[ethereal music]
Why do some worlds have
rings and others don't?
Why no rings for Earth or Mars?
We wouldn't recognize
Saturn without them.
He looks naked
without his rings.
But how did he get them
in the first place?
This is exactly what the French
astronomer douard Roche asked
himself when he looked at Saturn
through his telescope in 1848.
Roche speculated that Saturn's
rings were the debris of a moon
or moons that had
ventured too close
and were pulled apart
by the massive planet.
[orchestral music]
Roche was able to
devise an equation
that applies to all worlds.
It tells you how closely a
body can come to a planet
before it's pulled
apart by the planet's
tidal forces of gravity
and is turned into a ring.
That's the Roche limit.
But until NASA's Cassini
spacecraft executed a series
of daredevil maneuvers
in the Saturn system,
there was a vigorous,
scientific debate
about when his rings formed.
Some astronomers
suggested they were nearly
as old as the planet itself.
More than 4 billion years ago
when the planet coalesced out
of the disk of gas and dust
that surrounded the newborn sun,
a moon or moons likely
violated Saturn's Roche limit.
Others thought the rings
to be fairly recent,
perhaps only 100
million years old or so,
and the Cassini spacecraft
proved them right.
What is Earth's own Roche limit?
If the moon were ever to come
closer than 12,000 miles,
which, by the way,
it's absolutely
in no danger of doing.
[orchestral music]
And it's a good thing
too because I like
our moon right where it is.
There's only one other
moon in the solar system
that moves me like ours does.
Maybe it's because it's the
only one with a thick atmosphere
like Earth's and
the kind of surface
features, lakes and mountains,
that remind me of home.
All of this was hidden from view
by a dense layer of orange smog
until the European Space
Agency collaborated
with NASA to send a
spacecraft to land
on his mysterious surface.
Yes, that would be the one named
after you, Christiaan Huygens,
first to see that world
through your telescope.
[dramatic music]
After an interplanetary
voyage of seven years,
the Cassini-Huygens spacecraft
arrived in the Saturn system,
the fourth of our ships to
venture there but the first
to send a probe to explore
the surface of Saturn's moon,
Titan, and to
reveal a moon of far
greater complexity and
splendor than our own rather
dull and lifeless moon.
[soothing music]
As Carl Sagan had predicted
more than two decades before,
there were seas of
methane and ethane,
and there was water ice.
When Cassini first arrived
in 2004 at Saturn's Northern
Hemisphere, it was in
the depths of winter,
and the sun didn't come
out until five years later
when Saturn's
northern spring began.
Is it just me, or is
this whirling hexagon
at Saturn's north
pole every bit as
exotic as the fantasies
our ancient ancestors had
of these worlds?
The geometrically regular
hexagonal shape of this feature
brings to mind the
handiwork of intelligence,
terraforming,
reworking the surface
for some unknown purpose.
But it's actually the
result of the sudden change
in wind speeds as
vast upwellings
of ammonia rise near the poles.
It's the mother of all
hurricanes, a frenzy
of thunder and
lightning, containing
countless hurricanes within it.
Spring can be a violent,
stormy season on Earth too.
But it was during Saturn's
seven-year-long summer
that Cassini was commanded
to take her own life.
[ethereal music]
