0:00:08 > 0:00:10We live on a world of wonders.
0:00:10 > 0:00:16A place of astonishing beauty and complexity.
0:00:17 > 0:00:21There are vast oceans and incredible weather.
0:00:22 > 0:00:25Giant mountains
0:00:25 > 0:00:27and stunning landscapes.
0:00:30 > 0:00:37I'm a physicist, and I'm fascinated by the way that the universal laws of nature
0:00:37 > 0:00:42that made all this, also created such diverse and different worlds
0:00:42 > 0:00:45out there in the solar system.
0:00:47 > 0:00:52I think we're living through the greatest age of discovery our civilisation has known.
0:00:52 > 0:00:56We've voyaged to the farthest reaches of the solar system.
0:00:56 > 0:01:00We've photographed strange new worlds,
0:01:00 > 0:01:05stood in unfamiliar landscapes, tasted alien air.
0:01:08 > 0:01:12But the one thing we haven't found on those worlds
0:01:12 > 0:01:16is the thing that makes our planet unique.
0:01:19 > 0:01:21Life.
0:01:21 > 0:01:23But is that really true?
0:01:23 > 0:01:28Is the Earth the only place in the solar system that could support life?
0:01:30 > 0:01:37In this film we will search the solar system for worlds that harbour the conditions to support life.
0:01:40 > 0:01:47What we find on these worlds may help us answer the question, are we alone in the universe?
0:01:51 > 0:01:56That's not only one of the great fundamental questions for science,
0:01:56 > 0:02:01but one of the great unanswered questions in human history.
0:02:31 > 0:02:38Floating in the Sea of Cortez off the cost of Mexico is the research vessel Atlantis,
0:02:38 > 0:02:43the mother ship for the exploration of one of the most alien worlds we know.
0:02:48 > 0:02:50But it's an alien world on our planet.
0:03:05 > 0:03:08The Atlantis is the launch vessel for Alvin,
0:03:08 > 0:03:13one of the world's most rugged submarines.
0:03:13 > 0:03:15Built like a spacecraft,
0:03:15 > 0:03:19it's designed to explore the deepest depths of the ocean.
0:03:21 > 0:03:25And I'm lucky enough to have hitched a ride down to the sea floor,
0:03:25 > 0:03:28two kilometres beneath the surface.
0:03:32 > 0:03:36That has got to be the closest thing to going into space that you can do.
0:03:36 > 0:03:41And, given that I'm not going to go into space any time soon, I think it's the next best thing.
0:03:43 > 0:03:45See you in eight hours.
0:04:00 > 0:04:04Roger, Alvin. Your checks are good. Permission to dive.
0:04:05 > 0:04:07Roger. Alvin diving.
0:04:15 > 0:04:19The parallels to spaceflight are obvious.
0:04:19 > 0:04:24As the tiny capsule descends, we are leaving the familiar world
0:04:24 > 0:04:29of the surface of our planet, and entering a strange, hostile world.
0:04:34 > 0:04:39If anything goes wrong, we will be completely on our own.
0:04:46 > 0:04:50MACHINE BEEPS
0:04:53 > 0:04:55Beeping is never good.
0:04:57 > 0:05:02'Fortunately, Alvin is one of only a handful of submarines
0:05:02 > 0:05:06'that can withstand the colossal pressure of the deep ocean.'
0:05:08 > 0:05:13At the Earth's surface, we're used to one atmosphere of pressure.
0:05:13 > 0:05:20As we descend, the pressure increases by another atmosphere every ten metres.
0:05:20 > 0:05:22And it soon adds up.
0:05:28 > 0:05:33We're approaching a kilometre deep. The pressure outside there is now
0:05:33 > 0:05:39100 atmospheres, that's higher than the atmospheric pressure on the surface of Venus.
0:05:39 > 0:05:45Without knowing, if you were asked a question, could life exist down here, 100 atmospheres,
0:05:45 > 0:05:49cold, dark no sign of sunlight at all,
0:05:49 > 0:05:53it's pitch black there, you would say no.
0:05:53 > 0:05:54Well, I would say no.
0:06:00 > 0:06:04But the depths of the ocean are not lifeless.
0:06:04 > 0:06:07Illuminated by Alvin's lights,
0:06:07 > 0:06:12we find oases of life in the deserts of the ocean floor.
0:06:18 > 0:06:22So we have landed, after about an hour of descent.
0:06:22 > 0:06:25We've just stopped in the most incredible place.
0:06:25 > 0:06:28Look at those.
0:06:28 > 0:06:31We've landed on top of the tube worms.
0:06:32 > 0:06:34Amazing things.
0:06:42 > 0:06:47This underwater city is one of the most bizarre environments on our planet.
0:07:00 > 0:07:06It's built around a hydrothermal vent, a volcanic opening in the Earth's crust
0:07:06 > 0:07:14that pumps out clouds of sulphurous chemicals and water heated to nearly 300 Celsius.
0:07:14 > 0:07:21And somehow, life has found a way to thrive in these most extreme conditions.
0:07:29 > 0:07:32This is a genuinely remarkable place.
0:07:32 > 0:07:35There are mats,
0:07:35 > 0:07:40carpets of yellow bacteria.
0:07:42 > 0:07:45Look at that. It's not only just bacterial blobs,
0:07:45 > 0:07:48there is real complex organisms.
0:07:48 > 0:07:51Alien. I want to say that word, alien environment.
0:07:51 > 0:07:53It really is alien to us.
0:07:57 > 0:08:01For me, the fascinating thing about finding life down here
0:08:01 > 0:08:03is that the conditions on the
0:08:03 > 0:08:08deep ocean floor are more similar in many ways to the conditions on
0:08:08 > 0:08:12worlds hundreds of millions of kilometres away out there
0:08:12 > 0:08:18in the solar system than they are to the conditions just two kilometres from my head on the Earth's surface.
0:08:18 > 0:08:22It's incredibly dark, there is no sunlight,
0:08:22 > 0:08:26there's a brutal mixture of hot and cold water,
0:08:26 > 0:08:29and just rock and minerals.
0:08:29 > 0:08:35So, if life can not only survive but even flourish in these conditions,
0:08:35 > 0:08:39then you've got to feel that it's much more likely that life can
0:08:39 > 0:08:43also survive and flourish out there in the solar system.
0:08:52 > 0:08:56Ever since the invention of the telescope 400 years ago,
0:08:56 > 0:09:01we have looked to our neighbouring worlds for signs of life.
0:09:05 > 0:09:10As technology has improved, we've been able to search the planets in more and more detail,
0:09:10 > 0:09:13and we have found nothing.
0:09:14 > 0:09:18But that doesn't mean the rest of the solar system is dead,
0:09:18 > 0:09:22because we're only beginning to scratch the surface of what's out there.
0:09:27 > 0:09:31There are literally hundreds of other worlds.
0:09:31 > 0:09:35Planets and their moons which we have barely explored.
0:09:39 > 0:09:44Among them may be worlds that hold the conditions to support life.
0:09:48 > 0:09:51And the best way to find out what those conditions are
0:09:51 > 0:09:55is to look at the one place we know life flourishes.
0:09:58 > 0:10:00The Earth.
0:10:11 > 0:10:14Life is pretty much only chemistry.
0:10:14 > 0:10:18It's just the reactions between atoms and molecules.
0:10:18 > 0:10:21And so for life to exist, you only really need three things.
0:10:21 > 0:10:24First of all, you need the right chemistry set.
0:10:24 > 0:10:28Now, I'm made of something like 40 elements,
0:10:28 > 0:10:31almost half of the known elements, which is pretty complicated.
0:10:31 > 0:10:37But actually 96% of me is only made of four of them, carbon, nitrogen, oxygen and hydrogen.
0:10:37 > 0:10:40Secondly, you need a power source.
0:10:40 > 0:10:44You need a battery, something to make a flow of electrons
0:10:44 > 0:10:47that powers the processes of life.
0:10:47 > 0:10:53Now here on Earth, most life uses the power of the sun.
0:10:53 > 0:10:59And thirdly, you need some kind of medium for life to play itself out in,
0:10:59 > 0:11:01for processes to happen.
0:11:01 > 0:11:04And here on Earth,
0:11:04 > 0:11:07you don't have to look very far at all to find that medium, that solvent.
0:11:07 > 0:11:10Because it's this, water.
0:11:32 > 0:11:35If you want to see how important water is to life,
0:11:35 > 0:11:40there's no better place to come than the Atacama desert in Chile.
0:11:46 > 0:11:52The soil here is more sterile than a hospital operating theatre.
0:11:52 > 0:11:56In fact, scientists have looked for the most basic form of life, bacteria,
0:11:56 > 0:12:01in some parts of the Atacama, and they found absolutely nothing.
0:12:05 > 0:12:09All deserts are characterised by a lack of moisture.
0:12:09 > 0:12:12But the Atacama takes that to the extremes.
0:12:14 > 0:12:18The Sahara is 50 times wetter than the Atacama.
0:12:18 > 0:12:23There are weather stations here that have measured 1mm of rainfall in 10 years.
0:12:23 > 0:12:27There are river valleys that have been dry for 120,000 years.
0:12:27 > 0:12:33There are rocks that haven't seen rainfall for 20 million years.
0:12:39 > 0:12:44It's this dryness that explains why nothing can survive here.
0:12:44 > 0:12:49Even the most primitive form of life on Earth, the bacteria,
0:12:49 > 0:12:52need water for their survival.
0:12:52 > 0:12:54And there are no exceptions.
0:12:54 > 0:12:58And this seemingly fundamental link between water and life
0:12:58 > 0:13:02is driving the search for life out there in the solar system.
0:13:02 > 0:13:05Because, wherever we find water,
0:13:05 > 0:13:09that will be the best place to look for life beyond the Earth.
0:13:14 > 0:13:20The Earth is the only planet that currently has liquid water on its surface.
0:13:20 > 0:13:23The other planets are either too close to the sun,
0:13:23 > 0:13:27like Mercury, and baked dry.
0:13:27 > 0:13:30Or they are too far away.
0:13:30 > 0:13:38Saturn's rings are made of water, but in the depths of space, it's frozen into lumps of solid ice.
0:13:42 > 0:13:48But that doesn't mean that liquid water has never existed elsewhere in the solar system.
0:13:48 > 0:13:52And if it has, we should be able to find the evidence,
0:13:54 > 0:13:58because wherever water goes, it leaves its footprints.
0:14:09 > 0:14:14These are the Scablands, a remote part of the North Western United States.
0:14:14 > 0:14:20It's one of the most spectacular places to come to see how water
0:14:20 > 0:14:23carves its signature into the landscape.
0:14:30 > 0:14:34The largest flood on Earth went through this area here.
0:14:34 > 0:14:39Jim Rice is an astro-geologist. He believes that understanding the events that created this landscape
0:14:39 > 0:14:43can help in the search for water on other planets.
0:14:43 > 0:14:47We are kind of like CSI arriving at the scene of a crime, this is the evidence left here.
0:14:47 > 0:14:52- We've come to piece it together. - I can see this is not a normal river system.
0:14:52 > 0:14:54You can see, because it is so straight.
0:14:54 > 0:15:00There is no meandering of a river here, it's just a big hole.
0:15:07 > 0:15:12This entire landscape was created at the end of the last Ice Age.
0:15:15 > 0:15:23200 miles to the east lay a huge lake, held in place by a wall of glacial ice.
0:15:24 > 0:15:33When that wall ruptured, over 2,000 cubic kilometres of water swept out in a single catastrophic event.
0:15:38 > 0:15:40The flood waters were at least 400 feet deep here.
0:15:40 > 0:15:44But actually they were another 200 feet stacked on top of that, coming across here.
0:15:44 > 0:15:48So we would be under 200 feet of water standing right here.
0:15:48 > 0:15:51So am I to imagine a wave?
0:15:51 > 0:15:55Yeah, a massive wave rolling, rumbling, this water would
0:15:55 > 0:15:57be charged full of big chunks of ice from that ice dam.
0:15:57 > 0:16:03It would be loaded with big chunks of the salt bed rock being gouged, ripped out of here.
0:16:03 > 0:16:05It would be an impressive sight.
0:16:15 > 0:16:21As the floodwaters tore across the landscape, they carved out this 20 mile long canyon.
0:16:24 > 0:16:28And at its head, it left these giant horseshoes.
0:16:28 > 0:16:32At over 400 feet high and five miles across,
0:16:32 > 0:16:37this was the largest waterfall the world has ever known.
0:16:44 > 0:16:49The easiest way of thinking about it is if you took every river in the world, put them in
0:16:49 > 0:16:55the same location, had them flowing at the same time, these floods are 10 times larger than that.
0:16:55 > 0:17:00And how long do we think it took to sculpt this landscape?
0:17:00 > 0:17:0248 hours to a week.
0:17:02 > 0:17:04It's instantaneous, geologically.
0:17:07 > 0:17:11The Scablands reveal the characteristic signature
0:17:11 > 0:17:14that water carves into the landscape.
0:17:15 > 0:17:21It's a signature that can be seen from space, and not just on the Earth.
0:17:24 > 0:17:30When we turn our telescopes on our next door neighbour and prime candidate for finding
0:17:30 > 0:17:32alien life, the planet Mars,
0:17:32 > 0:17:37we find almost identical features cut into its surface.
0:17:44 > 0:17:49The Red Planet is covered in outflow channels.
0:17:49 > 0:17:53Straight, wide canyons, exactly like the Scablands.
0:17:55 > 0:18:00And they are filled with identical geological features.
0:18:10 > 0:18:17It all suggests that similar huge floods once tore across the surface of Mars.
0:18:17 > 0:18:21This is a picture of here from the air.
0:18:21 > 0:18:23I am sat somewhere around here.
0:18:23 > 0:18:31And here are the horseshoe shapes of the dry folds which are just over there.
0:18:31 > 0:18:36This is a picture taken of the surface of Mars,
0:18:36 > 0:18:41and you see those typical horseshoe shapes of the folds.
0:18:41 > 0:18:48Also, you see the structures upstream of the folds, these grooves cut into the landscape.
0:18:48 > 0:18:53And you see that here, grooves cut into the landscape as the water
0:18:53 > 0:18:57cascades down and then flows over the folds
0:18:57 > 0:19:01and cuts the gigantic valleys out as it moves downstream.
0:19:02 > 0:19:09So, all this adds up, I think, to an overwhelming smoking gun
0:19:09 > 0:19:14that there were vast amounts of water that flowed very quickly
0:19:14 > 0:19:17over the surface of Mars at some point in the past.
0:19:21 > 0:19:28But although the outflow channels are proof that liquid water once flowed across Mars,
0:19:28 > 0:19:30it may not point to the existence of life.
0:19:34 > 0:19:41Because if the Martian landscapes were formed by the same processes that formed the Scablands on Earth,
0:19:41 > 0:19:46the floods that created them may only have lasted a matter of days.
0:19:51 > 0:19:54For life to get a foothold, you need more than that.
0:19:54 > 0:19:59You need areas of standing water.
0:19:59 > 0:20:05Lakes and rivers that persist for millions of years.
0:20:05 > 0:20:11In order to look for evidence of that standing water, we've done the only thing we can,
0:20:11 > 0:20:16we have sent an army of robotic explorers to the surface of the planet.
0:20:17 > 0:20:19We have touch down, we have touch down.
0:20:25 > 0:20:27Over the last 35 years,
0:20:27 > 0:20:32we've landed six robot probes on Mars.
0:20:32 > 0:20:39And one of them, Opportunity, is still rolling across the surface, investigating the Martian geology.
0:20:50 > 0:20:54The Mars rovers has really captured our imaginations.
0:20:54 > 0:20:59I suppose, because they genuinely are explorers in the old-fashioned sense.
0:20:59 > 0:21:04They are the extension of our senses to the surface of another world.
0:21:04 > 0:21:08But they have also been very important scientifically, because
0:21:08 > 0:21:11you can't really get to know another planet from orbit.
0:21:11 > 0:21:15You have got to get down to the surface, you've got to touch it,
0:21:15 > 0:21:19you've got to dig down and examine it microscopically.
0:21:19 > 0:21:23And the Rovers really have, by doing that,
0:21:23 > 0:21:27made some extremely important scientific discoveries.
0:21:35 > 0:21:42One of the most significant of those discoveries was made in November 2004.
0:21:42 > 0:21:48The Opportunity rover was examining an impact feature called the Endurance crater,
0:21:48 > 0:21:52when it detected deposits of a remarkable mineral.
0:22:08 > 0:22:13This is the world's largest salt works on the Baha peninsula in Mexico.
0:22:13 > 0:22:20And what they do here is pump sea water into these lagoons and let it evaporate.
0:22:20 > 0:22:25What they're after is this stuff, which is sodium chloride, table salt.
0:22:25 > 0:22:29But, at different stages, different salts, different minerals, crystallise out.
0:22:29 > 0:22:34So all the things really that are in sea water emerge, crystallise out
0:22:34 > 0:22:37at different stages of the process.
0:22:46 > 0:22:53In one of the lagoons, pond number nine, the sea water is at exactly the right concentration
0:22:53 > 0:22:59to precipitate out these beautiful crystals that cover the entire floor of the lagoon.
0:23:08 > 0:23:11This is gypsum,
0:23:11 > 0:23:17and it's exactly the same stuff that Opportunity found on the surface of Mars.
0:23:17 > 0:23:22Now, what's interesting about that discovery is how you make gypsum.
0:23:22 > 0:23:26You see, its chemical formula is CaSO4.
0:23:29 > 0:23:33So it's calcium sulphate.
0:23:33 > 0:23:37Dihydrate, 2H2O.
0:23:37 > 0:23:39That's water.
0:23:39 > 0:23:47So, the only way we know of, the only way to make gypsum here on Earth, is to have calcium
0:23:47 > 0:23:51and sulphate ions in the presence of liquid water.
0:23:51 > 0:23:56So, large deposits of gypsum on the surface of Mars tells you
0:23:56 > 0:24:02that there must have been big areas of water present for a very long time.
0:24:05 > 0:24:08The discovery of gypsum has helped to build a picture
0:24:08 > 0:24:12of an ancient Mars that was much warmer and wetter.
0:24:14 > 0:24:19Subsequent discoveries of gypsum in networks of sand dunes
0:24:19 > 0:24:24suggest that large areas of Mars were once covered in standing water.
0:24:29 > 0:24:33And where there is standing water, there is the chance of life.
0:24:37 > 0:24:40This area of the salt flats is, we think,
0:24:40 > 0:24:44very similar to areas that have been seen on Mars.
0:24:44 > 0:24:48And it certainly looks extremely inhospitable.
0:24:48 > 0:24:51It's hard at first sight to see how anything could live here.
0:24:51 > 0:24:54But, if you just dig
0:24:54 > 0:24:56a tiny bit below the surface,
0:24:56 > 0:25:02then you see that this layer of gypsum is only a few millimetres thick,
0:25:02 > 0:25:07and then immediately the ground beneath it turns this greeny colour.
0:25:07 > 0:25:14It's green because that is bacteria that thrive in these seemingly inhospitable conditions.
0:25:14 > 0:25:17Now if these bacteria can survive here,
0:25:17 > 0:25:24then there seems to be no good reason why they couldn't also have survived and even flourished on Mars
0:25:24 > 0:25:28when there was water present at some point in the very distant past.
0:25:33 > 0:25:36But although it may once have been more hospitable,
0:25:36 > 0:25:41any liquid water has long since disappeared from the surface of Mars.
0:25:44 > 0:25:50About three billion years ago, it died as a planet.
0:25:50 > 0:25:56Its core froze and the volcanoes that had produced its atmosphere seized up.
0:25:59 > 0:26:03The solar wind stripped away the remains of that atmosphere.
0:26:03 > 0:26:06Any liquid water would have evaporated
0:26:06 > 0:26:10or soaked into the soil where it froze.
0:26:11 > 0:26:15It left the surface of Mars too cold, too exposed
0:26:15 > 0:26:18and too dry to support life.
0:26:24 > 0:26:29It's highly unlikely that there will be life on the surface of Mars today.
0:26:29 > 0:26:33But that's not to say that life couldn't exist somewhere on the Red Planet,
0:26:33 > 0:26:36maybe we're just looking in the wrong place.
0:26:39 > 0:26:42There are other potential habitats for life on Mars.
0:26:44 > 0:26:49Detailed pictures of the surface show the entrances to caves,
0:26:49 > 0:26:53revealing the existence of a world beneath the Martian surface.
0:26:57 > 0:26:59We know there may be water down there.
0:26:59 > 0:27:06Satellite data shows permafrost, ice frozen in the soil.
0:27:06 > 0:27:11Deep below the surface, that ice may melt to form liquid water.
0:27:16 > 0:27:21It all hints at an undiscovered subterranean world
0:27:21 > 0:27:25that may be a more likely place to find life.
0:27:46 > 0:27:50If you were to imagine the perfect habitat for life,
0:27:50 > 0:27:53then it would surely be somewhere like this.
0:27:53 > 0:27:55A warm climate, lots of liquid water,
0:27:55 > 0:28:00a beautiful, dense atmosphere.
0:28:00 > 0:28:04You see the results everywhere, just life everywhere you look.
0:28:07 > 0:28:11All the life we're familiar with thrives in pretty much the same
0:28:11 > 0:28:17conditions that we do, driven by the heat and light of the sun.
0:28:17 > 0:28:20But this is by no means the only life on Earth.
0:28:24 > 0:28:29There's another living planet hidden beneath the surface
0:28:29 > 0:28:32that exists in completely different conditions.
0:28:32 > 0:28:38It raises fascinating possibilities for the caves on Mars.
0:28:46 > 0:28:49This is the Cueva de Villa Luz in Tabasco, Mexico,
0:28:49 > 0:28:51the Cave of the House of Light.
0:28:51 > 0:28:56And it is the definition of a hostile environment to me.
0:28:56 > 0:29:01Because (HE SNIFFS) it's full of hydrogen sulphide gas, hence
0:29:01 > 0:29:07the gas monitor which says at the moment one part per million hydrogen sulphide, very toxic for me,
0:29:07 > 0:29:12which is why I have got this gas mask in case it all gets too much.
0:29:12 > 0:29:16So, it's a place where you, at first sight,
0:29:16 > 0:29:21would not expect a great many life forms to survive and flourish.
0:29:49 > 0:29:55Although the cave is a death-trap for us, that doesn't mean that nothing lives here.
0:29:55 > 0:29:57In fact, it's teeming with life.
0:30:00 > 0:30:05Look at these fish, just everywhere in the cave water. And they're
0:30:05 > 0:30:08adapted to live in these conditions.
0:30:08 > 0:30:10In fact, if you look at them closely,
0:30:10 > 0:30:12they're quite pink.
0:30:12 > 0:30:16That's thought to be because they've got lots of haemoglobin
0:30:16 > 0:30:18because there's not much oxygen down here,
0:30:18 > 0:30:23so they need to have an efficient way of moving oxygen around their bodies.
0:30:23 > 0:30:25Beautiful.
0:30:31 > 0:30:35But the really interesting life is found in the depths of the caves,
0:30:35 > 0:30:41where the concentration of poisonous gas is high enough to set off my alarm.
0:30:42 > 0:30:45Down here, far from the light of the sun,
0:30:45 > 0:30:50are organisms whose energy source comes from the air around them.
0:30:52 > 0:30:58They use the hydrogen sulphide gas bubbling up through these springs.
0:30:58 > 0:31:02The same gas that could be fatally poisonous to me
0:31:02 > 0:31:04is their source of life.
0:31:15 > 0:31:18These things are what I came deep underground to see.
0:31:18 > 0:31:24These are snottites. And you can see why they're called that.
0:31:24 > 0:31:29They're really one of the most alien life forms that I can conceive of
0:31:29 > 0:31:31on the Earth
0:31:31 > 0:31:36Because they metabolise hydrogen sulphide, so they metabolise this
0:31:36 > 0:31:40faintly acidic and nasty gas that I'm just breathing in now.
0:31:40 > 0:31:45You can almost feel it on your tongue, actually, the acidity of it.
0:31:45 > 0:31:50They metabolise it, they react it with oxygen, and they produce sulphuric acid.
0:31:50 > 0:31:55So their breathing process, if you like, their version of what I do,
0:31:55 > 0:31:59I breathe in oxygen, react that with sugars and breathe out CO2 and get energy
0:31:59 > 0:32:05these guys breathe in hydrogen sulphide and oxygen and produce sulphuric acid.
0:32:05 > 0:32:07In fact, I can test it here with this.
0:32:16 > 0:32:18Yes, you see, look at that.
0:32:18 > 0:32:24That, well, what looks like water, that secretion of dripping off the snottites, has actually got a pH...
0:32:24 > 0:32:29well, it's now about between 0.5 and 0.
0:32:29 > 0:32:30That's strong acid.
0:32:30 > 0:32:33That's as strong as battery acid.
0:32:33 > 0:32:37It's actually highly concentrated sulphuric acid.
0:32:37 > 0:32:41So, what a strange organism.
0:32:41 > 0:32:43Alien in every sense of the word.
0:32:43 > 0:32:48Except that it's present on, well, just below the surface, of our planet.
0:32:52 > 0:32:55And the snottites are not alone.
0:32:55 > 0:32:59Organisms that can extract energy from the minerals around them
0:32:59 > 0:33:01are found under the ground all over the world.
0:33:03 > 0:33:08In fact, this way of life is so successful that it's thought there
0:33:08 > 0:33:13may be more life living beneath the Earth's surface than there is on it.
0:33:15 > 0:33:18And that raises an intriguing possibility.
0:33:18 > 0:33:22If life can thrive below the Earth's surface,
0:33:22 > 0:33:27why couldn't organisms like snottites survive and flourish
0:33:27 > 0:33:29beneath the surface of Mars?
0:33:33 > 0:33:37If you think about it, living below the surface of Mars might actually
0:33:37 > 0:33:41be quite a good idea, because the surface is incredibly hostile.
0:33:41 > 0:33:45It's subjected to intense ultraviolet radiation from the sun.
0:33:45 > 0:33:48It's a very cold place, and the atmospheric pressure doesn't
0:33:48 > 0:33:52allow liquid water to exist on the surface.
0:33:52 > 0:33:57But, if there is life below the surface of Mars, then obviously we have a problem.
0:33:57 > 0:33:59How could you possibly detect it?
0:33:59 > 0:34:03Well, actually, there is a perhaps tantalising clue that
0:34:03 > 0:34:10there might be something interesting going on below the Martian surface.
0:34:21 > 0:34:25These are termites, or white ants.
0:34:25 > 0:34:31And they're very unusual animals because they eat wood.
0:34:31 > 0:34:33This is their food.
0:34:33 > 0:34:40There are many, many species of these, billions of individuals across the planet.
0:34:40 > 0:34:45And, in the process of digesting wood, they produce the gas methane.
0:34:45 > 0:34:49Because there are so many of them, they actually produce an estimated
0:34:49 > 0:34:5150 million tonnes of methane
0:34:51 > 0:34:56and pump it into the Earth's atmosphere every year.
0:34:56 > 0:34:59And it's not just termites.
0:34:59 > 0:35:03There's lots of methane naturally in our atmosphere.
0:35:03 > 0:35:06It's all produced either biologically...
0:35:08 > 0:35:13or by active geological processes like mud volcanoes.
0:35:19 > 0:35:22And that makes it all the more surprising that methane
0:35:22 > 0:35:28has been detected in the atmosphere of the supposedly dead planet Mars.
0:35:32 > 0:35:37It was telescopes on Earth, using infrared spectroscopy,
0:35:37 > 0:35:41that first identified methane in Mars's tenuous atmosphere.
0:35:46 > 0:35:51Those first measurements appeared to show only tiny amounts.
0:35:51 > 0:35:55But closer observations have revealed that the gas
0:35:55 > 0:36:01is concentrated in a handful of plumes that vary with the seasons.
0:36:02 > 0:36:04In the warmer summer months,
0:36:04 > 0:36:10thousands of tonnes of the gas is released from vents in the surface.
0:36:10 > 0:36:15Something under the surface of Mars must be producing it.
0:36:15 > 0:36:20It may be coming from previously unknown geological processes.
0:36:23 > 0:36:27But it could be that it's coming from a biological source.
0:36:28 > 0:36:32Now no-one, I don't think, is seriously suggesting that there
0:36:32 > 0:36:36are termites running around beneath the surface of Mars.
0:36:36 > 0:36:41But it's not actually the termites that are particularly interesting about this story.
0:36:41 > 0:36:44It's the way they digest the wood.
0:36:44 > 0:36:50You see, they use symbiotic bacteria, bacteria that live in their guts, called Archaea.
0:36:50 > 0:36:56And Archaea, these bacteria that can digest wood and produce methane,
0:36:56 > 0:37:01are the most common organisms beneath the surface of the Earth.
0:37:03 > 0:37:08The snottites are members of the Archaea,
0:37:08 > 0:37:16as are many of the microorganisms found living around deep-sea hydrothermal vents.
0:37:16 > 0:37:23In fact, it's Archaea that we find thriving in many of the Earth's most extreme environments.
0:37:26 > 0:37:32So I think it's quite a fascinating prospect that the methane we see
0:37:32 > 0:37:39in Mars's atmosphere might just be produced by organisms like Archaea,
0:37:39 > 0:37:42living below the Martian surface.
0:37:44 > 0:37:48But while Mars remains a tantalising possibility,
0:37:48 > 0:37:52it's no longer the only place in the solar system
0:37:52 > 0:37:54we think could harbour alien life.
0:38:00 > 0:38:04Far out, a billion kilometres from the sun,
0:38:04 > 0:38:08the solar system becomes a very different place.
0:38:10 > 0:38:14The planets, like Saturn, are made of gas, not rock.
0:38:18 > 0:38:21There's plenty of water out here, but it's frozen solid.
0:38:24 > 0:38:30The planets are surrounded by networks of moons, carved from ice.
0:38:30 > 0:38:33They're cold and desolate.
0:38:35 > 0:38:40They don't seem likely places to find life.
0:38:40 > 0:38:45Any places on Earth remotely similar are completely barren.
0:39:04 > 0:39:06This is central Iceland.
0:39:06 > 0:39:12And, at this time of year, in mid-November, it's an increasingly inhospitable place.
0:39:12 > 0:39:16It's about 3 o'clock in the afternoon, it's already well below freezing.
0:39:16 > 0:39:19The sun is dipping below the horizon.
0:39:19 > 0:39:22And it will stay this way for another six months.
0:39:22 > 0:39:29And there's pretty much no visible life here at all.
0:39:29 > 0:39:34There are no trees, no grass, and just listen.
0:39:34 > 0:39:36SILENCE
0:39:38 > 0:39:41No insects, no birds.
0:39:41 > 0:39:43Nothing.
0:39:47 > 0:39:51But it's because these places are so cold and inhospitable
0:39:51 > 0:39:55that they're of increasing interest to astro-biologists.
0:39:57 > 0:40:01Because discoveries in these frozen places of Earth have raised new hope
0:40:01 > 0:40:07of finding life among the icy worlds of the outer solar system.
0:40:09 > 0:40:11And in those frozen wastes
0:40:11 > 0:40:15we have found one world that is of particular interest.
0:40:16 > 0:40:18It's one of Jupiter's moons.
0:40:25 > 0:40:29Jupiter has a vast network of moons.
0:40:29 > 0:40:32The four largest have been known
0:40:32 > 0:40:36since they were discovered by Galileo in 1610.
0:40:36 > 0:40:38And they're a varied bunch.
0:40:41 > 0:40:45Closest to the planet is the tortured moon Io.
0:40:45 > 0:40:52It's torn apart by volcanoes that carpet its surface with bright yellow sulphur.
0:40:58 > 0:41:03In total contrast to the heat of Io comes its neighbour,
0:41:03 > 0:41:06the ice moon Europa.
0:41:09 > 0:41:13It's about the same size as our moon.
0:41:13 > 0:41:16And it's the smoothest body in the solar system.
0:41:19 > 0:41:25Its surface is made of an unbroken shell of ice.
0:41:25 > 0:41:30Though it's etched with a network of mysterious red markings.
0:41:34 > 0:41:39It exists at a chilly minus 160 Celsius.
0:41:39 > 0:41:43It seems an incredibly unlikely home for life.
0:41:50 > 0:41:54The photographs of Europa from space
0:41:54 > 0:41:59reveal a vast, icy wilderness.
0:42:00 > 0:42:06But, if you look more closely, then you start to see surface features.
0:42:06 > 0:42:13And those features tell you a lot about what's going on deep beneath the ice.
0:42:21 > 0:42:26Close-up, we can see deep cracks that criss-cross the surface of Europa.
0:42:30 > 0:42:32At higher magnification
0:42:32 > 0:42:36we see areas where the ice has been broken into icebergs
0:42:36 > 0:42:39and jumbled up before refreezing.
0:42:42 > 0:42:46We see the same formations in sea ice on Earth,
0:42:46 > 0:42:51where the movements of the ocean have caused the ice to bend and crack.
0:42:53 > 0:42:57It suggests something similar may be happening on Europa.
0:42:59 > 0:43:03But it's the way the cracks are broken and fractured that provide
0:43:03 > 0:43:08the compelling evidence that there is liquid water on Europa.
0:43:12 > 0:43:15You see, as Europa orbits around Jupiter,
0:43:15 > 0:43:20Jupiter's intense gravity stretches and squashes the moon.
0:43:20 > 0:43:25And that stresses the ice and causes it to fracture and crack.
0:43:25 > 0:43:30But the position of those cracks is not quite where you would expect it to be.
0:43:30 > 0:43:35And the explanation for that is that the icy surface of Europa
0:43:35 > 0:43:39has shifted, it's moved relative to the rocky core.
0:43:39 > 0:43:46And the only way that could happen is if there's a layer, or an ocean of liquid water,
0:43:46 > 0:43:52surrounding the rocky core that allows the outer ice surface to slip around.
0:43:56 > 0:44:01Measurements of Europa's magnetic field have confirmed that its icy shell
0:44:01 > 0:44:09is sitting on top of a salty ocean that may be a staggering 100km deep.
0:44:24 > 0:44:28That would mean that there is more than twice as much life-giving
0:44:28 > 0:44:34liquid water on this tiny moon than there is on planet Earth.
0:44:38 > 0:44:41But it's not just the discovery of the hidden ocean
0:44:41 > 0:44:47that makes us believe that Europa may be the most likely home to alien life.
0:44:48 > 0:44:55And that's why I've come to this spectacular ice cave in the Vatnajokull glacier.
0:44:55 > 0:44:58You see, the laws of nature are universal.
0:44:58 > 0:45:03That may not only apply to laws of physics, but also to the laws of biology as well.
0:45:03 > 0:45:05And if that's the case,
0:45:05 > 0:45:11then what we find in these ice caves of Iceland may tell us something
0:45:11 > 0:45:16about what we could expect to find below the frozen surface of Europa.
0:45:42 > 0:45:45It's hard to describe this place.
0:45:47 > 0:45:50It's absolutely magnificent.
0:45:50 > 0:45:55Visually, the quality of the ice, it's just completely
0:45:55 > 0:45:58transparent and clear.
0:45:58 > 0:46:01You can see straight through it.
0:46:03 > 0:46:07The cave tunnels into the heart of the glacier,
0:46:07 > 0:46:12where the ice has been frozen for a thousand years.
0:46:12 > 0:46:16It's what astro-biologists find in this ice
0:46:16 > 0:46:22that makes us think that Europa could be teeming with life.
0:46:30 > 0:46:33NASA scientist Richard Hoover
0:46:33 > 0:46:37has spent his career looking for life in unlikely places.
0:46:40 > 0:46:42Well, that went very well.
0:46:44 > 0:46:50- OK.- So, will any organisms that you find in that ice be living in a sense that I would understand it?
0:46:50 > 0:46:53They're actually alive now, and metabolising?
0:46:53 > 0:46:59For a long time it was thought that ice microorganisms
0:46:59 > 0:47:03were present only in a state of what is called deep anabiosis.
0:47:03 > 0:47:08Suspended animation. It's now becoming quite clear that that isn't necessarily
0:47:08 > 0:47:15the case for all the microorganisms, there may be others that are actually actively living in the ice.
0:47:15 > 0:47:19So in this glacier, the whole place, this whole cave
0:47:19 > 0:47:24may be populated by living things, not frozen things?
0:47:24 > 0:47:29Things existing, living, cell dividing, reproducing, all the things you do?
0:47:29 > 0:47:30All of this.
0:47:36 > 0:47:40It's this prospect of finding things living in solid ice
0:47:40 > 0:47:42that has had the greatest impact
0:47:42 > 0:47:47on our ideas of where life could survive in the solar system.
0:47:49 > 0:47:51OK, we're at lowest magnification.
0:47:51 > 0:47:53So, that is 100,000 millionths of a metre?
0:47:53 > 0:47:57Yes. We have bacteria.
0:47:59 > 0:48:04So, these are organisms that have been trapped in that glacier for thousands of years?
0:48:04 > 0:48:06Yes, look at this.
0:48:06 > 0:48:09Beautiful. You're seeing life in ice.
0:48:09 > 0:48:13We now know that some microorganisms
0:48:13 > 0:48:18are capable of actually causing the ice to melt,
0:48:18 > 0:48:23because they generate, essentially, anti-freeze proteins.
0:48:23 > 0:48:29They change the temperature at which ice goes from a solid state to a liquid state.
0:48:29 > 0:48:32And they could have been forming little tiny pockets,
0:48:32 > 0:48:34maybe only a few microns in diameter,
0:48:34 > 0:48:39but if he can make a two or three micron diameter ball of liquid water,
0:48:39 > 0:48:41and he has the ability to move,
0:48:41 > 0:48:47then that bacterium is now not in a glacier, but he's in an ocean.
0:48:47 > 0:48:51What are the implications of these discoveries?
0:48:51 > 0:48:58The fact that you've got living bacteria inside ice on Earth, what are the implications for Europa?
0:48:58 > 0:49:04You can clearly have bacteria like this in the frozen ice near the surface crust.
0:49:04 > 0:49:07And the thing that is most exciting to me,
0:49:07 > 0:49:12is that surface crust of Europa has a wide variety of colours
0:49:12 > 0:49:16that are highly suggestive of microbial life.
0:49:16 > 0:49:21And so there is a very, very strong possibility
0:49:21 > 0:49:26that the ice of Europa may contain viable, living microorganisms.
0:49:30 > 0:49:35It's a controversial idea, but it is a dizzying thought
0:49:35 > 0:49:39that the mysterious red stains on the surface of Europa
0:49:39 > 0:49:44could be the visible signs of alien life.
0:49:49 > 0:49:53The discovery of the huge ocean of liquid water
0:49:53 > 0:49:59under the surface of this tiny moon, combined with the potential for life in ice,
0:49:59 > 0:50:04and the intriguing red markings that criss-cross its surface,
0:50:04 > 0:50:10have made Europa the most fascinating and important alien world we know.
0:50:20 > 0:50:23A true wonder of the solar system,
0:50:23 > 0:50:29because it's our best hope of finding extraterrestrial life.
0:50:47 > 0:50:50That question, are we alone in the universe?
0:50:50 > 0:50:57Is this the only planet amongst the billions of planets in our galaxy,
0:50:57 > 0:51:02amongst the billions of galaxies in the universe, that harbours life?
0:51:02 > 0:51:07Is, I think, one of the most important questions,
0:51:07 > 0:51:10perhaps THE most important question that we can ask.
0:51:10 > 0:51:13Think about what it would mean for us
0:51:13 > 0:51:18if the answer was that there was no other life in the solar system,
0:51:18 > 0:51:21in our galaxy, perhaps even in the universe.
0:51:21 > 0:51:25How valuable would that make planet Earth?
0:51:25 > 0:51:27How valuable would that make us?
0:51:27 > 0:51:33But then imagine that the answer is that, on every moon of every planet
0:51:33 > 0:51:38where the conditions are right, then life survives and flourishes.
0:51:38 > 0:51:43That makes us part of a wider cosmic community,
0:51:43 > 0:51:47if the universe is teeming with life.
0:51:48 > 0:51:53If knowing the answer to the question is so profoundly important,
0:51:53 > 0:51:59then surely striving to find the answer should be of overwhelming importance.
0:51:59 > 0:52:03I believe it's the most important question you can possibly ask.
0:52:03 > 0:52:06Because we have a chance of answering it.
0:52:21 > 0:52:24What we've learned from the extreme places on Earth
0:52:24 > 0:52:30is that, if there is life out there in the solar system, it will almost certainly be simple.
0:52:30 > 0:52:38Single-celled organisms like bacteria eking out an existence in the most hostile of environments.
0:52:50 > 0:52:52One thing seems certain.
0:52:52 > 0:52:57The only place in the solar system where there is complex life,
0:52:57 > 0:53:00life that can build a civilisation,
0:53:00 > 0:53:03is here on planet Earth.
0:53:04 > 0:53:11But how did that happen? What is it that makes our world so special?
0:53:11 > 0:53:18Because, after all, everything in the solar system shares the same genesis.
0:53:20 > 0:53:29It was all created out of nothing more than a spinning cloud of gas and dust 4.5 billion years ago.
0:53:38 > 0:53:42Solid worlds condensed out of the swirling mists.
0:53:42 > 0:53:46But those worlds were radically different.
0:53:50 > 0:53:56Around the solar system, there are worlds that erupt with volcanoes of sulphur.
0:53:58 > 0:54:00And others with geysers of ice.
0:54:03 > 0:54:08There are worlds with rich atmospheres and swirling storms.
0:54:11 > 0:54:14And there are moons carved from ice
0:54:14 > 0:54:17that hide huge oceans of liquid water.
0:54:22 > 0:54:27But there's only one world where the laws of physics have conspired
0:54:27 > 0:54:30to combine all these features in one place.
0:54:33 > 0:54:38On Earth, the temperature and atmospheric pressure are just right
0:54:38 > 0:54:43to allow oceans of liquid water to exist on the surface of the planet.
0:54:46 > 0:54:51And it's big enough to have retained its molten core
0:54:51 > 0:54:54that not only powers geysers and volcanoes,
0:54:54 > 0:54:58but also produces our magnetic field
0:54:58 > 0:55:03that fends off the solar wind and protects our thick, nurturing atmosphere.
0:55:12 > 0:55:16It's the combination of all those wonders in one place
0:55:16 > 0:55:21that allowed life to begin and to get a foothold here on Earth.
0:55:21 > 0:55:27But, to allow that life to evolve into such complex creatures as ourselves
0:55:27 > 0:55:30requires one more ingredient.
0:55:30 > 0:55:33And that's time. Deep time.
0:55:33 > 0:55:41The kind of time over which mountains rise and fall, and planets are formed and stars live and die.
0:55:41 > 0:55:48And it's perhaps that that makes the earth so rare and so precious in the cosmos.
0:55:48 > 0:55:53Because it's been stable enough for long enough for life to evolve
0:55:53 > 0:55:56into such magnificent complexity.
0:56:05 > 0:56:11The life we have on Earth today is the result of millions of years of stability.
0:56:13 > 0:56:17And the pinnacle of that is us, humankind.
0:56:19 > 0:56:23A species that has developed to the point where we can bend
0:56:23 > 0:56:28and shape and change the world around us.
0:56:30 > 0:56:34We have even left our own planet behind
0:56:34 > 0:56:38to begin exploring our cosmic surroundings.
0:56:49 > 0:56:54You could take the view that our exploration of the universe
0:56:54 > 0:56:57has made us somehow insignificant.
0:56:57 > 0:57:04One tiny planet around one star amongst hundreds of billions.
0:57:04 > 0:57:06But I don't take that view.
0:57:06 > 0:57:13Because we've discovered that it takes the rarest combination of chance, and the laws of nature,
0:57:13 > 0:57:18to produce a planet that can support a civilisation.
0:57:18 > 0:57:21That most magnificent structure
0:57:21 > 0:57:26that allows us to explore and understand the universe.
0:57:26 > 0:57:33And that's why, for me, our civilisation is THE wonder of the solar system.
0:57:34 > 0:57:38MUSIC: "Calling Occupants Of Interplanetary Craft" by the Carpenters
0:57:39 > 0:57:43# Calling occupants of interplanetary craft
0:57:46 > 0:57:48# Calling occupants...
0:57:48 > 0:57:54And if you were to be looking at the Earth from outside the solar system,
0:57:54 > 0:57:56that much would be obvious.
0:57:57 > 0:58:01# Calling occupants of interplanetary craft...
0:58:04 > 0:58:11We have written the evidence of our existence onto the surface of our planet.
0:58:11 > 0:58:18Our civilisation has become a beacon that identifies our planet as home to life.
0:58:18 > 0:58:23# We'd like to make a contact with you
0:58:29 > 0:58:36# Calling occupants of interplanetary, anti-adversary craft
0:58:41 > 0:58:45# We are your friends
0:58:49 > 0:58:52# We are your friends...#
0:58:57 > 0:59:01If you'd like to know more about the solar system,
0:59:01 > 0:59:06go to bbc.co.uk/science.
0:59:06 > 0:59:09Subtitles by Red Bee Media Ltd