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The night sky is ablaze with stars.
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Hundreds of billions
in our galaxy alone.
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Many larger, brighter
and more majestic than our Sun.
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On the scale of galaxies and stars,
the planets of our solar system
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are little more than grains of sand
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caught momentarily
in the light of the Sun.
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But on those motes of dust,
for over 4 billion years,
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great stories have played out unseen.
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Stories of worlds born...
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...and worlds lost.
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Planets forged amongst the calm...
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...and the chaos.
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Their destinies more entwined
than we ever imagined.
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We know this...
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...because in the last few decades,
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we've sent spacecraft
to all seven of the worlds beyond our own.
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These are the stories
that they return to Earth,
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the stories of the planets.
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For the first few million years
after the Sun formed,
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there were no planets to see it rise.
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Just clouds of dust and gas.
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The leftovers from the birth of the Sun.
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Over tens of millions of years,
the dust began to stick together
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and form the first rocks.
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Eventually, gravity assembled the rocks
to create planetary embryos...
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...that, in time, formed
the four closest planets to the Sun.
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Today, Mercury is the closest of all,
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enduring the Sun's full glare.
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Further out lies Venus...
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...choked by a thick atmosphere.
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Then Venus's neighbour - Earth.
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And farthest of all - Mars.
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A cold desert world.
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Together, they form the only rocky
so-called terrestrial planets
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in the solar system.
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And of the four, one is unique.
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Just look at this... and listen to it.
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This is what a planet looks
and sounds like
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after 4 billion years of evolution
by natural selection.
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There is nowhere else in the solar system
that looks and sounds like this,
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which is interesting
when you think about it,
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because all the planets and moons
are made out of the same stuff -
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the carbon, nitrogen, oxygen, iron -
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all those atoms were present
in the cloud that collapsed
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to form the solar system
4.5 billion years ago.
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And yet, Earth appears to be exceptional,
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a lone living planet
in an otherwise desolate solar system.
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So, what is it that makes this place
so special?
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Is it fate? Is it chance?
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 exist,
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the thing that brings meaning
to the universe - life.
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Earth is a special world
in our solar system,
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and perhaps even for thousands
of light years beyond.
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Our world certainly has unique properties.
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It's the right size and distance
from the Sun
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to have retained an atmosphere
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that's protected its oceans of life-giving
water for billions of years.
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But as we've left the Blue Planet
and explored our sister worlds...
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...we've discovered that each appears
to have had a moment
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when it's enjoyed
almost Earth-like conditions.
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Every one of our rocky neighbours
has a story of what might have been.
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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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Mercury is a world of mystery
and apparent contradictions.
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It's in quite an elliptical orbit,
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which means it can be as far away
from the Sun as 70 million kilometres
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but as close as 46 million.
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That means that temperatures at midday
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can rise to 430 degrees Celsius
on the surface, but at night,
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because it's a small planet
and it's got no atmosphere,
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temperatures fall to minus 170 degrees.
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It's also locked into what's called
a spin-orbit resonance,
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which means the planet spins precisely
three times on its axis
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for every two orbits,
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and that, in turn, means that its day
is twice as long as its year,
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and that means that I could walk over
the surface like this, 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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Mercury is the least explored
of the inner rocky worlds...
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...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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Five, four, three,
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main engines start, two, one and zero,
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and liftoff of Messenger,
on NASA's mission to Mercury.
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A planetary enigma
in our inner solar system.
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Now going through the sound barrier.
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A direct route to Mercury is impractical.
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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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that it would need vast amounts of fuel
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to slow down and enter orbit
around Mercury.
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We just had spacecraft separation.
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Ground-lit solids have jettisoned.
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So Messenger controlled its trajectory
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by stepping from one planet to the next,
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using gravity to slow itself,
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spiralling inwards towards its target.
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Even so, Messenger approached Mercury
at such high speed
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that it was forced to fly past the planet
three times...
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...slowing on each pass...
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...until, after almost seven years
of flawless navigation,
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it arrived safely in orbit.
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Messenger set about its mission
to map Mercury's surface...
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...and began revealing the secrets
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of the most-cratered planet
in the solar system
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in exquisite new detail.
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Messenger was able to do much more
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than just take images
of Mercury's surface.
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By tracking radio signals emitted
by the spacecraft,
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we're able to see very slight changes
in the orbital path around Mercury,
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as seen from Earth,
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and that allows us to map out
Mercury's gravitational field.
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There are also instruments that allow us
to see how the planet wobbles around
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as it spins on its axis,
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and putting all these measurements
together allows us
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to take a cross section
through the planet
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to see what it's made of.
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And when we do that,
we find something very strange.
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Mercury's core extends out about 85%
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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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and removed at some point in its past.
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And there was more.
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The tiny probe began detecting
chemical elements
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in concentrations that no-one had thought
possible this close to the Sun.
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The discovery
of relatively large concentrations
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of elements like sulphur and potassium
on Mercury's surface was a huge surprise.
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If you think back to the time
when the planets were forming,
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you don't expect high concentrations
of those elements
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close to the Sun,
where Mercury orbits today,
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because they're so-called
volatile elements -
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they boil away easily - so you'll only
find high concentrations further out,
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in the colder reaches of the solar system.
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So Mercury is an enigma,
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and discoveries like these have forced us
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to completely rethink our theories about
the formation of the planet.
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Just a few million years
after its formation,
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Mercury was still seething
with the heat of its violent birth.
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Slowly, it cooled, and a crust formed.
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Over time, the crust became enriched
in the volatile elements
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that were escaping Mercury's interior.
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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.
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We now think Mercury was born
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perhaps 170 million kilometres
further away,
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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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Today, it's hard to imagine
the planets in orbits
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other than the ones we see
in the night sky.
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They feel eternal, permanent.
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It's natural to think of the solar system
as a piece of celestial clockwork,
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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
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exactly where they're going
to be at any point in time.
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Now, that is true
if there's only one planet and one star.
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So imagine that's the Sun
and this is Mercury.
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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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But add in one more planet,
let's say Jupiter over there.
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Now there's a gravitational force between
all three of these objects
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and it turns out that, even in principle,
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it is not possible to calculate
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exactly 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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in our knowledge of the position
of the planets
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can lead to radically different
predictions.
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And that's because the system itself,
the orbits of the planets,
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are not stable over very long timescales.
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So planets don't necessarily remain
in the same orbits for ever.
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And the evidence we've gathered
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from the volatiles on Mercury's surface
and the unusual size of its core
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suggests that this may have been
what happened.
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If Mercury began its life
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170 million kilometres
further away from the Sun...
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...then it would have been
in a region of space
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where the young Mars was also forming.
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This region was full of scores
of planetary embryos,
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all fighting for position.
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Amongst the chaos,
something large kicked Mercury inwards,
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towards the Sun.
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Mercury collided with another embryo.
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A glancing blow saw much of its crust
and mantle lost to space.
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Much of this material remained behind,
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perhaps helping to form the early Venus.
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If the theory is correct, then Mercury,
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now little more than a planetary core,
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continued towards the Sun
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and ended up in the peculiar
elliptical orbit we see today.
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The idea that Mercury's outer layers
were stripped away
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in some violent collision
many billions of years ago
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is a superficially attractive one.
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But the theory does have problems.
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Any collision violent enough to do that
heats up the planet,
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and that boils away the volatiles.
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So you have to think
of a very specific kind of collision,
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or perhaps even multiple,
more delicate collisions,
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in order to fit the data.
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So, I think it's fair to say
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that the precise nature
of Mercury's formation
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00:20:24,320 --> 00:20:29,520
is still one of the great unsolved
mysteries in planetary science.
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After four years of observation
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and its discoveries that hint at
Mercury's turbulent past,
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Messenger finally ran out of fuel...
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00:20:53,480 --> 00:20:57,679
...and added yet another crater
to this tiny world that,
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00:20:57,800 --> 00:21:01,840
just perhaps,
could have had a different story to tell.
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50 million kilometres beyond Mercury,
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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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You see that bright point of light
out there in the evening sky?
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That's Venus.
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It's so bright
because it's quite a large planet,
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about the same size as the Earth,
it's not too far away,
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but in particular because it's shrouded in
highly reflective clouds.
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Now, that's the frustrating
but also tantalising thing about Venus.
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Even through a big telescope,
when you look at it,
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it is featureless -
you never see the surface.
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And that meant that,
even until the 1950s,
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00:22:25,920 --> 00:22:29,311
astronomers speculated
that it might be a living world
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with jungles and forests
and rivers and oceans.
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So much so, in fact,
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that when we first sent a spacecraft
to land on the surface of Venus,
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we prepared for a splash landing.
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Throughout the 1960s and '70s,
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00:23:25,120 --> 00:23:30,559
the Soviet Venera programme
sent multiple missions to explore Venus.
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00:23:34,280 --> 00:23:35,475
Many failed...
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...but with each attempt,
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we learned a little more
of the extreme conditions on the planet.
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After 20 years of trying,
Venera 13 began its perilous descent.
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The craft was prepared
to withstand pressures
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00:24:49,400 --> 00:24:51,517
that could crush a car in seconds...
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00:24:53,400 --> 00:24:56,313
...and temperatures that would melt lead.
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00:25:01,040 --> 00:25:03,430
On 1st March 1982...
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00:25:06,600 --> 00:25:09,752
...the Soviets took
the first full colour picture...
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00:25:11,760 --> 00:25:13,240
...of the Venusian surface.
249
00:25:34,520 --> 00:25:37,115
Even under the most extreme
of conditions,
250
00:25:37,240 --> 00:25:40,392
the probe sent its precious data
home to Earth...
251
00:25:49,400 --> 00:25:55,840
...until, 127 minutes after touchdown,
it finally succumbed.
252
00:25:57,120 --> 00:25:59,715
Far from a benign ocean world...
253
00:26:00,920 --> 00:26:02,752
...Venus is a vision of hell...
254
00:26:04,200 --> 00:26:06,032
...where no life can survive.
255
00:26:16,160 --> 00:26:18,550
So, where did it all go wrong for Venus?
256
00:26:18,680 --> 00:26:22,640
Well, that is a good question,
and it's an important one.
257
00:26:22,760 --> 00:26:25,116
It's been said
that we won't fully understand the Earth
258
00:26:25,240 --> 00:26:26,390
until we understand Venus,
259
00:26:26,520 --> 00:26:28,955
and that's because
the planets are so similar.
260
00:26:29,080 --> 00:26:32,152
Venus is the same size as the Earth,
261
00:26:32,280 --> 00:26:35,637
and it's the same composition,
as far as we know,
262
00:26:35,760 --> 00:26:39,800
and although it's closer to the Sun,
it's not as close as Mercury.
263
00:26:39,920 --> 00:26:43,630
So why is it
that one world remained heaven
264
00:26:43,760 --> 00:26:46,480
whilst the other became hell?
265
00:26:54,160 --> 00:26:55,833
Counterintuitively,
266
00:26:55,960 --> 00:27:01,319
the surface temperatures today on Venus
are hotter than those on Mercury.
267
00:27:08,080 --> 00:27:11,676
And the story of Venus's climate
is further complicated
268
00:27:11,800 --> 00:27:15,430
by the fact that,
over the lifetime of the planet,
269
00:27:15,560 --> 00:27:18,632
the Sun itself has been evolving.
270
00:27:21,600 --> 00:27:27,073
As the Sun gets older, the star burns
hotter and hotter and hotter.
271
00:27:27,200 --> 00:27:30,113
That means that, in the past,
when the Sun was younger,
272
00:27:30,240 --> 00:27:32,152
it must have been cooler.
273
00:27:32,280 --> 00:27:34,431
It's called the faint young Sun.
274
00:27:34,560 --> 00:27:36,711
And that has a big impact
on the planets.
275
00:27:36,840 --> 00:27:40,959
At the time when life
was just about beginning on the Earth,
276
00:27:41,080 --> 00:27:43,595
3.5 to 4 billion years ago,
277
00:27:43,720 --> 00:27:44,915
the Sun was fainter.
278
00:27:45,040 --> 00:27:47,555
And that means that Venus was cooler.
279
00:27:47,680 --> 00:27:50,036
In fact, temperatures on Venus
at that time
280
00:27:50,160 --> 00:27:54,040
would have been like a pleasant spring day
here on Earth.
281
00:28:05,280 --> 00:28:07,920
Within a few millions years
of its formation,
282
00:28:08,040 --> 00:28:10,350
the surface of Venus had cooled.
283
00:28:14,240 --> 00:28:17,438
The planet now found itself
at just the right distance
284
00:28:17,560 --> 00:28:19,836
from the faint young Sun
285
00:28:19,960 --> 00:28:25,194
for Venus to experience
a sight familiar to us here on Earth.
286
00:28:43,800 --> 00:28:45,075
The heavens opened.
287
00:29:02,920 --> 00:29:06,516
Great torrents flooded the surface.
288
00:29:08,320 --> 00:29:10,755
Rivers of water flowed.
289
00:29:19,240 --> 00:29:21,994
Venus became an ocean world.
290
00:29:39,720 --> 00:29:43,714
The planet's atmosphere
allowed it to hold on to the oceans...
291
00:29:47,640 --> 00:29:52,157
...by acting as a blanket,
keeping the surface temperate...
292
00:29:54,400 --> 00:29:56,392
...thanks to the greenhouse effect.
293
00:30:02,800 --> 00:30:05,793
The greenhouse effect
is pretty simple physics.
294
00:30:05,920 --> 00:30:08,833
Gases like carbon dioxide
and water vapour
295
00:30:08,960 --> 00:30:12,192
in a planetary atmosphere
are transparent to visible light.
296
00:30:12,320 --> 00:30:15,711
That is obvious,
because there's a source of visible light,
297
00:30:15,840 --> 00:30:17,797
the Sun, and I can see it.
298
00:30:17,920 --> 00:30:21,357
So that radiation falls
onto the surface of the planet
299
00:30:21,480 --> 00:30:22,914
and it heats it up.
300
00:30:23,040 --> 00:30:27,273
The rocks then re-radiate that out into
the atmosphere again,
301
00:30:27,400 --> 00:30:30,552
but this time not as visible light,
but as infrared,
302
00:30:30,680 --> 00:30:32,080
which my eyes can't see.
303
00:30:32,200 --> 00:30:36,831
Now, carbon dioxide and water vapour
absorb infrared,
304
00:30:36,960 --> 00:30:40,192
and so they trap that energy
and the planet heats up.
305
00:30:40,320 --> 00:30:43,313
Now, that's not necessarily a bad thing.
306
00:30:43,440 --> 00:30:45,671
The Earth would be
at an average temperature
307
00:30:45,800 --> 00:30:50,397
of around minus 18 degrees Celsius
without the greenhouse effect.
308
00:30:50,520 --> 00:30:56,198
But there is a thin line between heating
a planet up and frying it.
309
00:31:02,000 --> 00:31:04,913
Gradually, over 2 billion years,
310
00:31:05,040 --> 00:31:07,874
the young Sun grew brighter.
311
00:31:17,680 --> 00:31:19,797
Temperatures began to rise,
312
00:31:19,920 --> 00:31:23,277
lifting more and more water vapour
into the atmosphere.
313
00:31:30,600 --> 00:31:33,274
The greenhouse effect
grew more intense.
314
00:31:54,280 --> 00:31:58,194
Rain evaporated
long before reaching the ground.
315
00:32:05,440 --> 00:32:08,353
Venus had reached a tipping point.
316
00:32:08,480 --> 00:32:12,315
A runaway greenhouse effect
had taken hold.
317
00:32:31,760 --> 00:32:35,071
Venus's moment in the sun was over.
318
00:32:43,120 --> 00:32:47,876
Its cracked surface today
is even hotter than Mercury's,
319
00:32:48,000 --> 00:32:51,630
making Venus the hottest
of all the planets.
320
00:33:02,240 --> 00:33:05,836
As the young Sun's brightness
continued to increase,
321
00:33:05,960 --> 00:33:09,158
the effects were felt across
all the terrestrial planets.
322
00:33:22,000 --> 00:33:25,152
Mars, much further out than Venus,
323
00:33:25,280 --> 00:33:28,478
enjoyed its moment in the sun, too.
324
00:33:34,280 --> 00:33:38,274
With an atmosphere rich
in greenhouse gases,
325
00:33:38,400 --> 00:33:42,758
rivers flowed across its surface
for hundreds of millions of years.
326
00:33:48,960 --> 00:33:52,795
But Mars, being smaller than Venus,
327
00:33:52,920 --> 00:33:55,037
couldn't hold on to its atmosphere.
328
00:34:06,280 --> 00:34:08,192
Much of its water evaporated...
329
00:34:10,680 --> 00:34:12,512
...and escaped into space...
330
00:34:21,440 --> 00:34:25,036
...leaving only small traces behind,
331
00:34:25,160 --> 00:34:29,074
frozen in patches across the planet,
332
00:34:29,200 --> 00:34:35,117
where missions continue to search for the
first signs of extra-terrestrial life.
333
00:34:42,440 --> 00:34:45,512
There's a crater on Mars called
the Hellas Basin,
334
00:34:45,640 --> 00:34:50,635
which is 1,500 kilometres across
and 9 kilometres deep.
335
00:34:50,760 --> 00:34:53,514
That means you could put Everest
on the floor
336
00:34:53,640 --> 00:34:55,950
and the summit would not reach the rim.
337
00:34:56,080 --> 00:35:00,597
The air pressure is so high down there
that liquid water can exist.
338
00:35:03,440 --> 00:35:07,559
So, I suppose it's not impossible to
imagine microbes coming up
339
00:35:07,680 --> 00:35:11,560
from deep below the surface
to bask in the midday sun
340
00:35:11,680 --> 00:35:17,358
before disappearing back down below again
to survive the cold of the Martian night.
341
00:35:17,480 --> 00:35:23,272
But if life does exist out there,
it will certainly only be simple life.
342
00:35:23,400 --> 00:35:27,633
There will be nothing anywhere near
as complex as you or me,
343
00:35:27,760 --> 00:35:29,080
or even this plant.
344
00:35:58,480 --> 00:36:01,120
The story of the solar system is,
in a sense,
345
00:36:01,240 --> 00:36:03,755
a story of instability
and constant change,
346
00:36:03,880 --> 00:36:05,951
at least for the inner rocky worlds.
347
00:36:06,080 --> 00:36:09,232
Mercury has changed its position
radically -
348
00:36:09,360 --> 00:36:12,751
its orbit now takes it close
to the searing heat of the Sun.
349
00:36:12,880 --> 00:36:16,794
Venus probably had water on its surface
350
00:36:16,920 --> 00:36:21,711
for around 2 billion years
before it became hotter than Mercury.
351
00:36:21,840 --> 00:36:26,960
And Mars lost its oceans and rivers
perhaps 3.5 billion years ago.
352
00:36:27,080 --> 00:36:30,357
But unique amongst those worlds is Earth,
353
00:36:30,480 --> 00:36:35,714
because it's remained pretty much
like this - liquid water on the surface -
354
00:36:35,840 --> 00:36:41,472
for 4 billion years, and that has allowed
complex carbon chemistry to develop.
355
00:36:47,440 --> 00:36:50,000
Today, our planet is dominated by life.
356
00:36:50,120 --> 00:36:53,670
It's in every nook and cranny.
I mean, look at this place.
357
00:36:53,800 --> 00:36:56,872
This is a volcano
in the middle of the Atlantic Ocean,
358
00:36:57,000 --> 00:36:59,640
and it is literally teeming with life.
359
00:36:59,760 --> 00:37:04,915
And think of all the chance events
that had to happen over 4 billion years
360
00:37:05,040 --> 00:37:08,954
just to produce the little creatures
in this rock pool.
361
00:37:13,480 --> 00:37:17,872
Life has woven itself
into the fabric of the planet.
362
00:37:23,440 --> 00:37:28,560
It's an integral part
of every continent and every ocean.
363
00:37:33,280 --> 00:37:37,957
It plays a crucial role in maintaining
the balance of our atmosphere
364
00:37:38,080 --> 00:37:40,834
that keeps our planet temperate.
365
00:37:45,880 --> 00:37:47,951
Of all the terrestrial planets,
366
00:37:48,080 --> 00:37:51,790
Earth has enjoyed
the longest moment of them all.
367
00:38:00,840 --> 00:38:02,274
But it can't last.
368
00:38:04,360 --> 00:38:11,073
Earth will ultimately follow the fate
of the other rocky planets,
369
00:38:11,200 --> 00:38:13,795
because even though
we don't feel it day to day,
370
00:38:13,920 --> 00:38:17,675
the Sun's ageing process is relentless.
371
00:38:19,720 --> 00:38:22,758
We can say with confidence
what's going to happen to the Sun
372
00:38:22,880 --> 00:38:24,200
towards the end of its life,
373
00:38:24,320 --> 00:38:26,437
partly because we understand physics
374
00:38:26,560 --> 00:38:30,315
and the nuclear physics of what happens
inside the cores of stars,
375
00:38:30,440 --> 00:38:35,913
but also because the life cycle of stars
is written across the night sky.
376
00:38:36,040 --> 00:38:38,839
Take that bright star there, for example.
377
00:38:38,960 --> 00:38:40,758
It's called Arcturus.
378
00:38:40,880 --> 00:38:44,794
It's around the mass of the Sun,
perhaps a little bit heavier,
379
00:38:44,920 --> 00:38:48,755
but it's between
6 and 8 billion years old,
380
00:38:48,880 --> 00:38:51,759
perhaps 3 billion years older
than the Sun,
381
00:38:51,880 --> 00:38:54,759
and it is now a red giant star.
382
00:38:54,880 --> 00:38:57,600
It's exhausted the hydrogen fuel
in its core
383
00:38:57,720 --> 00:39:00,155
and it's swollen up and cooled.
384
00:39:01,760 --> 00:39:04,719
And that is what we think will happen
to the Sun
385
00:39:04,840 --> 00:39:06,797
in about 5 billion years' time.
386
00:39:17,200 --> 00:39:20,671
As the Sun exhausts
its hydrogen fuel in the core,
387
00:39:20,800 --> 00:39:23,269
its outer edge will inflate.
388
00:39:28,480 --> 00:39:30,836
It will enter a red giant phase,
389
00:39:30,960 --> 00:39:34,636
expanding millions of kilometres
out into space.
390
00:39:39,240 --> 00:39:42,153
Mercury will be the first to be engulfed.
391
00:39:53,320 --> 00:39:56,916
Then Venus's fate will be sealed.
392
00:40:09,720 --> 00:40:14,112
Earth may just escape
the fiery fate of its neighbours...
393
00:40:16,920 --> 00:40:21,631
...hanging on with Mars,
beyond the edge of the dying star.
394
00:40:35,360 --> 00:40:40,071
The era of the four terrestrial planets
will be over.
395
00:40:42,760 --> 00:40:45,480
The lives lived on the surface
of one of them
396
00:40:45,600 --> 00:40:47,557
nothing more than a distant memory.
397
00:40:56,480 --> 00:41:00,360
But that's not quite the end of the story.
398
00:41:02,360 --> 00:41:06,240
Right at the end of the Sun's life,
something wonderful will happen.
399
00:41:10,240 --> 00:41:12,835
A collection of icy worlds
that have lain dormant
400
00:41:12,960 --> 00:41:16,192
for the entire history of the solar system
will awake.
401
00:41:16,320 --> 00:41:19,199
These are the worlds
that orbit the outer planets -
402
00:41:19,320 --> 00:41:22,358
the moons of Jupiter and Saturn.
403
00:41:27,880 --> 00:41:31,794
These distant worlds
that circle the outer gas giants
404
00:41:31,920 --> 00:41:33,639
will begin to warm.
405
00:41:39,280 --> 00:41:42,034
Like Saturn's moon - Enceladus.
406
00:41:44,960 --> 00:41:47,316
Or Jupiter's moon - Europa.
407
00:41:54,280 --> 00:41:55,555
Amongst all these moons,
408
00:41:55,680 --> 00:41:58,991
there is one above all others
that we think, perhaps,
409
00:41:59,120 --> 00:42:03,114
has the best chance of becoming
a place that we'd recognise.
410
00:42:15,720 --> 00:42:19,919
Way out, in the cold, distant reaches
of the solar system...
411
00:42:21,640 --> 00:42:22,869
...past Jupiter...
412
00:42:25,360 --> 00:42:28,637
...around the icy ringed-planet Saturn,
413
00:42:28,760 --> 00:42:30,592
orbits a gem.
414
00:42:31,720 --> 00:42:34,554
And the Cassini spacecraft
is on its way to Saturn.
415
00:42:42,120 --> 00:42:43,600
Titan.
416
00:42:55,760 --> 00:42:59,197
A planet-sized moon bigger than Mercury...
417
00:43:04,880 --> 00:43:09,159
...surrounded by a thick atmosphere
of nitrogen and methane...
418
00:43:10,640 --> 00:43:14,680
...with a surface
that has long remained a mystery.
419
00:43:20,520 --> 00:43:25,834
The Huygens probe was our first chance
to explore beneath the clouds...
420
00:43:25,960 --> 00:43:31,194
...and its camera sent back
these first glimpses of the distant moon.
421
00:44:07,760 --> 00:44:12,073
Wonderfully, the craft made a soft landing
422
00:44:12,200 --> 00:44:16,114
and continued to beam back what it saw.
423
00:44:20,560 --> 00:44:24,349
This is a remarkable photograph,
and as is always the case in science,
424
00:44:24,480 --> 00:44:27,552
the more you know about it,
the more wonderful it gets.
425
00:44:27,680 --> 00:44:30,673
This is a photograph
from the surface of a moon
426
00:44:30,800 --> 00:44:34,999
orbiting around a planet
over a billion kilometres away.
427
00:44:35,120 --> 00:44:38,431
So we got a camera down
onto the surface of a world
428
00:44:38,560 --> 00:44:41,871
in the frozen far reaches
of the solar system.
429
00:44:42,000 --> 00:44:44,879
What we see here is something
that looks like a flood plain
430
00:44:45,000 --> 00:44:47,959
or a riverbed -
very much like this, actually -
431
00:44:48,080 --> 00:44:51,278
and we can say that this is a riverbed,
or a flood plain,
432
00:44:51,400 --> 00:44:55,360
because these rocks on the surface
look like this.
433
00:44:55,480 --> 00:44:59,793
They've been smoothed and eroded
by flowing liquid.
434
00:45:02,440 --> 00:45:07,595
We know these are, in fact,
boulders of frozen water.
435
00:45:07,720 --> 00:45:10,872
They're frozen solid
because the temperature
436
00:45:11,000 --> 00:45:16,120
on the surface of this moon
is minus 180 degrees Celsius.
437
00:45:16,240 --> 00:45:18,152
And that raises an interesting question -
438
00:45:18,280 --> 00:45:22,718
if it's so cold,
then what was the flowing liquid?
439
00:45:31,000 --> 00:45:35,040
Huygens detected significant amounts
of methane,
440
00:45:35,160 --> 00:45:37,072
a flammable gas on Earth.
441
00:45:39,240 --> 00:45:41,755
But the relatively high
atmospheric pressure
442
00:45:41,880 --> 00:45:45,396
and cold temperatures
at the surface of Titan
443
00:45:45,520 --> 00:45:48,991
means that this methane
exists as a liquid.
444
00:46:07,520 --> 00:46:09,398
Titan could be wet...
445
00:46:11,120 --> 00:46:14,796
...not with water,
but with liquid methane,
446
00:46:14,920 --> 00:46:19,073
driving rock-like chunks of ice
down mountain channels
447
00:46:19,200 --> 00:46:21,590
and out into open flood plains.
448
00:46:27,720 --> 00:46:30,872
Huygens survived for just a few hours,
449
00:46:31,000 --> 00:46:35,631
but didn't detect any trace
of liquid methane at its landing site.
450
00:46:42,800 --> 00:46:45,679
But the probe's mother ship, Cassini,
451
00:46:45,800 --> 00:46:47,678
remained in orbit around Saturn.
452
00:46:49,240 --> 00:46:51,471
A year after Huygens landed,
453
00:46:51,600 --> 00:46:55,958
Cassini again flew high above
Titan's north pole
454
00:46:56,080 --> 00:47:02,759
and discovered something seen nowhere else
in the solar system beyond Earth.
455
00:47:11,600 --> 00:47:18,552
Liquid pooling into not just one
but scores of great lakes.
456
00:47:41,640 --> 00:47:45,919
Cassini discovered lakes
of liquid methane.
457
00:47:51,000 --> 00:47:54,789
Earth has a strange cold twin.
458
00:48:03,400 --> 00:48:07,792
In some ways, you could just imagine
floating in a boat on those lakes,
459
00:48:07,920 --> 00:48:13,279
and it would look something like this,
except this would be liquid methane gas,
460
00:48:13,400 --> 00:48:18,156
and those mountains there
would be mountains of frozen water ice,
461
00:48:18,280 --> 00:48:19,714
as hard as rock.
462
00:48:22,280 --> 00:48:24,556
What's also fascinating,
and in fact tantalising,
463
00:48:24,680 --> 00:48:27,878
is that Titan has a complex chemistry,
464
00:48:28,000 --> 00:48:32,279
and that chemistry is carbon chemistry,
the chemistry of life.
465
00:48:32,400 --> 00:48:35,359
So we found molecules
like hydrogen cyanide,
466
00:48:35,480 --> 00:48:37,631
which are the building blocks
of amino acids.
467
00:48:37,760 --> 00:48:41,037
We found molecules
called vinyl cyanides,
468
00:48:41,160 --> 00:48:46,792
which chemists and biologists speculate
could form some sort of cell membranes.
469
00:48:46,920 --> 00:48:50,800
And so, all the ingredients for life
are present on Titan.
470
00:48:53,680 --> 00:48:56,878
Now, very few scientists think there
will be life on Titan today.
471
00:48:57,000 --> 00:49:01,950
It is, after all,
minus 180 degrees Celsius at the surface,
472
00:49:02,080 --> 00:49:05,232
but because of the presence
of all those ingredients,
473
00:49:05,360 --> 00:49:09,718
it might be a very different story
if you warmed Titan up.
474
00:49:27,080 --> 00:49:29,720
In the light of the old, expanding Sun...
475
00:49:32,280 --> 00:49:36,354
...the far reaches of the solar system
will receive more solar energy.
476
00:49:38,200 --> 00:49:41,511
Titan's atmosphere will begin to warm.
477
00:49:56,960 --> 00:50:02,718
Mountains of ice will shrink and melt
as temperatures rise...
478
00:50:06,200 --> 00:50:11,434
...the frozen water they contain replacing
the liquid methane.
479
00:50:42,760 --> 00:50:47,471
Mountains will become oceans... of water.
480
00:50:56,440 --> 00:51:02,198
In a strange twist of fate,
at the end of the life of the Sun,
481
00:51:02,320 --> 00:51:06,519
the solar system's last ocean world
will wake up
482
00:51:06,640 --> 00:51:09,599
to its own biological possibilities.
483
00:51:15,600 --> 00:51:21,039
This distant moon will enjoy
its brief moment in the sun.
484
00:51:28,000 --> 00:51:32,199
It's easy to think of habitability
as a permanent feature of worlds,
485
00:51:32,320 --> 00:51:34,198
a defining characteristic, if you like.
486
00:51:34,320 --> 00:51:37,870
So the Earth is a living planet
because it's in the Goldilocks zone
487
00:51:38,000 --> 00:51:41,198
around the Sun -
not too close and not too far away.
488
00:51:41,320 --> 00:51:43,357
But things are more complicated than that.
489
00:51:43,480 --> 00:51:45,676
Solar systems are dynamic places.
490
00:51:45,800 --> 00:51:49,999
Planetary orbits can change,
and stars can vary in brightness.
491
00:51:50,120 --> 00:51:54,239
So planets that were once heaven
can become hell.
492
00:52:00,720 --> 00:52:04,077
We now understand
that the Earth has been a fortunate world,
493
00:52:04,200 --> 00:52:07,079
an oasis of calm
in an ever-changing solar system
494
00:52:07,200 --> 00:52:10,876
that's maintained a stable climate,
perhaps against the odds,
495
00:52:11,000 --> 00:52:15,517
for the 4 billion years
it took complex living things to evolve.
496
00:52:23,760 --> 00:52:27,993
We don't know how many planets like Earth
there are out there amongst the stars,
497
00:52:28,120 --> 00:52:32,478
places where the ingredients
of solar systems have assembled themselves
498
00:52:32,600 --> 00:52:35,877
into structures
that can dream of other worlds.
499
00:52:36,000 --> 00:52:40,995
But we have to take the possibility
very seriously that there might be few,
500
00:52:41,120 --> 00:52:45,956
and that would make Earth - and us -
extremely rare and precious.
501
00:53:17,520 --> 00:53:23,073
Mercury is the most enigmatic
of all the planets and difficult to study.
502
00:53:27,280 --> 00:53:31,559
The real problem is that it's really hot.
It's the planet closest to the Sun,
503
00:53:31,680 --> 00:53:34,514
and obviously
that makes it challenging.
504
00:53:36,320 --> 00:53:38,994
So you have to protect the spacecraft
505
00:53:39,120 --> 00:53:41,640
from the heat from the Sun, and the way
Messenger addressed this
506
00:53:41,720 --> 00:53:45,316
was to put the whole spacecraft behind
a giant ceramic sunshade.
507
00:53:45,440 --> 00:53:48,114
It had this sunshade.
If it didn't face the Sun,
508
00:53:48,240 --> 00:53:50,516
you would've melted the instruments,
literally.
509
00:53:53,600 --> 00:53:57,276
The other major challenge
is you had to protect the spacecraft
510
00:53:57,400 --> 00:53:59,710
from the heat reflected from the planet,
511
00:53:59,840 --> 00:54:03,993
and the way we dealt with that was to be
in an extremely elliptical orbit,
512
00:54:04,120 --> 00:54:07,830
where we flew in very close
over the north pole and took observations.
513
00:54:07,960 --> 00:54:09,952
The instruments would heat up,
and then we'd fly,
514
00:54:10,080 --> 00:54:13,391
like, 10,000 kilometres
farther out from the planet
515
00:54:13,520 --> 00:54:14,749
while we cooled off.
516
00:54:14,880 --> 00:54:17,236
And in this way - heat up, cool down -
517
00:54:17,360 --> 00:54:19,750
and kept everything below
the danger temperatures,
518
00:54:19,880 --> 00:54:21,837
where instruments could be damaged.
519
00:54:32,640 --> 00:54:37,999
I was in charge of the camera team. I
don't think I slept much the night before.
520
00:54:39,240 --> 00:54:42,312
I was really anxious
to get that first image back.
521
00:54:44,840 --> 00:54:46,399
When that image came back,
522
00:54:46,520 --> 00:54:48,216
we just started pointing
at all the features
523
00:54:48,240 --> 00:54:51,120
that had never been seen before
and saying, "Look at this, look at that."
524
00:54:55,200 --> 00:54:59,831
Over time, the close-up flights
of Mercury's north pole
525
00:54:59,960 --> 00:55:04,876
allowed the team to peer deep
into the shadows of one particular crater.
526
00:55:09,560 --> 00:55:14,874
We realised that we could actually design
a way to take a picture
527
00:55:15,000 --> 00:55:18,357
using a very long exposure
to see inside these dark craters.
528
00:55:18,480 --> 00:55:21,040
Messenger was designed with instruments
529
00:55:21,160 --> 00:55:23,595
that could specifically look
at their reflectance
530
00:55:23,720 --> 00:55:25,552
and also measure hydrogens.
531
00:55:25,680 --> 00:55:28,320
So we thought,
"OK, there's a lot of hydrogen there,"
532
00:55:28,440 --> 00:55:33,799
and there was, like, this fantastic case
built for these ice deposits.
533
00:55:35,400 --> 00:55:37,835
Incredibly, Messenger detected
534
00:55:37,960 --> 00:55:41,795
hundreds of billions of tonnes
of frozen water ice...
535
00:55:43,680 --> 00:55:47,640
...scattered in the permanent shadows
of the polar craters.
536
00:55:50,040 --> 00:55:53,670
The fact that water ice can survive
for a very long period of time
537
00:55:53,800 --> 00:55:56,713
is a reflection of the fact
that Mercury is rotating
538
00:55:56,840 --> 00:55:58,354
almost perfectly straight up,
539
00:55:58,480 --> 00:56:02,030
so that there are craters
near the pole that are so deep
540
00:56:02,160 --> 00:56:05,437
and completely shaded
from sunlight ever hitting them.
541
00:56:05,560 --> 00:56:08,120
So ice could be stable
in those polar regions
542
00:56:08,240 --> 00:56:10,675
that are permanently shadowed
for billions of years.
543
00:56:15,640 --> 00:56:18,109
Messenger had confirmed
544
00:56:18,240 --> 00:56:22,075
the existence of an essential
ingredient for life...
545
00:56:24,640 --> 00:56:28,429
...on the surface
of the closest planet to the Sun.
546
00:56:30,880 --> 00:56:34,396
During Messenger's final orbit,
all the fuel was depleted.
547
00:56:34,520 --> 00:56:36,079
There was nothing that we could do,
548
00:56:36,200 --> 00:56:40,080
and every orbit just brought it
slightly closer to the planet.
549
00:56:40,200 --> 00:56:41,793
One of our engineers realised
550
00:56:41,920 --> 00:56:44,799
that we still had some helium
on board the spacecraft,
551
00:56:44,920 --> 00:56:47,640
and if you blew helium out the back,
it would work like fuel,
552
00:56:47,760 --> 00:56:50,992
and that managed to extend us
for a few extra weeks.
553
00:56:55,520 --> 00:56:58,319
But, of course,
all things have to come to an end.
554
00:57:06,560 --> 00:57:09,234
The last day,
many of us watched the signal
555
00:57:09,360 --> 00:57:11,397
as it went behind the planet
and never came back,
556
00:57:11,520 --> 00:57:12,600
and we knew it had crashed.
557
00:57:12,720 --> 00:57:16,157
And it left us
with a real bittersweet feeling
558
00:57:16,280 --> 00:57:20,991
because we were happy at the success
but of course sad that it was over.
559
00:57:21,120 --> 00:57:25,034
We all took so much pride
in this amazing spacecraft
560
00:57:25,160 --> 00:57:29,120
that lasted way longer
than any of us had planned for it to last
561
00:57:29,240 --> 00:57:31,277
and that had told us so much about Mercury
562
00:57:31,400 --> 00:57:34,791
and really changed the way
that we looked at this planet.