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The night sky is ablaze with
stars...
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00:00:26,900 --> 00:00:29,980
..hundreds of billions in our
galaxy alone...
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00:00:31,900 --> 00:00:35,940
..many larger, brighter
and more majestic than our sun.
4
00:00:42,460 --> 00:00:46,260
On the scale of galaxies and stars,
the planets of our solar system
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00:00:46,260 --> 00:00:49,260
are little more than grains of sand
caught momentarily
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00:00:49,260 --> 00:00:51,060
in the light of the sun.
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00:00:51,060 --> 00:00:55,300
But on those motes of dust,
for over four billion years,
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00:00:55,300 --> 00:00:57,860
great stories have played
out unseen.
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00:01:01,900 --> 00:01:03,820
Stories of worlds born...
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00:01:05,460 --> 00:01:07,180
..and worlds lost.
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00:01:12,900 --> 00:01:15,220
Planets forged amongst the calm...
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..and the chaos.
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00:01:22,460 --> 00:01:26,380
Their destinies more entwined than
we ever imagined.
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00:01:30,340 --> 00:01:32,860
We know this...
15
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..because in the last few decades...
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..we've sent spacecraft to all
seven...
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..of the worlds beyond our own.
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These are the stories
that they return to Earth,
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00:01:48,620 --> 00:01:50,580
the stories of the planets.
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00:02:17,540 --> 00:02:20,780
For the first few million years
after the sun formed,
21
00:02:20,780 --> 00:02:23,500
there were no planets to see it
rise...
22
00:02:30,900 --> 00:02:34,580
..just clouds of dust and the gas...
23
00:02:38,900 --> 00:02:41,540
..the leftovers from the birth
of the sun.
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00:02:46,780 --> 00:02:52,300
Over tens of millions of years,
the dust began to stick together...
25
00:02:52,300 --> 00:02:54,260
..and form the first rocks.
26
00:02:59,980 --> 00:03:02,900
Eventually, gravity assembled
the rocks
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00:03:02,900 --> 00:03:05,180
to create planetary embryos...
28
00:03:08,620 --> 00:03:12,860
..that in time formed the four
closest planets to the sun.
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00:03:31,700 --> 00:03:35,060
Today, Mercury is the closest of
all,
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00:03:35,060 --> 00:03:37,380
enduring the sun's full glare.
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00:03:40,180 --> 00:03:42,020
Further out...
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00:03:42,020 --> 00:03:43,380
..lies Venus...
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00:03:46,140 --> 00:03:49,100
..chocked by a thick atmosphere.
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00:03:51,140 --> 00:03:53,700
Then Venus's neighbour, Earth.
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00:03:56,620 --> 00:03:59,740
And farthest of all, Mars...
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00:04:04,220 --> 00:04:06,620
..a cold desert world.
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00:04:09,180 --> 00:04:13,500
Together, they formed the only rocky
so-called terrestrial planets
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00:04:13,500 --> 00:04:14,820
in the solar system.
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00:04:16,300 --> 00:04:18,020
And of the four...
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00:04:18,020 --> 00:04:20,300
..one is unique.
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00:04:33,460 --> 00:04:34,860
Just look at this...
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..and listen to it.
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00:04:37,620 --> 00:04:40,180
This is what a planet looks
and sounds like
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after four billion years of
evolution by natural selection.
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There is nowhere else in the solar
system
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that looks and sounds like this,
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00:04:48,620 --> 00:04:51,300
which is interesting,
when you think about it,
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00:04:51,300 --> 00:04:54,940
because all the planets and moons
and are made out of the same stuff -
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they're carbon, nitrogen, oxygen,
iron.
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00:04:58,420 --> 00:05:01,980
All those atoms were present in the
cloud that collapsed
51
00:05:01,980 --> 00:05:05,380
to form the solar system,
four-and-a-half billion years ago.
52
00:05:06,860 --> 00:05:10,300
And yet Earth appears
to be exceptional -
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00:05:10,300 --> 00:05:14,460
a lone living planet in an otherwise
desolate solar system.
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00:05:16,020 --> 00:05:20,100
So what is it that makes
this place so special?
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Is it fate? Is it chance?
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00:05:22,500 --> 00:05:24,300
These are important questions
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because Earth is the only place
we know of
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where the most complex
phenomena in the universe exists,
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00:05:30,460 --> 00:05:34,340
the thing that brings meaning
to the universe - life.
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00:05:56,980 --> 00:06:00,860
Earth is a special world
in our solar system
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00:06:00,860 --> 00:06:04,500
and perhaps even for thousands of
light years beyond.
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00:06:12,420 --> 00:06:15,700
Our world certainly has unique
properties.
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00:06:17,140 --> 00:06:20,180
It's the right size and distance
from the sun
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00:06:20,180 --> 00:06:24,300
to have retained an atmosphere
that's protected its oceans
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00:06:24,300 --> 00:06:27,380
of life-giving water for billions of
years.
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00:06:32,740 --> 00:06:35,620
But as we've left the blue
planet,
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00:06:35,620 --> 00:06:37,860
and explored our sister worlds...
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00:06:42,580 --> 00:06:47,500
..we've discovered that each
appears to have had a moment
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00:06:47,500 --> 00:06:50,900
when it enjoyed almost
Earth-like conditions.
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00:07:00,900 --> 00:07:06,580
Every one of our rocky neighbours
has a story of what might have been.
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00:07:26,820 --> 00:07:29,980
Mercury is a small, tortured
world.
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More than any other planet,
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it's endured the unflinching glare
of the sun for billions of years.
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00:07:45,500 --> 00:07:49,700
Mercury is a world of mystery
and apparent contradictions.
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00:07:49,700 --> 00:07:51,420
It's in quite an elliptical orbit,
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00:07:51,420 --> 00:07:55,300
which means it can be as far away
from the sun as 70 million km,
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but as close as 46 million.
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00:07:58,300 --> 00:08:01,100
That means that temperatures
at midday
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00:08:01,100 --> 00:08:04,020
can rise to 430 degrees
Celsius on the surface,
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00:08:04,020 --> 00:08:08,020
but, at night, because it's a small
planet and it's got no atmosphere,
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00:08:08,020 --> 00:08:11,740
temperatures fall to minus 170
degrees.
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00:08:11,740 --> 00:08:14,980
It's also locked into what's called
the spin-orbit resonance,
83
00:08:14,980 --> 00:08:19,980
which means the planet spins
precisely three times on its axis
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00:08:19,980 --> 00:08:21,820
for every two orbits
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00:08:21,820 --> 00:08:26,900
and that in turn means that its day
is twice as long as its year.
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00:08:26,900 --> 00:08:29,580
That means that I could walk
over the surface like this
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about 2mph
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and keep the sun at the same point
in the sky.
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I could stroll in eternal twilight.
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00:08:41,380 --> 00:08:45,420
Mercury is the least explored
of the inner rocky worlds...
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00:08:48,100 --> 00:08:52,820
..because getting to a planet in
such a strange oval-shaped orbit,
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so close to the sun...
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..is a tremendous challenge.
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00:08:58,100 --> 00:09:01,260
Five, four, three,
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00:09:01,260 --> 00:09:04,780
main engine start,
two, one and zero.
96
00:09:04,780 --> 00:09:08,620
And lift off of Messenger on Nasa's
mission to Mercury...
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00:09:12,500 --> 00:09:15,340
..a planetary enigma in our
inner solar system.
98
00:09:20,780 --> 00:09:23,020
Now going through the sound
barrier.
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00:09:24,300 --> 00:09:28,180
A direct route to Mercury is
impractical.
100
00:09:28,180 --> 00:09:32,780
Now going through the period of
maximum dynamic pressure.
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A spacecraft would arrive with so
much speed
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00:09:35,180 --> 00:09:38,620
that it would need vast amounts of
fuel to slow down
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00:09:38,620 --> 00:09:40,860
and enter orbit around Mercury.
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We just had spacecraft separation.
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00:09:49,580 --> 00:09:52,420
So Messenger controlled
its trajectory
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00:09:52,420 --> 00:09:56,100
by stepping from one planet to the
next,
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00:09:56,100 --> 00:09:58,420
using gravity to slow itself,
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00:09:58,420 --> 00:10:01,540
spiralling inwards towards its
target.
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00:10:16,580 --> 00:10:21,060
Even so, Messenger approached
Mercury at such high speed
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00:10:21,060 --> 00:10:24,940
that it was forced to fly past the
planet three times...
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..slowing on each pass...
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00:10:38,580 --> 00:10:42,900
..until, after almost seven years
of flawless navigation,
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00:10:42,900 --> 00:10:45,100
it arrived safely in orbit.
114
00:11:07,580 --> 00:11:11,420
Messenger set about its mission
to map Mercury's surface...
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..and began revealing the secrets
of the most cratered planet
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00:11:19,060 --> 00:11:23,060
in the solar system in exquisite
new detail.
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00:11:49,660 --> 00:11:51,700
Messenger was able to do much
more
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than just take images of Mercury's
surface.
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00:11:56,020 --> 00:11:58,620
By tracking radio signals emitted
by the spacecraft,
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00:11:58,620 --> 00:12:02,940
we're able to see very slight
changes in the orbital path
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00:12:02,940 --> 00:12:05,540
around Mercury, as seen from Earth,
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00:12:05,540 --> 00:12:08,820
and that allows us to map out
Mercury's gravitational field.
123
00:12:08,820 --> 00:12:10,500
There are also instruments
that allow us
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00:12:10,500 --> 00:12:13,300
to see how the planet wobbles around
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00:12:13,300 --> 00:12:14,860
as it spins on its axis.
126
00:12:14,860 --> 00:12:17,060
And putting all these measurements
together
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00:12:17,060 --> 00:12:20,940
allows us to take a cross section
through the planet,
128
00:12:20,940 --> 00:12:22,860
to see what it's made of.
129
00:12:22,860 --> 00:12:25,980
And when we do that,
we find something very strange.
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00:12:25,980 --> 00:12:29,220
Mercury's core extends out about
85%
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00:12:29,220 --> 00:12:32,860
from the centre of the planet
to the surface.
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It's almost entirely an exposed
planetary core.
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It's as if the rocks of the surface
were smashed away
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00:12:41,340 --> 00:12:43,420
and removed at some
point in its past.
135
00:12:47,940 --> 00:12:49,340
And there was more.
136
00:12:50,500 --> 00:12:55,060
The tiny probe began detecting
chemical elements in concentrations
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that no-one had thought possible
this close to the sun.
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00:13:05,660 --> 00:13:09,180
The discovery of relatively
large concentrations of elements
139
00:13:09,180 --> 00:13:12,420
like sulphur and potassium on
Mercury's surface
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00:13:12,420 --> 00:13:14,340
was a huge surprise.
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00:13:14,340 --> 00:13:17,820
If you think back to the time
when the planets were forming,
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00:13:17,820 --> 00:13:21,820
you don't expect high concentrations
of those elements close to the sun,
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where Mercury orbits today, because
they're so-called volatile elements.
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They boil away easily,
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so you only find high concentrations
further out,
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00:13:30,900 --> 00:13:33,580
in the colder reaches of
the solar system.
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So Mercury is an enigma
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00:13:36,460 --> 00:13:40,180
and discoveries like these have
forced us to completely rethink
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00:13:40,180 --> 00:13:42,940
our theories about the formation of
the planet.
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00:13:55,300 --> 00:13:58,340
Just a few million years
after its formation,
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00:13:58,340 --> 00:14:02,580
Mercury was still seething with the
heat of its violent birth.
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00:14:13,140 --> 00:14:16,500
Slowly, it cooled and a crust
formed.
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Over time, the crust became enriched
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00:14:27,980 --> 00:14:31,780
in the volatile elements that were
escaping Mercury's interior.
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00:14:34,180 --> 00:14:38,300
But this could only happen
if Mercury started out
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not in the position we see it
today...
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..but much further out.
158
00:14:45,340 --> 00:14:51,340
We now think Mercury was born
perhaps 170 million km
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further away,
close to the orbit of Mars...
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..a place where,
if it had stayed,
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its destiny could have been very
different.
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But it wasn't to be.
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The young planetary embryo was
ripped from its promising position
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long before it could mature.
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00:15:39,580 --> 00:15:42,140
Today, it's hard to imagine
the planets
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in orbits other than the ones we see
in the night sky.
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They feel eternal, permanent.
168
00:15:50,500 --> 00:15:52,540
It's natural to think of the solar
system
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as a piece of celestial clockwork,
almost like a Swiss watch.
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So if we knew where all the planets
were at some point in time,
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let's say today, then we could
imagine calculating
172
00:16:03,260 --> 00:16:07,260
exactly where they're going to be at
any point in time.
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00:16:07,260 --> 00:16:11,860
Now, that is true if there's only
one planet and one star.
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00:16:11,860 --> 00:16:14,740
So imagine that's the sun
and this is Mercury.
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00:16:14,740 --> 00:16:18,660
Now, we know the gravitational force
between Mercury and the sun.
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And, indeed, if that's all there is,
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then we can calculate its orbit
around the sun
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with essentially
infinite precision.
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00:16:27,340 --> 00:16:32,460
But add in one more planet,
let's say Jupiter over there.
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00:16:32,460 --> 00:16:36,900
Now there's a gravitational force
between all three of these objects
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00:16:36,900 --> 00:16:40,220
and it turns out that,
even in principle,
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it is not possible to calculate
exactly
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where they're all going to
be in the future
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or where they were at some
point in the past.
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This means that any uncertainty,
even of a few metres
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00:16:54,180 --> 00:16:57,020
in our knowledge of the position of
the planets,
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00:16:57,020 --> 00:16:59,700
can lead to radically
different predictions.
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00:17:01,380 --> 00:17:05,940
And that's because the system
itself, the orbits of the planets,
189
00:17:05,940 --> 00:17:08,900
are not stable over very long
timescales.
190
00:17:12,700 --> 00:17:17,260
So planets don't necessarily remain
in the same orbits forever.
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And the evidence we've gathered
192
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from the volatiles on Mercury's
surface,
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00:17:28,620 --> 00:17:31,060
and the unusual size of its core,
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00:17:31,060 --> 00:17:34,580
suggests that this may have been
what happened.
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00:17:43,580 --> 00:17:45,780
If Mercury began its life
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00:17:45,780 --> 00:17:49,300
170 million km further away
from the sun...
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..then it would have been in a
region of space
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00:17:53,660 --> 00:17:55,900
where the young Mars was also
forming.
199
00:18:02,780 --> 00:18:06,780
This region was full of scores
of planetary embryos,
200
00:18:06,780 --> 00:18:08,700
all fighting for position.
201
00:18:17,540 --> 00:18:22,260
Amongst the chaos, something
large kicked Mercury inwards
202
00:18:22,260 --> 00:18:23,860
towards the sun.
203
00:18:48,180 --> 00:18:51,140
Mercury collided with another
embryo.
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00:19:00,820 --> 00:19:03,980
A glancing blow saw much
of its crust
205
00:19:03,980 --> 00:19:06,020
and mantle lost to space.
206
00:19:15,220 --> 00:19:18,260
Much of this material remained
behind,
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perhaps helping to form
the early Venus.
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00:19:25,140 --> 00:19:28,140
If this theory is correct,
then Mercury,
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00:19:28,140 --> 00:19:33,220
now little more than a planetary
core, continued towards the sun
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00:19:33,220 --> 00:19:38,140
and ended up in the peculiar
elliptical orbit we see today.
211
00:19:49,900 --> 00:19:52,940
The idea that Mercury's outer layers
were stripped away
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00:19:52,940 --> 00:19:56,020
in some violent collision many
billions of years ago
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00:19:56,020 --> 00:19:58,340
is a superficially attractive one,
214
00:19:58,340 --> 00:20:01,300
but the theory does have problems.
215
00:20:01,300 --> 00:20:04,980
Any collision violent enough to do
that heats up the planet
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00:20:04,980 --> 00:20:07,500
and that boils away the volatiles.
217
00:20:07,500 --> 00:20:11,740
So you have to think of
a very specific kind of collision,
218
00:20:11,740 --> 00:20:15,180
or perhaps even multiple
more delicate collisions,
219
00:20:15,180 --> 00:20:17,660
in order to fit the data.
220
00:20:17,660 --> 00:20:21,940
So I think it's fair to say
that the precise nature
221
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of Mercury's formation is still one
of the great unsolved mysteries
222
00:20:26,220 --> 00:20:27,900
in planetary science.
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00:20:34,300 --> 00:20:36,620
After four years of observation,
224
00:20:36,620 --> 00:20:41,420
and its discoveries that hint
at Mercury's turbulent past,
225
00:20:41,420 --> 00:20:43,900
Messenger finally ran out of fuel...
226
00:20:52,500 --> 00:20:56,140
..and added yet another crater
to this tiny world
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00:20:56,140 --> 00:21:00,580
that just perhaps could've had
a different story to tell.
228
00:21:25,100 --> 00:21:28,540
50 million km beyond Mercury,
229
00:21:28,540 --> 00:21:33,060
shrouded by an unbroken
blanket of cloud...
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..lies a world which,
at first sight,
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has the potential to be far more
Earth-like.
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00:21:55,220 --> 00:21:59,300
See that bright point of light out
there in the evening sky?
233
00:21:59,300 --> 00:22:01,100
That's Venus.
234
00:22:01,100 --> 00:22:03,740
It's so bright because it's quite
a large planet,
235
00:22:03,740 --> 00:22:06,900
about the same size as the Earth.
It's not too far away.
236
00:22:06,900 --> 00:22:08,500
But, in particular,
237
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because it's shrouded in highly
reflective clouds.
238
00:22:12,660 --> 00:22:17,020
That's the frustrating but also
tantalising thing about Venus.
239
00:22:17,020 --> 00:22:19,700
Even through a big telescope,
when you look at it,
240
00:22:19,700 --> 00:22:22,580
it is featureless.
You never see the surface.
241
00:22:22,580 --> 00:22:24,820
And that meant that,
even until the 1950s,
242
00:22:24,820 --> 00:22:28,940
astronomers speculated that it might
be a living world,
243
00:22:28,940 --> 00:22:32,420
with jungles and forests
and rivers and oceans.
244
00:22:32,420 --> 00:22:36,060
So much so, in fact, that when
we first sent a spacecraft to land
245
00:22:36,060 --> 00:22:39,700
on the surface of Venus,
we prepared for a splash landing.
246
00:23:22,100 --> 00:23:24,540
Throughout the 1960s and '70s,
247
00:23:24,540 --> 00:23:29,740
the Soviet's Venera programme sent
multiple missions to explore Venus.
248
00:23:33,180 --> 00:23:34,460
Many failed...
249
00:23:36,780 --> 00:23:39,540
..but, with each attempt,
we learned a little more
250
00:23:39,540 --> 00:23:42,220
of the extreme conditions on the
planet.
251
00:24:13,340 --> 00:24:18,540
After 20 years of trying, Venera 13
began its perilous descent.
252
00:24:45,820 --> 00:24:48,220
The craft was prepared to withstand
pressures
253
00:24:48,220 --> 00:24:50,660
that could crush a car in seconds...
254
00:24:52,700 --> 00:24:55,340
..in temperatures that would melt
lead.
255
00:24:59,900 --> 00:25:02,740
On March 1st 1982...
256
00:25:05,660 --> 00:25:08,980
..the Soviets took the first
full colour picture...
257
00:25:10,860 --> 00:25:12,300
..of the Venusian surface.
258
00:25:33,420 --> 00:25:36,300
Even under the most extreme
of conditions,
259
00:25:36,300 --> 00:25:39,660
the probe sent its precious data
home to Earth...
260
00:25:49,060 --> 00:25:54,860
..until, 127 minutes after
touchdown, it finally succumbed.
261
00:25:56,300 --> 00:25:58,460
Far from a benign ocean world...
262
00:25:59,820 --> 00:26:01,700
..Venus is a vision of hell...
263
00:26:02,980 --> 00:26:05,340
..where no life can survive.
264
00:26:15,540 --> 00:26:17,820
So where did it all go wrong
for Venus?
265
00:26:17,820 --> 00:26:21,900
Well, that is a good question
and it's an important one.
266
00:26:21,900 --> 00:26:24,380
It's been said that we won't fully
understand the Earth
267
00:26:24,380 --> 00:26:25,980
until we understand Venus
268
00:26:25,980 --> 00:26:29,220
and that's because the planets
are so similar.
269
00:26:29,220 --> 00:26:31,660
Venus is the same size as the Earth.
270
00:26:31,660 --> 00:26:35,500
It's the same composition,
as far as we know.
271
00:26:35,500 --> 00:26:39,420
And although it's closer to the sun,
it's not as close as Mercury.
272
00:26:39,420 --> 00:26:42,740
So why is it that one world
remained heaven
273
00:26:42,740 --> 00:26:45,180
whilst the other became hell?
274
00:26:53,300 --> 00:26:57,980
Counterintuitively, the surface
temperatures today on Venus
275
00:26:57,980 --> 00:27:00,700
are hotter than those on Mercury...
276
00:27:07,380 --> 00:27:10,980
..and the story of Venus's climate
is further complicated
277
00:27:10,980 --> 00:27:14,580
by the fact that,
over the lifetime of the planet,
278
00:27:14,580 --> 00:27:17,620
the sun itself has been evolving.
279
00:27:20,540 --> 00:27:26,460
As the sun gets older, the star
burns hotter and hotter and hotter.
280
00:27:26,460 --> 00:27:29,220
That means that, in the past,
when the sun was younger,
281
00:27:29,220 --> 00:27:31,380
it must have been cooler.
282
00:27:31,380 --> 00:27:33,420
It's called the faint young sun
283
00:27:33,420 --> 00:27:36,020
and that has a big impact
on the planets.
284
00:27:36,020 --> 00:27:40,060
At the time, when life was just
about beginning on the Earth,
285
00:27:40,060 --> 00:27:42,580
three-and-a-half to four billion
years ago,
286
00:27:42,580 --> 00:27:46,660
the sun was fainter and
that means that Venus was cooler.
287
00:27:46,660 --> 00:27:49,340
In fact, temperatures on Venus
at that time
288
00:27:49,340 --> 00:27:53,420
would have been like a pleasant
spring day here on Earth.
289
00:28:04,700 --> 00:28:07,300
Within a few million years
of its formation,
290
00:28:07,300 --> 00:28:09,500
the surface of Venus had cooled.
291
00:28:13,380 --> 00:28:16,900
The planet now found itself
at just the right distance
292
00:28:16,900 --> 00:28:22,060
from the faint young sun for Venus
to experience a sight
293
00:28:22,060 --> 00:28:24,420
familiar to us here on Earth.
294
00:28:42,860 --> 00:28:44,300
The heavens opened.
295
00:29:02,300 --> 00:29:04,820
Great torrents flooded the surface.
296
00:29:07,740 --> 00:29:09,980
Rivers of water flowed.
297
00:29:18,460 --> 00:29:21,460
Venus became an ocean world.
298
00:29:38,980 --> 00:29:42,780
The planet's atmosphere allowed
it to hold on to the oceans...
299
00:29:46,820 --> 00:29:50,660
..by acting as a blanket, keeping
the surface temperate...
300
00:29:53,060 --> 00:29:55,900
..thanks to the greenhouse effect.
301
00:30:01,980 --> 00:30:05,140
The greenhouse effect is pretty
simple physics.
302
00:30:05,140 --> 00:30:09,620
The gases like carbon dioxide, water
vapour and the planetary atmosphere
303
00:30:09,620 --> 00:30:11,780
are transparent to visible light
304
00:30:11,780 --> 00:30:14,940
and it's obvious because there's
a source of visible light,
305
00:30:14,940 --> 00:30:16,940
the sun, and I can see it.
306
00:30:16,940 --> 00:30:20,580
So that radiation falls onto the
surface of the planet
307
00:30:20,580 --> 00:30:24,700
and it heats it up.
The rocks then re-radiate that
308
00:30:24,700 --> 00:30:26,300
out into the atmosphere again,
309
00:30:26,300 --> 00:30:29,820
but, this time, not as visible
light, but as infrared,
310
00:30:29,820 --> 00:30:31,860
which my eyes can't see.
311
00:30:31,860 --> 00:30:35,620
Now, carbon dioxide and water vapour
absorb infrared,
312
00:30:35,620 --> 00:30:39,540
and so they trap that energy and the
planet heats up.
313
00:30:39,540 --> 00:30:42,660
Now, that's not necessarily
a bad thing.
314
00:30:42,660 --> 00:30:45,340
The Earth would be at an average
temperature
315
00:30:45,340 --> 00:30:49,700
of around minus 18 degrees Celsius
without the greenhouse effect,
316
00:30:49,700 --> 00:30:55,060
but there is a thin line between
heating the planet up and frying it.
317
00:31:01,260 --> 00:31:06,820
Gradually, over two billion years,
the young sun grew brighter.
318
00:31:16,820 --> 00:31:21,340
Temperatures began to rise,
lifting more and more water vapour
319
00:31:21,340 --> 00:31:22,820
into the atmosphere.
320
00:31:29,580 --> 00:31:32,660
The greenhouse effect grew
more intense.
321
00:31:53,460 --> 00:31:57,100
Rain evaporated long before reaching
the ground.
322
00:32:04,540 --> 00:32:07,020
Venus had reached a tipping point.
323
00:32:08,380 --> 00:32:11,500
A runaway greenhouse effect
had taken hold.
324
00:32:30,660 --> 00:32:34,420
Venus's moment in the sun was over.
325
00:32:42,180 --> 00:32:47,220
Its cracked surface today
is even hotter than Mercury's,
326
00:32:47,220 --> 00:32:50,580
making Venus the hottest
of all the planets.
327
00:33:01,540 --> 00:33:04,860
As the young sun's brightness
continued to increase,
328
00:33:04,860 --> 00:33:08,220
the effects were felt across all the
terrestrial planets.
329
00:33:21,020 --> 00:33:27,140
Mars, much further out than Venus,
enjoyed its moment in the sun too.
330
00:33:34,420 --> 00:33:37,260
With an atmosphere rich
in greenhouse gases,
331
00:33:37,260 --> 00:33:41,780
rivers flowed across its surface for
hundreds of millions of years.
332
00:33:49,060 --> 00:33:54,020
But Mars, being smaller than Venus,
couldn't hold on to its atmosphere.
333
00:34:05,500 --> 00:34:07,900
Much of its water evaporated and...
334
00:34:09,740 --> 00:34:11,420
..and escaped into space...
335
00:34:21,340 --> 00:34:24,540
..leaving only small traces behind,
336
00:34:24,540 --> 00:34:27,140
frozen in patches across the
planet...
337
00:34:28,380 --> 00:34:30,940
..where missions continue to search
338
00:34:30,940 --> 00:34:34,060
for the first signs of
extraterrestrial life.
339
00:34:41,660 --> 00:34:46,180
There's a crater on Mars called
the Hellas Basin,
340
00:34:46,180 --> 00:34:49,660
which is 1,500 km across and 9km
deep.
341
00:34:49,660 --> 00:34:52,580
That means you could put Everest
on the floor
342
00:34:52,580 --> 00:34:55,260
and the summit would
not reach the rim.
343
00:34:55,260 --> 00:34:57,460
The air pressure is so high
down there
344
00:34:57,460 --> 00:34:59,380
that liquid water can exist.
345
00:35:02,500 --> 00:35:06,060
So I suppose it's not impossible
to imagine microbes
346
00:35:06,060 --> 00:35:10,820
coming up from deep below the
surface to bask in the midday sun
347
00:35:10,820 --> 00:35:13,940
before disappearing back
down below again
348
00:35:13,940 --> 00:35:16,980
to survive the cold
of the Martian night.
349
00:35:16,980 --> 00:35:19,540
But if life does exist out
there,
350
00:35:19,540 --> 00:35:22,540
it will certainly only
be simple life.
351
00:35:22,540 --> 00:35:26,900
There will be nothing anywhere
near as complex as you or me,
352
00:35:26,900 --> 00:35:28,780
or even this plant.
353
00:35:57,340 --> 00:36:00,180
The story of the solar system is,
in a sense,
354
00:36:00,180 --> 00:36:02,900
a story of instability and constant
change,
355
00:36:02,900 --> 00:36:05,180
at least for the inner rocky
worlds.
356
00:36:05,180 --> 00:36:08,140
Mercury has changed its position
radically.
357
00:36:08,140 --> 00:36:11,980
Its orbit now takes it close to the
searing heat of the sun.
358
00:36:11,980 --> 00:36:15,460
Venus probably had water on its
surface
359
00:36:15,460 --> 00:36:20,820
for around two billion years before
it became hotter than Mercury.
360
00:36:20,820 --> 00:36:23,620
And Mars lost its oceans and rivers
361
00:36:23,620 --> 00:36:26,060
perhaps three-and-a-half billion
years ago.
362
00:36:26,060 --> 00:36:29,020
But unique amongst those worlds is
Earth
363
00:36:29,020 --> 00:36:32,700
because it's remained pretty
much like this,
364
00:36:32,700 --> 00:36:36,900
liquid water on the surface,
for four billion years,
365
00:36:36,900 --> 00:36:40,420
and that has allowed complex carbon
chemistry to develop.
366
00:36:46,540 --> 00:36:49,140
Today, our planet is dominated
by life.
367
00:36:49,140 --> 00:36:52,780
It's in every nook and cranny.
I mean, look at this place.
368
00:36:52,780 --> 00:36:56,140
This is a volcano in the middle
of the Atlantic Ocean
369
00:36:56,140 --> 00:36:58,660
and it is literally teeming
with life.
370
00:36:58,660 --> 00:37:02,260
And think about all the chance
events that had to happen
371
00:37:02,260 --> 00:37:03,900
over four billion years
372
00:37:03,900 --> 00:37:07,340
just to produce the little creatures
in this rock pool.
373
00:37:12,660 --> 00:37:16,460
Life has woven itself into
the fabric of the planet.
374
00:37:23,100 --> 00:37:27,220
It's an integral part of every
continent and every ocean.
375
00:37:32,500 --> 00:37:34,140
It plays a crucial role
376
00:37:34,140 --> 00:37:37,420
in maintaining the balance
of our atmosphere...
377
00:37:37,420 --> 00:37:39,740
..that keeps our planet temperate.
378
00:37:44,980 --> 00:37:47,380
Of all the terrestrial planets,
379
00:37:47,380 --> 00:37:50,900
Earth has enjoyed the longest
moment of them all...
380
00:37:59,740 --> 00:38:01,300
..but it can't last.
381
00:38:04,540 --> 00:38:09,460
Earth will ultimately follow the
fate of the other rocky planets
382
00:38:09,460 --> 00:38:12,820
because, even though we don't feel
it day-to-day,
383
00:38:12,820 --> 00:38:16,580
the sun's ageing process is
relentless.
384
00:38:18,980 --> 00:38:21,860
We can say with confidence what's
going to happen to the sun
385
00:38:21,860 --> 00:38:23,380
towards the end of its life,
386
00:38:23,380 --> 00:38:25,700
partly because we understand physics
387
00:38:25,700 --> 00:38:29,780
and the nuclear physics of what
happens inside the cores of stars,
388
00:38:29,780 --> 00:38:32,820
but also because the life cycle of
stars
389
00:38:32,820 --> 00:38:35,340
is written across the night sky.
390
00:38:35,340 --> 00:38:38,180
Take that bright star there,
for example.
391
00:38:38,180 --> 00:38:40,060
It's called Arcturus.
392
00:38:40,060 --> 00:38:43,980
It's around the mass of the sun,
perhaps a little bit heavier,
393
00:38:43,980 --> 00:38:47,780
but it's between six and eight
billion years old,
394
00:38:47,780 --> 00:38:51,180
perhaps three
billion years older than the sun -
395
00:38:51,180 --> 00:38:53,980
and it is now a red giant star.
396
00:38:53,980 --> 00:38:56,820
It's exhausted the hydrogen fuelled
in its core,
397
00:38:56,820 --> 00:38:58,900
and it's swollen up and cooled.
398
00:39:01,020 --> 00:39:03,940
And that is what we think
will happen to the sun
399
00:39:03,940 --> 00:39:05,980
in about five billion years' time.
400
00:39:16,340 --> 00:39:19,860
As the sun exhausts its hydrogen
fuel in the core,
401
00:39:19,860 --> 00:39:22,220
its outer edge will inflate.
402
00:39:27,660 --> 00:39:30,020
It will enter a red giant phase,
403
00:39:30,020 --> 00:39:33,860
expanding millions of kilometres
out into space.
404
00:39:38,300 --> 00:39:41,260
Mercury will be the first
to be engulfed.
405
00:39:52,540 --> 00:39:55,500
Then Venus's fate will be sealed.
406
00:40:09,020 --> 00:40:12,900
Earth may just escape the fiery
fate of its neighbours...
407
00:40:16,140 --> 00:40:20,540
..hanging on with Mars beyond
the edge of the dying star.
408
00:40:34,340 --> 00:40:38,820
The era of the four terrestrial
planets will be over.
409
00:40:42,020 --> 00:40:44,580
The lives lived on the surface
of one of them
410
00:40:44,580 --> 00:40:47,380
nothing more than a distant memory.
411
00:40:55,540 --> 00:40:59,020
But that's not quite the end
of the story.
412
00:41:01,620 --> 00:41:05,460
Right at the end of the sun's life,
something wonderful will happen.
413
00:41:08,860 --> 00:41:11,620
A collection of icy worlds that have
lain dormant
414
00:41:11,620 --> 00:41:15,500
for the entire history of the solar
system will awake.
415
00:41:15,500 --> 00:41:18,620
These are the worlds that orbit
the outer planets,
416
00:41:18,620 --> 00:41:20,700
the moons of Jupiter and Saturn.
417
00:41:26,660 --> 00:41:31,060
These distant worlds that circle
the outer gas giants
418
00:41:31,060 --> 00:41:32,900
will begin to warm...
419
00:41:37,940 --> 00:41:40,860
..like Saturn's moon Enceladus...
420
00:41:44,260 --> 00:41:46,460
..or Jupiter's moon Europa.
421
00:41:53,140 --> 00:41:56,580
Amongst all these moons,
there is one above all others
422
00:41:56,580 --> 00:41:59,340
that we think perhaps has the best
chance
423
00:41:59,340 --> 00:42:02,140
of becoming a place that we'd
recognise.
424
00:42:14,980 --> 00:42:19,020
Way out in the cold, distant
reaches of the solar system...
425
00:42:20,740 --> 00:42:22,180
..past Jupiter...
426
00:42:24,500 --> 00:42:27,860
..around the icy-ringed planet
Saturn,
427
00:42:27,860 --> 00:42:29,340
orbits a gem...
428
00:42:30,700 --> 00:42:33,500
And the Cassini spacecraft
is on its way to Saturn.
429
00:42:41,660 --> 00:42:43,060
..Titan...
430
00:42:55,060 --> 00:42:58,580
..a planet-sized moon bigger
than Mercury...
431
00:43:04,100 --> 00:43:08,220
..surrounded by a thick atmosphere
of nitrogen and methane...
432
00:43:09,900 --> 00:43:13,340
..with a surface that has long
remained a mystery.
433
00:43:19,220 --> 00:43:22,380
The Huygens probe was our
first chance to explore
434
00:43:22,380 --> 00:43:24,100
beneath the clouds...
435
00:43:25,260 --> 00:43:29,860
..and its camera sent back these
first glimpses of the distant moon.
436
00:44:07,060 --> 00:44:11,020
Wonderfully, the craft made a soft
landing
437
00:44:11,020 --> 00:44:14,540
and continued to beam back
what it saw.
438
00:44:19,980 --> 00:44:23,180
This is a remarkable photograph and,
as is always the case in science,
439
00:44:23,180 --> 00:44:26,620
the more you know about it,
the more wonderful it gets.
440
00:44:26,620 --> 00:44:29,820
This is a photograph from
the surface of the moon
441
00:44:29,820 --> 00:44:33,900
orbiting around a planet over a
billion kilometres away,
442
00:44:33,900 --> 00:44:37,660
so we've got a camera down onto the
surface of a world
443
00:44:37,660 --> 00:44:41,140
in the frozen far reaches
of the solar system.
444
00:44:41,140 --> 00:44:44,260
What you see here is something
that looks like a flood plain,
445
00:44:44,260 --> 00:44:47,300
or a riverbed,
very much like this, actually.
446
00:44:47,300 --> 00:44:50,340
And we can say that this is
a riverbed or a flood plain
447
00:44:50,340 --> 00:44:53,220
because these rocks on the
surface...
448
00:44:53,220 --> 00:44:54,580
..look like this.
449
00:44:54,580 --> 00:44:58,820
They've been smoothed and eroded
by flowing liquid.
450
00:45:01,620 --> 00:45:07,300
We know these are in fact
boulders of frozen water.
451
00:45:07,300 --> 00:45:11,100
They're frozen solid because
the temperature on the surface
452
00:45:11,100 --> 00:45:15,500
of this moon is minus 180
degrees Celsius.
453
00:45:15,500 --> 00:45:17,460
That raises an interesting question.
454
00:45:17,460 --> 00:45:19,540
If it's so cold...
455
00:45:19,540 --> 00:45:21,380
..then what was the flowing liquid?
456
00:45:30,060 --> 00:45:34,260
Huygens detected significant
amounts of methane...
457
00:45:34,260 --> 00:45:36,100
..a flammable gas on Earth.
458
00:45:38,460 --> 00:45:41,100
But the relatively high atmospheric
pressure
459
00:45:41,100 --> 00:45:44,060
and cold temperatures at the surface
of Titan
460
00:45:44,060 --> 00:45:48,380
means that this methane
exists as a liquid.
461
00:46:06,500 --> 00:46:08,500
Titan could be wet...
462
00:46:09,860 --> 00:46:14,060
..not with water, but with liquid
methane...
463
00:46:14,060 --> 00:46:18,580
..driving rock-like chunks of ice
down mountain channels
464
00:46:18,580 --> 00:46:20,860
and out into open flood plains.
465
00:46:26,780 --> 00:46:29,820
Huygens survived for just a few
hours,
466
00:46:29,820 --> 00:46:32,140
but didn't detect any trace
467
00:46:32,140 --> 00:46:34,540
of liquid methane
at its landing site.
468
00:46:41,900 --> 00:46:46,980
But the probe's mother ship Cassini
remained in orbit around Saturn.
469
00:46:48,500 --> 00:46:51,100
A year after Huygens landed,
470
00:46:51,100 --> 00:46:55,220
Cassini again flew high
above Titan's North Pole
471
00:46:55,220 --> 00:46:59,180
and discovered something seen
nowhere else in the solar system
472
00:46:59,180 --> 00:47:00,900
beyond Earth.
473
00:47:10,820 --> 00:47:17,140
Liquid pooling into not just one,
but scores of great lakes.
474
00:47:40,660 --> 00:47:42,780
Cassini discovered lakes...
475
00:47:43,940 --> 00:47:45,260
..of liquid methane.
476
00:47:49,820 --> 00:47:53,940
Earth has a strange cold twin.
477
00:48:02,340 --> 00:48:07,060
In some ways, you could just imagine
floating in a boat on those lakes
478
00:48:07,060 --> 00:48:08,820
and it'll look something like this,
479
00:48:08,820 --> 00:48:12,780
except this would be liquid
methane gas
480
00:48:12,780 --> 00:48:17,340
and those mountains there would be
mountains of frozen water ice
481
00:48:17,340 --> 00:48:18,820
as hard as rock.
482
00:48:20,740 --> 00:48:23,660
What's also fascinating, and in fact
tantalising,
483
00:48:23,660 --> 00:48:26,860
is that Titan has a complex
chemistry
484
00:48:26,860 --> 00:48:31,700
and that chemistry is carbon
chemistry - the chemistry of life.
485
00:48:31,700 --> 00:48:34,020
So we found molecules like hydrogen
cyanide,
486
00:48:34,020 --> 00:48:37,100
which are the building
blocks of amino acids.
487
00:48:37,100 --> 00:48:40,020
We found molecules called vinyl
cyanides,
488
00:48:40,020 --> 00:48:42,780
which chemists and biologists
speculate
489
00:48:42,780 --> 00:48:46,060
could form some sort of
cell membranes.
490
00:48:46,060 --> 00:48:50,020
And so all the ingredients
for life are present on Titan.
491
00:48:52,580 --> 00:48:56,340
Now, very few scientists think
there will be life on Titan today.
492
00:48:56,340 --> 00:49:01,260
It is, after all, minus 180
degrees Celsius at the surface,
493
00:49:01,260 --> 00:49:04,700
but, because of the presence
of all those ingredients,
494
00:49:04,700 --> 00:49:08,540
it might be a very different story
if you warm Titan up.
495
00:49:26,100 --> 00:49:29,100
In the light
of the old expanding sun...
496
00:49:31,020 --> 00:49:33,580
..the far reaches of the solar
system
497
00:49:33,580 --> 00:49:35,860
will receive more solar energy.
498
00:49:37,180 --> 00:49:40,540
Titan's atmosphere will begin
to warm.
499
00:49:55,580 --> 00:50:01,060
Mountains of ice will shrink
and melt as temperatures rise...
500
00:50:05,340 --> 00:50:10,500
..the frozen water they contain
replacing the liquid methane.
501
00:50:42,140 --> 00:50:46,260
Mountains will become oceans
of water.
502
00:50:55,740 --> 00:51:00,020
In a strange twist of fate,
at the end the life of the sun...
503
00:51:01,500 --> 00:51:04,740
..the solar system's last
ocean world
504
00:51:04,740 --> 00:51:08,660
will wake up to its own biological
possibilities.
505
00:51:14,540 --> 00:51:20,220
This distant moon will enjoy
its brief moment in the sun.
506
00:51:27,100 --> 00:51:30,180
It's easy to think of habitability
as a permanent feature
507
00:51:30,180 --> 00:51:33,620
of a world's defining
characteristic, if you like.
508
00:51:33,620 --> 00:51:37,140
So the Earth is a living planet
because it's in the Goldilocks zone
509
00:51:37,140 --> 00:51:40,780
around the sun - not too close
and not too far away.
510
00:51:40,780 --> 00:51:42,660
But things are more complicated
than that.
511
00:51:42,660 --> 00:51:45,060
Solar systems are dynamic places.
512
00:51:45,060 --> 00:51:49,500
Planetary orbits can change
and stars can vary in brightness,
513
00:51:49,500 --> 00:51:53,220
so planets that were once
heaven can become hell.
514
00:51:59,820 --> 00:52:03,260
We now understand that the Earth
has been a fortunate world,
515
00:52:03,260 --> 00:52:06,060
an oasis of calm in an ever-changing
solar system,
516
00:52:06,060 --> 00:52:09,940
that's maintained a stable climate,
perhaps against the odds,
517
00:52:09,940 --> 00:52:14,540
for the four billion years it took
complex living things to evolve.
518
00:52:22,500 --> 00:52:25,500
We don't know how many planets
like Earth there are
519
00:52:25,500 --> 00:52:27,340
out there amongst the stars,
520
00:52:27,340 --> 00:52:30,220
places where the ingredients of
solar systems
521
00:52:30,220 --> 00:52:32,940
have assembled themselves into
structures
522
00:52:32,940 --> 00:52:35,180
that can dream of other worlds,
523
00:52:35,180 --> 00:52:38,300
but we have to take the possibility
very seriously
524
00:52:38,300 --> 00:52:40,300
that there might be few.
525
00:52:40,300 --> 00:52:44,900
And that would make Earth, and us,
extremely rare and precious.
526
00:53:16,500 --> 00:53:21,860
Mercury is the most enigmatic of all
the planets and difficult to study.
527
00:53:26,540 --> 00:53:28,780
The real problem is that it's really
hot.
528
00:53:28,780 --> 00:53:31,340
It's the planet closest to the sun
529
00:53:31,340 --> 00:53:34,300
and obviously that makes it
challenging.
530
00:53:35,860 --> 00:53:37,580
So you have to protect the
spacecraft
531
00:53:37,580 --> 00:53:40,380
from the heat from the sun, and the
way Messenger addressed this
532
00:53:40,380 --> 00:53:44,540
was to put the whole spacecraft
behind a giant ceramic sunshade.
533
00:53:44,540 --> 00:53:47,060
It had this sunshade.
If it didn't face the sun,
534
00:53:47,060 --> 00:53:50,100
it would have melted the
instruments, literally.
535
00:53:52,540 --> 00:53:56,460
The other major challenge is, you
have to protect a spacecraft
536
00:53:56,460 --> 00:53:58,780
from the heat reflected
from the planet
537
00:53:58,780 --> 00:54:00,500
and the way we dealt with that
538
00:54:00,500 --> 00:54:02,940
was to be in an extremely elliptical
orbit,
539
00:54:02,940 --> 00:54:07,380
where we flew in very close over the
North Pole and took observations.
540
00:54:07,380 --> 00:54:10,300
The instruments would heat up and
then we'd fly like 10,000km
541
00:54:10,300 --> 00:54:13,300
farther out from the planet...
542
00:54:13,300 --> 00:54:15,380
..while we cooled off,
and, in this way,
543
00:54:15,380 --> 00:54:17,940
heat up, cool down and kept
everything below
544
00:54:17,940 --> 00:54:21,100
the danger temperatures, where
instruments could be damaged.
545
00:54:31,860 --> 00:54:34,580
I was in charge of the camera team.
546
00:54:34,580 --> 00:54:36,900
I don't think I slept much
of the night before.
547
00:54:38,420 --> 00:54:41,500
I was really anxious to get
that first image back.
548
00:54:43,820 --> 00:54:45,220
When that image came back,
549
00:54:45,220 --> 00:54:47,220
we just started pointing
at all the features
550
00:54:47,220 --> 00:54:50,860
that had never been seen before and
saying, "Look at this, look at that."
551
00:54:55,340 --> 00:54:59,260
Over time, the close-up flights
of Mercury's North Pole
552
00:54:59,260 --> 00:55:02,460
allowed the team to peer deep into
the shadows
553
00:55:02,460 --> 00:55:04,340
of one particular crater.
554
00:55:08,180 --> 00:55:12,140
We realised that we could actually
design a way
555
00:55:12,140 --> 00:55:15,540
to take a picture using a very long
exposure
556
00:55:15,540 --> 00:55:18,580
to see inside these dark craters.
557
00:55:18,580 --> 00:55:20,420
Messenger was designed with
instruments
558
00:55:20,420 --> 00:55:22,700
that could specifically look at
their reflectance
559
00:55:22,700 --> 00:55:24,900
and also measure hydrogen.
560
00:55:24,900 --> 00:55:28,020
So we saw, "OK, there's a lot
of hydrogen there,"
561
00:55:28,020 --> 00:55:32,820
and there was like this fantastic
case built for these ice deposits.
562
00:55:34,740 --> 00:55:38,380
Incredibly, Messenger detected
hundreds of billions
563
00:55:38,380 --> 00:55:40,700
of tonnes of frozen water ice...
564
00:55:42,860 --> 00:55:46,900
..scattered in the permanent
shadows of the polar craters.
565
00:55:48,900 --> 00:55:52,860
The fact that water ice can survive
for a very long period of time
566
00:55:52,860 --> 00:55:56,020
is a reflection of the fact
that the Mercury is rotating
567
00:55:56,020 --> 00:55:59,340
almost perfectly straight up, so
that there are craters near the pool
568
00:55:59,340 --> 00:56:02,140
that are so deep and completely
shaded
569
00:56:02,140 --> 00:56:04,500
from sunlight ever hitting them.
570
00:56:04,500 --> 00:56:06,980
So ice could be stable in those
polar regions
571
00:56:06,980 --> 00:56:09,780
that are permanently shadowed for
billions of years.
572
00:56:15,780 --> 00:56:18,140
Messenger had confirmed
the existence
573
00:56:18,140 --> 00:56:20,620
of an essential ingredient for
life...
574
00:56:23,860 --> 00:56:27,340
..on the surface at the closest
planet to the sun.
575
00:56:30,020 --> 00:56:33,500
During Messenger's final orbits,
all the fuel was depleted -
576
00:56:33,500 --> 00:56:35,340
there was nothing that we could do -
577
00:56:35,340 --> 00:56:39,300
and every orbit just brought
it slightly closer to the planet.
578
00:56:39,300 --> 00:56:40,900
One of our engineers realised
579
00:56:40,900 --> 00:56:43,740
that we still had some helium
on board the spacecraft
580
00:56:43,740 --> 00:56:47,140
and, if you blew helium out the
back it would work like fuel,
581
00:56:47,140 --> 00:56:50,460
and that managed to extend us
for a few extra weeks.
582
00:56:54,580 --> 00:56:58,020
But, of course, all things
have to come to an end.
583
00:57:05,780 --> 00:57:08,580
The last day,
many of us watched the signal
584
00:57:08,580 --> 00:57:10,820
as it went behind the planet and
never came back
585
00:57:10,820 --> 00:57:12,100
and we knew it had crashed
586
00:57:12,100 --> 00:57:15,380
and it left us with a real
bittersweet feeling,
587
00:57:15,380 --> 00:57:18,380
because we were happy
at the success,
588
00:57:18,380 --> 00:57:20,300
but, of course, sad that it was
over.
589
00:57:20,300 --> 00:57:24,260
We all took so much pride
in this amazing spacecraft
590
00:57:24,260 --> 00:57:28,260
that lasted way longer than any
of us had planned for it to last
591
00:57:28,260 --> 00:57:30,660
and that had told us so much
about Mercury
592
00:57:30,660 --> 00:57:33,660
and really changed the way that we
looked at this planet.
593
00:57:45,420 --> 00:57:48,980
Touchdown on the red planet...
594
00:57:48,980 --> 00:57:50,540
..to uncover...
595
00:57:51,620 --> 00:57:52,940
..the story of Mars.
596
00:57:59,340 --> 00:58:01,780
Journey through our solar system
with this free poster
597
00:58:01,780 --> 00:58:03,980
produced by the Open University
598
00:58:03,980 --> 00:58:07,220
and discover more about its planets
and moons.
599
00:58:08,340 --> 00:58:11,460
Order your free copy by calling...
600
00:58:15,300 --> 00:58:17,460
Or go to...
601
00:58:21,380 --> 00:58:24,580
..and follow the links
to the Open University.
50853
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