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00:00For thousands of years, planet Mercury has baffled astronomers.
00:08But now its secrets have been revealed.
00:11It's a bizarre world, unlike any other.
00:15When you look at the family of planets that make up our solar system,
00:19you know, Mercury does seem to be a little bit of a weirdo.
00:23This is a tough world.
00:27One surviving brutal attacks from comets, the sun and even other planets.
00:35You can see by the surface of Mercury that it has a lot of battle scars.
00:40The solar system did not treat it well.
00:43Yet this world has been brought to life with water ice, volcanism and tectonic activity.
00:52If I had to describe Mercury in a word, it would be surprising.
00:56At first it may seem that you could write this off as a dull little dead rock close to the sun.
01:02But Mercury has a story to tell.
01:04And this story could end with Mercury threatening the very existence of planet Earth.
01:12Ignore it at your peril because there may come a day when Mercury makes its presence very well known indeed.
01:23NASA's Jet Propulsion Laboratory, California
01:35In April 2015, the Messenger spacecraft completed its final orbit around planet Mercury.
01:44The images sent back to Earth during its mission stunned scientists.
01:52Our view of Mercury from before and after the Messenger probe is like having terrible vision your whole life
01:58and then finally going to the optometrist and getting glasses that clear everything up.
02:04And it completely opened our eyes to what this planet looks like.
02:08Mercury orbits in the innermost solar system, the closest planet to the sun,
02:14around three times closer than Earth, in a scorching environment of lethal heat and radiation.
02:25Capturing images was a monumental challenge for the Messenger mission team led by Sean Solomon.
02:32Mercury has been among the most difficult planets in the solar system to study.
02:37The Hubble Space Telescope is forbidden from viewing planet Mercury
02:43because it's too close to the sun and their optics would be severely damaged.
02:49Messenger looked closer than we ever had before.
02:53It revealed a strange world of black holes,
02:57closer than we ever had before.
03:00It revealed a strange world, just 5% the mass of the Earth.
03:06In fact, closer in size to our moon.
03:10At first glance, Mercury and Earth, they're nothing alike, right?
03:13They're both made of rocks, that's about it. They're both orbiting the same sun.
03:17But when you start looking a little bit closer at Mercury,
03:20you start seeing some really surprising similarities to Earth.
03:25When scientists looked at Mercury, they noticed something that shouldn't be there.
03:31Mercury has water on it, which, if you were to make a long list of all the discoveries about Mercury,
03:37I think water would be right at the top of, what? What? Seriously?
03:46It got even stranger. The water exists in the form of ice.
03:54There could even be a trillion tons of it,
03:57enough to encase Washington, D.C. in a frozen block three kilometers thick.
04:05So how do you get frozen water on the closest planet to the sun?
04:13Mercury is a world of brutal extremes.
04:18The sun-facing side is blasted by solar radiation,
04:22with temperatures rocketing to over 400 degrees Celsius,
04:27hot enough to melt lead.
04:31But the side facing away plummets to minus 180.
04:36It's all because of Mercury's almost non-existent atmosphere.
04:42Planetary scientist Dan Derder demonstrates the effect this has using a campfire.
04:49So right now, with this jacket on, holding the warmth in, I'm a bit like a planet with an atmosphere.
04:55The atmosphere of a planet, it's a thermal blanket, a thermal insulator,
04:58that helps make more uniform the temperature of the entire planet.
05:01So at the moment I'm pretty warm uniformly, but if I take this, if I take my jacket off altogether,
05:07well, you know, I can already feel my back cooling off.
05:12Sitting here without my jacket, without an atmosphere, if you will,
05:16I'm like the planet Mercury, which has no effective atmosphere,
05:20and therefore the only warmth that it can hold is the warmth radiating directly on it from the sun itself.
05:26Any parts of Mercury facing away from the sun very rapidly radiate that heat off to space
05:32and cool to very, very chilly temperatures.
05:38So could water ice survive on the side of Mercury facing away from the sun?
05:44The nighttime side is very bitterly cold.
05:47If that were the end of the story, you might be able to have, you know,
05:52frozen water, icy water on the side of Mercury facing away from the sun.
05:57But Mercury doesn't keep one side always locked to the sun and one side always facing away.
06:02It actually does rotate.
06:04It rotates three times for every two trips it takes around the sun.
06:08As Mercury rotates, the sun would vaporize any water ice falling under its glare.
06:15But are there any hiding places where the sun cannot reach?
06:21Planetary scientist Nina Lanza searches fissures in Iceland to find the answer.
06:28So here we can see at the surface there's no ice, it's too warm.
06:32But if we look down in this fissure here, it's only about 20 feet deep,
06:38but at the bottom there's actually some ice.
06:42So if we measure a rock that's been in the sun, let's say this one,
06:47we can see it's actually pretty warm, it's about 61 degrees Fahrenheit.
06:52But if we aim now for the bottom of this hollow, we can see now it's about 33,
06:59no, it's dropping, it's 32 degrees Fahrenheit.
07:02And this is because the sun doesn't really get to the bottom of this place.
07:06And that's the key.
07:10While this ice is hidden in a fissure,
07:13on Mercury the ice survives in craters on the planet's north pole,
07:20forever safe from the glare of direct sunlight,
07:24because these craters sit in a perfect spot.
07:29Mercury's axis doesn't tilt very much compared to the sun.
07:33The Earth's axis tilts about 23 degrees,
07:35but Mercury is pretty much straight up and down.
07:37And that means that as large as the sun is in the sky,
07:40there are craters on the poles that sunlight never gets to, they're always cold.
07:46So the surface of Mercury is heated and then cooled as it moves around the sun,
07:51but there are craters that are always dark and always cold.
07:55Against all the odds,
07:57despite everything that suggests this shouldn't happen,
08:01Mercury is a safe haven for water ice.
08:06The bottoms of these craters are called cold traps
08:09because they're so cold that any water that gets there stays there basically forever,
08:14can last literally for billions of years.
08:16But where would it have come from?
08:20It probably came from comets and asteroids.
08:24Comets are giant chunks of ice.
08:26So if a comet hits Mercury, that's going to deliver a lot of ice.
08:30And in fact, we know a lot of asteroids have ice on them as well.
08:33So both of these things could deliver water to Mercury.
08:38Comets and asteroids brought water ice to the innermost planets of our solar system.
08:47This same water delivery system gave Earth the elements needed for life.
08:53And though Mercury has pockets of frozen water,
08:57the planet can never use them to develop living organisms.
09:02What Mercury is really showing us is that you can start with the same basic building blocks for planets, right?
09:09You can have essentially the same materials, but end up with very different environments.
09:15Compared to the other planets, Mercury ended up as the runt of the litter.
09:21But there's evidence this wasn't always the case.
09:25Because this little world was once much, much bigger.
09:46Although we have theories for how the planets first formed in our solar system,
09:53Mercury just doesn't fit them.
09:57When you look at the family of planets that make up our solar system,
10:01you know, Mercury does seem to be a little bit of a weirdo.
10:05On Earth, we have a relatively small core and a thick mantle and a thin crust.
10:10So you compare the core of the Earth to the size of the Earth, it's relatively small.
10:14If you look at Mercury, it's not that way at all.
10:17The core is absolutely huge compared to the planet itself.
10:22In 1974, Mariner 10 became the first spacecraft to fly past Mercury.
10:29And when it did, it found something completely unexpected.
10:34A magnetic field.
10:36Mars does not have a magnetic field. It did once.
10:39Venus does not have a magnetic field.
10:41And so it was a surprise that Mercury had a magnetic field.
10:44Now, Earth has a global magnetic field that arises from convective motions
10:49in the metallic fluid core deep in the Earth's interior.
10:54And it's a hundred times stronger at the surface than Mercury's magnetic field.
11:01To generate a magnetic field, Mercury's core must be huge.
11:06You would think that for a planet the size of Mercury,
11:09it wouldn't be able to sustain a magnetic field.
11:11But because the core is so large and dense, some of the center is still molten,
11:17and thus it can sustain that magnetic field.
11:21Mercury's huge iron core is surrounded by an unusually thin mantle of rock.
11:28It just looks odd.
11:31It's funny. You don't expect to get a core that large.
11:35In a planet that small.
11:37It's almost as if Mercury lost some of its mantle somewhere.
11:41I mean, maybe it left it behind the couch.
11:43Who knows? But it's gone now.
11:47But how can this material just vanish?
11:51Mercury could be the way it is now.
11:53Because it started as a much larger planet,
11:56and then something happened to strip away the top layer of it.
12:00And the only way we know how to do that on a planetary scale
12:04is with planetary impacts.
12:10Picture the early solar system, when Mercury was still forming.
12:16It was completely different from the little world we know now.
12:20Up to four times more massive.
12:23Twice the mass of Mars.
12:26But it was orbiting in a shooting gallery.
12:29Impacts were inevitable.
12:31And before long, an object the size of our moon smashed into Mercury,
12:37in a gigantic impact event.
12:43This is apocalyptic. This is the sweatiest nightmare you can have.
12:47You're resurfacing an entire planet.
12:51The energies are vast.
12:53One of these giant impacts probably would have remelted it all the way through.
12:57What's going to remain after it's done is completely different
13:00than what started in the first place.
13:02It's hard to overstate just how impactful these events are.
13:10Huge chunks of Mercury's mantle were flung into space.
13:14The result?
13:16Two-thirds of its mass is now made up by its core.
13:21But where did the rest of it go?
13:25Billions of years ago, there was a planet that we will never know.
13:28A planet that was destroyed when the current Mercury was formed.
13:32Is it possible that some of it may still be out there?
13:39This lost mantle debris could still exist in the innermost solar system.
13:45Scientists call these hypothetical objects
13:48vulcanoids.
13:50Vulcanoids can be very valuable for understanding the formation of Mercury
13:54because some of these vulcanoids may be pieces of Mercury's missing mantle.
14:00But in order to survive,
14:02these vulcanoids would have to orbit on a gravitational tightrope.
14:07You know, like this marshmallow,
14:09vulcanoids exist in a kind of a precarious position in the solar system.
14:13A little too close to the fire, a little too close to the sun,
14:18those objects actually would vaporize away.
14:21Kind of like that.
14:24If you orbit too far from the sun,
14:26you're going to approach too closely to Mercury,
14:28have gravitational encounters,
14:30maybe get flung out of the solar system,
14:32or maybe just impact Mercury itself and get eaten, if you will.
14:39But in between those two extremes,
14:41at just the right distance from the sun,
14:43a little closer to the sun than Mercury,
14:45but not so close that you get fried,
14:47is this vulcanoid region where objects in orbit around the sun
14:51could remain gravitationally stable over the entire age of the solar system.
14:55That's the place to look for this potential population
14:58of little asteroid-like objects.
15:04Across the galaxy, we're finding that many objects
15:07can survive in the inner regions around stars.
15:11Most solar systems out there
15:13have a lot more planets closer in towards their star,
15:16even closer than where Mercury is.
15:18For some reason, our inner solar system is surprisingly quiet.
15:24Furthermore, finding Mercury's vulcanoid debris
15:27would replicate what we found all over our solar system.
15:31We find debris, the fossils of planetary formation,
15:35scattered throughout the solar system.
15:37Between Mars and Jupiter, we have a huge population of asteroids.
15:40We look around the orbit of Jupiter and Saturn,
15:43and we find the Trojan asteroids orbiting near Jupiter.
15:46We look beyond Neptune, and we see the whole Kuiper Belt population
15:49all the way out to the Oort Cloud.
15:51When you look at our solar system, there are several gaps.
15:54Whenever we see a gap in the planets, we see debris.
15:57Well, there's a gap between the sun and Mercury.
16:00So is there any debris there?
16:03The existence of these vulcanoids would certainly make a lot of sense.
16:07The first time I heard about vulcanoids,
16:09I just really was enthralled by this whole idea.
16:12And this is so cool because it could be an entire part of the solar system
16:16right in front of us. I mean, this is really close, closer than the sun is,
16:20that we have not discovered, that we know nothing about, basically.
16:24So everything about this I love.
16:28A whole new unseen realm of the solar system,
16:32one that may contain the remnants of Mercury's missing mantle.
16:37So we know where they should be.
16:39Yet there's a problem.
16:41You may wonder, why haven't we seen these vulcanoids if they actually exist?
16:45If they're actually orbiting, why don't we just look for them?
16:48Well, it's because they're really close to the sun.
16:52Unlike the asteroids or the Kuiper Belt objects,
16:54where we're looking out away from the sun,
16:56from our perspective here on the Earth,
16:58we're looking out into the dark nighttime sky.
17:00In the case of the vulcanoids, we're looking for something very, very close to the sun,
17:04in close to that really brilliant light source.
17:10Our sun is around 1.4 million kilometres across.
17:14Mercury is just 4,800 kilometres.
17:19When our telescopes see Mercury passing in front of the sun,
17:23it's little more than a pinprick.
17:26With vulcanoids, we're searching for something thousands of times smaller than Mercury.
17:33If vulcanoids exist, they orbit in a blind spot.
17:40But scientists will keep searching for them,
17:42because they could be the last surviving remnants of Mercury's lost mantle.
17:49If we were to discover vulcanoids,
17:51that would offer us an entirely new population of objects to study.
17:56This material could offer some rather unique insights into the formation of Mercury itself.
18:02A giant head-on impact had been our best explanation for Mercury's weird structure.
18:11But now, there's a startling new theory for Mercury's missing mantle.
18:18It was stolen.
18:20But who was the thief?
18:32THE MISSING MANTLE
18:42For years, we assumed that Mercury lost most of its mantle in a giant head-on collision.
18:52But the Messenger spacecraft threw this theory into question.
18:57One of the exciting things that we've learned about Mercury recently,
19:01by sending probes to study its surface,
19:03is that its surface is littered with material that we didn't think should be there.
19:07Things that we'd call volatiles.
19:11The volatiles are chemical elements like potassium.
19:17They're called volatile because they evaporate easily in high temperatures.
19:22The sorts of temperatures generated during a giant impact.
19:27If I had to describe Mercury in a word, it would be surprising.
19:31The idea was that a smaller object ran into Mercury and knocked off material.
19:36Well, that actually would work to make it lose material,
19:39but it would also generate a lot of heat.
19:42And because of that, the volatile materials on Mercury would also have been lost.
19:46But today, we see that they're still there.
19:48So it meant that most of the basic scenarios
19:52that had been laid out for how Mercury was assembled had been disproven.
19:58We had to go back to the drawing board
20:00and rethink how Mercury was assembled and how Mercury evolved.
20:06How did Mercury lose its mantle, but retain these volatiles?
20:11Mercury is like a detective case.
20:14We have the body, we have some clues, but we really have to piece it together.
20:18Planetary scientist Eric Asporg tackled this giant riddle.
20:23The original idea was that Mercury was hit by something smaller than itself.
20:28But whatever process made Mercury somehow preserved all these volatiles
20:34that should have vaporised and gone.
20:37Eric proposed a bold alternative, a hit-and-run collision.
20:43In the hit-and-run collision idea,
20:45you actually have Mercury hitting something bigger than itself
20:48without losing all of its volatiles.
20:54Hitting something bigger may sound much worse.
20:58But it all depends on how you hit it.
21:03In the early solar system, there were a lot of players.
21:06And just like in an ice hockey match, things became brutal.
21:11You would have had planetesimals growing through collisions.
21:15The bigger objects would have dominated
21:17because not only are they running into smaller objects,
21:20they're drawing them to themselves gravitationally.
21:23And two objects will sweep up most of the matter.
21:26Those two objects became Venus and Earth.
21:31Earth and Venus were the enforcers of the inner solar system,
21:35destroying most of the competition.
21:37Little Mercury was one of the last players left.
21:41And it could have had a showdown with Earth.
21:45Mercury is the little guy in this thing.
21:47And this hockey puck is our volatiles.
21:54The little guy comes in at a high speed
21:56and has a head-on collision with the big guy.
22:00It is catastrophic.
22:03But suppose now, instead of hitting him in a head-on collision,
22:06he hits him in a glancing blow.
22:16So he might knock off a little bit of equipment,
22:18but Mercury just keeps on going and keeps his hockey puck.
22:25Mercury is the last player left.
22:28Mercury just keeps on going and keeps his hockey puck.
22:34A head-on impact would have sent a shockwave across Mercury,
22:38melting the entire surface.
22:43But a glancing blow would have been less ferocious.
22:47The more grazing you hit something,
22:49the less energy you bring to bear,
22:51the less violent it is.
22:54The grazing collision is where a piece of the surface
22:57is grazed off and blasted into space,
22:59and the other hemisphere of the body is largely unaffected
23:02and allowed some primordial material to remain on the surface.
23:07Most of Mercury's mantle was stripped away.
23:12But this grazing impact didn't send a shockwave through the planet.
23:18And so the volatiles remained,
23:21along with enough of the mantle to reshape this world
23:24with a thin outer layer of rock.
23:28Mercury was transformed into the smallest planet in the solar system,
23:34with its lost mantle being stolen by an unexpected thief.
23:40That mantle accretes onto the biggest object around,
23:43but the biggest object around isn't Mercury.
23:47When Mercury's mantle got knocked off in this collision,
23:50it had to go somewhere.
23:52So if you're looking for Mercury's missing mantle,
23:54look no further than right under your feet.
23:58To this day, part of Mercury could be part of Earth.
24:03Stolen in a hit-and-run collision.
24:08Mercury was able to survive the formation of the solar system,
24:11but it paid a cost. It was battered.
24:15From the outset, Mercury has had a tough ride.
24:20It's been pounded by the sun,
24:22and the planets,
24:24and things have not improved since.
24:27You can see by the surface of Mercury that it has a lot of battle scars.
24:31The solar system did not treat it well.
24:35The planet has been bombarded and fried.
24:41But these events could help explain one of Mercury's biggest mysteries.
24:47Because when compared to the other,
24:50Because when compared to the other,
24:52beautiful, coloured planets in our solar system,
24:58Mercury is different.
25:01Its dark grey surface baffles scientists.
25:06One of the most intriguing things is how dark the surface of Mercury is.
25:10A big mystery in the solar system is why.
25:13Why is Mercury like that?
25:16The rocky inner planets all formed in the same region of the solar system,
25:20from similar materials.
25:23Yet Mercury is darker than all of them.
25:28Some of the darkest surfaces here on Earth are lava fields.
25:33Planetary scientist Nina Lanza visits one in Iceland
25:36to find some common ground.
25:40The surface of a planet records the history
25:43of all the processes that have acted upon it.
25:46So when we look at the surface of Mercury,
25:48we can piece together that story.
25:50Right now, we're standing on a basalt lava flow.
25:55Basalt is a type of volcanic rock that's the building block of all planets.
25:59So we know, just by seeing this basalt here,
26:02that there was volcanic activity.
26:08On Earth, most basaltic rock is covered by oceans.
26:12On Mercury, however, the basalt is exposed,
26:16and the old liquid lava flows are visible as smooth channels of solid rock.
26:23For a billion years after Mercury's formation,
26:26lava exploded from volcanic vents
26:30and leaked out from fissures.
26:34Could this volcanism explain the dark world we see today?
26:39Basalt is a pretty dark rock already.
26:42But what's so interesting about Mercury
26:44is that it's actually darker than basalt.
26:47So from the recent messenger mission to Mercury,
26:51we have some ideas about what may be making the surface so dark.
26:55And it's really strange.
26:56It's actually carbon in the form of graphite,
26:59the mineral graphite, which is what you find in a pencil.
27:02So that material is all over the surface of Mercury,
27:06and it's incorporated into the rocks.
27:09Carbon doesn't come from basaltic rock.
27:13But there is something else that could have brought carbon to Mercury.
27:19Comets.
27:21These dirty ice balls contain carbon.
27:26And they've bombarded the planet for billions of years.
27:30And the evidence of these attacks is etched into the surface.
27:36It's hard to look at Mercury and not wince on occasion.
27:39It is really covered with craters.
27:42It has been battered and bruised.
27:45It really has just been terribly, terribly mistreated over its lifetime.
27:51Mercury has been hit hard and often.
27:56Mercury is moving more rapidly around the Sun than the Earth is.
27:59So a head-on impact is going to be faster
28:02than a head-on impact of a comet in the Earth.
28:04So pound for pound, a comet impact on Mercury is much more energetic,
28:08will do much more damage than an impact on Earth.
28:14If you've ever been to, like, Meteor Crater in Arizona in the United States,
28:18it's around a mile across. It's this huge crater.
28:21If you were to stick that crater on Mercury, it would disappear.
28:25Mercury has so many craters that are so much bigger than a mile across.
28:30Mercury's biggest impact site is Caloris Basin.
28:35Over 1,000 times larger than Meteor Crater, it's so big,
28:40there are now other, newer craters inside it.
28:44And Caloris Basin is coated in carbon.
28:48Comets have a lot of carbon in them.
28:50And we know that comets hit planets.
28:52So it's kind of obvious to say, where did the carbon come from?
28:55Well, comets. But maybe not.
28:58In 2016, the Messenger team revealed an exciting new theory.
29:05Are the largest impact craters actually exposing something deeper?
29:10Impact craters are windows into the lower parts of the crust of a planet.
29:15And the larger the impact crater, the deeper.
29:17The impact event has excavated material from death
29:21and brought it to where we can see it at the surface.
29:25The craters reveal a twist.
29:29The carbon that makes Mercury so dark had been there all along,
29:33as part of the material that first formed the planet.
29:41When the young Mercury was hit,
29:43parts of the surface were transformed into an ocean of molten rock.
29:47As this rock ocean cooled,
29:49it solidified, forming a crust.
29:54And what ended up on top of this crust
29:57was graphite, the crystallized form of carbon.
30:01Carbon-containing minerals like graphite
30:03would end up near the surface of Mercury, at the top of that magma ocean,
30:06because those minerals are a lot less dense than the surface of Mercury.
30:10And so the surface of Mercury,
30:12the surface of the planet,
30:14the surface of the ocean,
30:16because those minerals are a lot less dense
30:18than the conventional rocky minerals
30:20that contain a lot of iron and nickel.
30:22Those are more dense.
30:24They tend to sink to the centre of the planet.
30:26Those lighter elements, like graphite, would tend to float to the top.
30:32But volcanism then covered the planet in new basalt lava flows,
30:38coming from deep below the surface, where the carbon didn't sink.
30:44This lava buried Mercury's ancient crust.
30:48Over time, that carbon is covered up by subsequent lava flows.
30:52But there was this layer of carbon waiting underneath the surface of Mercury.
30:56And as objects hit Mercury and gouged out holes in the surface,
31:00it exposed this hidden, darker layer underneath.
31:05Covered by a billion years of lava flows
31:08and revealed by giant impact events,
31:12one thing's for certain.
31:14Mercury's surface has been a truly hellish landscape.
31:18Imagine that we're on the surface of Mercury
31:20during that first billion years when volcanism was very active.
31:26Raining down from the sky,
31:28we'll have all these comets and meteorites
31:30just pelting the surface, mercilessly.
31:33And then beneath your feet,
31:36there'll be all this molten rock bubbling up,
31:40really an awful place to be in that first billion years on Mercury.
31:48Over billions of years,
31:50comets and volcanism reshaped the surface of Mercury.
31:55But that wasn't the end for this planet.
31:59As scientists discovered giant cliffs
32:02stretching for hundreds of kilometres,
32:06it seems Mercury is still alive.
32:11EXPLOSIONS
32:25Mercury.
32:27A small, dark world covered in craters and ancient lava flows.
32:33But scientists have found something else on the surface.
32:37Towering cliffs over three kilometres high,
32:41known as fault scarps.
32:44If you were walking along one of these scarps on the surface of Mercury,
32:47there would be this giant cliff face
32:49that would go on for miles and miles, well over the horizon.
32:52So you would be walking next to an almost endless cliff.
32:57The largest fault scarp is Enterprise Rupes.
33:00At around 1,000 kilometres long,
33:03it would span the vertical length of mainland Britain.
33:09We see fault scarps on Earth,
33:11and they are evidence of tectonic activity.
33:17Planetary scientist Janie Radebaugh is at a scarp in Death Valley to explain.
33:24This long, straight line of shadows behind me
33:27is formed because Death Valley is still spreading apart
33:30and the material on the right has dropped down from the material on the left
33:34and left a really sharp fault scarp.
33:37The fault scarp you see behind me and the ones that we see on Mercury
33:40are formed by tectonic forces.
33:43And this just means that there are forces inside of the planets
33:46and those forces cause breaks in the crust.
33:53On Earth, the transfer of heat from the mantle
33:56drives the movement of continental plates.
34:01These plates move around the surface of the planet,
34:04interacting with each other,
34:08building mountains, rift valleys, and even continents.
34:16Do the fault scarps on Mercury mean it also has plate tectonics?
34:22When you think about the Earth's plate tectonics,
34:24there are multiple plates of rock
34:26that are moving around on a layer of liquid rock below.
34:29Mercury, however, has basically just one big plate.
34:32There's a solid surface that covers the entire planet.
34:38Mercury has a different type of tectonics.
34:43Over billions of years, its liquid core cools.
34:48And as the interior cools,
34:50the planet shrinks by around 14 kilometres in diameter.
34:55When something cools, it contracts.
34:57It actually becomes smaller.
35:00Because of this contraction, the rock in the crust wrinkles,
35:05creating massive scarps.
35:07You can imagine if you take a balloon and cover it in mud
35:10and let the mud dry,
35:12and then you let a little bit of air out of the balloon,
35:14what's going to happen?
35:15Well, that mud's going to try to contract as well, but it can't,
35:18and so it'll crack and snap like that,
35:21and you'll get these thrust faults, these scarps.
35:24But for Mercury, that isn't the end of the story.
35:30We understand that the large fault scarps on Mercury
35:33formed a long time ago when the planet was cooling
35:36and the crust was shrinking and buckling.
35:38But more recently,
35:40MESSENGER has come very close to Mercury
35:42and has found really small fault scarps.
35:45Unlike these giant scarps,
35:47these small ones are less than 60 metres in length.
35:51What does this mean?
35:53These are small scarps.
35:55And the thing is, if those were really old,
35:58impacts would have erased them,
36:00and so these should be long gone off the surface.
36:03So that means that they're recent, they're new.
36:06Right now, Mercury has the record
36:08of being the smallest official planet in the solar system.
36:11But is it possible the smallest planet is still shrinking?
36:15Throughout its life, Mercury has been bombarded.
36:19Impacts have covered the large old fault scarps and craters,
36:24and so the small scarps should be covered in impact scars too.
36:30But they're pristine, which means that they're young.
36:34And that means Mercury is still shrinking.
36:39Mercury is actually still tectonically active,
36:41so the Earth is no longer the only tectonically active planet
36:44in the solar system.
36:46We've seen that Mercury has a surprising history.
36:49The thing is, the planet itself is still cooling,
36:52still contracting,
36:54even after all of these billions of years after it formed.
36:57That's amazing.
36:59At first, it may seem that you could write this off
37:02as a dull little dead planet,
37:04but Mercury has a story to tell.
37:07A story that continues to this day,
37:10despite everything the planet has been through.
37:15Mercury's had it pretty rough.
37:17It's had a tough time over the history of the solar system.
37:20It has been tossed around by planets.
37:22It has been impacted by gigantic asteroids and comets.
37:25It is shrinking.
37:27It's bombarded by solar storms.
37:29It's been destroyed.
37:31And it's entirely possible that in the future,
37:34Mercury will get its revenge.
37:46Once MESSENGER completed its final orbit of Mercury,
37:52it crashed into the planet's surface.
37:56A fitting end for a world with a history of impacts.
38:01At the end of the MESSENGER mission,
38:04our view of Mercury had been substantially changed.
38:08We suddenly had a picture of a complete world,
38:11a place that had hellishly high temperatures
38:14and yet ices of water,
38:16an evolution that didn't match that of any of its sibling planets.
38:20Could Mercury's future be just as unpredictable as its past?
38:25Right now, the future of Mercury looks bright.
38:29Very, very bright.
38:33And that's because of Mercury's massive neighbor.
38:37The sun is a giant ball of hydrogen and helium,
38:40and in the core, there's a nuclear fusion reaction.
38:43But over time, this nuclear fusion reaction
38:46and in the core, there's a nuclear fusion reaction.
38:49But over time, the fuel that the sun runs on, hydrogen,
38:52will begin to die out.
38:54It'll actually burn through all of its fuel.
38:56When that happens, in the final phases of its life,
39:00the sun will bloat up to become a red giant star,
39:03hundreds of times the size that it currently is.
39:07As the sun expands, it will engulf Mercury.
39:12Being on the surface of Mercury is bad enough right now.
39:15Now put it inside of a star.
39:21All of that heat bombarding the planet will literally boil it away.
39:28But is there a chance that Mercury could escape this roasting?
39:34There's not just the evolution of the sun that we have to take into account.
39:38The planets themselves and their orbits are evolving.
39:41They look stable, but over long time periods,
39:44they can change drastically.
39:50The sun exerts the strongest gravitational pull
39:53of any object in our solar system.
39:57It's why the planets orbit it.
40:00But planets can also pull on each other.
40:05And because Mercury is tiny,
40:07it's the most vulnerable to these gravitational tugs.
40:11After the sun, the next biggest object is Jupiter.
40:17We can take computer models
40:19and simulate Mercury's orbit as it's affected by Jupiter
40:22to see what happens to Mercury.
40:24And depending on initial conditions, a lot of different things can happen.
40:27But in some percentage of the models,
40:29what can happen is Mercury's orbit changes so much
40:33that it actually swings out and can reach the orbit of the Earth.
40:42Jupiter is often seen as the bully of the solar system.
40:46But in this instance, it's Mercury's saviour.
40:51Over time, its gravitational influence
40:53stretches Mercury's orbit out further and further.
40:59And the little world escapes the sun's clutches
41:02long before it expands to a red giant.
41:07But this tale has one final twist.
41:11It's easy to dismiss Mercury in the pantheon of planets,
41:15but ignore it at your peril,
41:17because there may come a day
41:19when Mercury makes its presence very well known indeed.
41:24I'd like to think that Mercury does not bear the solar system
41:27any ill will for all the hard time it's been given
41:30over the last 4.5 billion years.
41:33But, you know, that's a long time to build up a grudge.
41:37Mercury has had such a difficult past.
41:39It's been beat on by the sun, collided with other planets.
41:42Now Mercury could come in and start wreaking havoc with us.
41:45So maybe we can think of this as sort of Mercury's revenge.
41:49Billions of years ago,
41:52Earth could well have collided with a young Mercury,
41:56changing Mercury forever.
42:03And in the future,
42:05it could come face to face with Earth once again.
42:10Smashing into our planet in one final impact.
42:17Wiping out any trace of the world we know and love.
42:23It would melt our entire crust.
42:25It would wipe out all life on our planet.
42:32So how worried should we be about this threat?
42:35The odds of this happening aren't that high.
42:38And if it happens, it's going to happen billions of years in the future.
42:42So I'm not terribly worried about it on a personal scale.
42:45But as an astronomer and as a scientist, that's fascinating.
42:51Mercury may continue to surprise us even to its dying day.
42:57A world born in fire that might also end in flames.
43:02But the fact that it made it this far is nothing short of remarkable.
43:07Living in the toughest place imaginable,
43:09Mercury has made it through the history of the solar system.
43:13Battered, bruised, beaten, but definitely not broken.
43:20Mercury is one of the solar system's great survivors.
43:23Despite all of the things that happened to it, it's still hanging in there.
43:27Mercury, you know, has been attacked from all sides, right?
43:31It's been roasted by the sun.
43:33It's been pummeled by impactors.
43:35Yet it's still there, still being a great planet.
43:39This plucky little survivor who's still orbiting,
43:43despite everything it's gone through.