that's time. Deep time.

563
00:55:33,040 --> 00:55:41,800
The kind of time over which mountains rise and fall, and planets are formed and stars live and die.

564
00:55:41,840 --> 00:55:48,280
And it's perhaps that that makes the earth so rare and so precious in the cosmos.

565
00:55:48,320 --> 00:55:53,640
Because it's been stable enough for long enough for life to evolve

566
00:55:53,680 --> 00:55:56,600
into such magnificent complexity.

567
00:56:05,240 --> 00:56:11,360
The life we have on Earth today is the result of millions of years of stability.

568
00:56:13,600 --> 00:56:17,320
And the pinnacle of that is us, humankind.

569
00:56:19,320 --> 00:56:23,320
A species that has developed to the point where we can bend

570
00:56:23,360 --> 00:56:28,560
and shape and change the world around us.

571
00:56:30,520 --> 00:56:34,080
We have even left our own planet behind

572
00:56:34,120 --> 00:56:38,360
to begin exploring our cosmic surroundings.

573
00:56:49,560 --> 00:56:54,200
You could take the view that our exploration of the universe

574
00:56:54,240 --> 00:56:57,080
has made us somehow insignificant.

575
00:56:57,120 --> 00:57:04,600
One tiny planet around one star amongst hundreds of billions.

576
00:57:04,640 --> 00:57:06,200
But I don't take that view.

577
00:57:06,240 --> 00:57:13,080
Because we've discovered that it takes the rarest combination of chance, and the laws of nature,

578
00:57:13,120 --> 00:57:18,200
to produce a planet that can support a civilisation.

579
00:57:18,240 --> 00:57:21,360
That most magnificent structure

580
00:57:21,400 --> 00:57:26,720
that allows us to explore and understand the universe.

581
00:57:26,760 --> 00:57:33,560
And that's why, for me, our civilisation is THE wonder of the solar system.

582
00:57:34,720 --> 00:57:38,600
MUSIC: "Calling Occupants Of Interplanetary Craft" by the Carpenters

583
00:57:39,400 --> 00:57:43,920
# Calling occupants of interplanetary craft

584
00:57:46,160 --> 00:57:48,520
# Calling occupants...

585
00:57:48,560 --> 00:57:54,120
And if you were to be looking at the Earth from outside the solar system,

586
00:57:54,160 --> 00:57:56,360
that much would be obvious.

587
00:57:57,120 --> 00:58:01,600
# Calling occupants of interplanetary craft...

588
00:58:04,560 --> 00:58:11,240
We have written the evidence of our existence onto the surface of our planet.

589
00:58:11,280 --> 00:58:18,640
Our civilisation has become a beacon that identifies our planet as home to life.

590
00:58:18,680 --> 00:58:23,280
# We'd like to make a contact with you

591
00:58:29,400 --> 00:58:36,840
# Calling occupants of interplanetary, anti-adversary craft

592
00:58:41,960 --> 00:58:45,720
# We are your friends

593
00:58:49,480 --> 00:58:52,200
# We are your friends...#

594
00:58:57,560 --> 00:59:01,440
If you'd like to know more about the solar system,

595
00:59:01,480 --> 00:59:06,480
go to bbc.co.uk/science.

596
00:59:07,480 --> 00:59:17,480
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