0:00:06 > 0:00:10We live on a world of wonders,
0:00:10 > 0:00:15a place of astonishing beauty and complexity.
0:00:17 > 0:00:22We have vast oceans, incredible weather,
0:00:22 > 0:00:26giant mountains and spectacular landscapes.
0:00:26 > 0:00:28If you think that this
0:00:28 > 0:00:33is all there is, that our planet exists in magnificent isolation,
0:00:33 > 0:00:34then you're wrong.
0:00:35 > 0:00:40We're part of a much wider ecosystem that extends way beyond
0:00:40 > 0:00:42the top of our atmosphere.
0:00:47 > 0:00:53I think we're living through the greatest age of discovery our civilisation has known.
0:00:53 > 0:00:57We've voyaged to the farthest reaches of the solar system.
0:00:57 > 0:01:01We've photographed strange new worlds,
0:01:01 > 0:01:05stood in unfamiliar landscapes, tasted alien air.
0:01:08 > 0:01:13Now, the laws of physics are simple and they're universal.
0:01:13 > 0:01:17What applies here applies everywhere else up there.
0:01:18 > 0:01:22But it's fascinating that they can manifest themselves
0:01:22 > 0:01:26in so many different ways across the solar system.
0:01:27 > 0:01:33In this programme, I'm going to look at how the universal forces of nature
0:01:33 > 0:01:39that created all this can also wreak devastation across the solar system.
0:01:41 > 0:01:44How they can be the death of a planet...
0:01:46 > 0:01:50..and how they can keep other worlds alive.
0:01:56 > 0:02:01I can just see pieces of molten rock rising up just below my chin.
0:02:04 > 0:02:09These forces are so far reaching, they bridge the depths of space
0:02:09 > 0:02:12and transform a world long thought dead
0:02:12 > 0:02:17into a world of perpetual change and everlasting youth.
0:02:18 > 0:02:24The intense source of heat that powers that eternal change also
0:02:24 > 0:02:28drives one of the most spectacular sights in the solar system.
0:03:17 > 0:03:22I've come to one of Earth's natural wonders,
0:03:22 > 0:03:25the Grand Canyon in Arizona.
0:03:28 > 0:03:32It's a place to be humbled by nature.
0:03:34 > 0:03:36Well, this is undoubtedly
0:03:36 > 0:03:39one of the most beautiful places I've seen on Earth.
0:03:41 > 0:03:43From sunrise to sunset,
0:03:43 > 0:03:49the changing light brings this immense landscape to life.
0:03:49 > 0:03:53I hear stories of people coming here at sunrise with tears in their eyes
0:03:53 > 0:03:56at the majesty of the view, and I can see why.
0:04:00 > 0:04:03It's incredible to think that this enormous valley
0:04:03 > 0:04:05was etched and carved
0:04:05 > 0:04:10by the action of running water over just a few million years.
0:04:14 > 0:04:17Our knowledge of natural wonders like the Grand Canyon
0:04:17 > 0:04:20was once limited to our own planet.
0:04:22 > 0:04:27But the space age has brought new worlds into view.
0:04:33 > 0:04:36This is Mars, the Red Planet.
0:04:36 > 0:04:41Fourth rock out from the sun, it has a canyon so vast
0:04:41 > 0:04:44you could fit our own Grand Canyon into one of its side channels.
0:04:50 > 0:04:53Named after the space probe that first saw it,
0:04:53 > 0:04:56this is the Valles Marineris.
0:05:00 > 0:05:068km deep and over 3,000km long,
0:05:06 > 0:05:10on Earth it would run all the way from Los Angeles to New York.
0:05:17 > 0:05:22We're beginning to get a deep and quite profound understanding
0:05:22 > 0:05:26of the way that Mars has evolved geologically because
0:05:26 > 0:05:30we are now there, because we now have eyes and ears on the surface.
0:05:30 > 0:05:35And there really is no substitute for actual exploration, for actually
0:05:35 > 0:05:38going somewhere and touching it and taking pictures of it.
0:05:42 > 0:05:44Look at this picture.
0:05:45 > 0:05:48This is a picture of a sunset.
0:05:49 > 0:05:50These are pictures of clouds.
0:05:53 > 0:05:57It's amazing to think these incredibly familiar-looking photographs
0:05:57 > 0:06:00were taken on the surface of Mars.
0:06:02 > 0:06:03This picture,
0:06:03 > 0:06:06or this amazing colour picture...
0:06:06 > 0:06:09I could have got a camera here and just snapped any picture
0:06:09 > 0:06:12and it would have looked exactly like this one.
0:06:12 > 0:06:15In the Grand Canyon, you can see the Colorado River
0:06:15 > 0:06:17running in the bottom of the valley
0:06:17 > 0:06:20so you can understand how this landscape was made.
0:06:20 > 0:06:23Whereas here on Mars, there's no sign of any water.
0:06:24 > 0:06:28So Mars is a puzzling place.
0:06:31 > 0:06:34Despite all the similarities between Mars and Earth,
0:06:34 > 0:06:39it's the differences between these two planets that are most telling.
0:06:39 > 0:06:43Mars is now a desolate dead wasteland,
0:06:43 > 0:06:47a world where the processes that sculpted its familiar landscapes
0:06:47 > 0:06:49seized up long ago.
0:06:53 > 0:06:56It's fascinating for me as a physicist to see how
0:06:56 > 0:07:02the same basic simple laws of nature can play out
0:07:02 > 0:07:08in such radically different ways, and produce such astonishingly
0:07:08 > 0:07:14varied and beautiful and violent and dead worlds
0:07:14 > 0:07:17out there, across the solar system.
0:07:36 > 0:07:40The Big Island of Hawaii, in the middle of the Pacific Ocean,
0:07:40 > 0:07:42holds the key to what happened to Mars.
0:07:44 > 0:07:48This is the perfect place to witness how a planet can be kept alive
0:07:48 > 0:07:52by nothing more than the simple flow of heat.
0:07:55 > 0:07:58You can smell the volcanic ash coming into the...
0:07:58 > 0:08:02into the helicopter. And everywhere you look, it looks like a...
0:08:02 > 0:08:05It's almost like an apocalyptic scene.
0:08:08 > 0:08:13This volcano is Kilauea, which means spewing.
0:08:13 > 0:08:18It's been erupting almost continuously since 1983.
0:08:18 > 0:08:22In fact, you can see molten rock flowing down the side
0:08:22 > 0:08:25of the mountain just cutting a swathe through the trees.
0:08:27 > 0:08:31This might look like widespread destruction,
0:08:31 > 0:08:35but volcanic eruptions are Earth's geological heartbeat.
0:08:35 > 0:08:40Active volcanoes make our planet a vibrant, living world.
0:08:40 > 0:08:43That is the most spectacular demonstration
0:08:43 > 0:08:47of our planet being geologically alive that I've ever seen.
0:08:56 > 0:08:58A few kilometres north of Kilauea,
0:08:58 > 0:09:02you can see just what volcanic action can produce,
0:09:02 > 0:09:03given enough time.
0:09:06 > 0:09:12The islands of Hawaii were built entirely by volcanoes.
0:09:12 > 0:09:18Today, these mountains are the largest volcanoes on our planet,
0:09:18 > 0:09:22and we've seen landscapes just like this, on Mars.
0:09:26 > 0:09:29It's quite an experience being 4km high.
0:09:29 > 0:09:34It makes you out of breath and you sniff a lot because your nose becomes...
0:09:34 > 0:09:36And makes you feel like you've had a few drinks.
0:09:38 > 0:09:39This is Mauna Kea,
0:09:39 > 0:09:43one of the five volcanoes that make up the Big Island of Hawaii.
0:09:43 > 0:09:45I've known about it since I was very little
0:09:45 > 0:09:48because it's one of the most famous observatories on the planet.
0:09:48 > 0:09:55Everywhere you look, surrounded by our eyes to the cosmos.
0:09:55 > 0:09:59Although this mountain is 4km above the surface of the Pacific,
0:09:59 > 0:10:04it's actually 10km above the surface of the Pacific floor.
0:10:04 > 0:10:07That makes it the highest mountain on Earth,
0:10:07 > 0:10:09over a kilometre higher than Mount Everest.
0:10:09 > 0:10:17But it is a tiny volcano compared to the biggest volcano on the surface of Mars.
0:10:28 > 0:10:30This is Olympus Mons,
0:10:30 > 0:10:35named after Mount Olympus, the mythical home of the Greek gods.
0:10:35 > 0:10:43This vast outpouring of lava stretches over 550km across,
0:10:43 > 0:10:46but it's the height of this volcano that is breathtaking.
0:10:48 > 0:10:52It soars 25km into the Martian sky,
0:10:52 > 0:10:57nearly three times the full height of Mauna Kea.
0:10:57 > 0:11:02But Olympus Mons isn't just the tallest volcano in the solar system,
0:11:02 > 0:11:05it's the highest mountain we know.
0:11:07 > 0:11:11There are striking similarities between the volcanic landscapes
0:11:11 > 0:11:15here on Hawaii and the giant volcanoes found on Mars.
0:11:15 > 0:11:20These similarities can be traced back billions of years
0:11:20 > 0:11:22to the fiery birth of the solar system.
0:11:25 > 0:11:30All this heat you can see driving this spectacular volcanic activity
0:11:30 > 0:11:33is a relic, a hangover of the Earth's formation.
0:11:33 > 0:11:40Now, all the rocky planets - Earth, Mars, Venus, Mercury - were formed in the same way.
0:11:40 > 0:11:46They came from a collapsing dust cloud over 4.5 billion years ago.
0:11:53 > 0:11:57With the ignition of the sun, our solar system was born.
0:11:59 > 0:12:02Little by little, the rocky bodies grew,
0:12:02 > 0:12:05falling together under their own gravity.
0:12:05 > 0:12:09This process not only generated immense amounts of heat,
0:12:09 > 0:12:13it also delivered radioactive material to the cores.
0:12:17 > 0:12:24These two ancient sources of heat power Earth's volcanoes to this day,
0:12:24 > 0:12:31but the volcanoes on Mars are little more than a petrified memory of a distant past.
0:12:31 > 0:12:34When we look down on the surface of Mars,
0:12:34 > 0:12:37we see no evidence of this kind of volcanic activity.
0:12:37 > 0:12:41As far as we can tell, Mars is a dead world.
0:12:43 > 0:12:48Mars must have had similar inner heat to Earth to build its volcanoes.
0:12:48 > 0:12:51Yet something obviously happened
0:12:51 > 0:12:54to stop the Red Planet's geological heartbeat.
0:12:56 > 0:13:00Now, as everyone who's left a hot cup of tea sat on the kitchen table knows,
0:13:00 > 0:13:04hot things lose heat to their cooler surroundings,
0:13:04 > 0:13:09and what's true for cups of tea, in physics is also true for planets.
0:13:09 > 0:13:15This is hot and that up there - space - is cold.
0:13:15 > 0:13:17So planets lose heat to space.
0:13:21 > 0:13:23Mars is a much smaller planet than the Earth.
0:13:23 > 0:13:29It's about half the diameter, it's an eighth of the volume,
0:13:29 > 0:13:33so there was much less heat trapped in there to begin with.
0:13:33 > 0:13:37Now, planets lose heat to space through their surfaces,
0:13:37 > 0:13:41and smaller things have a larger surface area
0:13:41 > 0:13:45in relation to their volume than big things.
0:13:45 > 0:13:51So that means that Mars will lose its heat to space much quicker than the Earth does.
0:13:55 > 0:13:58When the interior of Mars grew cold,
0:13:58 > 0:14:01the mighty volcanoes lost their lifeblood.
0:14:01 > 0:14:05The Red Planet's geological heart died,
0:14:05 > 0:14:10and millions of years ago, the surface of Mars ground to a halt.
0:14:11 > 0:14:14The fate of a whole planet was destined
0:14:14 > 0:14:17by the simplest of laws of physics.
0:14:31 > 0:14:37Since the dawn of human history, we've been able to gaze up into the night sky,
0:14:37 > 0:14:40but we're lucky because we're the first generation that's been able
0:14:40 > 0:14:45to build machines to actually go to those planets and moons.
0:14:45 > 0:14:49And we've found that they're more beautiful, more violent,
0:14:49 > 0:14:51more magnificent and fascinating
0:14:51 > 0:14:53than we could have possibly imagined.
0:14:56 > 0:14:59The more worlds we study, the more we realise
0:14:59 > 0:15:03that our solar system is a cosmic laboratory.
0:15:03 > 0:15:07Even the slightest differences in size or position
0:15:07 > 0:15:11can create a world radically different from its neighbours.
0:15:15 > 0:15:19So here on Earth, we have one beautiful manifestation of how those
0:15:19 > 0:15:25simple laws of nature can play out, how they can build a planet.
0:15:25 > 0:15:27In Mars we have another example.
0:15:27 > 0:15:31What happens when you take a planet that's smaller than Earth
0:15:31 > 0:15:33and move it further away from the sun?
0:15:33 > 0:15:37It loses its heat more quickly and it becomes geologically inactive.
0:15:37 > 0:15:41But what would happen if you took a planet just like the Earth
0:15:41 > 0:15:43and moved it a little closer to the sun?
0:15:46 > 0:15:49Well, we know of such a planet.
0:15:51 > 0:15:54It's the brightest point of light in our night sky.
0:15:56 > 0:15:59So similar in size to our own world,
0:15:59 > 0:16:03this planet has been called Earth's twin.
0:16:05 > 0:16:06This is Venus.
0:16:08 > 0:16:14Orbiting closer to the sun, Venus was named for its shining beauty.
0:16:16 > 0:16:20But our planetary twin hides its true identity
0:16:20 > 0:16:23beneath a thick blanket of cloud.
0:16:33 > 0:16:35Over the last 4.5 billion years,
0:16:35 > 0:16:41Venus has turned into an unimaginably oppressive world.
0:16:41 > 0:16:45The atmosphere is so dense that the pressure is crushing.
0:16:45 > 0:16:47It's 90 times atmospheric pressure here on Earth.
0:16:50 > 0:16:54Now, Venus takes 243 days to rotate once on its axis.
0:16:54 > 0:16:58That means its day is longer than its year.
0:17:01 > 0:17:04So Venus has the hottest average surface temperature,
0:17:04 > 0:17:08other than the sun's, anywhere in the solar system - 470 Celsius.
0:17:16 > 0:17:20I've come to India, to a place called the Deccan Traps.
0:17:24 > 0:17:28Hidden in this lush green landscape are tantalising clues
0:17:28 > 0:17:33to understanding how immense heat caused Venus to choke to death.
0:17:34 > 0:17:37When you look at this landscape today,
0:17:37 > 0:17:41it's incredibly peaceful and beautiful, rolling green hills,
0:17:41 > 0:17:45but I think it's astonishing to think
0:17:45 > 0:17:48that everything you see down there is lava.
0:17:48 > 0:17:53This whole landscape, half a million square kilometres of it, was created
0:17:53 > 0:17:59in an ongoing volcanic eruption that lasted for a million years.
0:18:01 > 0:18:06If you take away the green foliage, the underlying landscape of lava
0:18:06 > 0:18:10is actually very similar to what we see on Venus.
0:18:13 > 0:18:17Using radar to peer down through the clouds,
0:18:17 > 0:18:20the surface of Venus was finally revealed.
0:18:20 > 0:18:26It's covered with floods of solid lava, just like we see in India,
0:18:26 > 0:18:30but on a scale many thousands of times larger.
0:18:30 > 0:18:34We've also counted over 50,000 volcanoes,
0:18:34 > 0:18:38the most on any planet in the solar system.
0:18:38 > 0:18:41Venus is a similar size to Earth,
0:18:41 > 0:18:44so it may still have a hot geological heart
0:18:44 > 0:18:50powering its volcanoes but as yet we haven't witnessed any eruptions.
0:18:53 > 0:18:56The ancient floods of lava we see on Venus
0:18:56 > 0:19:00and here in India were created in much the same way.
0:19:02 > 0:19:05For both planets, this was volcanic activity in overdrive.
0:19:05 > 0:19:09The eruptions here in India 65 million years ago
0:19:09 > 0:19:13affected the Earth's climate so much that they're thought to have played
0:19:13 > 0:19:17a major role in the mass extinction events at the end of the cretaceous period,
0:19:17 > 0:19:21which wiped out over two-thirds of the species on Earth.
0:19:21 > 0:19:26Now, life on Earth recovered but Venus wasn't so lucky.
0:19:30 > 0:19:33The intense volcanic activity on both planets
0:19:33 > 0:19:35didn't just blast out molten rock.
0:19:35 > 0:19:42It also released copious amounts of gases, like carbon dioxide.
0:19:44 > 0:19:48But slight differences in the way the laws of physics played out
0:19:48 > 0:19:54on Venus helped push our cosmic twin down a path of no return.
0:19:56 > 0:20:01Venus and Earth reacted very differently to the same kind of volcanic cataclysm,
0:20:01 > 0:20:05and the reason for that is something that happens so often on Earth
0:20:05 > 0:20:07that we take it for granted, and just moan about it.
0:20:16 > 0:20:21Rain plays a significant role in keeping our planet a pleasant place to live.
0:20:24 > 0:20:27Acting as part of a global recycling system,
0:20:27 > 0:20:30rain keeps our atmosphere in balance,
0:20:30 > 0:20:35washing out the potent greenhouse gas, carbon dioxide,
0:20:35 > 0:20:39ready to be locked away in rocks in our oceans.
0:20:42 > 0:20:46But on Venus, the laws of physics have made it impossible
0:20:46 > 0:20:48for rainfall to cleanse its atmosphere.
0:20:48 > 0:20:52In fact, there's no liquid water at all.
0:20:54 > 0:20:59Venus lost its water essentially because it's hotter than the Earth.
0:20:59 > 0:21:03You see, temperature is just a measure of how fast things are moving around.
0:21:03 > 0:21:07So on Venus the oceans would have evaporated into the atmosphere
0:21:07 > 0:21:10and that water in the atmosphere would be moving around
0:21:10 > 0:21:13extremely quickly, simply because it's hot.
0:21:13 > 0:21:16And Venus is so close to the sun and so hot,
0:21:16 > 0:21:20those water molecules are moving so fast that the gravity of the planet
0:21:20 > 0:21:23can't continue to hold them in the atmosphere,
0:21:23 > 0:21:27and so they simply escape off into space.
0:21:30 > 0:21:34With no water, there is no rain on Venus.
0:21:34 > 0:21:38For billions of years, there has been nothing
0:21:38 > 0:21:42to temper the build-up of volcanic gases in its atmosphere.
0:21:45 > 0:21:50Venus ended up cocooned in a thick, high-pressure,
0:21:50 > 0:21:52dense blanket of carbon dioxide,
0:21:52 > 0:21:57and that made the temperature rise and rise and rise,
0:21:57 > 0:22:02turning Venus into the hell-like world we see today.
0:22:15 > 0:22:18Compared to scorched Venus and frozen Mars,
0:22:18 > 0:22:21our home is a very special ball of rock.
0:22:23 > 0:22:27Although governed by the same universal set of rules,
0:22:27 > 0:22:31our planet is not too big, not too small,
0:22:31 > 0:22:36not too hot, not too cold.
0:22:36 > 0:22:39Earth has been called the Goldilocks planet
0:22:39 > 0:22:41because everything is just right.
0:22:45 > 0:22:50Our world is unique but it doesn't exist in splendid isolation.
0:22:50 > 0:22:55It is intimately connected with its cosmic neighbours.
0:22:55 > 0:23:01Earth is not the master of its own destiny and it never has been.
0:23:01 > 0:23:05The life and death of our planet is influenced by forces
0:23:05 > 0:23:08emanating from the very depths of space.
0:23:14 > 0:23:17As we better understand our place in space,
0:23:17 > 0:23:22we've come to realise that our sharing of the same physical laws
0:23:22 > 0:23:24with the other worlds in the solar system
0:23:24 > 0:23:27isn't the only connection we have with them,
0:23:27 > 0:23:35because those same laws lead to a direct physical connection
0:23:35 > 0:23:38between the Earth and the other worlds out there
0:23:38 > 0:23:46that is subtle, is complicated, but can sometimes be extremely powerful.
0:23:52 > 0:23:57Out in the farthest reaches of the solar system are vast worlds
0:23:57 > 0:24:00that have a direct impact on our very existence.
0:24:04 > 0:24:09This is our sun from ten billion kilometres away -
0:24:09 > 0:24:12just another star in a sea of stars.
0:24:12 > 0:24:17But as you head towards the light, you enter a realm of giants.
0:24:20 > 0:24:24The furthest planet from the sun is icy Neptune,
0:24:24 > 0:24:27with its thick, blue methane atmosphere.
0:24:31 > 0:24:36Uranus comes next, the only planet that lies on its back.
0:24:40 > 0:24:45Further in towards the sun, and the planets get even bigger.
0:24:45 > 0:24:49Saturn, with its beautiful rings of ice.
0:24:54 > 0:24:58Finally we reach the king of the giants - Jupiter.
0:25:01 > 0:25:04Jupiter is the largest planet in the solar system,
0:25:04 > 0:25:10so big you could fit Earth inside it over 1,000 times.
0:25:10 > 0:25:15It's made up of the same stuff as our sun - hydrogen and helium -
0:25:15 > 0:25:18the most common elements in the universe.
0:25:18 > 0:25:20In its thick churning atmosphere,
0:25:20 > 0:25:24gigantic storms have raged for centuries.
0:25:27 > 0:25:32Now, astrologers have said for years that Jupiter influences our lives,
0:25:32 > 0:25:37but we now have scientific evidence that this mighty planet does have
0:25:37 > 0:25:42a significant connection with our own small world.
0:25:42 > 0:25:45Jupiter is so different to our planet -
0:25:45 > 0:25:49you know, a big ball of gas, half a billion kilometres away -
0:25:49 > 0:25:54it's difficult to see how it could have anything to do with us at all.
0:25:54 > 0:25:59But despite the fact that astrology is a load of rubbish,
0:25:59 > 0:26:03Jupiter can, in fact, have a profound influence on our planet
0:26:03 > 0:26:09and it's through a force that, well, surrounds us and penetrates us
0:26:09 > 0:26:13and binds the galaxy together - gravity.
0:26:18 > 0:26:23Gravity is one of the fundamental forces of nature.
0:26:23 > 0:26:26It exists between all objects, and the effects
0:26:26 > 0:26:32of a gravitational field extend way beyond the planet that creates it.
0:26:36 > 0:26:40Gravity is by far the weakest force of nature. But it's the only force
0:26:40 > 0:26:43that has an influence across the entire solar system,
0:26:43 > 0:26:47and that's because, although it's weak, it has an infinite range.
0:26:47 > 0:26:48It never quite goes away.
0:26:48 > 0:26:53So, no matter how far you go, you feel the force of gravity
0:26:53 > 0:26:57as a planet, although it drops and drops and drops away.
0:27:00 > 0:27:04Jupiter has the most powerful gravitational field of all the planets,
0:27:04 > 0:27:09and it's the gas giant's gravity that can directly influence
0:27:09 > 0:27:14the orbits of asteroids and other wandering space debris.
0:27:15 > 0:27:19Jupiter is so massive, by far the most massive planet in the solar system,
0:27:19 > 0:27:25that its gravitational field can have a profound influence on passing interplanetary stuff.
0:27:25 > 0:27:26It can do three things.
0:27:26 > 0:27:30Firstly, it can capture the stuff, literally hoover it up.
0:27:30 > 0:27:33Secondly, it can deflect the stuff,
0:27:33 > 0:27:36such that it throws it out of the solar system.
0:27:36 > 0:27:39But thirdly, and most importantly for us,
0:27:39 > 0:27:45it can deflect stuff onto a direct collision course with our planet.
0:27:52 > 0:27:55Influenced by Jupiter's gravity,
0:27:55 > 0:28:00today, right now, we're highly vulnerable from asteroid impacts.
0:28:01 > 0:28:04But we do have sentinels standing guard.
0:28:07 > 0:28:12On top of the mountain of Heleakala on the Hawaiian honeymoon island of Maui,
0:28:12 > 0:28:16I've come to see Professor Nick Kaiser, who's searching
0:28:16 > 0:28:20our solar system for potentially hostile space debris.
0:28:22 > 0:28:26The prime task is to try and find killer asteroids,
0:28:26 > 0:28:28things that are out there in the solar system
0:28:28 > 0:28:30that might hit the Earth.
0:28:30 > 0:28:33There's an air of Hollywood about it, isn't there, in some sense?
0:28:33 > 0:28:37Well, that's right. I would say a lot more resources have been spent
0:28:37 > 0:28:41on making movies about killer asteroids than actually finding them.
0:28:41 > 0:28:46Anything that's a kilometre in size, if it hit the Earth it would be devastating.
0:28:46 > 0:28:49It would probably kill nearly everyone on the planet.
0:28:54 > 0:28:59Each night, using a revolutionary billion-pixel digital sensor,
0:28:59 > 0:29:02the team scans a vast swathe of the sky.
0:29:02 > 0:29:04Mind your head!
0:29:04 > 0:29:10They're looking for any unidentified objects that might be heading our way.
0:29:10 > 0:29:13So any one of these points of light could in fact be an asteroid?
0:29:13 > 0:29:15That's right. And it's very likely,
0:29:15 > 0:29:20in fact, it's almost certain that there are asteroids in that image.
0:29:20 > 0:29:23The problem is, how do you figure out which ones they are?
0:29:23 > 0:29:24Yeah.
0:29:26 > 0:29:32The camera captures several images of the same patch of sky taken minutes apart.
0:29:32 > 0:29:35The team can then see if anything has moved,
0:29:35 > 0:29:37relative to the background of stars.
0:29:37 > 0:29:41What we've done here is taken two images and subtracted them,
0:29:41 > 0:29:44and you see the stars have nearly all gone away.
0:29:44 > 0:29:47There's a couple of interesting things left.
0:29:47 > 0:29:51If you look over here, you see a dark thing and a white thing,
0:29:51 > 0:29:55so that's something which was THERE in the first image
0:29:55 > 0:29:59and THERE in the second image. And there's another one here.
0:29:59 > 0:30:03So even on this tiny patch of sky, we've already detected two objects.
0:30:03 > 0:30:06What that means is, when we do that kind of analysis
0:30:06 > 0:30:10on a whole field of view, we'll detect hundreds of objects.
0:30:14 > 0:30:20The beauty of our night sky belies the potential danger it holds in store.
0:30:20 > 0:30:26Over 2,000 objects have been identified that pass close to the Earth,
0:30:26 > 0:30:31with something like 400 that could be on a future collision course.
0:30:33 > 0:30:37And all of these menacing lumps of rock, at some point,
0:30:37 > 0:30:41come under Jupiter's gravitational influence.
0:30:41 > 0:30:48Now, if you ever needed a demonstration of how congested the space is near the Earth,
0:30:48 > 0:30:53just look at this movie of the near-Earth objects,
0:30:53 > 0:30:58so here's Mercury, Venus, the Earth, Mars and out here is Jupiter.
0:30:58 > 0:31:02Here's the asteroid belt, but look at the congestion in there.
0:31:02 > 0:31:08Every one of those points of light is an asteroid that we know of.
0:31:08 > 0:31:13Just look at the Earth swimming through them, so when you look up into the nice clear night sky and
0:31:13 > 0:31:16you want to be reassured that we're all nice and safe,
0:31:16 > 0:31:18just remember this movie.
0:31:22 > 0:31:26Our planet is on a deadly journey.
0:31:26 > 0:31:31Earth is trapped in a cosmic game of dodgeball as it orbits the sun,
0:31:31 > 0:31:39a game where the gravitational stranglehold of Jupiter regularly throws asteroids our way.
0:31:52 > 0:31:56Jupiter's gravitational influence on passing space debris
0:31:56 > 0:32:00has made our planet a world under constant bombardment.
0:32:02 > 0:32:07One of the most famous meteorite impact sites is the Barringer crater in Arizona.
0:32:08 > 0:32:1250,000 years ago,
0:32:12 > 0:32:19a 300,000 ton, 50 metre in diameter lump of iron and nickel entered the Earth's atmosphere
0:32:19 > 0:32:23and made this crater, and it should remind us
0:32:23 > 0:32:27that our environment doesn't just stop at the top of our atmosphere.
0:32:27 > 0:32:31Our environment stretches out into the solar system,
0:32:31 > 0:32:34to the very edges of the solar system, to wherever this rock came from.
0:32:38 > 0:32:40In the grand scheme of things,
0:32:40 > 0:32:46the asteroid that struck here was relatively small and innocuous.
0:32:57 > 0:33:01But there are much larger impact craters hidden in Earth's landscapes
0:33:01 > 0:33:05that have far more devastating tales to tell.
0:33:10 > 0:33:17200 years ago, white settlers crossed the ancient Appalachian mountains here in America
0:33:17 > 0:33:20to seek new land out west.
0:33:20 > 0:33:23Little did they know what they were walking into.
0:33:25 > 0:33:30Well, this place to me feels like the very definition of small town America.
0:33:30 > 0:33:34It's on the border between Virginia, Kentucky and Tennessee.
0:33:34 > 0:33:37It's the kind of town where you feel that
0:33:37 > 0:33:40nothing much has changed for the last 100 years.
0:33:42 > 0:33:45But this place was the site,
0:33:45 > 0:33:49way back in history, of a violent cosmic intervention.
0:33:53 > 0:33:56This is Middlesboro, Kentucky.
0:33:56 > 0:34:01It's a town built inside a meteorite impact crater.
0:34:06 > 0:34:12The asteroid that struck here would have been huge, about half a kilometre across,
0:34:12 > 0:34:16hitting the Earth well over 200 million years ago.
0:34:19 > 0:34:25I find it fascinating that when you look out of this view, you don't really see an obvious crater.
0:34:25 > 0:34:30And indeed, it wasn't until the 1960s that anybody had any idea that there was
0:34:30 > 0:34:33a colossal impact from space
0:34:33 > 0:34:38just over there, centred right on the 18th hole of the golf course.
0:34:43 > 0:34:48We now know where the giant asteroid that struck here could have come from.
0:34:56 > 0:35:04Located between Jupiter and Mars is a vast reservoir of rocky debris that forms the asteroid belt,
0:35:04 > 0:35:10and it's this ancient rubble that Jupiter, our neighbourhood giant, can nudge towards the Earth.
0:35:17 > 0:35:20Well, here's my model of the solar system.
0:35:20 > 0:35:23There's the sun in the middle,
0:35:23 > 0:35:29then the Earth, Mars, Jupiter,
0:35:29 > 0:35:33and the asteroids sort of scattered in between the big region
0:35:33 > 0:35:35between Mars and Jupiter.
0:35:35 > 0:35:38In fact they extend over 150 million miles,
0:35:38 > 0:35:43which is further than the distance from the Earth to the sun.
0:35:44 > 0:35:48This is my coffee, by the way, which doesn't represent anything.
0:35:50 > 0:35:53I'll put it over there. But now and again,
0:35:53 > 0:35:56because of collisions in the asteroid belt, a stray asteroid
0:35:56 > 0:36:02will get thrown into the position where they keep rhythmically meeting Jupiter over and over again.
0:36:02 > 0:36:04And because Jupiter is such a massive planet,
0:36:04 > 0:36:08that means that it gets a kick, it gets a gravitational kick.
0:36:08 > 0:36:12And that changes the orbit of these asteroids and over time, their orbit
0:36:12 > 0:36:17can become, well, elongated or elliptical rather than circular.
0:36:17 > 0:36:19That means that they can get thrown
0:36:19 > 0:36:23into the inner solar system and cross the orbits
0:36:23 > 0:36:27of the inner planets, including the orbit of the Earth.
0:36:27 > 0:36:30And you get a potentially catastrophic collision.
0:36:37 > 0:36:41Jupiter was once thought to be our protector,
0:36:41 > 0:36:47its enormous gravity swallowing up dangerous asteroids.
0:36:47 > 0:36:53Yet we now realise its gravitational influence can propel some of those asteroids in our direction.
0:36:55 > 0:37:01But surprisingly, catastrophic impacts with space debris might not be a bad thing,
0:37:01 > 0:37:04at least, in Earth's past.
0:37:07 > 0:37:10Impacts from space shaped our planet.
0:37:10 > 0:37:12They made our world what it is today.
0:37:12 > 0:37:15Take life on Earth, for example.
0:37:15 > 0:37:18Now, it's possible, or probable even,
0:37:18 > 0:37:23that impacts on a colossal scale changed the climate so much
0:37:23 > 0:37:26that huge swathes of life on Earth were wiped out,
0:37:26 > 0:37:32creating ecological niches into which other species could evolve -
0:37:32 > 0:37:33us, for example.
0:37:36 > 0:37:39It's incredible to think that a planet
0:37:39 > 0:37:45half a billion kilometres away could dictate the fate of our world.
0:37:45 > 0:37:50Jupiter's immense gravity bridges the depths of space
0:37:50 > 0:37:54and even though its power could one day devastate Earth,
0:37:54 > 0:38:02that same gravitational field breathes life into other corners of the solar system.
0:38:02 > 0:38:03For better or worse,
0:38:03 > 0:38:09Jupiter's powerful gravitational field has had a direct influence on our planet.
0:38:09 > 0:38:15We're part of a much wider ecosystem that extends to the very edges of the solar system,
0:38:15 > 0:38:20and that ecosystem is bound together by the force of gravity,
0:38:20 > 0:38:25and it's gravity that has power to bring worlds to life.
0:38:34 > 0:38:39Our understanding of the solar system began much closer to home.
0:38:41 > 0:38:46Gazing down at us, it was our moon, with its regularly changing face,
0:38:46 > 0:38:51that first fired our interest in worlds beyond our own.
0:38:53 > 0:38:58When we could look further out, we discovered the solar system was full
0:38:58 > 0:39:04of moons, each invisibly connected to their parent planets by gravity.
0:39:09 > 0:39:15Our moon is a cold, geologically dead world, but the powerful gravitational bond
0:39:15 > 0:39:23that exists between another moon and its parent planet has done something astonishing.
0:39:23 > 0:39:25It has brought this moon to life,
0:39:25 > 0:39:30making it the most violent place in the solar system.
0:39:31 > 0:39:38400 years ago, it was Galileo who first looked up at the night sky through a telescope.
0:39:38 > 0:39:45Turning his attention to Jupiter, he noticed that this giant planet was not alone.
0:39:48 > 0:39:50Oh, yeah!
0:39:50 > 0:39:52Absolutely magnificent.
0:39:53 > 0:39:56You see a disc surrounded by...
0:39:56 > 0:39:58Well, I can see three points of light.
0:39:58 > 0:40:01Galileo, over several nights, saw four points of light.
0:40:01 > 0:40:07And he correctly surmised that those are actually moons, other worlds in orbit around Jupiter.
0:40:13 > 0:40:19Jupiter's four largest moons are named after the four lovers of the Greek god Zeus.
0:40:21 > 0:40:24Furthest out is Callisto.
0:40:24 > 0:40:29Then there's huge Ganymede, the largest moon in the solar system.
0:40:31 > 0:40:34Next is icy Europa,
0:40:34 > 0:40:40and finally, the small, yellow-tinged moon nearest to Jupiter, Io.
0:40:43 > 0:40:48So the legend goes, Zeus tried to protect his lover, Io, by turning her into a cow
0:40:48 > 0:40:52to hide her from the jealous gaze of his wife, Hera.
0:40:52 > 0:40:58But it didn't work, and Hera sent a gadfly to torment Io.
0:40:58 > 0:41:05And it was prescient in a way to name that satellite Io, the tormented moon.
0:41:05 > 0:41:10Because we've since learned that it is indeed an incredibly tormented world.
0:41:14 > 0:41:19For 400 years, we expected Io to be as dead as our own moon.
0:41:20 > 0:41:26But in the late 1970s, when the first spacecraft passed by Jupiter,
0:41:26 > 0:41:29we finally saw Io up close.
0:41:32 > 0:41:39But it didn't make any sense, as here was a moon with no meteorite impact craters.
0:41:42 > 0:41:46Now, it's impossible to believe that Io could have escaped
0:41:46 > 0:41:50the bombardment that we see on our moon and practically every
0:41:50 > 0:41:55other body in the solar system, so the only explanation is that that surface is young.
0:41:55 > 0:42:01It must have been recently produced, and that in turn means that Io,
0:42:01 > 0:42:08that tiny moon of Jupiter out there, must be a geologically active world.
0:42:10 > 0:42:14The truth about Io would blow us away.
0:42:23 > 0:42:30We may not have stood on Io, but there are places we can go here on Earth to help unlock its secrets.
0:42:33 > 0:42:35This is Ethiopia in East Africa.
0:42:35 > 0:42:41We're being flown out by military helicopter to the very hot,
0:42:41 > 0:42:46very inhospitable, Afar region in the north-east of the country.
0:42:51 > 0:42:54And this is what I've come to see.
0:43:03 > 0:43:08It's one of the rarest geological phenomena on our planet,
0:43:08 > 0:43:11a volcano with a lake of molten lava.
0:43:14 > 0:43:19As the sun goes down, the lava lake comes to life.
0:43:25 > 0:43:30This volcano is called Erta Ale by the local Afar people.
0:43:30 > 0:43:32It means "smoking mountain".
0:43:36 > 0:43:40For many, this place is a vision of hell.
0:43:46 > 0:43:53Yet it holds the key to understanding Io, a world over half a billion kilometres away.
0:43:55 > 0:44:01This hot, tortured landscape of lava will be our home for the next three days.
0:44:01 > 0:44:08Temperatures here reach 50 Celsius in the shade and we're camping right on top of the volcano.
0:44:12 > 0:44:17But we're in good hands, as Io specialist Dr Ashley Davies is part of the team.
0:44:21 > 0:44:24It is absolutely spectacular, isn't it?
0:44:24 > 0:44:29- Extraordinarily beautiful. - You feel like you're looking into the core of the planet.
0:44:29 > 0:44:34It's a window into the interior of the Earth, so magma is rising up
0:44:34 > 0:44:39from some kilometres down, circulating through the surface and then sinking back down again.
0:44:39 > 0:44:43It is very difficult to breathe, it's very acidic and very bitter.
0:44:43 > 0:44:47The magma has gases in it,
0:44:47 > 0:44:50and as it comes up to the surface, just like when you pour
0:44:50 > 0:44:54out a bottle of Coca-Cola, the gases come out.
0:44:54 > 0:44:58So what we have here is sulphur dioxide, hydrogen sulphide,
0:44:58 > 0:45:03water vapour, carbon dioxide, coming out of the magma
0:45:03 > 0:45:08before the magma then cools and sinks down, and that's what we're breathing now, it's incredibly unpleasant.
0:45:13 > 0:45:16This volcanic phenomenon is a product of immense amounts
0:45:16 > 0:45:21of primordial heat escaping from inside our planet.
0:45:23 > 0:45:28Yet we have seen something similar in the far reaches of the solar system.
0:45:32 > 0:45:37Io is the same size as our moon and should be a cold, dead world.
0:45:42 > 0:45:49Yet our first glimpses of Io revealed it as seething with heat, alive with volcanic activity.
0:45:51 > 0:45:58Just one of the many lava lakes on Io releases more heat than all Earth's volcanoes put together.
0:46:05 > 0:46:07If we were to stand on the surface of Io now,
0:46:07 > 0:46:11what would be the similarities and what would be the differences?
0:46:11 > 0:46:15The lake would probably appear very, very similar to this, except for the scale.
0:46:15 > 0:46:18The lava lakes on Io are vastly larger.
0:46:18 > 0:46:22The biggest one, we think, is 180km in diameter.
0:46:22 > 0:46:26180km? So that would stretch way... obviously way beyond the horizon on Earth!
0:46:26 > 0:46:29Yeah, it's almost beyond description,
0:46:29 > 0:46:34to see something that size and it's just this huge pool of molten lava.
0:46:39 > 0:46:43Io is the most volcanic place in the solar system,
0:46:43 > 0:46:49endlessly pumping out heat into the cold vacuum of space.
0:46:55 > 0:47:01But what's really interesting is that it's so small that it shouldn't be volcanic at all.
0:47:08 > 0:47:11It was one of the greatest surprises of planetary science
0:47:11 > 0:47:14when these massive volcanoes were discovered on Io.
0:47:14 > 0:47:17Mighty planets like Mars - has big volcanoes.
0:47:17 > 0:47:21They're not erupting anymore, they haven't erupted for a long time.
0:47:21 > 0:47:26Venus, lots of volcanoes there, but they haven't been erupting in a long time, probably.
0:47:26 > 0:47:29And here we have Io, which is just insanely volcanic.
0:47:29 > 0:47:32It's just pumping out vast amounts of energy in a zone, in a part of
0:47:32 > 0:47:36the solar system where it was thought that everything was dead.
0:47:43 > 0:47:48Everything we now know about Io comes from looking at it from a distance,
0:47:48 > 0:47:51but measuring the heat pumping out of this lava lake
0:47:51 > 0:47:56will give us a better idea of the true scale of the heat loss on Io.
0:47:59 > 0:48:01Far from being a benign, bubbling cauldron,
0:48:01 > 0:48:07this volcano has the power to kick off at a moment's notice.
0:48:11 > 0:48:17I can just see pieces of molten rock rising up just below my chin,
0:48:17 > 0:48:21and with it a cloud of heat, absolutely overpowering heat.
0:48:21 > 0:48:26There must be a hell... a hell of an eruption going on.
0:48:31 > 0:48:35Seeing active volcanism like this on such a small moon like Io
0:48:35 > 0:48:38changed our view of the workings of the solar system.
0:48:40 > 0:48:47A world like Io, having such a powerful internal heat source, cries out for an explanation.
0:48:47 > 0:48:54You see, Io is far too small a world to have retained any internal heat
0:48:54 > 0:49:00to have the same heat source that powers the Earth's volcanoes.
0:49:00 > 0:49:06So something else must be driving that powerful volcanism on Io.
0:49:10 > 0:49:17New images sent back from recent space probes confirm that Io is a surprising and bizarre world.
0:49:23 > 0:49:27Being so far from the sun, Io's surface is mostly very cold.
0:49:29 > 0:49:32It is covered in frozen sulphur, which gives it its yellow colour.
0:49:35 > 0:49:40Yet Io is pockmarked by cauldrons of molten lava.
0:49:42 > 0:49:47You know, I think it is remarkable, and fortunate in a sense,
0:49:47 > 0:49:52that you can come to a place like this on our planet and just
0:49:52 > 0:49:56get the tiniest sense of what it must be like
0:49:56 > 0:50:01to stand on the edge of one of those magnificent lava lakes on Io.
0:50:04 > 0:50:07When we first saw hot volcanic action on Io,
0:50:07 > 0:50:11it was quite a surprise for most planetary scientists.
0:50:11 > 0:50:16But by considering simple laws of physics, it didn't surprise everyone.
0:50:16 > 0:50:22Just weeks before Voyager arrived at Jupiter, three scientists made a prediction,
0:50:22 > 0:50:29and it was one of those predictions that, when you see it, is almost obvious.
0:50:29 > 0:50:33It was using physics that had been known for hundreds of years, but nobody had thought of it.
0:50:33 > 0:50:40They predicted that Io should have an intense internal heat source because of its unique position
0:50:40 > 0:50:47in the solar system, very close to a giant planet and surrounded by other large moons.
0:50:52 > 0:50:58Io sits about the same distance from Jupiter as our own moon does from Earth,
0:50:58 > 0:51:04but don't forget that orbiting outside Io are its sister moons, Europa and Ganymede.
0:51:06 > 0:51:10Io is under the influence not just of the massive gravitational pull
0:51:10 > 0:51:15of Jupiter, but also the additional pull of its neighbouring moons.
0:51:15 > 0:51:22It's this gravitational tug of war that conspires to breathe life into Io.
0:51:24 > 0:51:29Now, Io has a very interesting relationship with Europa and Ganymede,
0:51:29 > 0:51:34because for every four orbits that Io makes around the planet,
0:51:34 > 0:51:39Europa goes around almost exactly twice
0:51:39 > 0:51:43and Ganymede goes around just once.
0:51:43 > 0:51:48Periodically, they line up together, bang, bang, bang, and Io gets
0:51:48 > 0:51:54a powerful gravitational kick on a very regular basis.
0:51:54 > 0:52:00And that has the effect of moving Io out of the nice circular orbit
0:52:00 > 0:52:03into an elliptical or an eccentric orbit.
0:52:03 > 0:52:09Io comes close to Jupiter and then far away from Jupiter, and then close to Jupiter again.
0:52:09 > 0:52:15And because Jupiter's gravity is so big, that has the effect of stretching and squashing Io.
0:52:15 > 0:52:20Now, imagine it was a squash ball. If you stretch and squash and stretch and squash,
0:52:20 > 0:52:23then it gets hot by friction, and the same thing happens to this moon.
0:52:23 > 0:52:30The power of the gravitational interaction between Jupiter and Io is extraordinary.
0:52:30 > 0:52:37It contorts the shape of this tiny moon, moving rock as if it were nothing more than water.
0:52:39 > 0:52:44Now, this crater is about, what, 30 metres from the base
0:52:44 > 0:52:48that you can see down there up to the edge of the rim.
0:52:48 > 0:52:53Now, Io, when it orbits around Jupiter every 1.8 days,
0:52:53 > 0:52:56flexes by something like 100 metres.
0:52:56 > 0:53:00That's three times the height of that crater.
0:53:00 > 0:53:04Remember, Io's surface is pretty much like this,
0:53:04 > 0:53:09solid rock, so imagine how much energy that takes,
0:53:09 > 0:53:13and all that energy comes from Jupiter's gravitational field,
0:53:13 > 0:53:16and that is the energy that powers the volcanoes.
0:53:27 > 0:53:30Io is a world beyond our imagination.
0:53:30 > 0:53:34Its unique gravitational connections
0:53:34 > 0:53:38provide a seemingly inexhaustible supply of heat.
0:53:38 > 0:53:42As well as its huge lava lakes, the heat also powers
0:53:42 > 0:53:46the largest volcanic eruptions in the solar system.
0:53:54 > 0:53:59Molten rock and gas blasts out from the frigid surface.
0:53:59 > 0:54:06The gas expands, shattering lava into a giant fountain of fine particles.
0:54:12 > 0:54:16With weak gravity and a sparse atmosphere,
0:54:16 > 0:54:21Io's volcanic plumes can reach 500 km above the moon's surface.
0:54:41 > 0:54:49This incredible phenomenon, volcanism, comes from the simplest of laws of physics,
0:54:49 > 0:54:53the law that says that heat contained in a planet
0:54:53 > 0:54:58must eventually find a way to escape into the coldness of space.
0:54:58 > 0:55:03But what a spectacular way for the laws of physics to play out.
0:55:05 > 0:55:08In the most unexpected of places,
0:55:08 > 0:55:11in the coldest reaches of the solar system,
0:55:11 > 0:55:16the laws of physics created a fiery world of wonder,
0:55:16 > 0:55:18and Io is not alone.
0:55:18 > 0:55:22Many of the hundreds of moons in the solar system
0:55:22 > 0:55:27are not dead, barren and uninteresting worlds,
0:55:27 > 0:55:34but active, often violent and always beautiful worlds of wonder.
0:55:38 > 0:55:41Io is fascinating.
0:55:41 > 0:55:46It doesn't derive its energy from an internal heart source in the same way that the Earth does.
0:55:46 > 0:55:51It extracts energy from its orbit around its giant parent planet,
0:55:51 > 0:55:57Jupiter, and for all those reasons, Io is a wonder of the solar system.
0:56:22 > 0:56:26Our exploration of the planets and moons orbiting our star
0:56:26 > 0:56:30has given us valuable insights into the nature of our own world.
0:56:33 > 0:56:39Our view of the Earth's place in space has been turned on its head.
0:56:39 > 0:56:46Out there are many truly violent and hostile worlds,
0:56:46 > 0:56:53but they're driven by the same laws that shape and control our own world.
0:56:53 > 0:56:55And so, I suppose,
0:56:55 > 0:57:00it's in many ways a miracle that we exist at all.
0:57:02 > 0:57:07Our solar system is like a cosmic laboratory.
0:57:07 > 0:57:13Until we went there, we had no idea of what the laws of nature could produce.
0:57:14 > 0:57:17I think one of the most important lessons that our exploration
0:57:17 > 0:57:23of the solar system has taught us is that the laws of nature
0:57:23 > 0:57:29can create vastly different worlds with the tiniest of changes.
0:57:32 > 0:57:37We now see how the life and death of planets and moons
0:57:37 > 0:57:42is governed by interconnections which span the solar system,
0:57:42 > 0:57:46and we wouldn't be here if it wasn't for those connections.
0:58:16 > 0:58:19Subtitles by Red Bee Media Ltd
0:58:19 > 0:58:22E-mail subtitling@bbc.co.uk