How the Universe Works (2010) s06e06 Episode Script

Secret History of Mercury (62 min)

For thousands of years, planet Mercury has baffled astronomers, but now its secrets have been revealed.
It's a bizarre world unlike any other.
When you look at the family of planets that make up our solar system, you know, Mercury does seem to be a little bit of a weirdo.
This is one tough world, surviving brutal attacks from comets, the Sun, and even other planets.
You could see by the surface of Mercury that it has a lot of battle scars.
The solar system did not treat it well.
And yet, this world has been brought to life with water ice, volcanism, and tectonic activity.
If I had to describe Mercury in a word, it would be "surprising.
" At first, it may seem that you could write this off as a dull, little dead rock close to the Sun, but Mercury has a story to tell.
And this story could end with Mercury threatening the very existence of planet Earth.
Ignore it at your peril because there may come a day when Mercury makes its presence very well known indeed.
captions paid for by discovery communications completes its final orbit around planet Mercury.
And the images sent back to Earth during its mission left scientists stunned.
Our view of Mercury from before and after the messenger probe is like having terrible vision your whole life and then finally going to the optometrist and getting glasses that clear everything up, and it completely opened our eyes to what this planet looks like.
Mercury orbits the innermost solar system, the closest planet to the Sun around three times closer than Earth, in a scorching environment of lethal heat and radiation.
Capturing images was a monumental challenge for the messenger mission team led by Sean Soloman.
Mercury has been among the most difficult planets in the solar system to study.
The Hubble space telescope is forbidden from viewing planet Mercury because it's too close to the Sun, and their optics would be severely damaged.
Messenger looks closer than we ever have before.
It reveals a strange world, just 5% the mass of the Earth, closer in size to our moon.
At first glance, Mercury and Earth, they're nothing alike, right? They're both made of rocks.
That's about it.
They're both orbiting the same sun, but when you start looking a little bit closer at Mercury, you start to see some really surprising similarities to Earth.
When scientists look at Mercury, they spot something that shouldn't be there.
Mercury has water on it, which if you were going to make a long list of all the discoveries about Mercury, I think water would be right at the top of "what? What? Seriously?" If you thought in advance of the last place in the world to find water, you'd think of Mercury because it's so close to the Sun.
It gets even stranger.
The water exists in the form of ice.
There could even be a trillion tons of it, enough to encase Washington, D.
C.
, in a frozen block 2 miles thick.
So how do you get frozen water onto the closest planet to the Sun? For me, I think the most interesting thing about Mercury is the fact that what should be the hottest planet in the solar system is, in some parts, among the coldest, and I love that paradox because I love the unexpected.
Mercury is a world of brutal extremes.
The Sun-facing side is blasted by solar radiation With temperatures rocketing to 800 degrees Fahrenheit, hot enough to melt lead, but the side facing away plummets to -300.
It's all because of Mercury's almost nonexistent atmosphere.
Planetary scientist Dan Durda demonstrates the effect this has using a campfire.
So right now, with this jacket on, holding the warmth in, I'm a bit like a planet with an atmosphere.
The atmosphere of a planet, it's a thermal blanket, a thermal insulator, that helps make more uniform the temperature of the entire planet.
So at the moment, I'm pretty warm uniformly, but if I take my jacket off altogether, well, you know, I can already feel my back cooling off.
Sitting here without my jacket, without an atmosphere, if you will, I'm like the planet Mercury which has no effective atmosphere, and therefore the only warmth that it can hold is the warmth radiating directly on it from the Sun itself.
Any parts of Mercury facing away from the Sun very rapidly radiate that heat off to space and cool to very, very chilly temperatures.
So could water ice survive on the side of Mercury facing away from the Sun? The nighttime side is very bitterly cold.
If that were the end of the story, you might be able to have, you know, frozen water, icy water, on the side of Mercury facing away from the Sun.
But Mercury doesn't keep one side always locked to the Sun and one side always facing away.
It actually does rotate.
It rotates three times for every two trips it takes around the Sun.
As Mercury turns, the Sun would vaporize any water ice facing it.
But are there secret hiding places where the Sun cannot reach? To find the answer, planetary scientist Nina Lanza searches fissures in Iceland.
So here we can see, at the surface, there's no ice.
It's too warm, but if we look down in this fissure here, it's only about 20 feet deep, but at the bottom, there's actually some ice.
So if we measure a rock that's been in the Sun, let's say this one, we can see it's actually pretty warm.
It's about 61 degrees Fahrenheit, but if we aim, now, for the bottom of this hollow, we can see now, it's about 33 nope.
It's dropping.
It's 32 degrees Fahrenheit, and this is because the Sun doesn't really get to the bottom of this place, and that's the key.
While this ice is hidden in a fissure, on Mercury the ice survives in craters on the planet's north pole, forever safe from the glare of direct sunlight, because these craters sit in a perfect spot.
Mercury's axis doesn't tilt very much compared to the Sun.
The Earth's axis tilts about 23 degrees, but Mercury is pretty much straight up and down, and that means that as large as the Sun is in the sky, there are craters on the poles that sunlight never gets to.
They're always cold.
So the surface of Mercury is heated and then cooled as it moves around the Sun, but there are craters that are always dark and always cold.
Against all the odds, despite everything that suggests this shouldn't happen, Mercury is a safe haven for this water ice.
The bottoms of these craters are called cold traps because they're so cold that any water that gets there stays there basically forever.
It can last literally for billions of years.
But where would it have come from? It probably came from comets and asteroids.
Comets are giant chunks of ice.
So if a comet hits Mercury, that's going to deliver a lot of ice, and, in fact, we know a lot of asteroids have ice on them as well.
So both of these things could deliver water to Mercury.
Comets and asteroids brought water ice to the innermost planets of the solar system.
This same water delivery system gave Earth the elements needed for life.
And though Mercury has pockets of frozen water, the planet can never use it to develop living organisms.
What Mercury is really showing us is that you can start with the same basic building blocks for planets, right? You can have essentially the same materials, but end up with very different environments.
Compared to the other planets, Mercury ended up as the runt of the litter.
But there's evidence this wasn't always the case.
Could this little world have once been much, much bigger? We have theories for how the planets first formed in our solar system, but Mercury just doesn't fit in.
When you look at the family of planets that make up our solar system, you know, Mercury does seem to be a little bit of a weirdo.
On Earth, we have a relatively small core and a thick mantle and a thin crust.
So you compare the core of the Earth to the size of the Earth, it's relatively small.
If you look at Mercury, it's not that way at all.
The core is absolutely huge compared to the planet itself.
Mercury's huge iron core is surrounded by an unusually thin mantle of rock.
It just looks odd.
It's funny.
You don't expect to get a core that large in a planet that small.
It's almost as if Mercury lost some of its mantle somewhere.
I mean, maybe it left it behind the couch.
Who knows? But it's gone now.
But how can this material just vanish? Mercury could be the way it is now because it started as a much larger planet and then something happened to strip away the top layer of it, and the only way we know how to do that on a planetary scale is with planetary impacts.
Picture the early solar system when Mercury is still forming.
It's completely different from the little world we know now.
Up to four times more massive, twice the mass of Mars, but it orbits in a shooting gallery.
Impacts are inevitable, and before long, an object the size of our moon smashes into Mercury in a giant impact event.
This is apocalyptic.
This is the sweatiest nightmare you can have.
You're resurfacing an entire planet.
The energies are vast.
One of these giant impacts probably would have remelted it all the way through.
What's going to remain after it's done is completely different than what started in the first place.
It's hard to overstate just how impactful these events are.
Huge chunks of Mercury's mantle are flung into space.
The result? Two-thirds of its mass are now made up by its core, but where did the rest of it go? Billions of years ago, there was a planet that we will never know, a planet that was destroyed when the current Mercury was formed.
Is it possible that some of it may still be out there? The lost mantle debris could still exist in the innermost solar system.
Scientists call these hypothetical objects vulcanoids.
Vulcanoids can be very valuable for understanding the formation of Mercury because some of these vulcanoids may be pieces of Mercury's missing mantle.
But in order to survive, these vulcanoids would have to orbit on a gravitational tightrope.
You know, like this marshmallow, vulcanoids exist in kind of a precarious position in the solar system, a little too close to the fire, a little too close to the Sun, those objects actually would vaporize away, kind of like that.
If you orbit too far from the Sun, you're going to approach too closely to Mercury and have gravitational encounters, maybe get flung out of the solar system or maybe just impact Mercury itself and get eaten, if you will.
But in-between those two extremes, at just the right distance from the Sun, a little closer to the Sun than Mercury, but not so close that you get fried, is this vulcanoid region where objects in orbit around the Sun could remain gravitationally stable over the entire age of the solar system.
That's the place to look for this potential population of little asteroid-like objects.
We know where they should be, but there's a problem.
You may wonder why haven't we seen these vulcanoids if they actually exist? If they're actually orbiting, why don't we just look for them? Well, it's because they're really close to the Sun.
Unlike the asteroids or the Kuiper belt objects where we're looking out away from the Sun from our perspective here on the Earth, we're looking out into the dark nighttime sky, in the case of the vulcanoids, we're looking for something very, very close to the Sun, in close to that really brilliant light source.
Our sun is around Mercury is just 3,000.
When our telescopes see Mercury passing in front of the Sun, it's little more than a pinprick.
With vulcanoids, we're searching for something thousands of times smaller than Mercury.
If vulcanoids exist, they orbit in a blind spot, but scientists will keep searching for them because they could be the last surviving remnants of Mercury's lost mantle.
If we were to discover vulcanoids, that would offer us an entirely new population of objects to study.
This material could offer some rather unique insight into the formation of Mercury itself.
A giant head-on impact has been our best explanation for Mercury's weird structure.
But now there's a startling new theory for Mercury's missing mantle.
It was stolen, but who is the thief? For years, we assumed that Mercury lost most of its mantle in a giant head-on collision.
But the messenger spacecraft turned everything on its head.
One of the exciting things that we've learned about Mercury recently by sending probes to study its surface is that its surface is littered with material that we didn't think should be there, things that we'd call volatiles.
The volatiles are chemical elements like potassium.
They're called volatile Because they evaporate easily in high temperatures, just the sorts of temperatures generated by a giant impact.
If I had to describe Mercury in a word, it would be "surprising.
" The idea was that a smaller object ran into Mercury and knocked off material.
Well, that actually would work to make it lose material, but it would also generate a lot of heat, and because of that, the volatile materials on Mercury would also have been lost.
But today, we see that they're still there.
So it meant that most of the basic scenarios that have been laid out for how Mercury was assembled had been disproven.
We had to go back to the drawing board and rethink how Mercury was assembled and how Mercury evolved.
How did Mercury lose its mantle but retain these volatiles? Mercury is like a detective case.
We have the body.
We have some clues, but we really have to piece it together.
Planetary scientist Erik Asphaug tackles this giant riddle.
The original idea was that Mercury was hit by something smaller than itself.
But whatever process made Mercury somehow preserved all these volatiles that should have vaporized and gone.
Erik has a bold alternative, a hit-and-run collision.
In the hit-and-run collision idea, you actually have Mercury hitting something bigger than itself without losing all of its volatiles.
Hitting something bigger may sound even worse, but it all depends on how you hit it.
In the early solar system, there were a lot of players, and just like a hockey match, things got brutal.
You would have had planetesimals growing through collisions.
The bigger objects would have dominated because not only are they running into smaller objects, they're drawing them to themselves gravitationally, and two objects will sweep up most of the matter.
Those two objects became Venus and Earth.
Earth and Venus are the enforcers of the inner solar system, wiping out most of the competition.
Whoo! Little Mercury is one of the last players left, and it could have a faceoff with Earth.
Mercury is the little guy in this thing, and this hockey puck is our volatiles.
The little guy comes in at a high speed and has a head-on collision with the big guy.
It is catastrophic.
But suppose now, instead of hitting him in a head-on collision, he hits him in a glancing blow.
So he might knock off a little bit of equipment, but Mercury just keeps on going and keeps his hockey puck.
A head-on impact would send a shockwave across Mercury, melting the entire surface.
But a glancing blow is less ferocious.
The more grazing you hit something, the less energy you bring to bear, the less violent it is.
In the grazing collision is where piece of the surface has grazed off and blasted into space, and the other hemisphere of the body is largely unaffected and allowed some primordial material to remain on the surface.
Most of Mercury's mantle is stripped away, but this grazing impact doesn't send a shockwave through the planet, and so the volatiles remain along with enough of the mantle to reshape this world with a thin outer layer of rock.
Mercury was transformed into the smallest planet in the solar system, and its lost mantle was stolen by an unexpected thief.
That mantle accretes onto the biggest object around, but the biggest object around isn't Mercury.
When Mercury's mantle got knocked off in this collision, it had to go somewhere.
So if you're looking for Mercury's missing mantle, look no further than right under your feet.
To this day, part of Mercury could be part of Earth, stolen in a hit-and-run collision.
Mercury was able to survive the formation of the solar system, but it paid a cost.
It was battered.
Since day one, Mercury has had a tough ride.
It's been pounded by the Sun and planets, and things have not improved since.
You can see by the surface of Mercury that it has a lot of battle scars.
The solar system did not treat it well.
The planet has been bombarded and fried, but these events could help explain one of Mercury's biggest mysteries.
Why is the planet so dark? The solar system is full of beautiful, colorful planets but Mercury is different.
Its dark gray surface baffles scientists.
One of the most intriguing things is how dark the surface of Mercury is.
A big mystery in the solar system is why? Why is Mercury like that? The rocky inner planets all formed in the same region of the solar system from similar materials, and yet Mercury is darker than all of them.
Some of the darkest surfaces here on Earth are lava fields.
Planetary scientist Nina Lanza visits one in Iceland to find some common ground.
The surface of a planet records the history of all the processes that have acted upon it.
So when we look at the surface of Mercury, we can piece together that story.
Right now, we're standing on a basalt lava flow.
Basalt is a type of volcanic rock that's the building block of all planets, so we know just by seeing this basalt here, there was volcanic activity.
On Earth, most basaltic rock is covered by oceans.
On Mercury, the basalt is exposed.
Old liquid lava flows are visible as smooth channels of solid rock.
For a billion years after Mercury's formation, lava explodes from volcanic vents and leaks out from fissures.
Could the volcanism explain the dark world we see today? Basalt is a pretty dark rock already, but what's so interesting about Mercury is that it's actually darker than basalt.
So from the recent messenger mission to Mercury, we had some ideas about what may be making the surface so dark, and it's really strange.
It's actually carbon in the form of graphite, the mineral graphite, which is what you find in a pencil.
So that material is all over the surface of Mercury, and it's incorporated into the rocks.
Carbon doesn't come from basaltic rock, but there is something else that could have brought carbon to Mercury Comets.
These dirty ice balls contain carbon, and they've bombarded the planet for billions of years.
The evidence of these attacks is etched into the surface.
It's hard to look at Mercury and not wince on occasion.
It is really covered with craters.
It has been battered and bruised.
It really has just been terribly, terribly mistreated over its lifetime.
Mercury has been hit often and hard.
Mercury is moving more rapidly around the Sun than the Earth is, so a head-on impact is going to be faster than a head-on impact of a comet in the Earth.
So pound for pound, a comet impact on Mercury is much more energetic, will do much more damage than an impact on Earth.
If you've ever been to, like, meteor crater in Arizona in the United States, it's around a mile across.
It's this huge crater.
If you were to stick that crater on Mercury, it would disappear.
Mercury has so many craters that are so much bigger than a mile across.
Mercury's biggest impact site is Caloris basin.
Over 1,000 times larger than meteor crater, it's so big there are now other, newer craters inside it And Caloris basin is coated in carbon.
Comets have a lot of carbon in them, and we know that comets hit planets.
So it's kind of obvious to say, "where did the carbon come from?" Well, comets, but maybe not.
In 2016, the messenger team reveals an exciting new theory.
Are the largest impact craters actually exposing something deeper? Impact craters are windows into the lower parts of the crust of a planet, and the larger the impact crater, the deeper the impact event has excavated material from depth and brought it to where we can see it at the surface.
The craters reveal a twist.
The carbon that makes Mercury so dark had been there all along as part of the material that first formed the planet.
When young Mercury is hit, parts of the surface are transformed into an ocean of molten rock.
As this magma ocean cools, it solidifies, forming a crust, and sitting on top of this crust, graphite, the crystallized form of carbon.
Carbon containing minerals like graphite would end up near the surface of Mercury at the top of that magma ocean because those minerals are a lot less dense than the conventional rocky minerals that contain a lot of, you know, iron and nickel.
Those are more dense.
They tend to sink to the center of the planet.
Those lighter elements like graphite would tend to float to the top.
But volcanism covers the planet in new basalt lava flows coming from deep below the surface where the carbon didn't sink, and this lava buries Mercury's ancient crust.
Over time, that carbon is covered up by subsequent lava flows, but there was this layer of carbon waiting underneath the surface of Mercury, and as objects hit Mercury and gouged out holes in the surface, it exposed this hidden darker layer underneath.
Covered by a billion years of lava flows and revealed by giant impact events, one this is for sure Mercury's surface has been a truly hellish landscape.
Imagine that we're on the surface of Mercury during that first billion years when volcanism was very active.
Raining down from the sky, all these comets and meteorites just pelting the surface mercilessly, and then, beneath your feet, there'll be all this molten rock bubbling up.
Really an awful place to be in that first billion years on Mercury.
Over billions of years, comets and volcanism reshaped the surface of Mercury, and this planet is not done yet.
Scientists find giant cliffs stretching hundreds of miles.
It seems Mercury is alive.
Mercury, a small, dark world covered in craters and ancient lava flows, but scientists find something else on the surface.
Towering cliffs around 2 miles high known as fault scarps.
If you were walking along one of these scarps on the surface of Mercury, there would be this giant cliff face that would go on for miles and miles well over the horizon, so you would be walking next to an almost endless cliff.
The largest fault scarp is enterprise Rupes.
At over 600 miles long, it would span the width of Texas.
We see fault scarps on Earth, evidence of tectonic activity.
Planetary scientist Jani Radebaugh visits a scarp in death valley to demonstrate.
This long, straight line of shadows behind me formed because death valley is still spreading apart and the material on the right has dropped down from the material on the left and left a really sharp fault scarp.
The fault scarp you see behind me, and the ones we see on Mercury, are formed by tectonic forces, and this just means that there are forces inside of the planets, and those forces cause breaks in the crust.
On Earth, transfer of heat from the mantle drives the movement of continental plates.
These plates move around the surface of the planet, interacting with each other Building mountains, rift valleys, even continents.
Do the fault scarps on Mercury mean it also has plate tectonics? When you think about the Earth's plate tectonics, there are multiple plates of rock that are moving around on a layer of liquid rock below.
Mercury, however, has basically just one big plate.
There's a solid surface that covers the entire planet.
Mercury has a different kind of tectonics.
Over billions of years, its liquid core cools, and as the interior cools, the planet shrinks around 9 miles.
When something cools, it contracts.
It actually becomes smaller.
Because of this contraction, the rock and the crust wrinkles, creating massive scarps.
You can imagine, if you take a balloon and cover it in mud and let the mud dry, and then you let a little bit of air out of the balloon, what's going to happen? Well, that mud is going to try to contract as well, but it can't, and so it'll crack and snap like that, and you'll get these thrust faults, these scarps.
But for Mercury, the story doesn't end there.
We understand that the large fault scarps on Mercury formed a long time ago when the planet was cooling and the crust was shrinking and buckling, but more recently, messenger has come very close to Mercury and has found really small fault scarps.
Unlike the giant mile-high scarps, these small ones are less than 200 feet in length.
What does this mean? These are small scarps, and the thing is, if those were really old, impacts would have erased them, and so these should be long gone off the surface.
So that means that they're recent.
They're new.
Right now, Mercury has the record of being the smallest official planet in the solar system, but is it possible the smallest planet is still shrinking? Throughout its life, Mercury has been bombarded.
Impacts have cratered the large, old fault scarps.
The small scarps should be covered in impact scars as well, but they're pristine, which means that they're young, and that means Mercury is still shrinking.
Mercury is actually still tectonically active.
So the Earth is no longer the only tectonically active planet in the solar system.
We've seen that Mercury has a surprising history.
The thing is, the planet itself is still cooling, still contracting, even after all of these billions of years after it formed.
That's amazing.
At first, it may seem that you could write this off as a dull, little, dead rock close to the Sun, but Mercury has a story to tell.
A story that continues to this day despite everything the planet has gone through.
Mercury has had it pretty rough.
It's had a tough time over the history of the solar system.
It has been tossed around by planets.
It has been impacted by gigantic asteroids and comets.
It is shrinking.
It's bombarded by solar radiation.
That would tick anybody off, and it's entirely possible that in the future, Mercury will get its revenge.
Mercury could have a final trick up its sleeve, one that threatens our very existence.
Messenger completes its final orbit of Mercury and crashes into the planet's surface, a fitting conclusion for a world with a history of impacts.
At the end of the messenger mission, our view of Mercury had been substantially changed.
We suddenly had a picture of a complete world, a place that had hellishly high temperatures and yet ices of water, an evolution that didn't match that of any of its sibling planets.
Could Mercury's future be just as unpredictable as its past? Right now, the future of Mercury looks bright Very, very bright.
It's all thanks to Mercury's massive neighbor.
The Sun is a giant ball of hydrogen and helium, and in the core, there's a nuclear fusion reaction, but over time, the fuel that the Sun runs on, hydrogen, will begin to die out.
It will actually burn through all of its fuel.
When that happens, in the final phases of its life, the Sun will bloat up to become a red giant star, hundreds of times the size that it currently is.
As the Sun expands, it will engulf Mercury.
Being on the surface of Mercury is bad enough right now.
Now put it inside of a star.
All of that heat bombarding the planet will literally boil it away.
But is there a chance that Mercury could escape this roasting? There's not just the evolution of the Sun that we have to take into account.
The planets themselves and their orbits are evolving.
They look stable, but over long time periods, they can change drastically.
The Sun exerts the strongest gravitational pull of any object in the solar system.
It's why the planets orbit it, but planets can also pull on each other.
Mercury is tiny, making it most vulnerable to these gravitational tugs.
After the Sun, the next biggest object is Jupiter.
We can take computer models and simulate Mercury's orbit as it's affected by Jupiter to see what happens to Mercury, and depending on initial conditions, a lot of different things can happen.
But in some percentage of the models, what can happen is Mercury's orbit changes so much that it actually swings out and can reach the orbit of the Earth.
Jupiter is often seen as the bully of the solar system.
Here, it's Mercury's bodyguard.
Over time, its gravitational influence stretches Mercury's orbit out farther and farther, and the little world escapes the Sun's clutches long before it expands to a red giant.
But this tale has one final twist.
It's easy to dismiss Mercury in the pantheon of planets, but ignore it at your peril because there may come a day when Mercury makes its presence very well known indeed.
I'd like to think that Mercury does not bear the solar system any ill will for all the hard time it's been given over the last 4.
5 billion years, but, you know, that's a long time to build up a grudge.
Mercury has had such a difficult past.
It's been beat on by the Sun, collided with other planets.
Now Mercury could come in and start wreaking havoc with us, so maybe we can think of this as sort of Mercury's revenge.
Billions of years ago, Earth could well have collided with the young Mercury, changing Mercury forever.
And now, it comes face-to-face with Earth once again Smashing into our planet in one, final impact, wiping out any trace of the world we know and love.
It would melt our entire crust.
It would wipe out all life on our planet.
So how worried should we be about this threat? The odds of this happening aren't that high, and if it happens, it's going to happen billions of years in the future.
So I'm not terribly worried about it on a personal scale, but as an astronomer and as a scientist, that's fascinating.
Mercury may continue to surprise us, even to its dying day, a world born in fire that might also go down in flames.
But the fact that it made it this far is nothing short of remarkable.
Living in the toughest neighborhood imaginable, Mercury has made it through the history of the solar system, beaten, but not broken.
Mercury is one of the solar system's great survivors.
Despite all of the things that happened to it, it's still hanging in there.
It's been subject to the harshest environment in the solar system, and it's still around.
Mercury, you know, has been attacked from all sides, right? It's been roasted by the Sun.
It's been pummeled by impactors, and yet it's still there, still being a great planet, this plucky, little survivor who's still orbiting despite everything it's gone through.
This plucky, little survivor who's still orbiting