0:00:07 > 0:00:09Across the world,
0:00:09 > 0:00:13a daring and far-fetched experiment is under way.
0:00:15 > 0:00:17I'm going to increase more.
0:00:19 > 0:00:24It's very risky, but it's worth doing, and also,
0:00:24 > 0:00:30if I succeeded, I will be the king. HE LAUGHS
0:00:31 > 0:00:35Scientists are attempting a journey that previous generations
0:00:35 > 0:00:38have only dreamed of.
0:00:39 > 0:00:42I can't imagine a less hospitable place for people.
0:00:42 > 0:00:45High pressures, white hot temperatures.
0:00:45 > 0:00:47Nasty place.
0:00:51 > 0:00:57They are trying to reach the centre of the Earth.
0:00:57 > 0:00:593, 2, 1...
0:01:03 > 0:01:07What they are glimpsing is a bizarre and alien world.
0:01:09 > 0:01:14We're at a golden age, in terms of the real discovery of the bulk of the deep Earth.
0:01:19 > 0:01:22It's really almost a planet within our own big planet.
0:01:24 > 0:01:27It's like a forest.
0:01:29 > 0:01:31It looks very interesting.
0:01:32 > 0:01:35Their work is opening up a window
0:01:35 > 0:01:38on one of the great mysteries of the solar system.
0:01:40 > 0:01:43The Earth's core.
0:01:44 > 0:01:51A hidden world, 4,000 miles, deep beneath your feet.
0:02:05 > 0:02:07The Goddard Space Flight Centre
0:02:07 > 0:02:12is NASA's mission control for unmanned spacecraft.
0:02:16 > 0:02:21From here, scientists manage many of its most important telescopes and satellites.
0:02:25 > 0:02:28Space engineer Ken LaBel has devoted his career
0:02:28 > 0:02:33to perfecting the smooth running of these explorations of the stars.
0:02:36 > 0:02:41But in February 1997, he was thrown into a space mystery,
0:02:41 > 0:02:46that would offer clues to what is happening deep within our own Earth's core.
0:02:49 > 0:02:52The Hubble Space Telescope was in trouble.
0:02:56 > 0:03:01It's a Friday afternoon. I'm at the office, and the phone rings.
0:03:01 > 0:03:04Engineer who I've been working with called me and said,
0:03:04 > 0:03:07"Well, you know, we newly launched last month, two new instruments.
0:03:07 > 0:03:12"We're seeing some problems we weren't anticipating."
0:03:15 > 0:03:19Two ground-breaking new instruments had been installed on Hubble.
0:03:19 > 0:03:22They were designed to peer into deepest space,
0:03:22 > 0:03:25and to find black holes.
0:03:26 > 0:03:30But as Hubble criss-crossed the Earth,
0:03:30 > 0:03:34the highly sensitive multi-million dollar equipment was malfunctioning.
0:03:36 > 0:03:40The event they were seeing were these current spikes.
0:03:40 > 0:03:43If a signal is just moving along,
0:03:43 > 0:03:47all of a sudden you get some injection of noise so you get a spike.
0:03:48 > 0:03:54The issue for this particular device was that the error
0:03:54 > 0:03:57could end up being deadly.
0:03:57 > 0:04:00It could really take out their system. That was the fear.
0:04:00 > 0:04:05They'd lose this big scientific instrument that people spent
0:04:05 > 0:04:07most of their lives working on.
0:04:10 > 0:04:14What made finding the cause of these potential fatal spikes
0:04:14 > 0:04:17so urgent was that they were happening almost every day.
0:04:19 > 0:04:23One of the first tasks was to plot just where.
0:04:28 > 0:04:32And it soon became clear these weren't random events.
0:04:35 > 0:04:39They were tightly clustered across the centre of South America
0:04:39 > 0:04:40and the south Atlantic.
0:04:41 > 0:04:45It's an excellent indicator that our problem was being
0:04:45 > 0:04:49induced by that specific environment, and not because of a thermal issue,
0:04:49 > 0:04:52or a potentially a power system issue
0:04:52 > 0:04:56or some other type of spacecraft system not working appropriately.
0:04:56 > 0:05:00In fact this region of space has developed
0:05:00 > 0:05:04a reputation at NASA as a place of strange events.
0:05:07 > 0:05:11Astronauts have reported seeing flashes of light there.
0:05:13 > 0:05:17Satellites and space shuttle computers malfunction.
0:05:17 > 0:05:22It's even become known as space's Bermuda Triangle.
0:05:24 > 0:05:27Calling it the Bermuda Triangle is actually a good analogy.
0:05:27 > 0:05:32It's been called that several times over the past 20 or more years.
0:05:32 > 0:05:35It's a known hazard for spacecraft.
0:05:35 > 0:05:39The challenge was now figuring out what in the system was causing it,
0:05:39 > 0:05:41and what we could do about it.
0:05:46 > 0:05:50But this region of space wasn't only of interest to NASA,
0:05:50 > 0:05:55it also held important clues to what's happening in the deep Earth.
0:06:07 > 0:06:10It may be hidden 4,000 miles beneath our feet,
0:06:10 > 0:06:14but the core of our planet is central to life on Earth.
0:06:22 > 0:06:26Because it creates Earth's magnetic field.
0:06:30 > 0:06:35A tool for navigation that's vital for some of nature's greatest spectacles.
0:06:35 > 0:06:38The mass migrations that take place around the world.
0:06:42 > 0:06:45And a tool that helps us explore the planet, too.
0:06:47 > 0:06:52But most importantly of all, it helps protect life itself.
0:06:54 > 0:06:58Because the magnetic field it generates forms a vital barrier
0:06:58 > 0:07:00between us and the dangers of space.
0:07:03 > 0:07:08The core of the Earth and its magnetic field certainly played
0:07:08 > 0:07:11a role in the evolution of life on Earth.
0:07:11 > 0:07:14It shields us from the solar wind, and particles,
0:07:14 > 0:07:17boiling off the surface of the sun.
0:07:19 > 0:07:26Yet, for all the core's importance, though we have travelled high above our planet,
0:07:26 > 0:07:29we've never made it down to reach its heart.
0:07:33 > 0:07:40And that's because the barriers to a physical journey to the core are truly formidable.
0:07:52 > 0:07:55One place on the American continent,
0:07:55 > 0:07:59where you begin to get a sense of the challenges of descending
0:07:59 > 0:08:02into the Earth, is a deserted gold mine, Homestake.
0:08:08 > 0:08:11Dr Bill Roggenthen is a geologist
0:08:11 > 0:08:15and explorer of the subterranean planet.
0:08:16 > 0:08:22He's on the 12-minute journey down, to what is now the deepest laboratory in the USA.
0:08:29 > 0:08:31There's snow at the surface but underground,
0:08:31 > 0:08:34conditions are very different.
0:08:35 > 0:08:37It was chilly at the top.
0:08:37 > 0:08:40It's still early spring here in South Dakota,
0:08:40 > 0:08:42and as we go down, steadily,
0:08:42 > 0:08:46the temperature, at least the rock temperature
0:08:46 > 0:08:52increases at a rate of around over 22 degrees C per kilometre.
0:08:58 > 0:09:03Bill is travelling through the first barrier on any journey to the core.
0:09:08 > 0:09:11The Earth's crust.
0:09:11 > 0:09:16A shell of rock, typically around 35 kilometres thick.
0:09:22 > 0:09:27OK, so now we're standing on the 4,850 foot level,
0:09:27 > 0:09:31almost one-and-a-half kilometres below the surface.
0:09:32 > 0:09:36Homestake mine is the deepest anyone's managed to dig in the USA.
0:09:45 > 0:09:49Here, the rock temperature is around 29 degrees C.
0:09:52 > 0:09:55Another kilometre down and it would be hotter than the highest temperature
0:09:55 > 0:09:57ever recorded at the surface.
0:10:00 > 0:10:03And, right now, we are doing experiments
0:10:03 > 0:10:07and getting experiments going at this very deep location.
0:10:12 > 0:10:16As scientists probe the inner workings of our planet,
0:10:16 > 0:10:20it's not just the temperature they're contending with...
0:10:22 > 0:10:23..it's also the pressure.
0:10:31 > 0:10:33Any time when you're in the sub-surface
0:10:33 > 0:10:37and you make an opening, nature wants to close it.
0:10:37 > 0:10:39The deeper you are,
0:10:39 > 0:10:44the harder that nature works to try to get rid of that opening.
0:10:46 > 0:10:50Here, the rocks are particularly strong.
0:10:50 > 0:10:53But such is the weight of the ground above, even at Homestake,
0:10:53 > 0:10:55nothing is quite rock solid.
0:10:57 > 0:11:01So these instruments here are measuring the movement
0:11:01 > 0:11:06of the free surface of the rock way back into the rock itself.
0:11:06 > 0:11:10So, even though that movement is very minor, we need to
0:11:10 > 0:11:14monitor it to make sure these excavations are remaining stable.
0:11:16 > 0:11:21In some areas of the crust, it would be impossible to keep an excavation open at this depth
0:11:21 > 0:11:25because, at this intense pressure,
0:11:25 > 0:11:27solid rocks can behave like elastic
0:11:27 > 0:11:31and even change their constituency to become plastic.
0:11:34 > 0:11:41Yet, we're only 0.02% of the way to the centre of the Earth.
0:11:45 > 0:11:48So, even at this depth, why it's a huge amount,
0:11:48 > 0:11:52the pressure at the Earth's core is 50 million lbs per square inch.
0:11:52 > 0:11:54Truly immense.
0:11:59 > 0:12:01That means that at the centre of the Earth,
0:12:01 > 0:12:05the pressure is three million times that at the surface.
0:12:07 > 0:12:11The temperature is over 4,000 degrees Celsius.
0:12:11 > 0:12:14As hot as the surface of the sun.
0:12:17 > 0:12:20We're an ant, if you will, as we, kind of,
0:12:20 > 0:12:22burrow around this part of the world.
0:12:22 > 0:12:26Having said that, it's a tremendous opportunity to go down inside
0:12:26 > 0:12:29and see at least what this small part of the world
0:12:29 > 0:12:33looks like in three dimensions, and that's really exciting.
0:12:35 > 0:12:40The vastly increasing pressures and temperatures mean man will
0:12:40 > 0:12:43never be able to physically dig to the core.
0:12:44 > 0:12:50Scientists have had to search for other means to penetrate any further into the Earth.
0:13:18 > 0:13:21People thought I was a little nuts coming here.
0:13:21 > 0:13:26Famous people bet me money that I wouldn't stay for more than two years, and they all had to pay.
0:13:29 > 0:13:32Professor Rick Aster is one of America's leading explorers
0:13:32 > 0:13:38of the inner planet, though he has barely travelled below the surface.
0:13:40 > 0:13:44As former President of the Seismological Society of America,
0:13:44 > 0:13:45he doesn't need to.
0:13:48 > 0:13:53Next to his lab in New Mexico is the university's test site.
0:13:53 > 0:13:58It has provided him with a perfect landscape for an alternative way of seeing into the underworld.
0:14:01 > 0:14:07Earth tremors - natural and manmade.
0:14:09 > 0:14:11Seismology really is the killer application,
0:14:11 > 0:14:12when you get right down to it.
0:14:12 > 0:14:15It's the only methodology that we have
0:14:15 > 0:14:19to remotely study the deep interior of the Earth with any kind of resolution.
0:14:22 > 0:14:28Today, Rick is blowing up a tonne of high explosives to generate seismic waves.
0:14:34 > 0:14:38Although it's on a small scale, it's exactly the type of thing
0:14:38 > 0:14:41that we would need to look through the interior of the entire planet.
0:14:42 > 0:14:44WARNING ALARM WAILS
0:14:47 > 0:14:50Three seismographs have been set up.
0:14:50 > 0:14:53One, just meters away from the explosives.
0:14:53 > 0:14:57Another, at one kilometre distance.
0:14:57 > 0:15:01And the third, two kilometres from the blast.
0:15:01 > 0:15:05They'll measure how the Earth moves in response to the detonation.
0:15:05 > 0:15:09Particularly with these shallow explosions,
0:15:09 > 0:15:11most of the energy actually goes into the air.
0:15:11 > 0:15:14What I'm interested in is how much goes into the Earth.
0:15:15 > 0:15:185...
0:15:18 > 0:15:214...
0:15:21 > 0:15:233...
0:15:23 > 0:15:252...
0:15:25 > 0:15:261.
0:15:35 > 0:15:39- Always impressive. - HE CHUCKLES
0:15:39 > 0:15:42There's no doubt that that sent a lot of energy into the ground.
0:15:42 > 0:15:47I think we'll see a very strong signal from this explosion.
0:15:47 > 0:15:52This is what you see when you set off one tonne of explosives.
0:15:56 > 0:15:58A supersonic shockwave.
0:15:59 > 0:16:00But, hidden from view,
0:16:00 > 0:16:04a second pressure wave is travelling through the Earth.
0:16:06 > 0:16:07A seismic wave.
0:16:14 > 0:16:18And, it's what happens to this wave, underground,
0:16:18 > 0:16:20that Rick is interested in.
0:16:21 > 0:16:25Very close to the explosion we see a very simple signal.
0:16:25 > 0:16:31We see the seismic waves generated as a very sharp impulse
0:16:31 > 0:16:35travelling through the Earth, passing the seismograph, and it's over in just a second or two.
0:16:35 > 0:16:38At one and two kilometres,
0:16:38 > 0:16:43we see the development of a very rich wave train of scattered energies,
0:16:43 > 0:16:45scattering off the topography, the landscape,
0:16:45 > 0:16:51and scattering off the interior of the Earth, so that the signal is drawn out from this strong signal
0:16:51 > 0:16:53that was generated at the site.
0:16:57 > 0:17:02The further seismic waves travel, the more revealing they can be.
0:17:03 > 0:17:08Because the speed at which they move through the ground changes
0:17:08 > 0:17:13depending on the constituency of the material they pass through.
0:17:13 > 0:17:17The speed of the wave tells us, basically, how stiff the rocks are.
0:17:17 > 0:17:21That can tell us a lot about what's going on within the Earth.
0:17:21 > 0:17:23If you're studying a volcano, for instance,
0:17:23 > 0:17:26the speed of seismic waves slows down tremendously
0:17:26 > 0:17:30when it goes through magma, as opposed to rock.
0:17:35 > 0:17:41But to create seismic waves, which are able to pass all the way through the centre of the Earth,
0:17:41 > 0:17:43and out the other side...
0:17:43 > 0:17:46you need seismic events bigger than this one.
0:17:54 > 0:17:55Earthquakes.
0:18:01 > 0:18:07The shockwaves of major Earthquakes radiate through the globe.
0:18:07 > 0:18:11Scientists have gained a form of X-ray vision into the heart of the Earth
0:18:11 > 0:18:14by analysing the speed at which they travel.
0:18:17 > 0:18:22It's revealed that we aren't simply living on one solid chunk of rock.
0:18:25 > 0:18:28The Earth is made up of different layers.
0:18:30 > 0:18:33First, is the Earth's thin crust.
0:18:35 > 0:18:37The Earth's crust is really, really thin.
0:18:37 > 0:18:42It's about 0.3% of the way to the centre of the Earth.
0:18:43 > 0:18:45Then there's the mantle,
0:18:45 > 0:18:50made of rocks turned malleable by the extreme heat and pressure.
0:18:55 > 0:19:00The Earth's mantle is made of rocks that are, in some ways, similar to what we see at the crust,
0:19:00 > 0:19:03although their chemistry is a little different.
0:19:03 > 0:19:09But then the waves hit something else and, crucially,
0:19:09 > 0:19:10they slow down.
0:19:21 > 0:19:25To a seismologist, that could only mean one thing.
0:19:28 > 0:19:31The fact that seismic waves travel down through
0:19:31 > 0:19:35the mantle in a certain manner and then they hit the outer core, which
0:19:35 > 0:19:40has a much slower seismic velocity, indicated the Earth had a core.
0:19:40 > 0:19:44Indeed, it had an enormous core and it's molten.
0:19:44 > 0:19:48It has a viscosity that's not much greater than water.
0:19:48 > 0:19:54So it's an enormous ocean of white-hot, molten metal.
0:20:00 > 0:20:04Seismology has managed to reveal the Earth's core.
0:20:05 > 0:20:10A huge sea inside our planet, the size of Mars.
0:20:15 > 0:20:19But that wasn't all seismology detected.
0:20:22 > 0:20:28Scientists found signals of something else inside this sea of molten metal -
0:20:28 > 0:20:29an inner core.
0:20:33 > 0:20:36But for years, quite what it's like remained an enigma.
0:20:44 > 0:20:47CLASSICAL MUSIC
0:20:49 > 0:20:52The biggest breakthrough into the nature of this elusive inner core
0:20:52 > 0:20:58has come from a seismologist working as far away from the violence of Earthquakes as you can imagine.
0:20:58 > 0:21:03CLASSICAL MUSIC
0:21:03 > 0:21:08Dr Arwen Deuss took on a puzzle that had baffled every previous seismologist.
0:21:14 > 0:21:16So we have this mystery.
0:21:16 > 0:21:21We have an Earth which has a solid mantle and a fluid core.
0:21:22 > 0:21:28And people have discovered that there was actually an inner core inside this fluid outer core,
0:21:28 > 0:21:32but people didn't know for sure if this inner core was solid or fluid.
0:21:33 > 0:21:37It was a very difficult problem to solve.
0:21:42 > 0:21:45Arwen wasn't to be deterred.
0:21:45 > 0:21:50She suspected that the inner core was solid and was determined to prove it.
0:21:54 > 0:21:59If you want to prove that the inner core is solid,
0:21:59 > 0:22:02there's one specific wave you need to find, which is a tiny wave,
0:22:02 > 0:22:05and a really difficult wave to observe in seismograms,
0:22:05 > 0:22:09which we would call the shear wave, which can only travel through solid material.
0:22:13 > 0:22:17If Arwen could find a shear wave that had passed through
0:22:17 > 0:22:20the centre of the Earth, she'd prove the inner core was solid.
0:22:24 > 0:22:26But there was a major problem.
0:22:26 > 0:22:30How do you differentiate a tiny inner core shear wave,
0:22:30 > 0:22:36from the cacophony of other waves reverberating through the Earth?
0:22:37 > 0:22:38It's like a needle in a haystack.
0:22:38 > 0:22:42It's not something that pops out of the piece of paper
0:22:42 > 0:22:43when you look out the seismogram.
0:22:43 > 0:22:47So we realised we had to do something different if you want to find it.
0:22:47 > 0:22:49We couldn't repeat what other people had done.
0:22:52 > 0:22:54The hunt was on.
0:22:55 > 0:22:59A new approach to finding an inner core shear wave was needed.
0:23:02 > 0:23:05And Arwen found it in an incredible property of the Earth...
0:23:10 > 0:23:12The way in which the whole planet resonates
0:23:12 > 0:23:16when it has been struck by an Earthquake.
0:23:23 > 0:23:27Now when the Earth is hit by a major Earthquake, it's like a big hammer
0:23:27 > 0:23:31hits a string of a musical instrument,
0:23:31 > 0:23:34and that will start playing all the different tones of the Earth.
0:23:35 > 0:23:37NOTES RESONATE
0:23:37 > 0:23:41Now if we know that there's all these thousands of different tones
0:23:41 > 0:23:43happening in the Earth,
0:23:43 > 0:23:47what we can do is we can propose two different hypotheses.
0:23:50 > 0:23:55We can calculate what all these tones would look like for an Earth with a fluid inner core.
0:23:58 > 0:24:03And we can calculate what all these tones look like for an Earth with a solid inner core.
0:24:05 > 0:24:10By comparing the predictions, she finally knew where to look for the elusive shear wave.
0:24:13 > 0:24:18If she found it, she'd prove the inner core was solid.
0:24:18 > 0:24:22So this little peak here is our needle in the haystack.
0:24:22 > 0:24:26That is the thing we are going to be looking for in the real data.
0:24:27 > 0:24:31All she needed now was an Earthquake to test her theory on.
0:24:35 > 0:24:40A perfect candidate was a magnitude 7.9 quake that had
0:24:40 > 0:24:43occurred in 1996, under the Flores Sea in Indonesia.
0:24:47 > 0:24:50It was big.
0:24:50 > 0:24:52And it was deep.
0:24:54 > 0:24:58So it's one of the ideal Earthquakes to look for these inner core shear waves.
0:25:00 > 0:25:03She started collating the data of this quake
0:25:03 > 0:25:06from 47 different seismic stations across the world.
0:25:07 > 0:25:11This red box is where you would expect a wave from the inner core to arrive.
0:25:14 > 0:25:18Her hope was that the signal would eventually emerge
0:25:18 > 0:25:22through the noise created by thousands of other seismic waves.
0:25:25 > 0:25:30When we get to 40, we can see a little peak starting to appear.
0:25:30 > 0:25:33The question is, when we start adding more stations,
0:25:33 > 0:25:35is that bump going to grow or not?
0:25:36 > 0:25:39Arwen added station after station.
0:25:41 > 0:25:44This is what we're looking for. By adding more stations,
0:25:44 > 0:25:46the peak gets bigger, so this is quite exciting.
0:25:48 > 0:25:54She was on the brink of answering this fundamental question about the Earth's inner core.
0:25:55 > 0:26:00We add our last station, 47, and now we have a really large signal there.
0:26:04 > 0:26:06That's our needle in the haystack.
0:26:06 > 0:26:09We've got a really nice, strong, big arrival,
0:26:09 > 0:26:11proving that the inner core is solid.
0:26:14 > 0:26:18This shear wave, which could only pass through solid material,
0:26:18 > 0:26:22had travelled through the centre of the Earth.
0:26:24 > 0:26:28Arwen had discovered that sitting inside our planet
0:26:28 > 0:26:31is a solid metal ball, almost the size of the moon.
0:26:39 > 0:26:45But our solid inner core is proving stranger than Arwen could ever have imagined.
0:26:46 > 0:26:52Say you had an Earthquake at the North Pole and a seismometer at the South Pole,
0:26:52 > 0:26:56then a wave that would travel from the North Pole to the South Pole
0:26:56 > 0:27:01would arrive up to five seconds faster than from east to west if they go through the inner core.
0:27:03 > 0:27:06And we had no idea how to explain that.
0:27:08 > 0:27:12Seismology on its own simply can't unlock all the inner core's secrets
0:27:12 > 0:27:17but it seemed to be the only real way scientists could reveal them.
0:27:20 > 0:27:21Until now.
0:27:30 > 0:27:37In Japan, one man has pioneered a new technique to investigate the mysterious inner core.
0:27:38 > 0:27:46Because Kei Hirose is a scientist determined to leave the surface world behind,
0:27:46 > 0:27:50and complete an impossible mission to see the centre of the Earth.
0:27:53 > 0:27:57We cannot go into the centre of the Earth,
0:27:57 > 0:28:01but we can recreate the conditions
0:28:01 > 0:28:07corresponding at the centre of the Earth in my own laboratory,
0:28:07 > 0:28:10and it's a kind of journey to the centre of the Earth.
0:28:10 > 0:28:14I'll try to be the first person to reach there.
0:28:14 > 0:28:17It's very risky but it's worth doing,
0:28:17 > 0:28:22and also, if I succeeded...
0:28:24 > 0:28:26..I'll be the king! HE LAUGHS
0:28:33 > 0:28:37This is the SPring-8 Synchrotron Radiation Facility.
0:28:39 > 0:28:41Kei's using its powerful equipment
0:28:41 > 0:28:44in his attempt to recreate the immense temperatures
0:28:44 > 0:28:47and pressures found at the inner core.
0:28:47 > 0:28:50Somewhere rather more convenient to study.
0:28:53 > 0:28:55Oops.
0:28:57 > 0:29:00OK. So, as a diamond it looks beautiful,
0:29:00 > 0:29:04and then I put it on to the seed.
0:29:09 > 0:29:12The first part of his mission is to simulate the pressures
0:29:12 > 0:29:14found at the centre of the Earth.
0:29:16 > 0:29:23It took him ten years and hundreds of shattered diamonds to design an enormously powerful vice,
0:29:23 > 0:29:25using the tips of the jewels.
0:29:26 > 0:29:31Next, we can load the samples, and these are very tiny.
0:29:33 > 0:29:37Between the points Kei puts a sample...
0:29:37 > 0:29:39a shard of iron nickel alloy.
0:29:39 > 0:29:42The material scientists believe makes up the inner core.
0:29:45 > 0:29:50At the Earth's surface, it's composed of lots of tiny crystals.
0:29:50 > 0:29:53Kei hopes to show what happens to its structure
0:29:53 > 0:29:57under the extreme conditions found at the inner core.
0:29:59 > 0:30:02OK, that's fine.
0:30:05 > 0:30:09He uses the vice to raise the sample pressure to that of the inner core.
0:30:11 > 0:30:16It's equivalent to three medium times, atmospheric pressure,
0:30:16 > 0:30:22so it is very high pressure. But we just use a screwdriver
0:30:22 > 0:30:25to increase the pressure, to such extreme conditions.
0:30:25 > 0:30:28It's very simple.
0:30:37 > 0:30:42Part one of the mission complete. Now for stage two.
0:30:42 > 0:30:47Kei has to heat the sample to 4,700 Kelvin.
0:30:47 > 0:30:53A temperature found at the inner core, and on the surface of the sun.
0:31:01 > 0:31:05The beam of an infrared laser will be focused on the sample to raise its temperature.
0:31:12 > 0:31:18At the beginning of the experiment, Kei shines X-rays through the sample to create an image.
0:31:21 > 0:31:25The iron nickel crystals form a pattern of two concentric rings.
0:31:26 > 0:31:29So this image tells us what is going on inside the sample,
0:31:29 > 0:31:32under high pressure and high temperature.
0:31:34 > 0:31:36OK, so let's go.
0:31:39 > 0:31:41As the power of the laser is increased,
0:31:41 > 0:31:44the temperature of the sample rises.
0:31:47 > 0:31:52OK, so the sample is already about 1,500 Kelvins.
0:31:55 > 0:31:58Let's take the X-ray defraction images.
0:32:00 > 0:32:01And as the temperature grows,
0:32:01 > 0:32:05the iron nickel crystal structure begins to change.
0:32:10 > 0:32:16And now the temperature is about, OK, it's about 3,000 Kelvins.
0:32:16 > 0:32:19A uniform circular structure has all but gone,
0:32:19 > 0:32:22and crystals appear to be clumping together.
0:32:29 > 0:32:35Oh, now, the temperature is very high. It's almost close to the temperature at the core.
0:32:35 > 0:32:38You know, I'm very nervous at this moment.
0:32:40 > 0:32:43I'm going to increase more, OK?
0:32:48 > 0:32:49Oh, look at this.
0:32:49 > 0:32:54It is already 4,000 Kelvins, which is the real core temperature,
0:32:54 > 0:32:57and take another pattern here.
0:33:02 > 0:33:05Welcome to Kei's inner core of the Earth.
0:33:05 > 0:33:09For the first time, he has shown how iron nickel alloy crystals
0:33:09 > 0:33:14undergo a dramatic transformation under the pressures and temperatures
0:33:14 > 0:33:15found at the inner core.
0:33:20 > 0:33:26I think we should stop here. It's successful, we are very fortunate.
0:33:26 > 0:33:33We sometimes fail the experiment, but this time we are very lucky.
0:33:33 > 0:33:34Good.
0:33:38 > 0:33:42These X-ray images give us a real insight into the physical nature of the inner core.
0:33:44 > 0:33:49It's iron nickel alloy, but not as we know it.
0:33:49 > 0:33:51So, this is the first image.
0:33:51 > 0:33:58We have rings and it became spotty during heating.
0:33:58 > 0:34:02And the size of the crystal of iron nickel alloy increased
0:34:02 > 0:34:09by 1,000 times at core pressure and temperature, in our experiment,
0:34:09 > 0:34:11just in ten minutes.
0:34:15 > 0:34:18Over millions of years, under the extreme heat and pressure
0:34:18 > 0:34:22found at the core, these crystals could have grown to huge lengths.
0:34:25 > 0:34:29We may have very big crystals at the centre of the Earth.
0:34:29 > 0:34:33Maybe up to ten kilometres.
0:34:33 > 0:34:34It's like a forest.
0:34:37 > 0:34:39It looks very interesting.
0:34:44 > 0:34:48Kei believes this forest of crystals makes up the solid inner core of our Earth,
0:34:50 > 0:34:53with the crystals all pointing in the direction of the north pole.
0:34:55 > 0:35:01This could now explain why seismic waves travel through the core faster north to south,
0:35:01 > 0:35:03along the grain of the crystals,
0:35:03 > 0:35:05than east to west, across them.
0:35:07 > 0:35:13We tried many, many times, but we always failed.
0:35:13 > 0:35:20But we finally did and, you know, I realise how important it is.
0:35:20 > 0:35:27And, you know, probably it's a big achievement in my life.
0:35:30 > 0:35:35Kei's discovery is a significant step forward in our understanding of the core.
0:35:40 > 0:35:44But scientists' revelation of a white-hot metallic inner world
0:35:44 > 0:35:47raises another, more fundamental question.
0:35:50 > 0:35:54Why is the core of our planet so very different from everything
0:35:54 > 0:35:56we know at the rocky surface?
0:35:58 > 0:36:04The answer would ultimately turn out to be central to the story of life on Earth.
0:36:13 > 0:36:17Professor Dave Stevenson has made a career out of studying what lies
0:36:17 > 0:36:19beneath the surface of all planets in the solar system.
0:36:22 > 0:36:26I love looking at things that are difficult to understand,
0:36:26 > 0:36:32that are difficult to get to. So I've always been fascinated by cores.
0:36:32 > 0:36:36But one aspect of why
0:36:36 > 0:36:39I find Earth's core so fascinating,
0:36:39 > 0:36:41is that it - I believe -
0:36:41 > 0:36:47contains a memory of what happened in the history of the Earth.
0:36:56 > 0:37:02He believes to truly understand our core, we need to look up to the stars...
0:37:07 > 0:37:11..and go back to our planet's birth, in the violent collisions that
0:37:11 > 0:37:16happened during the formation of the solar system billions of years ago.
0:37:21 > 0:37:26The Earth's core formed through
0:37:26 > 0:37:31a very energetic set of events.
0:37:31 > 0:37:33Let's go back to the beginning.
0:37:33 > 0:37:40Imagine that you were bashing together bodies that were about the size of Mars.
0:37:40 > 0:37:45And when you do that, you produce an enormous amount of heat.
0:37:54 > 0:37:56EXPLOSIVE BOOM
0:37:56 > 0:38:00The early solar system was a brutal and chaotic place.
0:38:04 > 0:38:10But out of this fury, the conditions needed to forge our core were created.
0:38:17 > 0:38:18Heat.
0:38:20 > 0:38:28When you heat a mixture of solid material that is in the form of rock and iron,
0:38:28 > 0:38:34to very high temperatures, the iron will separate.
0:38:34 > 0:38:40It is heavy, and so it will sink under gravity to the centre
0:38:40 > 0:38:42of the Earth and the core will be formed.
0:38:47 > 0:38:54It's this separation of molten rock and metal that makes the outer layers of the Earth so different...
0:38:56 > 0:38:57..from the core inside.
0:39:05 > 0:39:08And the Earth's baptism of fire had another legacy.
0:39:10 > 0:39:14As the intense heat at the centre of our planet escaped,
0:39:14 > 0:39:17it caused the liquid metal within the core to move.
0:39:22 > 0:39:26This ceaseless motion in the depths of the Earth is what creates
0:39:26 > 0:39:30the magnetic field we experience at the surface.
0:39:35 > 0:39:40If you want to generate a magnetic field, the way the Earth does it,
0:39:40 > 0:39:44you need a metal. That's fine, iron is a metal, it needs to be liquid,
0:39:44 > 0:39:49that means it has to be hot. But you also need a temperature difference.
0:39:52 > 0:39:57As the heat flows from the hot inner core to the cooler mantle, it causes
0:39:57 > 0:40:01convection currents to form within the molten metal of the outer core.
0:40:07 > 0:40:13Those motions, through the process of electromagnetic induction,
0:40:13 > 0:40:17is the way in which the magnetic field is generated.
0:40:19 > 0:40:26And it's the generation of this magnetic field that is so vital to life on Earth.
0:40:31 > 0:40:35Because as charged particles are blown off the sun,
0:40:35 > 0:40:39the magnetosphere deflects them,
0:40:39 > 0:40:42creating a safe haven for our planet.
0:40:49 > 0:40:53This magnetic field is providing scientists with new insights
0:40:53 > 0:40:57into what's happening at the centre of the Earth...
0:40:57 > 0:40:58moment by moment.
0:41:13 > 0:41:18Geophysicist, Dan Lathrop, is on a mission to build a remarkable machine.
0:41:21 > 0:41:25He hopes it will do something no supercomputer has managed.
0:41:27 > 0:41:32Recreate the motions of molten metal in the core, to generate a magnetic field.
0:41:34 > 0:41:35He started small,
0:41:35 > 0:41:40but in his search for answers, the models have just got bigger...
0:41:40 > 0:41:42and bigger.
0:41:51 > 0:41:56What we don't know about the core is really details about the flows,
0:41:56 > 0:42:00and details about the magnetic fields inside the core.
0:42:00 > 0:42:02We know a bit about what happens at the surface,
0:42:02 > 0:42:05but this is a very thick layer of liquid metal
0:42:05 > 0:42:09and what happens underneath the surface is really a mystery to us.
0:42:16 > 0:42:19The experiment is fraught with danger.
0:42:19 > 0:42:24Dan plans to fill his core with 12 tons of molten sodium metal -
0:42:24 > 0:42:28a highly volatile element -
0:42:28 > 0:42:31and then spin it at up to 85 miles an hour.
0:42:40 > 0:42:44This is really as close to a model of the Earth as we're going to have.
0:42:44 > 0:42:48This device sets up a swirling mass of liquid metal as a mimic
0:42:48 > 0:42:51of what happens in deep Earth, but in a way that we
0:42:51 > 0:42:55can directly probe the flows, the rotating motions,
0:42:55 > 0:42:58and look at them in more detail than we could ever do for the Earth's core.
0:42:58 > 0:43:03He hopes this gargantuan model of the core will help explain
0:43:03 > 0:43:07something strange about the behaviour of Earth's magnetic field.
0:43:10 > 0:43:14It's never fixed, but constantly fluctuating.
0:43:14 > 0:43:18So, while most people think of the Earth's magnetic field
0:43:18 > 0:43:24as just being a simple north and south, it's really very complicated.
0:43:24 > 0:43:28There are patches of weaker field, patches of stronger field,
0:43:28 > 0:43:32all those are moving about the planet, some becoming weaker,
0:43:32 > 0:43:34some becoming stronger, in a very complex way.
0:43:37 > 0:43:39One thing is clear though.
0:43:39 > 0:43:42If the magnetic field is continually changing,
0:43:42 > 0:43:47then that must be caused by how the metal moves within the outer core.
0:43:53 > 0:43:58Early experiments have already hinted at what could be happening.
0:44:04 > 0:44:08Dan injected fluorescent dye into the rotating machine.
0:44:12 > 0:44:15The results suggest the core is place of great turbulence,
0:44:15 > 0:44:18filled with eddies and currents.
0:44:22 > 0:44:26You might think of the core, like the atmosphere of the Earth,
0:44:26 > 0:44:30being a very restless place with storms and fronts and bad weather.
0:44:30 > 0:44:36Very complicated turbulent motions, very complicated sets of vortices,
0:44:36 > 0:44:38all interacting with each other.
0:44:38 > 0:44:43And those drive motions like the convection we see in the atmosphere,
0:44:43 > 0:44:45billowing upwards motions in clouds.
0:44:45 > 0:44:50All of those then are shaped by the rotation of the core.
0:44:50 > 0:44:54And these deep motions interact with electric currents,
0:44:54 > 0:44:58drive electric currents and cause the Earth's main magnetic field.
0:45:03 > 0:45:07Dan's model is opening up a new window on the inner Earth.
0:45:09 > 0:45:14Our core may be a dynamo, but it's no simple one.
0:45:16 > 0:45:21Vast vortices and whirlpools create a magnetic field constantly in flux.
0:45:21 > 0:45:29And that causes unexpected phenomena that scientists are only now beginning to understand.
0:45:42 > 0:45:45Dr Jack Connerney has devoted his career at NASA
0:45:45 > 0:45:48to studying the magnetic fields of planets
0:45:48 > 0:45:50right across the solar system.
0:45:52 > 0:45:55Here at NASA's test facility,
0:45:55 > 0:46:01he's even got the ability to recreate the magnetic field of any heavenly body.
0:46:03 > 0:46:06But something that's really fascinated him
0:46:06 > 0:46:09are the changes that are happening to Earth's magnetosphere.
0:46:09 > 0:46:13And how they're related to the turbulent molten metal dynamo
0:46:13 > 0:46:15that is our core.
0:46:19 > 0:46:23The dynamo is electrically conducting fluid in motion,
0:46:23 > 0:46:26so when you have motion of that fluid,
0:46:26 > 0:46:29it carries with it the magnetic field.
0:46:29 > 0:46:33So, if you can look at how the magnetic field evolves in time,
0:46:33 > 0:46:38you are actually looking at how the fluid motion
0:46:38 > 0:46:41on the dynamo surface is evolving in time.
0:46:41 > 0:46:45So, by tracking the change in the magnetic field,
0:46:45 > 0:46:50we can essentially image the fluid motion on the surface of the core.
0:46:52 > 0:46:55By collating thousands of observations
0:46:55 > 0:46:57and the data from many satellites,
0:46:57 > 0:47:00scientists have been able to piece together a map
0:47:00 > 0:47:05of how Earth's magnetic field has been changing over the centuries.
0:47:09 > 0:47:12What they've discovered is that, over the last 180 years,
0:47:12 > 0:47:14it's been steadily weakening.
0:47:16 > 0:47:20Right now, the Earth field is decreasing fairly significantly,
0:47:20 > 0:47:21fairly rapidly.
0:47:24 > 0:47:26But, for Jack,
0:47:26 > 0:47:30there's one area of the magnetic field that particularly stands out.
0:47:35 > 0:47:38It's a region in our magnetosphere
0:47:38 > 0:47:41that's been weakening faster than any other.
0:47:45 > 0:47:49This is a map of the magnetic field, a contour map,
0:47:49 > 0:47:53and what you see here evolving in time, over hundreds of years,
0:47:53 > 0:47:57is a patch of very weak field in blue
0:47:57 > 0:48:00that slowly expands in size,
0:48:00 > 0:48:05becomes progressively weaker and weaker in field magnitude,
0:48:05 > 0:48:09and, as it does so, it's going to drift westward, slowly.
0:48:11 > 0:48:14This is the map scientists have created
0:48:14 > 0:48:18that shows just how a weakness in the Earth's magnetic field
0:48:18 > 0:48:20has been growing over 400 years.
0:48:23 > 0:48:29The blue patch of field is half the strength of that towards the poles.
0:48:29 > 0:48:31And scientists have given it a name.
0:48:33 > 0:48:36That weak field is the South Atlantic Anomaly.
0:48:41 > 0:48:44This region is still growing and, in just 200 years,
0:48:44 > 0:48:47it may cover the entire Southern Hemisphere.
0:48:53 > 0:48:56It's evidence that something truly remarkable is happening
0:48:56 > 0:48:58deep beneath our feet in the core.
0:49:02 > 0:49:07The first place the effects of it are felt aren't here on Earth,
0:49:07 > 0:49:09but high in space.
0:49:13 > 0:49:17And that's why NASA is so interested in the South Atlantic Anomaly.
0:49:17 > 0:49:18And in the core.
0:49:22 > 0:49:25It was the South Atlantic Anomaly that was to prove key
0:49:25 > 0:49:28to the space emergency that threatened the Hubble telescope.
0:49:30 > 0:49:34Two new multimillion-dollar instruments
0:49:34 > 0:49:36were repeatedly malfunctioning.
0:49:36 > 0:49:40And the upsets were occurring in just one area.
0:49:41 > 0:49:45Right in the heart of the South Atlantic Anomaly.
0:49:51 > 0:49:54Ken LaBel and his team needed to find out
0:49:54 > 0:49:56how the two phenomena could be related.
0:49:59 > 0:50:02They knew that the weak field at the South Atlantic Anomaly
0:50:02 > 0:50:07has one very significant effect on the structure of the magnetosphere.
0:50:07 > 0:50:09In that region of the South Atlantic,
0:50:09 > 0:50:13the Earth's magnetic field has a dip in it.
0:50:13 > 0:50:17In that region, the magnetic field changes its shape.
0:50:17 > 0:50:19It comes closer to the Earth.
0:50:19 > 0:50:24As the magnetic shield protects Earth from solar radiation,
0:50:24 > 0:50:28then in this dip charged particles like protons
0:50:28 > 0:50:31must be able to travel closer to our planet.
0:50:36 > 0:50:40Could these protons be causing the trouble with Hubble?
0:50:42 > 0:50:44Within two weeks,
0:50:44 > 0:50:45we had a test set built,
0:50:45 > 0:50:50and we went to one of the cyclotrons in the US to do some testing.
0:50:50 > 0:50:54And lo and behold, this part was quite susceptible to protons.
0:50:54 > 0:50:59The very culprit we'd expect to see issues with in the South Atlantic Anomaly.
0:51:03 > 0:51:07Every time Hubble passed through the South Atlantic Anomaly,
0:51:07 > 0:51:10it entered an exposed region of space.
0:51:12 > 0:51:16It was bombarded by charged particles.
0:51:19 > 0:51:22So, each of these events that we're seeing,
0:51:22 > 0:51:25those nine events in the first ten days,
0:51:25 > 0:51:31was a single proton hitting the sensitive portion of these devices.
0:51:31 > 0:51:36But, making the equipment completely proton proof
0:51:36 > 0:51:39was simply too difficult, even for NASA.
0:51:39 > 0:51:42Something else needed to be done.
0:51:42 > 0:51:45It was determined after a lot of work,
0:51:45 > 0:51:49both in testing and in environmental predictions,
0:51:49 > 0:51:52trying to come up with risk analyses,
0:51:52 > 0:51:56that, every time instruments pass the South Atlantic Anomaly,
0:51:56 > 0:51:59they turn off.
0:52:02 > 0:52:05It's never an even battle when you are dealing with
0:52:05 > 0:52:09something on as large a scale as the core and the magnetic field.
0:52:11 > 0:52:15So, a good story in the end for those instruments.
0:52:18 > 0:52:22Hubble's delicate sensors were now safe
0:52:22 > 0:52:26from the strange behaviour of the core deep under the South Atlantic.
0:52:30 > 0:52:34But the Anomaly is evidence of changes deep within the Earth
0:52:34 > 0:52:39that could ultimately have consequences for more than just satellites.
0:52:47 > 0:52:49To understand what these changes might be,
0:52:49 > 0:52:55scientists began mapping the magnetic field far below the ground.
0:52:55 > 0:53:00As we step down and look deeper and deeper inside the Earth,
0:53:00 > 0:53:03the field both grows in magnitude
0:53:03 > 0:53:07and it becomes more complex in structure and polarity.
0:53:09 > 0:53:12Scientists discovered that the simple North-South divide
0:53:12 > 0:53:14we experience at the surface
0:53:14 > 0:53:17breaks down at the level of the core.
0:53:19 > 0:53:23Under the South Atlantic, there are patches, indicated in green,
0:53:23 > 0:53:28where the magnetic field has actually flipped and points North.
0:53:31 > 0:53:34The combined effect of these patches,
0:53:34 > 0:53:37where the polarity of the field is reversed,
0:53:37 > 0:53:40is such to weaken the field over the South Atlantic.
0:53:40 > 0:53:43That weak field is the South Atlantic Anomaly.
0:53:52 > 0:53:54What could be happening
0:53:54 > 0:54:00in the molten metal ocean of the outer core to create these patches?
0:54:00 > 0:54:03Dan Lathrop thinks he knows.
0:54:08 > 0:54:12It's really the moving liquid metal's ability
0:54:12 > 0:54:15to drag and stretch and twist the magnetic field.
0:54:18 > 0:54:21In the same sense as we talk about a storm,
0:54:21 > 0:54:25when the air is being a particularly violent or unusual patch of weather,
0:54:25 > 0:54:29then there's some sort of flow structure down in the core under the South Atlantic
0:54:29 > 0:54:34that changed in such a way as to forcibly reverse the magnetic field.
0:54:36 > 0:54:42When scientists looked at the Earth's entire magnetic field at the level of the core,
0:54:42 > 0:54:47they discovered this perfect storm under the South Atlantic wasn't a one-off event.
0:54:50 > 0:54:54In fact, there are multiple patches where the field has flipped.
0:54:56 > 0:55:01Could these changes be harbingers of an even bigger shift?
0:55:03 > 0:55:08So, there's a very good chance that that South Atlantic Anomaly,
0:55:08 > 0:55:12that reversal at the level of the core, could deepen and spread,
0:55:12 > 0:55:15and that these small reversed patches in the Northern Hemisphere
0:55:15 > 0:55:16could also deepen and spread,
0:55:16 > 0:55:20and result in an overall reversal of the North-South pattern,
0:55:20 > 0:55:22the biggest structure in the magnetic field.
0:55:24 > 0:55:28So, if enough of these storms joined forces
0:55:28 > 0:55:31in the molten metal of the outer core,
0:55:31 > 0:55:35the Earth's magnetic field could reach a tipping point...
0:55:36 > 0:55:38..and flip.
0:56:06 > 0:56:09It's not a change that would happen overnight.
0:56:09 > 0:56:12The shifting flows of the core
0:56:12 > 0:56:17could take between 1,000 and 10,000 years to reverse our field.
0:56:21 > 0:56:23During this period, though,
0:56:23 > 0:56:28there would be some intriguing phenomena that we would all notice.
0:56:28 > 0:56:32During the reversal, the structure of the Earth's magnetic field
0:56:32 > 0:56:36could be more complicated than what we have now.
0:56:36 > 0:56:38So, instead of a north and south main pole,
0:56:38 > 0:56:41one could have two north poles and two south poles,
0:56:41 > 0:56:44or poles occurring at the Equator.
0:56:46 > 0:56:50The animals that rely on the core's magnetic field to navigate
0:56:50 > 0:56:54would have to find some other means to guide their migrations.
0:56:57 > 0:57:01And, the wandering magnetic poles would bring the Northern lights
0:57:01 > 0:57:04to unexpected locations.
0:57:05 > 0:57:09It wouldn't be the first time the flows of the outer core
0:57:09 > 0:57:12have undergone a dramatic change.
0:57:16 > 0:57:21Magnetised rocks contain a history of the core's turbulent past.
0:57:21 > 0:57:25We have very solid evidence that the Earth's magnetic field
0:57:25 > 0:57:29has reversed many hundreds of times in the Earth's history.
0:57:31 > 0:57:35So, the fact that we've seen so many changes and reversals,
0:57:35 > 0:57:38and so many changes in the historical times of the field,
0:57:38 > 0:57:42really gives us a view of the outer core being a very active place.
0:57:42 > 0:57:46It's not a question of IF the Earth is going to reverse its magnetic field, but WHEN.
0:57:50 > 0:57:55How soon this might be is one of the many mysteries of the core.
0:57:56 > 0:57:59But these remarkable experiments
0:57:59 > 0:58:03are now creating a real picture of the deep Earth
0:58:03 > 0:58:07to replace the fantasies of science fiction.
0:58:11 > 0:58:13We may never be able to go there.
0:58:15 > 0:58:19But we have a sense of what a journey might be like.
0:58:22 > 0:58:24One thing is certain, though...
0:58:26 > 0:58:31..this strange inner world is only STARTING to reveal its secrets.
0:58:48 > 0:58:51Subtitles by Red Bee Media Ltd
0:58:51 > 0:58:54E-mail subtitling@bbc.co.uk