Category
📺
TVTranscript
00:00Life on Earth depends on seas, rivers and rain.
00:11But is our blue planet unique?
00:16Or did the universe create countless other wet worlds just like it?
00:21Unlock the secrets of Earth's first oceans and we'll unlock the secrets of alien life.
00:47The Earth is the only planet we know of that has oceans of liquid water covering its surface.
00:54And it's the only planet we know of that has life.
00:58If you look at every living organism on Earth, you can see that each one has a fraction of
01:04water that makes up the system.
01:07We're basically bags of water that allow chemicals to move around and do things that we call
01:13life.
01:15On Earth, liquid water and life go hand in hand.
01:20But how lucky are we to have a watery oasis to call home?
01:24We have a lot of water.
01:26Was it made here when the Earth was made or was it brought here later by something from space?
01:32Is it a fluke to have a water world like this or is it inevitable?
01:38For decades, scientists have been trying to establish the origins of Earth's water.
01:43And they've come to a surprising conclusion.
01:46Our planet shouldn't be wet at all.
01:50The place where the Earth is right now seems very dry.
01:53So if the Earth formed as a dry rock around a hot young star, then how did this water get here?
02:01Every possibility has problems and we want to know the answer.
02:09Identifying the exact source of Earth's water is surprisingly complex.
02:18The journey starts over 4.6 billion years ago, during the formation of our solar system.
02:27A vast cloud of gas and dust hangs in space, teeming with vast quantities of hydrogen and oxygen.
02:39Oxygen is one of the most abundant atoms in the universe.
02:43Hydrogen is the most abundant atom in the universe.
02:46You're going to get a lot of whatever it is they form.
02:51Over millions of years, these highly reactive atoms bind together to form H2O, water.
03:00Water is a fairly simple molecule.
03:02It's made of two hydrogens and one oxygen.
03:08This newly formed water sticks to dust grains inside the gas cloud and freezes to form crystals of ice.
03:17Eventually the icy dust cloud becomes so dense that it starts to collapse under its own gravity.
03:24It's the start of a process that will create our entire solar system.
03:31There's enough water here to fill the Earth's oceans 3 million times over.
03:36When we see stars that are forming right now, and we study hundreds, thousands of them,
03:41we see disks of material beginning to orbit around the young stars.
03:45Gas, dust, and there's certainly quite a bit of water in that material.
03:50Gravity pulls more and more material into the center of the cloud, raising the pressure and temperature.
03:58Eventually the extreme forces spark nuclear fusion, and a protostar, our infant sun, bursts into life.
04:11It's bad news for the water surrounding the newly born star.
04:16The environment of a star when it forms is incredibly hot and violent.
04:22Any water that was existing in that region, because water is a volatile material, would be destroyed.
04:28Water cannot exist near a star early on during its formation.
04:36Astronomers believe the early sun may have sucked up much of the dust and water surrounding it,
04:43and then blasted this debris far out into space, in superheated jets of steam.
04:54In the Earth's most volcanic places, a similar process blasts hot water high into the air.
05:02Deep underground, the water is superheated, and there's nowhere for it to go when it turns into steam,
05:08and it's driven outward in these giant plumes.
05:11Protostars also have lots of water around them, and the magnetic fields around a protostar
05:16create basically tunnels that the water can escape from, and it's blown out along these tunnels
05:22in giant jets that spread water out all throughout the galaxy.
05:30In space, superheated water escapes through the magnetic weak spots at the poles of protostars.
05:37Pockets of water inside these vast, steamy jets eventually solidify in the cold of space
05:43to form ice pellets, and these speed away from the protostar, 80 times faster than machine-gun bullets.
05:52Is this really what happened in our young solar system?
05:57It would have been so awesome to be there to see that, but the lucky thing is that our galaxy continues to form stars.
06:03So we can study protostars all over the galaxy, and look at things that are a lot like what the sun experienced.
06:15In 2011, astronomers witnessed the formation of a star just like our own sun.
06:24Their telescopes reveal a central ball of gas, dragging in matter from the clouds surrounding it.
06:30Before blasting out water at a rate equal to 10 million Amazon rivers.
06:36It's believed a similar process ejected much of the water in our embryonic solar system.
06:48As the sun matures, the jets dry up, and a new threat to the remaining water emerges.
06:54A hot stream of charged particles known as solar wind.
07:00As the sun heats up, the ice nearby is turning into water.
07:06And as the sun heats up more, it's turning into water vapor.
07:12And then as the sun turns on its solar wind and becomes bright, it starts to blow that water out.
07:18The solar wind blows in a supersonic stream of plasma from the sun's outer layers.
07:28It strikes the surrounding cloud, blasting away most of the gas and water vapor.
07:34What's left behind is just dust with traces of water clinging to it.
07:40Further out from the sun, the solar wind has less impact, and it's also much colder.
07:48The result is a boundary of water ice half a billion miles from the sun, known as the snow line.
07:58When astronomers talk about the snow line, what they mean is how far away from the young sun
08:05was water able to condense?
08:09Where did it get cool enough for water to finally condense into droplets,
08:13get onto objects, become ice?
08:17Any closer than that, and you're just gas.
08:23The solar system's first and biggest planet is born at the snow line.
08:29Where the ice is thickest, clumps form,
08:33and then, attracted to each other by gravity, join up.
08:37A colossal snowball builds.
08:41It draws in all the matter around it,
08:45eventually creating the gas giant planet Jupiter.
08:51Where the snow line is thinner, a similar process forms the other gassy planets.
08:57Saturn, Uranus, and Neptune.
09:03On the inner, dry side of the snow line,
09:07dust clumps together, forming a family of small, rocky planets, including...
09:13Earth.
09:19Many astronomers believe our planet is fashioned from little more than arid rocks,
09:23with microscopic droplets of water sticking to them.
09:27But even this precious reserve of water is about to be threatened
09:31by the most violent event in our planet's history.
09:35Our solar system, 4.6 billion years ago.
09:43It was a giant planet.
09:47It was a giant planet.
09:51It was a giant planet.
09:55It was a giant planet.
09:59It was a giant planet.
10:037.5 billion years ago.
10:07Earth is just one of many large, rocky balls forming around the Sun,
10:11and our young planet's gravity continues to pull in chunks of debris
10:15from the surrounding dust cloud.
10:19These rocks hitting the Earth hold tiny amounts of water,
10:23remnants of a time before the Sun sparked into life.
10:27Planetary scientist Dan Derda believes this water had little chance of survival on the
10:36newborn Earth, thanks to the heating effects of multiple high-speed asteroid impacts.
10:43Building a planet is a very violent process.
10:46We can demonstrate pretty easily here with the high-speed impact of a bullet.
10:52OK, so, that's a single impact.
10:57When the bullet punches into the target, some of its kinetic energy is converted into heat.
11:04You can see this sudden hot burst using a thermal imaging camera.
11:13In the case of a real impact, a large asteroid impact, the energy is a lot greater.
11:18You're actually melting rock.
11:20And four and a half billion years ago, impacts like that were happening once a month.
11:32Let's go see what we got up there.
11:35A submachine gun demonstrates how this heat would have built up after multiple asteroid impacts.
11:41The cumulative effect of all these impacts is to heat the surface of the Earth
11:45to near magma, lava-like temperatures.
11:49The combined energy of the impacts boils the surface of the young Earth.
11:54There's a lot of impact and it's a very high-temperature activity.
11:58So, any water that would be present, it would be hard to hold on to it
12:02because of the heat and the energy.
12:04The water probably escaped.
12:07After 60 million years, the planet building stops
12:11and Earth's surface cools enough to form a crust,
12:14potentially trapping any remaining water inside it.
12:20But not for long.
12:22The crowded early solar system is home to more planets than exist today.
12:26And one, known as Theia, hurtles towards Earth.
12:34Smashing into Earth, Theia gouges out a huge chunk of our planet's crust.
12:40The rocky fragments create a colossal ring of debris
12:44that will eventually coalesce to form the Moon.
12:49Reeling from the impact, Earth reverts to a ball of lava
12:53and the heat drives off yet more water.
13:00The collision with Theia leaves the crust of the Earth bone dry.
13:05So where does the water that we see today come from?
13:09There are only two possibilities.
13:12In order for the water to survive, it either has to be embedded deeply enough in rocks
13:16that it isn't melted and evaporated,
13:19or it has to come to Earth after it forms.
13:24Was our planet originally formed from much wetter rock than the scientists had believed?
13:30Or were the oceans delivered to the Earth much later, from somewhere else?
13:37Initially, delivery seems the most likely possibility.
13:43Far out, beyond the orbit of Neptune,
13:47lies a vast band of icy material called the Kuiper Belt.
13:53It's made up of the leftover building blocks of the gas giant planets.
13:59Occasionally, chunks of this icy debris, known as comets,
14:03tumble into the inner solar system.
14:07Did such marauding comets bring water to the early Earth?
14:12Comets are basically big, dirty snowballs.
14:15They're giant balls of ice that have rock, pebbles, gravel, dust embedded in them.
14:23We think that comets are about 50% made of ice.
14:27So if you looked at everything in the solar system,
14:29trying to find a source for water on Earth,
14:32there's seemingly an obvious answer, and you'd look at comets.
14:36Passing comets present some of the most spectacular sights in the night sky.
14:42As they approach the sun,
14:44the solar wind blasts water from the surface of these dusty snowballs,
14:49generating a bright tail that can stretch for millions of kilometres.
14:55Today, comets are relatively rare visitors to the inner solar system,
15:00but four billion years ago, they were common,
15:03and Earth was in the firing line.
15:07It's completely reasonable to expect that icy bodies from the outer solar system
15:12came inward and hit the Earth.
15:14How much of a contribution is the question?
15:23Rocks dating to soon after the Moon formed prove the early Earth had vast oceans.
15:29But just how many comet impacts would it have taken to fill these seas?
15:34The answer is staggering.
15:36Comets come different sizes,
15:38and if you take the run-of-the-mill average comet,
15:40it might take 20 or 30 million comets to make the Earth's oceans.
15:46If millions of comets did bring water to the early Earth,
15:50they must have done it in a very short period,
15:53and just after the Moon formed.
15:57Astronomers scour the solar system for evidence of this rapid-fire, icy attack.
16:05And they find it in the most unlikely of places,
16:08our seemingly waterless Moon.
16:17In the 1970s, Apollo astronauts collect rocks from the Moon's largest craters
16:22to determine when they formed.
16:25They brought them back to our laboratories, and we could date them.
16:28When were those rocks actually made?
16:34Planetary geologists had assumed these craters had been blasted out
16:38around the time the Moon formed.
16:41They were in for a big surprise.
16:44We found that many of the big impact basins on the Moon
16:47were formed not in the earliest days of the accretion of the Moon,
16:50but several hundred million years later,
16:52during a period we call the Late Heavy Bombardment.
16:55Late because it happened several hundred million years
16:57after the Earth and Moon had formed.
17:03The Late Heavy Bombardment begins when the gas giants align.
17:08Their combined gravity disrupts a vast belt of asteroids lying close to Mars,
17:15sending a shower of rocks towards Earth, the Moon, and the inner planets.
17:23Then Neptune swings outwards,
17:26smashing into the comets of the Kuiper Belt
17:29and sending many of them hurtling inward too.
17:33All hell breaks loose.
17:3599% of the Kuiper Belt and the asteroid belt disappear.
17:37Lots of bodies get thrown every which way.
17:40We look at the Moon, we see that it is scarred, it is covered with craters.
17:44The Earth didn't somehow magically escape that same bombardment.
17:48For every crater you see on the Moon,
17:50the Earth is a bigger target out there in space.
17:52There are probably 20 or 30 craters formed on the Earth.
17:55We don't necessarily see them everywhere today
17:57because it's a lively planet with geologic processes that erase those craters.
18:03The Earth is pummeled by the Late Heavy Bombardment.
18:07But how many of these impacts were delivered by water-rich comets?
18:11We don't know, and that's one of the forefront science questions,
18:15is could the comets have come in to deliver ocean water at that time?
18:21If comets made our oceans,
18:24they should have left a unique chemical signature behind,
18:28because not all water is the same.
18:34On Earth, for every 10,000 drops of ordinary water,
18:38there exist three drops of semi-heavy water,
18:42a rare molecule made from deuterium instead of hydrogen.
18:48Deuterium is a normal hydrogen nucleus,
18:53except instead of just being one proton by itself,
18:56it's a proton and a neutron connected together.
19:02The extra neutron in deuterium adds weight,
19:05and that's why water made from these atoms is called heavy.
19:10Semi-heavy water forms more easily in cold conditions,
19:14so the edges of the solar system have more of it
19:17than regions closer to the sun.
19:21The ratio of deuterium to hydrogen is a very sensitive probe
19:25of where water formed in our solar system.
19:29And therefore we can look at the abundance of heavy water
19:32to determine where that water formed and how.
19:38In 1986, scientists get their first opportunity
19:42to test the chemistry of cometary water,
19:46when Halley's comet makes a fleeting return to the night sky.
19:51Astronomers look for the telltale signature of semi-heavy water,
19:56but the result is not what they're expecting.
20:00They measured the water for the first time
20:02and found it was about twice as heavy as Earth's oceans.
20:07Then in the 1990s,
20:09comets Hayakutake and Hale-Bopp pay a visit.
20:15Just like Halley's, these comets are as old as the ones
20:18that smashed into the Earth during the late heavy bombardment.
20:22But both Hayakutake and Hale-Bopp also turn out
20:25to have way more semi-heavy water than the Earth's oceans.
20:33People started to get worried
20:35because the entire Earth water budget, as measured by oceans,
20:39could not be made of just these comets.
20:44In 2015, the Rosetta space probe analyses a comet up close,
20:49and this time the data is indisputable.
20:53A semi-heavy water content three times greater than the water on Earth.
20:58For the most part, the chemistry of Earth's oceans and atmospheres
21:03is actually a very poor match for the chemistry of comets.
21:08When we look at the flavour of hydrogen in the water molecules
21:11that make up the comets that we've measured so far,
21:14it doesn't exactly match the flavour of the water that we find on our planet.
21:19It's clear in my mind that comets could not have brought all of Earth's water.
21:27But if the dirty snowballs weren't to blame, what was?
21:33An unexpected candidate begins to emerge.
21:38In 2011, the Dawn space probe flies by the giant asteroid Vesta.
21:46We used to think rocky objects like Vesta were completely dry.
21:51But the scientists see evidence of water on the surface.
21:55And Vesta's water turns out to be a perfect chemical match for Earth's oceans.
22:02The type of water is exactly what we think is contributing to the water on the Earth.
22:07So it looks like a really solid deal that those types of asteroids were putting the water on the Earth.
22:12Scientists turn their focus from comets to asteroids.
22:17But how could these dry-looking rocks have provided enough water to fill the Earth's oceans?
22:24The explanation could lie within the haunting remains of a failed planet.
22:33Mars
22:40Today, our solar system hosts four rocky planets.
22:45Mercury,
22:47Venus,
22:50Earth,
22:53and Mars.
22:56But there should have been five.
23:03Billions of years ago, planets were forming all over our solar system.
23:07But there was an area in between Mars and Jupiter
23:10where the gravity of Jupiter pretty much pulled apart anything that tried to form.
23:15The scattered remains of that failed rocky planet now fill this gravitational battleground.
23:22Its debris forms a vast band of rubble around the Sun called the asteroid belt.
23:29Rocks inside the asteroid belt range in size from grains of sand to giant boulders hundreds of kilometers wide.
23:38When I first started studying astronomy, we called them rocks, dry rocks.
23:43Now we understand that there may be a lot of water, maybe even liquid water, on some of the larger asteroids.
23:51Our newfound understanding of asteroid water comes from a study of meteorites.
23:55Tiny fragments from the asteroid belt that occasionally fall to Earth.
24:01I've got a sample of a meteorite called a carbonaceous chondrite.
24:05And it looks and feels rather dry to the touch,
24:10but I can tell you that that sample actually contains about 20% by weight water.
24:16Even crushing doesn't release the hidden moisture,
24:20because the water is chemically bound to the minerals that make up the rock.
24:26And let's see if we can get some heat going here on our burner.
24:34Heat allows the water molecules to break their chemical bonds and escape as vapor.
24:41Look at all that water coming out. Just this small sample of meteorite is driving off all of this water.
24:47So here is direct, tangible evidence of the amount of water,
24:51the astonishing amount of water that can be delivered to the Earth from the impact of asteroids.
24:58Four billion years ago, countless asteroids smash into Earth during the late heavy bombardment.
25:06Each impact generates an intense burst of heat that releases the water trapped inside the asteroid.
25:13This water vapor then falls back to the ground as rain.
25:18And this same water remains with us to this day in our oceans, our rivers,
25:28and even in our coffee cups.
25:31When we look at this fingerprint of deuterium on the Earth's water,
25:35it better matches meteorites and asteroids than it does comets.
25:39So, yes, certainly some of the water came from comets,
25:42but the majority of water in your body right now, amazingly, may have come from the asteroid belt.
25:48But water-bearing asteroids may not completely solve the mystery of Earth's first oceans.
25:57A remarkable new geological discovery suggests these impacts only tell part of the story.
26:05There's an amazing amount of water on the surface of the Earth.
26:09The Pacific Ocean has an area of roughly half the surface of the Earth.
26:13Millions and millions of cubic miles of water.
26:15And yet, that's not where all the water on Earth is.
26:20There's quite a bit of it under the surface.
26:24In recent years, geologists have made a stunning discovery.
26:28A layer of heated rock lying deep below the Earth's crust, which holds vast quantities of water.
26:38Seismologists stumble on the layer while analyzing the rumble of earthquakes.
26:46When a big earthquake strikes, low-frequency sound waves travel through the different layers of Earth's interior
26:53before reaching the crust on the other side of the planet.
26:58Studies of these long-range rumbles show some of the sound waves slow down
27:04when they reach a scorching layer of rock sitting 480 kilometers below the crust.
27:10And there's only one thing known to delay the passage of sound through rock.
27:16Water.
27:19Now, it's not like an ocean of water. It's water molecules bound up in minerals and with other molecules.
27:25But if you take all that water and put it all together,
27:28we think it actually would add up to more than all the water in all the oceans on the Earth combined.
27:34This vast underground reserve of water is a genuine puzzle.
27:40Because there's no way comets or asteroids could have penetrated so deeply below the Earth's crust.
27:49That's actually inside the Earth.
27:52It doesn't seem there's an easy way to get it from the surface down hundreds of miles into the mantle.
27:57So it seems far more likely that that water that exists, that was discovered, came with the Earth when it formed.
28:05Was the Earth born wet?
28:08It's a controversial idea, but the evidence is mounting.
28:13Steve Moyzisch believes this gray dust provides the most conclusive proof to date of the wet birth theory.
28:22This is a vial filled with little zircon minerals.
28:26These zircon minerals are amongst the oldest known substances that we have from our planet.
28:32And this sample here formed a mere 150 million years after our planet formed.
28:40And it's the very best record of the earliest Earth.
28:48Until recently, scientists believed Earth was a scorched, dry ball this early in its history.
28:56But ancient zircon samples paint a very different picture.
28:59Because the zircon contains the chemical signature of the Earth's first water oceans.
29:05The amazing find from samples such as these is that liquid water on our planet is a primordial phenomenon.
29:16These tiny zircon crystals are compelling evidence that the Earth was bathed in liquid water millions of years ago.
29:24Millions of years before the late heavy bombardment brought comets and asteroids to the Earth.
29:33Vast quantities of water must have been in the mix when the Earth was created.
29:40But this simple fact means everything we think we know about the birth of our planet is wrong.
29:48The heat of the sun evaporates water from the surface of the Earth's hottest places.
29:55Our vast, parched deserts are almost liquidless.
30:02Five billion years ago, the great cosmic desert that stretches from the young sun to the snowy mountains,
30:08is just as dry.
30:11How could the wet interior of our planet form out of this rocky, arid dust?
30:19We think that the materials that were forming in the solar system right where the Earth is today
30:24would have been much more dry than the Earth actually is.
30:27So we think that the Earth's moisture is much more dry than the Earth actually is.
30:31The materials that were forming in the solar system right where the Earth is today
30:34would have been much more dry than the Earth actually is.
30:37So we think that the Earth had to get an extra contribution of water-rich material.
30:44Where did all this extra cosmic water come from?
30:50Something must have transported it, in bulk, from the wet side of the snow line.
30:56A clue comes from observing distant exoplanets, being cooked alive by their parent stars.
31:13What we see a lot of are Jupiter-sized planets sitting really close to their star.
31:18Sometimes extremely close, sometimes much closer than Mercury is to the sun.
31:22Initially, these star-grazing giants were a mystery.
31:27How did they grow so big, so far away from the icy riches of the snow line?
31:34The only possibility is that these planets must have formed far out from their parent stars
31:39and then, later, migrated in.
31:44We know that those kind of planets can't form there. They're simply too big.
31:48They must have formed farther out and moved inward, migrated towards their star.
31:54And that is interesting because that makes you wonder,
31:57was our solar system always the configuration it is today,
32:01or have our planets moved back and forth?
32:05Exoplanet observations have forced astronomers to devise a radical new theory
32:10about the formation of our own solar system.
32:13Known as the Grand Tack Hypothesis, this theory suggests Jupiter radically altered its course.
32:21There was a time when the disk of dust and gas was very thick around the young sun.
32:27And that actually put a drag on planets as they orbited around.
32:32In the Grand Tack model, Jupiter forms on the outer, wet side of the snow line.
32:38But, slowed down by the matter around it, the gas giant's orbit spirals in closer to the sun.
32:45There's amazing evidence that Jupiter may have moved in as far as the orbit of Mars.
32:52As Jupiter moves in, it brings with it massive quantities of water from beyond the snow line.
33:00This is a chance to push material from much further out in the sky
33:03and throw it into the region where the Earth is forming.
33:05A chance to add a bunch of water-rich material to an otherwise dry Earth.
33:12It's kind of like a huge snow plow, just blasting this material and pushing it inwards
33:16so that while the Earth was forming, Jupiter could have been scattering a bunch of icy bodies
33:21from the outer part of the solar system into where the Earth was forming
33:25while the Earth was still being put together.
33:28Jupiter's inward spiral stops after 100,000 years, when Saturn forms.
33:36As the gravity of these two massive planets interact, they change tack, heading away from the sun.
33:45The water Jupiter leaves behind clumps together with dust to form Saturn.
33:50But how did this water, trapped inside the Earth, turn into the first oceans?
33:59Volcanoes may have played a crucial role.
34:06The first volcanoes were formed in the early 1,000s.
34:11The first volcanoes were formed in the early 1,000s.
34:14Volcanoes may have played a crucial role.
34:21Think about the very young Earth as a blister of volcanic activity.
34:25You see these giant clouds of ash and dust falling down,
34:29but in there, there also would have been water vapor.
34:32Water vapor that could have cooled and condensed in the atmosphere,
34:35built up clouds over hundreds or maybe even thousands of years,
34:39until there was a moment when there was enough water in the atmosphere to begin to rain.
34:44It really was a first rain, billions of years ago.
34:54As this volcanic water rains down on the surface of the Earth,
34:58the first rivers and oceans develop.
35:02This happens long before the late heavy bombardment that brings comets and asteroids to Earth.
35:08Based on evidence from the rocks,
35:10it appears that that liquid water is indigenous, native to our planet.
35:20Comets and asteroids brought some water to Earth.
35:24But if the Grand Tac hypothesis is correct,
35:27then Jupiter delivered most of the water we see filling our oceans today.
35:39And Earth wasn't the only planet watered by Jupiter's foray into the inner solar system.
35:46Both Mars and Venus may have once had oceans too.
35:52To truly appreciate how remarkable our living blue planet is,
35:57we need to find out why we remained watery,
36:01whilst our planetary neighbors dried up.
36:09Water
36:15Water defines the sights and sounds of our planet.
36:20As vapor, it paints the sky with rolling clouds.
36:25And as a liquid, it sculpts and shapes the Earth's surface.
36:30Water fills every cell of every living thing.
36:35And seen from space, our brilliant blue oceans are unique.
36:41A stark contrast to our drab planetary neighbors.
36:45Looking at our nearest neighbors, we see the catastrophe that happens when you lose water.
36:49Not only is water important for biological life,
36:52but the evolution of a planet really changes when you lose this particular molecule.
36:58All the inner planets were sculpted from the same materials.
37:02And there's good evidence that both Venus and Mars once had oceans too.
37:09Mars had a lot of water in its past.
37:12The whole surface is covered with these incredible rivers and streambeds
37:16that are empty now, but really look just like what we see on the Earth.
37:24And with Venus, we think we see evidence that it was also a water world when it was young too,
37:29although we still haven't explored Venus as well as we've explored Mars.
37:33So the best evidence we have suggests that all of these planets started out wet and went through a watery phase.
37:40Clearly, something is happening in the intervening billions of years
37:44that is erasing the water away from these planets.
37:48Why did our neighbors dry up?
37:53Around four and a half billion years ago, Mercury forms.
37:59The closest to the Sun of all the planets in our solar system.
38:03It's also the smallest.
38:06Tiny Mercury barely outsizes our Moon.
38:13And when it comes to holding on to surface water, size matters.
38:18If it's a small planet, it's actually going to lose water to space
38:22because it doesn't have the gravity to hold on to it.
38:29Next in line from the Sun sits Venus.
38:34Earth-sized Venus holds on to its water, at least for a while.
38:39Four billion years ago, Venus and Earth looked like twins.
38:44Both have oceans of liquid water and both are cloaked in thick atmospheres.
38:51But Venus takes up residence closer to the Sun and grows hotter.
38:57Its oceans evaporate, pumping the atmosphere full of water vapor,
39:02a powerful greenhouse gas.
39:05And Venus heats up even more.
39:08Venus got itself into a terrible, vicious cycle.
39:12Water got baked out of rocks.
39:14The minerals themselves were baked to such high temperatures,
39:16they released their water vapor.
39:19There was no way for the water to condense.
39:21It was too hot, so there were no rains.
39:23And so the water moved higher and higher into the atmosphere over time,
39:26where it got blown away.
39:29And now Venus is this hellish landscape, cooked under a heavy atmosphere.
39:44Of all the rocky planets, Mars sits furthest from the Sun.
39:50Billions of years ago, an ocean a mile deep covered half its northern hemisphere.
39:57But it wasn't heat or a lack of gravity that caused Mars to lose all its liquid water.
40:03Mars' oceans were blasted away by radiation.
40:08Mars does not have a magnetic field, and you need a magnetic field
40:11to protect yourself from the solar wind,
40:13these subatomic particles blasting away from the Sun.
40:17Earth has a magnetic field generated by its spinning molten iron core.
40:23This field protects our atmosphere from the solar wind.
40:29Mars, however, is smaller than Earth, and its core cooled,
40:33shutting down its magnetic field
40:35and exposing its atmosphere to the savagery of solar radiation.
40:40Mars' water didn't stand a chance.
40:43Incoming radiation split apart the hydrogen from the oxygen,
40:47so the hydrogen's very light, it just went to space, and it was gone.
40:50So then we were left on Mars with a lot of oxygen.
40:54This is why we hypothesized Mars is a red planet, because it's very rusty,
40:58and that's because all the oxygen that used to be in the water, it's now in the rocks.
41:06Mars' oceans evaporated, leaving behind traces of ice,
41:10and staining its landscape of vivid red.
41:25Four rocky planets created at the same time from the same building materials.
41:34But only one got lucky.
41:37Water is fundamental to life on Earth.
41:39It's the perfect solvent for organic molecules to let the machinery of life do what it does.
41:44You and I could not survive without water.
41:48Water enables the geology, it enables the climate, and it enables the biology of Earth.
41:53So I think that that marks it as a pretty special substance.
41:58It took 14 billion years, and a great deal of time,
42:03It took 14 billion years, and a great deal of luck for our watery Earth to form,
42:09and then to stay watery long enough for life to evolve.
42:16And the more we learn about water,
42:18the more we'll discover just how many other worlds in the universe got lucky too.
42:24And as we discover more and more exoplanets, and more and more planetary systems,
42:28we're going to get a better lay of the land,
42:30and a better view of whether or not our system is something that could be common,
42:33or whether something like the Earth is actually rare.
42:37It's going to require tomorrow's technology to get a better view.
42:42For now, our telescopes don't have the power to see exoplanets clearly enough to identify water.
42:48These places are still so far away that we're not going to be able to resolve
42:52pictures of oceans and continents and little clouds whipping around.
42:56But chemically, we could detect the signs not only of water vapor, but organic molecules.
43:02Scientists hope the next generation of telescopes will detect water on Earth-sized exoplanets
43:09by analyzing the chemical signatures of light passing through the atmospheres of these distant worlds.
43:17And when that happens, there's a good chance we'll discover a Milky Way packed full of watery worlds.
43:24And we've already discovered planets where, in principle, liquid water could exist.
43:29We don't yet know for certain. We will find out.
43:37I think in the next few decades, we will know.
43:39We will be able to identify a planet where we can say, yes, there's liquid water on the surface of that planet.
43:46That will be when the universe changes and we really grow up and realize
43:51we have brothers and sisters in the Milky Way galaxy.