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In the universe,

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everything seems
to orbit something.

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Planets orbit stars,
and moons orbit planets.

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Some moons are volcanic,
but the volcanoes are ice.

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Others are awash
with great oceans.

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There may be more
habitable moons in our galaxy

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than there are
habitable planets.

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Moons tell the unknown
stories of our solar system

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and show us how it all works.

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In our own solar system,
there are just eight planets.

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But orbiting six
of those planets are moons...

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...lots and lots of moons...
more than 300 of them.

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Each one is different...

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...each one a world all its own.

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Well, when we look
out on our solar system,

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we see a lot of planets.

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But even more than planets,
we see moons.

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And in many ways,
they're more interesting

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than the planets
that they go around.

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We have moons that are airless
and apparently dead, like ours.

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Then, out in
the outer solar system,

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we have moons
with oceans inside them

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and moons with atmospheres
around them.

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I'm for moons.
You can keep the planets.

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The biggest eruptions...

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...the coldest temperatures...

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...and the largest oceans
in the solar system...

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they're all on moons.

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There are moons
with ice volcanoes.

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There are moons with lakes of
methane and methane rainfall,

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smog clouds...

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...moons that are
so volcanically active

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that they keep remaking
their surface...

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Moons with all kinds of plumes
shooting off into space...

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really a much wider range
of environments

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than we ever
could have imagined.

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Often, when I'm describing

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to the general public,
or even to my fellow scientists,

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these moons of Saturn
and Jupiter,

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I call them "worlds"
because they really do have

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the complexity and mystery
of a whole world.

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Jupiter and Saturn
have over 60 moons each.

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These giant gas planets
and their moons

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are like mini solar systems,

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and each moon
has a distinct personality.

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Lapetus, a two-toned moon
in black and white.

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Titan, with a dense,
orange atmosphere.

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And icy Enceladus, blasting ice
geysers 200 miles into space.

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Each moon is unique.

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But they all have
one thing in common.

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All moons
are natural satellites,

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held in place by gravity.

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But moons do more
than just go around planets.

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They help stabilize the planets
in their orbits

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and keep the machinery
of the solar system

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running smoothly.

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The diversity of moons

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is an interesting combination
of predictable laws of science

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and then complete randomness
of just things smashing together

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and the chips kind of falling
where they did

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in a way that you could
never predict.

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Planets and moons
begin the same way.

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Once a star turns on,

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there's a lot of dust and gas
left over.

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Slowly, the dust particles clump
together, forming rocks.

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The rocks smash into each other
and form boulders.

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Slowly, the objects
get bigger and bigger.

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The process
is called accretion.

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One can think of it
as forming a snowball

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and rolling it down a hill.

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As it rolls down the hill,

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it collects and gathers up
yet more snow,

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which makes it roll
faster and harder.

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And so that process
of runaway accretion

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actually happens

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in the formation of the planets

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and in the formation
of moons, as well.

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It sounds simple enough,

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but nobody knew for sure
how it worked until 2003.

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On the International
Space Station,

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astronaut Don Pettit was
experimenting in zero gravity.

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He put grains of salt and sugar
inside a plastic baggie.

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Instead of floating apart,
they began to clump together.

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This is how both planets
and moons build up.

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But instead of taking shape
around stars,

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most big moons take shape
around planets.

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If the same process
makes them all,

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what makes all of them
so different from each other?

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Take two of Jupiter's moons,
Callisto and Ganymede...

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...two very different moons,

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each born from the same debris
when Jupiter was still young.

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Ganymede formed
close to Jupiter,

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where there was lots of debris.

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Because there was
so much material,

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it came together quickly...
in about 10,000 years...

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and it was hot.

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The heat separated the ice
from the rock.

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You can still see it in
Ganymede's distinct landscape.

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The primary factor that affects

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why moons are the way
they are today

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is energy...

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how much energy
was put into them

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as heat during accretion

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and how much energy
has been lost.

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All of those factors go
into telling us

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why moons behave the way they do

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and why they look the way
they do today.

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Callisto's surface
tells a different story.

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It formed much farther out,

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where there was less debris
and less heat.

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It took longer
and cooled faster.

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Unlike Ganymede,
Callisto's surface is uniform.

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Rock and ice never separated.

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Where a moon forms
can also mean the difference

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between survival
and destruction.

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Get too close,

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and a planet's gravity
will rip a moon to shreds.

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Scientists believe this
is what happened to many moons

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when Jupiter was young.

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And it's very likely
that Jupiter had

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an entire conveyor belt
of large moons

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that were wanting to form,

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only to be swallowed up
by the planet itself.

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The large moons we see today

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are only the last ones
that were able to stabilize

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right at the end
of that process,

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stop their death spiral,

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and survive into the position
we see today.

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But Jupiter keeps
trying to eat them.

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The gravity of the giant planet

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reaches out and pulls hard
on the orbiting moons.

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It transforms them
from lifeless balls of rock

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into strange and dramatic
worlds.

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Jupiter is the largest planet

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in our solar system.

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It has 63 moons.

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The four largest are called
the Galilean moons,

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named after the astronomer
Galileo,

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who discovered them in 1610.

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They show how gravity controls

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both what moons look like

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and how they behave.

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The first
of the Galilean moons, lo,

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orbits closest to the planet,

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just 260,000 miles
above Jupiter.

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That's about the same distance
as our Moon is from Earth.

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But unlike our Moon,

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the surface of lo
has no impact craters.

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Scientists realized that meant
the surface was new.

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But how could that be?

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Every time you look at lo,

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with a spacecraft
or even with a telescope,

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it's a little bit different.

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So the geology on lo changes

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like the weather
on other planets.

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It's that active.

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When NASA first sent
probes to fly past lo,

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they were shocked.

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They saw dozens
of active volcanoes.

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This is footage of an erupting
supervolcano on lo,

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blasting 200 miles into space.

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Everyone had
the same question...

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how could there be
active volcanoes on a moon?

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The answer was simple...
gravity.

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Jupiter's gravity is so huge

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that it reaches out
and crunches the moon.

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And it's not just Jupiter's
gravity pulling on lo.

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Other nearby moons
also pull on it as they pass by.

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So the core of the moon

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is being worked back and forth
all the time.

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It's called tidal friction

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and generates extreme heat
in lo's core.

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Almost like bending a
wire coat hanger until it breaks.

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And you feel the inside
of the coat hanger there...

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it feels rather warm.

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That tidal friction...
that internal friction...

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heats the interior of lo
until it's become,

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actually, one of the most
volcanically active worlds

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in the solar system.

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The constant
pushing and pulling

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generates temperatures
thousands of degrees high

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inside lo.

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It blasts out
in gigantic eruptions of lava.

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Io is the prime example

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of tidal forces
and gravitational interactions

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in the solar system.

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It is constantly being pulled
by Jupiter,

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and it's
constantly getting pulled

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by the other moons, as well.

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And so, as a result,

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there's a tremendous amount
of heat created.

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The floods of erupting lava

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constantly resurface lo,

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which is why there are
no visible impact craters

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on this moon.

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Gravity also heats
lo's neighbor, Europa.

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Europa's orbit is farther away
from Jupiter,

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so it's much colder.

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Instead of lava,
the surface of Europa is ice.

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The lowest recorded temperature
in Antarctica

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is minus-128 degrees.

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Europa's surface
is twice as cold.

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But underneath all the ice,

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there may be an ocean of water

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heated by
the same tidal friction

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that makes lo volcanic.

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Europa has a subsurface ocean,

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almost certainly.

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00:14:02,345 --> 00:14:07,442
And that subsurface ocean is in
contact with the rocky mantle,

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which provides heat

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and also provides, probably,

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appropriate nutrients
to sustain life.

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Someday we'll send a probe

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to explore beneath the ice
on Europa.

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00:14:25,735 --> 00:14:28,829
And maybe we'll discover
life-forms living there

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00:14:28,905 --> 00:14:32,671
in warm European oceans.

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00:14:37,247 --> 00:14:41,911
Out beyond lo and Europa
are nearly 60 more moons.

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They orbit much further away
from Jupiter,

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where the effects
of the giant planet's gravity

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are much weaker.

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Out here, it's too weak

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to generate tidal friction
and heat the moons.

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00:15:07,143 --> 00:15:10,635
So these remote worlds

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are cold and barren...

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But not featureless.

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They bear the scars
of countless collisions,

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00:15:19,756 --> 00:15:23,954
and scientists believe
it was collisions that created

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the most extraordinary
moon system of them all.

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The planet with
the most unusual moon system

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is Saturn.

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It's spread out
over more than 200,000 miles.

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Technically, there are
more than a billion moons.

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That's right...
a billion moons.

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00:15:55,191 --> 00:15:58,957
And all together,
they make up Saturn's rings.

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A moon can be a hunk of rock
or ice no bigger than a pebble,

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00:16:06,703 --> 00:16:08,796
as long as it orbits a planet.

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00:16:08,871 --> 00:16:10,839
The rings of Saturn are made

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of countless pieces
of rock and ice.

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They go from the size
of a pebble

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up to the size of a city.

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We don't refer
to all the ring particles

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that can get to be as big
as 10 or 20 meters across.

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We don't refer to them
as individual moons.

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But when we find a body

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that is maybe a kilometer
or two across,

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then you can start talking
about it as a moon or a moonlet.

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00:16:38,801 --> 00:16:40,234
Saturn's rings

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00:16:40,303 --> 00:16:43,067
are one of the oldest mysteries
of astronomy.

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00:16:43,139 --> 00:16:45,937
Where did they come from?

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To try and find out,

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00:16:49,812 --> 00:16:52,212
NASA sent the Cassini probe
on a 12-year mission

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00:16:52,281 --> 00:16:57,116
to study Saturn, its rings,
and its moons.

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00:17:03,192 --> 00:17:05,217
We took, with Cassini,

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probably the most beautiful
picture that's ever been taken,

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00:17:08,364 --> 00:17:11,424
and I'm not the only one
who has said this.

256
00:17:11,501 --> 00:17:15,335
Cassini was in the shadow
of Saturn, cast by the Sun,

257
00:17:15,405 --> 00:17:17,396
and so you don't see the Sun.

258
00:17:17,473 --> 00:17:21,671
You see the backlit planet of
Saturn and its beautiful rings.

259
00:17:21,744 --> 00:17:24,907
You see the refracted image
of the Sun

260
00:17:24,981 --> 00:17:27,950
poking out from the side
of Saturn.

261
00:17:28,017 --> 00:17:30,212
And nestled
in all of that splendor

262
00:17:30,286 --> 00:17:32,345
is this small little dot.

263
00:17:34,290 --> 00:17:37,259
That tiny dot is not a moon.

264
00:17:37,326 --> 00:17:40,056
That is
the distant planet Earth,

265
00:17:40,129 --> 00:17:42,893
nearly a billion miles away.

266
00:17:45,935 --> 00:17:48,165
Most of what we know
about Saturn,

267
00:17:48,237 --> 00:17:51,570
of its rings and moons,
comes from Cassini.

268
00:17:51,641 --> 00:17:55,338
Before Cassini, we thought
there were only eight rings.

269
00:17:55,411 --> 00:17:59,245
Today we can see over 30.

270
00:17:59,315 --> 00:18:01,545
What we have found at Saturn

271
00:18:01,617 --> 00:18:04,711
has been just literally
an embarrassment of riches.

272
00:18:04,787 --> 00:18:06,982
We're seeing something
that we had seen before,

273
00:18:07,056 --> 00:18:09,684
but now we're seeing it with
a level of detail and clarity

274
00:18:09,759 --> 00:18:11,158
that was just mind-blowing.

275
00:18:20,103 --> 00:18:22,196
Scientists used to think

276
00:18:22,271 --> 00:18:24,705
the rings were made
of the icy leftovers

277
00:18:24,774 --> 00:18:28,301
after Saturn was formed
about 4 billion years ago.

278
00:18:28,377 --> 00:18:30,038
But anything that old

279
00:18:30,113 --> 00:18:34,641
should be covered
with cosmic dust, and dirty.

280
00:18:36,486 --> 00:18:38,920
So why does Saturn's rings

281
00:18:38,988 --> 00:18:42,981
appear bright and clean,
almost new?

282
00:18:47,196 --> 00:18:48,686
To get the answer,

283
00:18:48,764 --> 00:18:52,791
Mission Control maneuvered
Cassini close to the rings.

284
00:18:55,171 --> 00:18:58,663
The probe saw that
all the ice pieces in the rings

285
00:18:58,741 --> 00:19:02,142
are constantly colliding
and breaking up.

286
00:19:06,816 --> 00:19:10,217
And each collision
exposes new surfaces

287
00:19:10,286 --> 00:19:12,754
that are clean and polished.

288
00:19:21,664 --> 00:19:25,225
This is what astronomers
think happened.

289
00:19:25,301 --> 00:19:27,098
When Saturn was young,

290
00:19:27,170 --> 00:19:31,038
it had no rings,
just lots of moons.

291
00:19:31,107 --> 00:19:33,667
At some point, an icy comet

292
00:19:33,743 --> 00:19:35,768
zoomed in from deep space

293
00:19:35,845 --> 00:19:38,405
and smashed
into one of those moons.

294
00:19:38,481 --> 00:19:42,383
The comet broke up
into billions of pieces.

295
00:19:46,022 --> 00:19:50,118
The impact also pushed
the moon closer to Saturn,

296
00:19:50,193 --> 00:19:53,651
where the planet's
enormous gravity broke it up.

297
00:20:00,903 --> 00:20:05,772
Now debris from the moon
and ice from the comet mixed.

298
00:20:08,010 --> 00:20:10,205
Gradually, Saturn's gravity

299
00:20:10,279 --> 00:20:14,807
pulled all those fragments
into rings around it.

300
00:20:17,620 --> 00:20:21,522
The story of moons
is the story of gravity.

301
00:20:21,591 --> 00:20:24,219
Gravity holds them in orbit.

302
00:20:24,293 --> 00:20:29,196
It heats up their insides
and shapes their surfaces.

303
00:20:29,265 --> 00:20:33,668
In the end, it controls
everything about moons,

304
00:20:33,736 --> 00:20:36,466
even their survival
and destruction.

305
00:20:39,308 --> 00:20:42,072
Gravity can even create
new moons

306
00:20:42,144 --> 00:20:48,640
by kidnapping asteroids,
comets, and even whole planets.

307
00:20:55,625 --> 00:20:58,219
We know
that gravity makes moons.

308
00:21:01,097 --> 00:21:03,759
The standard way
is to assemble them

309
00:21:03,833 --> 00:21:07,098
from debris left over
when planets are formed.

310
00:21:09,739 --> 00:21:12,970
But gravity makes moons
a second way, too.

311
00:21:13,042 --> 00:21:14,907
It captures them.

312
00:21:18,281 --> 00:21:21,546
Imagine a wandering comet
or asteroid.

313
00:21:21,617 --> 00:21:24,552
Somehow it gets knocked
off course.

314
00:21:24,620 --> 00:21:28,386
It wanders too close
to a planet.

315
00:21:28,457 --> 00:21:33,224
Gravity acts like
a science-fiction tractor beam

316
00:21:33,296 --> 00:21:34,490
and grabs it.

317
00:21:34,563 --> 00:21:37,691
Not quite enough gravity,
and it escapes.

318
00:21:39,302 --> 00:21:43,636
Too much gravity, and it
collides with the planet.

319
00:21:43,706 --> 00:21:47,403
Just enough,
and the comet or asteroid

320
00:21:47,476 --> 00:21:50,036
goes into orbit
around the planet

321
00:21:50,112 --> 00:21:52,342
and becomes a new moon.

322
00:21:57,820 --> 00:22:02,450
Mars has two tiny moons,
named Phobos and Deimos.

323
00:22:02,525 --> 00:22:06,188
Both are captured asteroids.

324
00:22:06,262 --> 00:22:09,754
One is pushing outward
as it circles the planet

325
00:22:09,832 --> 00:22:11,766
and will eventually break free

326
00:22:11,834 --> 00:22:14,826
and continue on its journey
through space.

327
00:22:14,904 --> 00:22:17,464
The other is circling inwards,

328
00:22:17,540 --> 00:22:20,509
a little closer to Mars
all the time.

329
00:22:20,576 --> 00:22:23,739
Eventually,
it'll smash into it.

330
00:22:31,387 --> 00:22:33,582
This is Cruithne.

331
00:22:33,656 --> 00:22:36,784
It's an asteroid, really,
just three miles across.

332
00:22:36,859 --> 00:22:41,660
But it's sometimes described
as Earth's second moon.

333
00:22:41,731 --> 00:22:44,291
With the little object Cruithne,

334
00:22:44,367 --> 00:22:46,597
which was discovered
back in 1986,

335
00:22:46,669 --> 00:22:49,331
we start to get
into this realm of...

336
00:22:49,405 --> 00:22:52,033
of what does it mean
to be a moon.

337
00:22:52,108 --> 00:22:55,339
Only a few thousand years ago,

338
00:22:55,411 --> 00:22:57,879
Cruithne was
an ordinary asteroid,

339
00:22:57,947 --> 00:23:00,814
orbiting the Sun
like billions of others.

340
00:23:00,883 --> 00:23:02,646
But eventually, it wobbled

341
00:23:02,718 --> 00:23:05,016
out of its orbit
in the Asteroid Belt

342
00:23:05,087 --> 00:23:07,487
and got snagged
by Earth's gravity.

343
00:23:09,925 --> 00:23:13,725
But then Cruithne
did something unusual.

344
00:23:13,796 --> 00:23:16,321
Instead of orbiting
around the Earth,

345
00:23:16,399 --> 00:23:17,889
like a normal moon,

346
00:23:17,967 --> 00:23:21,027
Cruithne began to follow
behind it.

347
00:23:21,103 --> 00:23:25,199
And so one might call it
a sort of a moon of the Earth...

348
00:23:25,274 --> 00:23:27,538
not exactly, though,
because that object is on...

349
00:23:27,610 --> 00:23:29,669
you know, it's
on its own independent orbit

350
00:23:29,745 --> 00:23:31,235
around the Sun, not the Earth.

351
00:23:35,384 --> 00:23:39,514
Sometimes asteroids
capture their own moons.

352
00:23:39,588 --> 00:23:42,682
In 1993, the Galileo spacecraft

353
00:23:42,758 --> 00:23:45,158
flew past the asteroid Ida

354
00:23:45,227 --> 00:23:48,321
and found something
nobody expected...

355
00:23:48,397 --> 00:23:51,889
a tiny half-mile-wide moon.

356
00:23:53,569 --> 00:23:55,867
The fact that we saw a satellite

357
00:23:55,938 --> 00:23:57,405
around only the second asteroid

358
00:23:57,473 --> 00:23:59,304
ever to be encountered
with a spacecraft

359
00:23:59,375 --> 00:24:00,672
immediately tells us

360
00:24:00,743 --> 00:24:03,906
that moons around asteroids
must be incredibly common.

361
00:24:08,017 --> 00:24:10,611
Not all
captured moons are small.

362
00:24:10,686 --> 00:24:14,247
The mother of all
captured moons is Triton.

363
00:24:14,323 --> 00:24:19,420
It orbits the planet Neptune,
and it is big...

364
00:24:19,495 --> 00:24:22,362
about 1,700 miles in diameter.

365
00:24:22,431 --> 00:24:26,697
But Triton is a moon
with an unusual story.

366
00:24:28,103 --> 00:24:30,435
Triton was
a very puzzling problem

367
00:24:30,506 --> 00:24:32,133
for planetary scientists,

368
00:24:32,208 --> 00:24:33,971
because our traditional view

369
00:24:34,043 --> 00:24:35,977
would tend to make
all the moons orbit

370
00:24:36,045 --> 00:24:38,536
in the same direction
that the planet itself spins.

371
00:24:38,614 --> 00:24:40,707
In the case of Triton
around Neptune,

372
00:24:40,783 --> 00:24:42,182
it's the other way around.

373
00:24:42,251 --> 00:24:43,775
Neptune is spinning this way.

374
00:24:43,853 --> 00:24:46,651
Triton is orbiting around
in the opposite direction.

375
00:24:46,722 --> 00:24:50,988
This means
it didn't form like most moons,

376
00:24:51,060 --> 00:24:54,120
out of the debris left over
from the birth of the planet,

377
00:24:54,196 --> 00:24:57,632
or it would orbit
in the same direction.

378
00:24:57,700 --> 00:25:00,168
So something wasn't right.

379
00:25:00,236 --> 00:25:04,468
Triton is huge,
and its orbit is funny.

380
00:25:04,540 --> 00:25:05,472
It's anomalous.

381
00:25:05,541 --> 00:25:07,372
It does not seem
as though it formed

382
00:25:07,443 --> 00:25:10,776
as a part of the Neptune system.

383
00:25:10,846 --> 00:25:15,442
It seems much more
like a captured planet.

384
00:25:15,518 --> 00:25:18,919
Scientists now think Triton

385
00:25:18,988 --> 00:25:21,513
was once a dwarf planet,
like Pluto.

386
00:25:21,590 --> 00:25:25,253
And a giant planet like Neptune
certainly has enough gravity

387
00:25:25,327 --> 00:25:29,058
to capture a moon
the size of Triton.

388
00:25:29,131 --> 00:25:31,622
Triton
was almost certainly formed

389
00:25:31,700 --> 00:25:33,531
way out
in the outer solar system

390
00:25:33,602 --> 00:25:36,093
and then at some point
was captured by Neptune.

391
00:25:36,171 --> 00:25:38,366
Perhaps Triton, early on,
had its own moon,

392
00:25:38,440 --> 00:25:39,771
they both were captured,

393
00:25:39,842 --> 00:25:43,141
and then that moon was destroyed
during the capture process.

394
00:25:45,214 --> 00:25:47,842
But Triton is in danger.

395
00:25:47,917 --> 00:25:51,819
Neptune is dragging it
closer and closer.

396
00:25:53,889 --> 00:25:57,848
Eventually,
it will get too close,

397
00:25:57,927 --> 00:26:02,364
and Neptune's immense gravity
will tear it apart.

398
00:26:14,176 --> 00:26:17,577
Triton the moon will be reborn

399
00:26:17,646 --> 00:26:20,877
as a ring system
around the planet.

400
00:26:33,128 --> 00:26:35,323
But what about our Moon?

401
00:26:35,397 --> 00:26:37,160
How did it get there?

402
00:26:37,232 --> 00:26:39,598
Was it captured?

403
00:26:42,905 --> 00:26:46,636
The truth
is even more extraordinary.

404
00:26:46,709 --> 00:26:50,941
It was born in extreme violence.

405
00:26:56,418 --> 00:26:58,477
Our Moon, like a lot of moons,

406
00:26:58,554 --> 00:27:04,015
is rocky, barren,
and pockmarked with craters.

407
00:27:04,093 --> 00:27:09,395
But in one way, our Moon
is unique in the solar system.

408
00:27:13,502 --> 00:27:14,833
For a long time,

409
00:27:14,903 --> 00:27:16,996
astronomers thought
the Moon formed

410
00:27:17,072 --> 00:27:20,371
from debris left over
from the birth of the Earth.

411
00:27:20,442 --> 00:27:22,239
But researchers in the 1960s

412
00:27:22,311 --> 00:27:25,439
came up with
a radically different idea.

413
00:27:25,514 --> 00:27:29,951
They suggested
it came from a giant impact.

414
00:27:44,099 --> 00:27:46,226
When we first had the idea

415
00:27:46,301 --> 00:27:49,498
of forming the Moon
from a giant impact,

416
00:27:49,571 --> 00:27:52,836
that was not
a terribly popular idea.

417
00:27:52,908 --> 00:27:55,968
And I actually did have good
science friends... colleagues...

418
00:27:56,045 --> 00:27:59,242
coming to me, saying, you know,
we really have to exhaust

419
00:27:59,314 --> 00:28:01,874
all the slow
evolutionary theories

420
00:28:01,950 --> 00:28:04,919
before we start talking
about cataclysms.

421
00:28:04,987 --> 00:28:08,218
The evidence
Bill Hartmann needed

422
00:28:08,290 --> 00:28:10,019
was on the Moon itself.

423
00:28:13,195 --> 00:28:14,719
And the proof had to wait

424
00:28:14,797 --> 00:28:19,962
until Apollo astronauts
finally went there in 1969.

425
00:28:22,237 --> 00:28:25,764
They brought back hundreds
of pounds of Moon rocks.

426
00:28:27,576 --> 00:28:31,444
Scientists analyzed the rocks
and were amazed.

427
00:28:31,513 --> 00:28:35,108
They were identical to rocks
in the Earth's crust,

428
00:28:35,184 --> 00:28:39,177
and they'd been superheated.

429
00:28:39,254 --> 00:28:43,315
So, how did
pieces of the Earth's crust

430
00:28:43,392 --> 00:28:45,792
become superhot
and wind up on the Moon?

431
00:28:45,861 --> 00:28:48,887
Hartmann was pretty sure
he knew.

432
00:28:48,964 --> 00:28:51,762
This whole idea
was that the Earth forms.

433
00:28:51,834 --> 00:28:53,426
Now you hit it with something.

434
00:28:53,502 --> 00:28:56,335
You blow all this light,
rocky material off the top.

435
00:28:56,405 --> 00:28:59,067
That material goes into orbit
and makes the Moon.

436
00:28:59,141 --> 00:29:02,076
The Moon's just made
out of rocky debris.

437
00:29:05,748 --> 00:29:09,115
Lmagine our
chaotic solar system

438
00:29:09,184 --> 00:29:11,015
4.5 billion years ago.

439
00:29:17,426 --> 00:29:19,656
The young Earth is just one

440
00:29:19,728 --> 00:29:23,323
of a hundred or so new planets
orbiting the Sun.

441
00:29:28,537 --> 00:29:32,974
One of them is a Mars-sized
planet called Theia,

442
00:29:33,041 --> 00:29:36,033
and it's on a collision course
with Earth.

443
00:29:40,949 --> 00:29:42,814
They smash into each other

444
00:29:42,885 --> 00:29:45,820
at many thousands of miles
an hour.

445
00:29:58,100 --> 00:30:02,298
Theia is destroyed,
and Earth barely survives.

446
00:30:02,371 --> 00:30:06,967
The impact blasts billions
of tons of debris into space.

447
00:30:07,042 --> 00:30:11,809
The Earth's gravity pulls it
into orbit around the planet.

448
00:30:11,880 --> 00:30:14,974
Now these hunks
of leftover Earth

449
00:30:15,050 --> 00:30:18,349
clump together
and form our Moon.

450
00:30:33,769 --> 00:30:38,433
That's the theory, anyway.
But how do you test it for real?

451
00:30:40,709 --> 00:30:42,734
Here at NASA's
Vertical Gun Range,

452
00:30:42,811 --> 00:30:47,248
they're re-creating
that ancient collision in a lab.

453
00:30:49,384 --> 00:30:52,353
This 30-foot-long gun
fires a tiny projectile

454
00:30:52,421 --> 00:30:54,548
at 18,000 miles an hour.

455
00:30:58,060 --> 00:31:00,460
The projectile is Theia.

456
00:31:00,529 --> 00:31:02,997
This ball represents the Earth.

457
00:31:03,065 --> 00:31:06,193
By changing
the angle of Theia's impact,

458
00:31:06,268 --> 00:31:08,532
the team can figure out
how precise

459
00:31:08,604 --> 00:31:12,165
the ancient collision had to be
in order to make the Moon.

460
00:31:12,241 --> 00:31:14,175
In the first shot,

461
00:31:14,243 --> 00:31:18,737
Theia hits the top of the Earth
with a glancing blow.

462
00:31:18,814 --> 00:31:21,647
So, here's the Earth,
if you will, suspended in space.

463
00:31:21,717 --> 00:31:22,979
And now it's gotten hit.

464
00:31:24,686 --> 00:31:27,985
So, now we see
the planet ejecta

465
00:31:28,056 --> 00:31:30,684
is being ripped
out of the Earth

466
00:31:30,759 --> 00:31:33,353
and is forming
this giant impact basin.

467
00:31:33,428 --> 00:31:35,225
And if this
really were the Earth,

468
00:31:35,297 --> 00:31:38,266
this basin would be
thousands of kilometers...

469
00:31:38,333 --> 00:31:40,597
thousands of miles... across.

470
00:31:40,669 --> 00:31:43,229
In this simulation,

471
00:31:43,305 --> 00:31:46,502
Theia only skims
off the surface of the planet,

472
00:31:46,575 --> 00:31:50,705
and very little debris
is thrown out into space...

473
00:31:50,779 --> 00:31:53,339
not nearly enough
to build our Moon.

474
00:31:59,488 --> 00:32:02,753
The second shot
is a head-on collision.

475
00:32:06,194 --> 00:32:07,525
Ka-pow!

476
00:32:07,596 --> 00:32:12,056
That's the end of planet Earth.
It's gone.

477
00:32:12,134 --> 00:32:14,864
Some of the debris is gonna go
out of the solar system.

478
00:32:14,937 --> 00:32:16,700
Some of the debris
will reaccrete

479
00:32:16,772 --> 00:32:19,366
to form small planetesimals
within the solar system.

480
00:32:27,616 --> 00:32:29,345
There's no Earth left,

481
00:32:29,418 --> 00:32:30,783
so there's no gravity

482
00:32:30,852 --> 00:32:33,412
to gather the debris
and form the Moon.

483
00:32:35,490 --> 00:32:39,551
Now the gun is set
to just the right angle...

484
00:32:39,628 --> 00:32:43,792
halfway between a glancing blow
and a direct hit.

485
00:32:43,865 --> 00:32:48,165
So we'll see what happens
if the Earth barely survives.

486
00:32:55,010 --> 00:32:59,208
Oh, oh, gorgeous!
Oh, my gosh!

487
00:32:59,281 --> 00:33:00,339
Ka-pow!

488
00:33:00,415 --> 00:33:02,906
Now we have the entire part
of the Earth

489
00:33:02,985 --> 00:33:04,418
being ripped apart,

490
00:33:04,486 --> 00:33:07,546
but the vapor plume is...
oh, my gosh.

491
00:33:07,622 --> 00:33:10,147
Aw, geez!

492
00:33:10,225 --> 00:33:12,159
That is gorgeous.

493
00:33:18,400 --> 00:33:23,997
But this was the beginning...
the beginning of our Moon.

494
00:33:25,807 --> 00:33:27,536
The experiment shows

495
00:33:27,609 --> 00:33:30,339
that Theia could have
smashed into the Earth

496
00:33:30,412 --> 00:33:33,245
and formed the Moon.

497
00:33:33,315 --> 00:33:36,807
But the collision
had to be just right.

498
00:33:36,885 --> 00:33:39,581
And lucky for us, it was.

499
00:33:45,594 --> 00:33:50,224
Today, the Moon orbits
250,000 miles from Earth.

500
00:33:51,767 --> 00:33:53,701
But when it first formed,

501
00:33:53,769 --> 00:33:56,704
the Moon orbited
just 15,000 miles

502
00:33:56,772 --> 00:33:58,706
above the Earth's surface.

503
00:34:01,376 --> 00:34:04,504
500 million years
after the Moon formed,

504
00:34:04,579 --> 00:34:05,944
if we looked up in the sky,

505
00:34:06,014 --> 00:34:08,983
the Moon would have comprised
a tremendous portion of the sky.

506
00:34:09,051 --> 00:34:10,450
It would have been enormous,

507
00:34:10,519 --> 00:34:12,783
because the Moon
would have been much closer.

508
00:34:14,656 --> 00:34:18,558
Back then,
the Earth was rotating so fast,

509
00:34:18,627 --> 00:34:20,527
a day lasted just six hours.

510
00:34:23,365 --> 00:34:28,632
But the Moon was so close,
its gravity acted like a brake.

511
00:34:32,274 --> 00:34:34,674
It slowed our planet down

512
00:34:34,743 --> 00:34:38,770
until a day now lasts 24 hours.

513
00:34:40,682 --> 00:34:44,277
The Moon's gravity
also created giant tides

514
00:34:44,352 --> 00:34:46,286
that surged across the planet,

515
00:34:46,354 --> 00:34:50,415
churning up the seas,
mixing minerals and nutrients.

516
00:34:50,492 --> 00:34:53,120
This created
the primordial soup

517
00:34:53,195 --> 00:34:56,289
from which the first forms
of life arose.

518
00:34:56,364 --> 00:35:00,130
Without our Moon, life on Earth
may never have happened.

519
00:35:03,205 --> 00:35:08,142
And there may be other moons
with a link to life, as well.

520
00:35:08,210 --> 00:35:12,909
Moons may be the great biology
experiments of the universe...

521
00:35:12,981 --> 00:35:17,714
the true laboratories
of life itself.

522
00:35:23,992 --> 00:35:27,257
Moons are full of surprises.

523
00:35:27,329 --> 00:35:31,095
There are moons
with giant volcanoes,

524
00:35:31,166 --> 00:35:35,933
moons with vast oceans
sealed under thick ice.

525
00:35:38,773 --> 00:35:43,904
And now we know a few
are rich in organic compounds.

526
00:35:43,979 --> 00:35:47,881
In the right combination,
they might even support life.

527
00:35:47,949 --> 00:35:50,543
In our solar system,
the biological window

528
00:35:50,619 --> 00:35:53,315
through which we can understand
the rest of the universe

529
00:35:53,388 --> 00:35:56,186
may be through these moons
of the outer solar system.

530
00:35:56,258 --> 00:35:59,125
That may be where we find
our second genesis,

531
00:35:59,194 --> 00:36:00,661
and that second genesis

532
00:36:00,729 --> 00:36:03,323
is really
our first deep understanding

533
00:36:03,398 --> 00:36:05,764
of the biological nature
of the universe.

534
00:36:14,376 --> 00:36:18,608
At first glance,
moons don't look ideal for life.

535
00:36:22,150 --> 00:36:25,244
Take Enceladus.

536
00:36:25,320 --> 00:36:29,518
It's a shiny ball of ice,
300 miles across,

537
00:36:29,591 --> 00:36:32,788
orbiting Saturn.

538
00:36:32,861 --> 00:36:35,091
It's the brightest object
in the solar system.

539
00:36:35,163 --> 00:36:37,757
It reflects 100% of the light
that hits it,

540
00:36:37,832 --> 00:36:39,060
so it's superbright,

541
00:36:39,134 --> 00:36:41,261
and that's because
it's water ice.

542
00:36:41,336 --> 00:36:43,770
In 2005, the Cassini probe

543
00:36:43,838 --> 00:36:48,707
spotted ice volcanoes erupting
from the surface of Enceladus.

544
00:36:48,777 --> 00:36:52,269
That meant there had to be
heat under all that ice...

545
00:36:52,347 --> 00:36:55,373
heat that created
oceans of water.

546
00:36:55,450 --> 00:36:59,978
And where there's water,
there's the possibility of life.

547
00:37:00,055 --> 00:37:04,116
So, this is Beehive Geyser
here in Yellowstone,

548
00:37:04,192 --> 00:37:06,717
and it is shooting water vapor
and water

549
00:37:06,795 --> 00:37:09,320
about 150 feet into the sky.

550
00:37:09,397 --> 00:37:11,763
And it's pretty incredible.

551
00:37:11,833 --> 00:37:14,427
So, now imagine if you're
on the surface of Enceladus.

552
00:37:14,502 --> 00:37:16,834
You would see geysers
that look a lot like this,

553
00:37:16,905 --> 00:37:21,501
and they are shooting ice grains
and water vapor into space

554
00:37:21,576 --> 00:37:24,136
thousands of times higher
than this geyser here.

555
00:37:24,212 --> 00:37:29,411
The ice volcanoes
are powered by gravity.

556
00:37:29,484 --> 00:37:30,815
Here's how.

557
00:37:30,885 --> 00:37:33,877
Saturn's gravity works
on the core of the moon,

558
00:37:33,955 --> 00:37:35,252
heating it up.

559
00:37:35,323 --> 00:37:37,314
The underground water expands

560
00:37:37,392 --> 00:37:41,226
and forces its way up through
cracks in the surface ice

561
00:37:41,296 --> 00:37:45,528
and blasts out into space
as ice crystals.

562
00:37:45,600 --> 00:37:49,297
These are some of
the most spectacular eruptions

563
00:37:49,371 --> 00:37:50,861
in our solar system.

564
00:37:50,939 --> 00:37:54,739
They make Beehive Geyser
look like a squirt gun.

565
00:37:54,809 --> 00:37:57,243
From the ice in the volcanoes,

566
00:37:57,312 --> 00:38:01,942
scientists have detected salt
and simple organic compounds.

567
00:38:02,017 --> 00:38:05,111
That means
the water under the ice

568
00:38:05,186 --> 00:38:08,314
is not only warm
but full of nutrients.

569
00:38:08,390 --> 00:38:10,551
Sound familiar?

570
00:38:10,625 --> 00:38:13,321
Heat, water, and nutrients...

571
00:38:13,395 --> 00:38:15,761
that's how life on Earth began.

572
00:38:15,830 --> 00:38:18,560
We realize
you could have all the things

573
00:38:18,633 --> 00:38:20,931
that we associate
with oceans on the Earth

574
00:38:21,002 --> 00:38:22,230
going on inside a moon.

575
00:38:22,304 --> 00:38:24,363
It's the discovery
of a lifetime.

576
00:38:24,439 --> 00:38:28,034
Saturn's
Enceladus has an ocean.

577
00:38:28,109 --> 00:38:30,202
So does Jupiter's Europa.

578
00:38:30,278 --> 00:38:35,011
But these aren't the only moons
where life could emerge.

579
00:38:35,083 --> 00:38:38,382
Saturn has another moon...
Titan...

580
00:38:38,453 --> 00:38:41,547
with an even greater potential
for life.

581
00:38:43,525 --> 00:38:48,360
In 2005, Cassini sent a probe,
called Huygens,

582
00:38:48,430 --> 00:38:50,398
on a one-way mission to Titan.

583
00:38:52,334 --> 00:38:55,064
For just 31/2 hours,

584
00:38:55,136 --> 00:38:57,366
Huygens transmitted
live pictures

585
00:38:57,439 --> 00:39:02,809
from the hostile surface,
nearly a billion miles away.

586
00:39:02,877 --> 00:39:06,142
Then the battery died.

587
00:39:06,214 --> 00:39:08,648
It was just incredible.

588
00:39:08,717 --> 00:39:12,380
This was the first time humans
had ever touched this moon

589
00:39:12,454 --> 00:39:14,149
with something
of our own making.

590
00:39:14,222 --> 00:39:15,348
It was just an event

591
00:39:15,423 --> 00:39:17,618
that should have been
celebrated the world over.

592
00:39:17,692 --> 00:39:19,751
We should have had
ticker-tape parades

593
00:39:19,828 --> 00:39:21,853
in every major city
across the U.S. And Europe

594
00:39:21,930 --> 00:39:23,124
to celebrate this.

595
00:39:23,198 --> 00:39:26,793
It was that history-making
and that astonishing.

596
00:39:33,908 --> 00:39:36,308
Raindrops on Titan

597
00:39:36,378 --> 00:39:38,938
are twice as big
as raindrops on Earth.

598
00:39:40,982 --> 00:39:43,815
But the rain isn't water.

599
00:39:43,885 --> 00:39:46,615
It's methane.

600
00:39:50,825 --> 00:39:53,760
On Earth, methane is a gas,

601
00:39:53,828 --> 00:39:58,288
but on Titan, it's a liquid
because the moon is so cold.

602
00:40:01,936 --> 00:40:04,200
There may be methane icebergs.

603
00:40:04,272 --> 00:40:06,706
There are certainly
methane lakes and rivers,

604
00:40:06,775 --> 00:40:09,107
and there's methane rain
and methane clouds

605
00:40:09,177 --> 00:40:11,145
and maybe bugs
swimming in methane.

606
00:40:11,212 --> 00:40:14,841
Bugs living in liquid methane

607
00:40:14,916 --> 00:40:17,043
may sound unbelievable.

608
00:40:17,118 --> 00:40:19,552
But scientists have discovered

609
00:40:19,621 --> 00:40:23,057
that Enceladus, Europa,
and Titan

610
00:40:23,124 --> 00:40:26,719
are all covered
with a substance called tholin.

611
00:40:26,795 --> 00:40:29,525
Tholin contains
the chemical building blocks

612
00:40:29,597 --> 00:40:31,690
for life to begin.

613
00:40:31,766 --> 00:40:36,567
So could life emerge
on any or all of these moons?

614
00:40:40,975 --> 00:40:43,671
We can't get our hands
on the tholin from the moons,

615
00:40:43,745 --> 00:40:46,714
so Chris McKay
makes it in the lab.

616
00:40:46,781 --> 00:40:51,411
He zaps a mixture of gases
found on Titan with electricity.

617
00:40:51,486 --> 00:40:56,753
What he gets
is a reddish-brown mud.

618
00:40:56,825 --> 00:40:58,725
So, this is
what we make... tholin,

619
00:40:58,793 --> 00:41:02,285
this sort of nonbiological
organic material.

620
00:41:02,363 --> 00:41:04,388
It's produced by chemical energy

621
00:41:04,466 --> 00:41:07,026
put into simple molecules,
like methane and nitrogen,

622
00:41:07,101 --> 00:41:08,534
and here we got it.

623
00:41:08,603 --> 00:41:11,436
And that's the material
we see on Titan.

624
00:41:11,506 --> 00:41:14,998
We see evidence for something
like this on Enceladus.

625
00:41:15,076 --> 00:41:16,373
We see it on the surface

626
00:41:16,444 --> 00:41:18,969
of many of the moons
in the outer solar system.

627
00:41:19,047 --> 00:41:21,345
This is nature's recipe

628
00:41:21,416 --> 00:41:25,284
for making the stuff that life
eventually emerges from.

629
00:41:25,353 --> 00:41:29,790
Somewhere in the outer
reaches of our solar system,

630
00:41:29,858 --> 00:41:32,292
on some remote moon,

631
00:41:32,360 --> 00:41:36,126
life may have already emerged.

632
00:41:36,197 --> 00:41:40,190
But it probably won't be life
as we know it.

633
00:41:40,268 --> 00:41:42,395
Life 2.0 doesn't
necessarily have to have

634
00:41:42,470 --> 00:41:44,461
the same genetics
as life 1.0, right?

635
00:41:44,539 --> 00:41:47,599
In fact, the more different it
is, the more interesting it is.

636
00:41:50,144 --> 00:41:53,136
Whether it's the same
or very different,

637
00:41:53,214 --> 00:41:56,741
the discovery of life
on the moons of our solar system

638
00:41:56,818 --> 00:42:00,015
will change the way
we look at the universe.

639
00:42:03,258 --> 00:42:06,386
I think that,
should we ever find

640
00:42:06,461 --> 00:42:07,723
that life had originated

641
00:42:07,795 --> 00:42:11,595
not once but twice
in our solar system,

642
00:42:11,666 --> 00:42:15,568
then you... you can
easily dismiss any arguments

643
00:42:15,637 --> 00:42:19,869
that say that life
is unique to the Earth.

644
00:42:22,010 --> 00:42:23,841
Moons are small,

645
00:42:23,912 --> 00:42:27,541
but they're still
diverse and dynamic worlds.

646
00:42:27,615 --> 00:42:31,551
They help us understand
how the universe works.

647
00:42:31,619 --> 00:42:35,282
They're essential cogs
in the cosmic machine.

648
00:42:35,356 --> 00:42:37,517
Without any moons,

649
00:42:37,592 --> 00:42:41,187
our solar system would be
a very different place.

650
00:42:41,262 --> 00:42:45,858
Without our Moon, life may
never have evolved on Earth.

651
00:42:45,934 --> 00:42:47,333
And who knows...

652
00:42:47,383 --> 00:42:51,933
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