NARRATOR (JACK FORTUNE): In the last few months a probe orbiting Mars has sent back astonishing pictures which have rekindled the search for life. Billions of years ago did a river flow through this canyon? If these gullies were created by water and if this was once a lake then there is every chance we will one day find signs of life on Mars. Ever since humans first realised there might be life elsewhere in the universe we've dreamed of aliens on Mars. Finding even a primitive life form would be one of the most important discoveries of all time.

PROF. WILLIAM HARTMANN (Planetary Science Institute, Arizona): I think the search for life on Mars is a perfect scientific question because either answer is so philosophically profound and interesting.

NARRATOR: If we found life on Mars it would mean there's likely to be life throughout the universe and if Mars is barren then maybe the rest of space is too. Maybe life on Earth is unique.

DR. MATTHEW GOLOMBEK (NASA Jet Propulsion Laboratory): Could life have started on Mars? If it didn't and the environment was right, why not, what, what, what was wrong with it? Did it take some chance in a billion random event to occur, or if it did happen what happened to it, how come there are not Martians running around.

NARRATOR: But there's an even more amazing possibility. Occasionally Mars is hit by such violent impacts that the debris is blasted right out into space. One of these rocks could have been the origin of life on Earth billions of years ago.

MATTHEW GOLOMBEK: Maybe actually life started on Mars and came to Earth via these meteorites. Maybe in fact we're all Martians. I mean you can ask one of the most fundamental questions that scientists can ask: are we alone in the universe? What's the frequency of life in the universe?

NARRATOR: For a planet to support life there is one vital ingredient it must have, one special substance which any alien anywhere will need. The search for life is the search for water. Life involves complex chemical reactions and as far as we know, complex chemistry needs water.

MATTHEW GOLOMBEK: Liquid water is the absolute key, fundamental, one and only must have it requirement for life. No liquid water, no life. It's just as simple as that.

NARRATOR: Mars seems an unlikely place to look for life. It's far too cold for water. Temperatures can be minus 100 degrees and there's virtually no atmosphere so every trace of moisture is sucked out into space. Unprotected a human being would last just a few seconds, but it seems that Mars was once far more hospitable. In 1969 NASA launched the first ever probe to go into orbit around Mars. When Mariner 9 arrived it found the planet completely shrouded in a massive dust storm. For a month they waited for the dust to clear.

PROF. JAMES HEAD (Department of Geological Sciences, Brown University): As soon as the images started coming back from Mariner 9 people began to get a really different picture of Mars.

NARRATOR: Everyone had expected to see a dead ball of rock like the Moon, but slowly they started seeing signs that Mars had once been very active.

JAMES HEAD: The first thing that emerged was this crater that must have been 20km above the surface because it was in the middle of the atmosphere, so it was like wow, there's something really big there that is sticking up into the atmosphere and above the atmosphere and in fact is volcanic so all of a sudden it wasn't like the Moon at all.

NARRATOR: And then they saw something even more amazing - what looked like dried up river valleys.

WILLIAM HARTMANN: That was a complete surprise. I mean this was supposed to be a dry, frozen planet and here we have evidence of things that look like old river beds.

JAMES HEAD: And it completely changed people's view of Mars in that sense back to something that might have been more Earth-like in the past and much more dynamic than the Moon.

NARRATOR: If these were dried up river beds it meant that Mars must once have had the perfect conditions for life. Rivers form from streams, streams are fed by rain, rain falls from clouds and clouds mean an atmosphere. Mars must once have been like the Earth is today, if these were rivers. The pictures from Mariner just weren't clear enough to be sure. Five years later the Viking orbiters arrived with better cameras. Some of these valleys had faint tributaries just like rivers on Earth.

DR. MICHAEL CARR (United States Geological Survey): And these really look like terrestrial river valleys when you look at them and you can see one here. You can see there's a, a valley through here and you can see it branches, there are tributaries. Here's one branch going off here with tributaries, so this looks very much like a terrestrial river system.

NARRATOR: But many scientists just wouldn't accept that rivers had once flowed on Mars. The Viking images still weren't good enough to settle the argument. Then in 1998 Mars Global Surveyor arrived equipped with the latest high resolution electronic cameras. Sections of the valleys were revealed in fantastic detail, but these amazing pictures were still no use. These valleys had all been eroded and filled in with sand. It was impossible to say how they'd been formed. Then, after they'd searched through thousands of images, they struck gold, a winding valley two kilometres wide and at a bend in the canyon a tiny channel, the unmistakable trace of an ancient river.

MICHAEL CARR: This is the best evidence that we have that climate in the past was different from what it is today, that there were warmer conditions on Mars.

NARRATOR: So Mars must once have been warm and wet like the Earth. It had rain and flowing rivers. Then at some point the atmosphere must have thinned and the planet cooled. The flowing rivers iced over and froze. But when did the rivers stop flowing? Was there time enough for life to emerge before the last traces of liquid water vanished from Mars? The river valleys are pockmarked with craters, so they must have formed billions of years ago during the beginning of the planet's history when Mars was still being bombarded with meteors, but there are other features carved into the surface more recently which tell a very different story about water on Mars - giant channels stretching for thousands of kilometres.

MICHAEL CARR: Well you see on this image a fairly typical example and you can see the channel on this picture here. Here's one side of the channel, here's the outline, you can see the sort of eastern shore of the channel here and there's the other side of the channel. These channels are enormous. This crater here is 60km across, so you can see that the, the channel down here is, is 200km across, so what happened, there was a lot of speculation as to how these channels formed. We, we looked into the possibility of lava, could lava cut a channel like this? Liquid hydrocarbons, liquid carbon dioxide, and of course water.

NARRATOR: Water seemed the most likely explanation, but how could anyone be sure what had carved these monster channels. For years scientists have been searching for clues on Earth. American geologist Jim Rice thinks he's found them, in Iceland. Buried under vast glaciers are some of the most active volcanoes in the world. It's a powerful combination which had shaped the entire landscape. When a volcano erupts lakes of steaming melt water form high up on top of the glacier until the enormous weight of water forces its way down bursting out from the base of the glacier. These are the biggest floods on earth, a surging wall of water stretching for kilometres. In their wake they leave vast plains littered with boulders. To a geologist these boulder fields contain unmistakable evidence of the flood waters that ripped through here.

DR. JIM RICE (Mars Exploration Programme, Arizona State University): OK, well this is really a classic textbook example. These two large boulders here. The size of them indicates this was an enormous flood, the deposit of these features, but also if you look at their geometry these boulders are kind of dipping back in this direction. This has given us information about the path the flood waters took that deposited these boulders. The flood waters were going in this direction where these rocks were deposited. Now there's other interesting things in the scene here. For instance you notice some of these smaller rocks are very well rounded and these little nicked corners of these boulders here. These are called percussion marks and these are produced in the water, highly turbulent water column when the boulders and rocks basically slam into one another knocking the corners off. These things all group together just tell you there's no doubt this was a catastrophic flood deposit.

NARRATOR: Four years ago the Americans sent a probe to land on Mars. It was aimed at the middle of one of these vast channels. For the first time we'd see what these strange features looked like from close up on the ground. The answer was obvious. The first view of the landing site sent back by the cameras on Pathfinder revealed a rock-strewn plain just like the boulder fields left by catastrophic floods on Earth. Here were the slanting boulders lined up by the flood waters and the cameras on the rover provided detailed views of the individual rocks with their telltale chipped edges.

MATTHEW GOLOMBEK: We saw perched rocks that were kind of sitting on a very small little, as if it had been deposited by this flood and actually scoured as water had run past it, so a whole suite of things like that. Observational evidence very, very strongly argues that, that this site is exactly what we thought it would be from orbit.

NARRATOR: Pathfinder confirmed that the vast Martian channels had been carved by enormous floods, so what does this mean for the chances of finding life? Some of these floods seem to be quite recent, so did Mars remain warm and wet for billions of years? Well that depends on what caused these giant floods.

MICHAEL CARR: They stop almost instantaneously in rubble-filled holes with a channel coming out and going off to the north and so that suggested that whatever caused the large channels it was, it erupted from the ground.

NARRATOR: Mike Carr's theory is that these floods are actually a sign that Mars was very cold. Deep underground geothermal heating melted ice. For millions of years the water built up trapped under pressure by a thick layer of permafrost, until something broke the crust. A vast reservoir of melt water is released and surges out of the ground.

MICHAEL CARR: As the water rushes out it would rush out so violent that it pulled a rock, the aquifer, with it so you'd have both rock and water coming out and then after the flood was over why the ground would collapse to form this rubble-filled hole that we see.

NARRATOR: To Mike Carr these floods are evidence that Mars has been deep frozen for billions of years.

MICHAEL CARR: I actually believe that cold climates are required to make these large flows. You can't get the large flows without this thick permafrost zone that keeps the water in the ground and doesn't allow it to get out.

NARRATOR: So Mars might have been frozen for almost its entire history with violent floods periodically bursting out from beneath the permafrost, so what does that mean for the chances of finding life? Well that depends on one other fundamental question.

JIM RICE: The question you have to ask is after these enormous floods what was the fate of that water?

NARRATOR: The floods on Mars were thousands of times bigger than anything on Earth today. Where did all the water go? The channels flow downhill into a giant basin around the north pole. Did the floods just fizzle out and freeze or were they big enough to fill this entire basin? Did they form an ice covered ocean which could have sheltered life for millions of years? NASA's Tim Parker is convinced there was an ocean on Mars. His evidence comes from features carved by a giant lake which once filled an enormous area of southern Utah.

DR. TIMOTHY PARKER (NASA Jet Propulsion Laboratory): We're flying over the, the floor of an ancient lake called Lake Bonneville that covered about half the state of Utah when it was at its maximum level about 10,000 years ago. The best evidence that there was a lake here is the erosion that occurred at the margin of the lake, the shorelines. Waves beating on the mountains and, and cutting cliffs and depositing ridges, beach ridges along the margin.

NARRATOR: One of the most dramatic features is the flat terrace in the side of this volcano cut by the water. It's a clear sign this was once an island in the middle of a giant lake. Tim believes he's seen similar features on Mars, flat terraces around rocky outcrops in the northern basin formed by the pounding of the ocean waves and around the edge of the basin Tim believes the old Viking images show two faint shorelines stretching for thousands of kilometres. He suggested these were cut by a shrinking ocean which might have existed for a billion years, but he had no way to prove it. Then last year Global Surveyor beamed back the first detailed contour map of Mars.

MICHAEL CARR: What we're looking at here is a view of the northern hemisphere. We're looking straight down on the pole and this is the pole. What the colours represent are elevations. This red here is the area of Tharsis where there are huge volcanoes and this one, Olympus Mons, is 2½ times the height of Mount Everest. It's a huge volcano. Now the large outflow channels, the largest ones are over here. The water came through here and down into the low areas here which are coloured blue.

NARRATOR: Now Jim Head could check the height of Tim Parker's proposed shorelines. A standing body of water forms a flat surface, so shorelines should be level along their entire length. The outer line, called contact 1, turned out to be extremely uneven, but contact 2 seemed much more like a shoreline.

JAMES HEAD: The younger contact, contact 2, was a much closer approximation to a flat line. It, it had bumps and wiggles as well, but in many places it was really straight and corresponded to the place where Parker had mapped the contact.

NARRATOR: But not everyone is persuaded. Last year the first close-up pictures of the edge of the basin came back from Mars and there's nothing that looks like a shoreline.

MICHAEL CARR: I do not find the evidence of these oceans convincing. I would expect to be able to see something because the original shorelines were depicted on the basis of imaging and yet when we go back and get better imaging of the, of the places where the shorelines have been mapped we're not seeing the shoreline there.

NARRATOR: But even on Earth evidence of shorelines can disappear when you get too close. The terrace cut into the volcano by Lake Bonneville just becomes a stretch of flat ground which means nothing this close in. The argument goes on, but perhaps life didn't need a whole ocean to get started. Just a few weeks ago NASA released one of the most important pictures ever taken of Mars. Layered rocks in a basin hundred of kilometres across, probably formed by the slow build up of sediments in a lake. Did this lake last long enough for life to evolve? Might these sediments contain fossilised Martians?

PROF. MICHAEL RUSSELL (Scottish Universities Environmental Research Centre): I think life would get started probably in a few hundred or perhaps 1,000 years so I don't think that we need huge amounts of time to, to worry about the origin of life. We don't, life kicks in I think pretty rapidly.

NARRATOR: On Earth it was about one billion years after the planet formed from a mass of molten rock that the first microscopic fossils appear, but no one actually knows how long it took the first living things to develop from the primordial soup.

MICHAEL RUSSELL: One needs millions to billions of years for evolution, but for the actual onset of life I don't think you need much time. I can look at Mars and think that this is a typical rocky, wet, sunny planet and life would have inevitably started on such a planet.

NARRATOR: Many scientists now believe that if life did emerge billions of years ago before Mars froze, craters once filled with water are the best place to look and there's one which stands out. 30 years ago NASA published pictures of a strange formation of white rock in the bottom of a crater 90km across.

MICHAEL RUSSELL: I just clocked the fact that it had been published and I thought it was a very interesting paper but I wasn't particularly interested in Mars at the time. It just rather made us think in a particular way.

NARRATOR: During a trip to central Turkey Mike Russell stumbled across a lake which made him think again about the white rocks on Mars. Near the southern shore Mike spotted strange white islands and decided to investigate further.

MICHAEL RUSSELL: Well these particular structures are probably only a few decades old and the lake has dropped level in the last 30 or 40 years or so. It's really exciting to come back here. I, I first came out here, I swam out here in 1992 and I couldn't believe my eyes because we'd been led to believe these were sand-dunes.

NARRATOR: Mike Russell discovered these islands were actually something far more extraordinary. These rocks had been built by microscopic organisms. Photosynthetic bacteria, the most primitive life form on earth, exactly the kind of creature you'd expect to find on Mars.

MICHAEL RUSSELL: From then on we started to think well, could there have been life on Mars in these craters and could it be that some of these white rocks could have been formed by bacteria.

NARRATOR: This lake showed that bacteria can build huge rock formations and as it happened these white rocks were made of magnesium carbonate, exactly what you'd expect on Mars which has magnesium-rich soil. So could this mysterious white formation be a sign that life evolved in this crater long ago?

MICHAEL RUSSELL: There will be vestiges of water in craters, in lakes and so forth on Mars for perhaps 500 million years and that is absolutely plenty of time to evolve photosynthetic bacteria, so I would expect us to find eventually evidence for photosynthetic bacteria on Mars.

NARRATOR: And wherever they first appeared they could have quickly colonised the entire planet.

MICHAEL RUSSELL: Of course that bacteria can get anywhere. It can last a long time, they can get blown by winds from here and there, so any standing body of water on Mars would have captured some of this, some of these early bacteria and they would have then colonised any seepages in any of these crater lakes on Mars.

NARRATOR: Last year the camera orbiting Mars sent back the first close-up pictures of the white rocks. Could these have been islands in a lake teeming with bacteria or are they just sand-dunes sculpted and compacted by the Martian winds? The only way we'll know for sure is when we land a probe right on them. As scientists have pieced together the tantalising clues it seems more and more likely that early in its history there could well have been life on Mars, but what chance is there that anything could still be alive there today?

MICHAEL CARR: If it ever started then the chance of it surviving up to today is pretty good because life somehow finds niches and adapts to whatever's there.

NARRATOR: Until a few years ago it didn't seem likely that anything could survive being deep frozen for billions of years, but research in one of the coldest places on earth has helped changed that. In northern Siberia the brief summer sun breaks up the pack ice and thaws the rivers, but just below the surface the ground stays permanently frozen. Every year a team of Russian scientists comes here to drill into the permafrost in search of micro organisms.

DR DAVID GILICHINSKY (Institute of Biological Problems in Soil Science): People found the micro-organisms in permafrost in the end of 19th century. It was done specially in Russia when people found mammoths.

NARRATOR: Back then it was the mammoth they were after, but David Gilichinsky's team have found bacteria and algae which have been frozen underground for an astonishing length of time. The deeper they go they older the permafrost. The frozen cores are taken to their laboratory near Moscow. A sample is taken from the centre of the core. Then they see if anything's still alive. They've discovered that bacteria can survive in the permafrost for far longer than anyone had thought possible.

DAVID GILICHINSKY: When they started it was surprise for us when they find even 10,000 years old bacteria, but step by step you can come in Arctic two or three million years old.

NARRATOR: These micro-organisms have been living off the minute amounts of liquid water which exist even in permafrost, so could they survive indefinitely? In Antarctica the ground is even colder than Siberia and it's been frozen for much longer. This is the closest place on earth to conditions to Mars. The Russians have now teamed up with NASA scientists to drill here. A few months ago they found bacteria which may turn out to have been at -20ºC for more than 10 million years.

DAVID GILICHINSKY: We have some data, but we are not sure 100% but probably we have now these alleged bacteria from Antarctic permafrost. It's between 8 and 15 million years old.

NARRATOR: Bacteria had been buried here in frozen ground since before the beginning of human evolution. If life can survive in Antarctica for 15 million years then something could be waiting to be revived on Mars and last summer Mars Global Surveyor sent back another extraordinary set of pictures which suggests that in places the Martian permafrost has been melting.

WILLIAM HARTMANN: The camera has detected hundreds of examples of cliff sides on Mars with little gullies running down the cliffs. Now these look exactly like gullies that would form on the Earth when, when an underground aquifer comes out the side of a cliff, a little bit of water comes out, runs down the gully, it makes a seep, it just keeps eroding down there. These cases on Mars you have a gully running down, then you, in many cases you even have a little mud-like deposit at the bottom.

NARRATOR: These gullies must be very recent. There's no sign of any erosion by the wind, so if they were carved by water our whole view of Mars would have to change. It means there could be liquid water just below the surface right now.

MICHAEL CARR: The temperature here typically is -60º, -70º, -80ºC. It's extremely cold, so it's hard to imagine how one could have seepages of water under these very cold, very cold conditions.

NARRATOR: Any water seeping from below would freeze long before it reached the surface, but perhaps the water could build up underground, trapped by a thick plug of ice until the pressure gets so great that the water forces a way out, gushing so fast that even in the icy Martian conditions it could carve a gully before freezing again. Bill Hartmann believes he's found signs of what could be melting the water underground. His team have discovered that volcanoes on Mars have erupted more recently than anyone had expected. The most recent lava flows may be less than a million years old.

WILLIAM HARTMANN: For every time that you heat up the crust of Mars enough to melt rock, that's 1200ºC, I mean there could be a thousand times when you heat it up to zero and melt ice. It's not out of the question in my mind that some of these seeps have been active 1,000 years ago, 10,000 years ago, 100,000 years ago, numbers like that, or maybe more recently, you know, maybe it was six months ago.

NARRATOR: If there is still water erupting on Mars the implications for the history of life are fantastic.

WILLIAM HARTMANN: I'll spin the, the most glorious optimistic scenario which would be that the geothermal heating from below has for 4 billion years, the entire history of Mars, has, has kept open some liquid water channels some place on Mars so there's always been some place you could swim from this place to that place.

NARRATOR: If there has been water on Mars for 4 billion years what strange creatures might be waiting for us?

PRESIDENT BILL CLINTON: If this discovery is confirmed, it will surely be one of the most stunning insights into our universe that science has ever uncovered.

DR MONICA GRADY (Department of Mineralogy, The Natural History Museum): The events of that week in August 1996 are burned indelibly on my brain and I will never forget them until my dying day.

NARRATOR: Four years ago a team of American scientists claimed they had found evidence of microscopic life in a meteorite that had come from Mars. Allan Hills 84001 has become the most intensively studied rock in the world because proving that this rock contained life from Mars has turned out to be very difficult.

DAVID McKAY (NASA Johnson Space Centre): What I would like to do this afternoon is lead you through our story which is a bit of a detective story.

NARRATOR: In 1994 geologists heard about a new meteorite which had come from Mars.

MONICA GRADY: We requested three or four small fragments, small chips about the size of my fingernail.

NARRATOR: Monica Grady's team in London found unusually large amounts of carbonate in the Allan Hills meteorite which formed orange globules in the rock. Carbonates can sometimes be produced by living things, but the finding didn't mean much on its own. Then David McKay's team at NASA took a closer look.

DAVID McKAY: We were looking, scanning around in our electron microscope looking at Allan Hills and we particularly looking at carbonates.

MONICA GRADY: There were a lot of rumours and, and mutterings in corners about the possibility of something unusual having been found in this meteorite, but nobody would say for definite.

DAVID McKAY: And I started to see some features in Allan Hills which were very strange, somewhat elongated features that had segments in them. The next day we found a few more of 'em and we really got excited.

NARRATOR: They thought they'd found a tiny fossilised Martian.

DAVID McKAY: This is perhaps the most controversial part of our presentation, but we'll show you those anyway.

MONICA GRADY: Anybody else who'd seen Allan Hills 84001, who'd analysed it, who'd even had a grandma who's seen 84001 was being interviewed to, to get their, their take on, on what had happened.

DAN GOLDIN (NASA): The scientists will lay out for you how an ancient rock found its way from Mars and it got to Earth.

MONICA GRADY: The first time I saw the worm-like structure was at the press conference that the NASA scientists had.

DAVID McKAY: Martian micro fossils.

MONICA GRADY: And I must admit I was pretty amazed at the structure.

EVERRETT GIBSON (NASA): It is 4½ billion years old.

MONICA GRADY: It looks like a segmented worm, it looks like you might think a fossilised bacterium would look like, but then you realise well, crumbs, it's very small.

DAN GOLDIN: It means we're right on the edge of a potential unbelievable discovery that's going to rock our world, rock our world, if it's true.

NARRATOR: When McKay's team analysed the rock surrounding their worm they found three different substances often produced by bacteria on earth. These substances could all be explained without life, but McKay had a theory which would explain why they were all there together.

DAVID McKAY: Ours was the only one that could explain everything simultaneously with the same simple explanation, that is life on Mars. (At Press Conference) They are the remains of Martian life.

DR KENNETH NEALSON (NASA Jet Propulsion Laboratory): Is the fact that six things are consistent with the presence of life enough to convince you that you're making one of the, the most sensational claims ever made and I would say no, that, that what you need is evidence that requires life to explain it.

NARRATOR: This may look like a Martian worm, but Ken Nealson has shown how deceptive appearances can be at this minute scale. The pictures of the Allan Hills meteorite were taken using an electron microscope and to get the best images the rock sample was coated with an extremely thin layer of gold. Nealson's team have been looking at rock with a new kind of electron microscope. This one can work without the gold coating.

KENNETH NEALSON: To my way of thinking it's very impressive how different the samples are when they're coated with gold or not coated.

NARRATOR: His uncoated rock looked jagged and crystalline at high magnification, but add the gold coating and tiny blobs appear which are about the same size as the famous Martian worm.

KENNETH NEALSON: The edges now can be rounded off with the gold and even an expert could be fooled. You'll look at it and you'll say wow, you know that could be life.

NARRATOR: So this might just be rock fragments made to look like a worm by a thin coating of gold. To prove it was once alive you have to find out what it's made of. In California Ken Nealson's team have been developing the kind of techniques which may one day be needed to probe Martian rocks for microscopic life. Their system works by firing a fine beam of electrons at the sample and it's astonishingly precise. They can find out what even the tiniest microscopic structures are made of, how much of each chemical element the sample contains. The first tests on the famous Allan Hills worm suggest it has exactly the same composition as the surrounding rock. If that's the case this was never living. The search for life on Mars began 25 years ago with the Viking missions. Two landers touched down and scooped up a sample of the Martian soil for chemical analysis. The billion dollar robots worked perfectly, but they failed to find proof of life. The Americans now realise that solving one of the greatest scientific mysteries could never be that straightforward.

MATTHEW GOLOMBEK: We sent two spacecraft which had extremely sophisticated life detection experiments and they came back negative. You don't need to go and land and poke your arm in and pull out soil and say is there a life eating chicken soup there? It's not, you know, that, we already did that and in a sense we didn't learn as much. We, maybe, maybe, and this is arguable certainly, maybe we asked too specific a question too soon.

NARRATOR: In 2003 the next missions will be heading for Mars. The Europeans have chosen to send the Beagle probe to scratch the surface looking for life, but NASA will be taking a giant leap backwards. The new rovers they're testing at the Mars yard will not be looking for life. They're just designed to gather basic information about the geological history of the planet.

MATTHEW GOLOMBEK: The rovers that we're sending have no life detection experiments. That's not the point. We've landed three places on Mars and those three places are all in the northern lowlands, they're all kind of the same. Well let's go see what the southern highlands are, what are they? Let's go land in places that have these unique signatures, let's go see what Mars is really about.

MAN: Rockets on the back shelf: fire!

NARRATOR: NASA is planning an armada of increasingly complex missions.

MAN: Three, two, one, release! Oh, sweet.

NARRATOR: When they've scoured every inch of Mars and they've identified the one place on the planet which is most likely to answer the question then they'll be ready to look for life.

MATTHEW GOLOMBEK: Get that kind of information, get that broad understanding first before you go in to ask the specific question, so I think that's, that's a noble question to try to answer - is there life on Mars today, but it's one that you dare not ask directly for maybe 20 years, maybe 30 years, maybe even more.

NARRATOR: The search for life on Mars may turn out to be one of the hardest problems science has ever faced. Perhaps it's a question no machine will ever be able to answer. Perhaps Mars will only give up its secrets when the first humans make the journey across the Solar System.

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