[music playing] NARRATOR: Lurking in the
shadows of the solar system are worlds so chemically active
and misshapen that they border on the bizarre. I think the most shocking
thing was how very different the solar system is. NARRATOR: These are the
moons surrounding the planets of the solar system. Moons that were once either
unknown or considered afterthoughts are now
on the cutting edge of astronomical exploration. What was surprising, that they
all didn't look like our moon. The so-called minor
members of the solar system are not of minor interest. NARRATOR: What surprises
await us on these alien moons? [theme music] [instrumental music] NARRATOR: Our solar system
has always been fertile ground for science fiction writers,
but with exponential advances in telescopic technology
and close encounters by unmanned probes the
curtain has now been lifted on a new ballet of moons. Most of which are going
one way around, some of which are going the other way
around, all at different rates, passing one another, the inner
ones passing the outer ones. NARRATOR: For nearly
half a century, it was believed the solar system
was home to only 32 moons. They ranged in size from
Jupiter's moon Ganymede, larger than the planet Mercury,
to small asteroid-shaped ones, like the Martian moons
Phobos and Deimos. That number has exploded. In 2007 alone, scientists
announced the discovery of 20 new moons around
Jupiter, one around Saturn, and three around Neptune. What happened is
astronomical telescopes had available to them what
are called CCD cameras. So these are digital cameras
that almost everybody has nowadays. [camera clicks] NARRATOR: It's difficult to hold
astronomers to an exact number of moons in the solar system. As cameras become more sensitive
and telescopes more powerful, more moons reveal themselves. [beeping] Moons are classified
in two distinct ways. Those like our moon travel
in nearly circular orbits above their planet's equators
and are called regular moons. While our moon formed
from an impact, all other regular moons
coalesced from the gaseous stew surrounding their
parent planets, a process known as accretion. The classic example
of regular moons would be the Galilean
moons of Jupiter-- Io, Europa, Ganymede,
and Callisto. The material that is
going to form Jupiter too but extended a little
bit, that material accumulates into the moons. NARRATOR: Moons that follow
elongated orbits highly tilted to their
planet's equators are called irregular moons. Most of these move in
retrograde orbits, clockwise, if their planet rotates
counterclockwise. Phoebe, the newly discovered
moon orbiting Saturn, is a perfect example. She began her celestial life
as an independent traveler, orbiting the sun before
being captured by the more massive Saturn. Whether regular or
irregular, moons must fall within the
gravitational reach of their parent planet. The limit of these orbits
is known as the Hill sphere. This phenomenon is named
after George William Hill, an American astronomer
from the mid-1800s. So a Hill sphere is this
region around the planet that moves along with the
planet, inside of which the gravity of the
planet overwhelms the gravity of the Sun. NARRATOR: The moons of
Mars, Deimos and Phobos, operate very differently
within the Hill sphere. If the planet is rotating
faster than the moon it orbits, like Deimos, the tidal
forces between the two actually shove Deimos
out further and further. Phobos, on the other
hand, is rotating faster than Mars rotates. NARRATOR: These small moons
were discovered by American astronomer Aspeth Hall in 1877. He named Phobos after the
Greek god of fear and Deimos for the god of terror. [snarling] Tom Duxbury was part of
the Mariner 9 mission that first photographed the two
potato-shaped moons in November of 1971. This was late at night on a
cold, rainy, dark, dreary day. And I looked at this, and I
turned the picture sideways. It looked like a skull. And it was such an eerie thing. NARRATOR: Phobos is
in a death spiral. It orbits just 3,700 miles
from the Martian surface, closer to its host planet than
any moon in our solar system. If our own moon were as close to
the Earth as Phobos is to Mars, it would look 20 times larger. Its orbital period
would be in hours, not days like it
is now, but hours. And at full moon, it
would fill the sky. The daily tides rise
and fall tens of feet, if not hundreds of feet. And so the Earth's moon would
eventually crash into the Earth in such a situation. [booming] [music playing] NARRATOR: Phobos's predicament
is caused by a process known as secular acceleration. As Phobos races faster
than Mars rotates, a tidal bump is raised
on the Martian surface. In the process, Mars yanks
Phobos closer to its surface with each orbit. The struggle between
Mars and Phobos is similar to the dynamics of
a simple game of tetherball. Imagine the ball as the
moon, the pole as the planet, and the rope between
the pole and the ball as the planet's
gravitational pull. What we see is that the
gravity would pull the moon in such a way that it speeds up. It goes faster and faster,
and it works its way in till it eventually
hits the pole. That's exactly what's
happening to Phobos. Phobos is going around Mars
faster than Mars rotates. That tidal interaction
is pulling Phobos in closer and closer and
speeding it up in its orbit. In about 50 million years, we
expect Phobos to be pulled in so closely it will
impact Mars and disappear as a moon of Mars. On the other hand, Deimos,
the further out moon is going slower
than Mars rotates. And so it's unwinding the
string in the opposite way. And what we see is Deimos is
going further and further away from Mars. And eventually Deimos will
be pulled away from Mars by the gravity of the Sun. So over time, Mars
will become moonless. [dramatic music] [instrumental music] NARRATOR: Because Phobos
outpaces the rotation of Mars, it appears to rise in the
west and set in the east. Instead of the planet turning
quickly under it like our moon and most other moons, and thus
having it rise in the east and set in the
west, it races ahead of the rotation of the planet. And so it comes up on the
western horizon and races ahead and sets on the eastern horizon. [music playing] NARRATOR: It will be another
50 million years before Phobos completely disappears. Before then, it may prove useful
for the eventual colonization of Mars. Science fiction writer Arthur C.
Clarke speculated on this idea in his book "The Sands of Mars." Although there is no real reason
to colonize Phobos itself, its close proximity to Mars
makes it a natural waystation. From a gravity
standpoint, it's much easier to go to Phobos,
which has no gravity, than it is to fight
the gravity of Mars to get down to its surface. NARRATOR: From Galileo
to Stanley Kubrick, the giant planets of
the outer solar system have tantalized our imagination
with their enormity. But in reality, exploring
them is tantamount to suicide. [booming] The overwhelming pressure from
Jupiter's massive atmosphere would make it almost
impossible to function. But the moons of these behemoths
may provide a more promising platform for exploration and
even future colonization. Are these prisoners of
Jupiter's gravity hostile worlds with little chance
of sustaining organic life? Or might they provide a safe
haven for future generations of planetary explorers? [music playing] NARRATOR: Until
recently, very little was known about the moons
of the gas and ice giants. Most of them remained hidden
in the glare of their parent planets. Today, modern telescopes and
unmanned space exploration reveal a realm populated
by a host of moons, from planet-like spherical
worlds to misshapen ones barely 30 miles across. Jupiter, the largest
planet in the solar system, is a moon magnet. Nearly four and 1/2
billion years ago, it began as a massive gas
cloud collapsing in on itself. This process,
known as accretion, formed the beginnings
of the Jovian system. While nearly all the gas
and spinning material went into forming
Jupiter itself, a very small percentage
clumped in small eddies within Jupiter's orbit. These miniature
accretions solidified into Jupiter's regular moons-- Io, Europa, Ganymede,
and Callisto. As Jupiter coalesced,
its massive gravity began adding to its
menagerie little remaining bits from the birth of
the early solar system. The giant planets formed early
in a gas-rich environment when there was lots of little flotsam
and jetsam around the solar system still to be
captured into orbit. NARRATOR: The number
of Jupiter's moons ranges from 60 to over 200,
depending on who's counting. What's clear is if you could
stand at the edge of Jupiter's gaseous atmosphere and
look to the heavens, you'd see a magnificent
dance of lunar objects. It would look pretty cool
to be able to see the moons. Every so often you'd
see Io come by. Every second time
Io comes by, you'd see Europa at the same time. And every four times Io
comes by, you'd see Ganymede. NARRATOR: One member
of this Jovian cast is so battered by
Jupiter's great mass that it's literally bursting
from the inside out. In February of 2007, the
New Horizons spacecraft, eventually bound for Pluto,
focused its cameras on Io. About the size of Earth's
moon, it orbits 263,000 miles from Jupiter's surface. What sets Io apart from
the other Jovian moons is its spectacular volcanism. New Horizons' cameras captured
detailed photos of glowing lava scattered across Io's surface. A huge, 200-mile-high dust
plume rose above the surface of the molten moon. Io, like all Jupiter's
regular moons, is named after a lover
of the god Jupiter from Roman mythology. It was discovered
by Galileo in 1610. First photographed
by Pioneer 1 in 1974 and again by Voyager 1 in
1979, its pulsating activity has puzzled and intrigued
scientists for decades. Leaving Io is about a ton per
second of material every second of every day. It's a phenomenal machine. I would like to go to
Io, even though it would be very dangerous, and
hot, and sulfurous. [laughs] [dramatic music] NARRATOR: Io is too small to
have maintained a molten core since its formation. Another mysterious process must
be responsible for its heating. Io's entire interior
may maybe molten because it's squeezed so much
as it's orbiting around Jupiter. NARRATOR: This process is
known as tidal heating. The massive gravity of
Jupiter is causing friction at the inner core of Io. Much like a sculptor
needs a cold lump of clay, Jupiter is endlessly
creating its own masterpiece. If a moon gets a little
bit of tidal heating, it becomes malleable. It can be stretched out
like clay and deformed by the gravity of
the parent planet. The surface of the moon is cold. It breaks, like pulling
clay apart quickly. It'll break. But the interior,
where it's warm, can literally flow, and stretch,
and be kneaded by the gravity of the parent planet. NARRATOR: Most regular
moons have circular orbits. To produce tidal heating, a moon
must be in a more oblong orbit, where the distance
from the host planet changes radically during
a single revolution. The only way to produce
these eccentric orbits is if another moon's
gravity disrupts it. When a moon is in an eccentric
orbit, a non-round orbit, it gets closer and farther
from its parent planet. When it does, it gets squeezed. It gets pulled apart
when it's closer. NARRATOR: Io is in orbital
resonance with its companion moons Europa and Ganymede. While Jupiter and
Io struggle to find a synchronistic relationship,
Europa and Ganymede are yanking Io in
opposite directions. Jupiter yanks back, and Io
gets stretched and squeezed in the process. The tidal heating
on Io, which is responsible for its
prodigious volcanoes, has a secondary side effect. It creates the largest
stationary visible object in the solar system,
a massive gas cloud 500 times the
size of Jupiter. In 1990, Professor Michael
Mendillo and his team from Boston University
discovered a large gas cloud spanning the huge distance
from one side of Jupiter to the other. They were the first to
photograph the entire nebula, discover its origins,
and the mechanism that keeps it growing. Now, Io is small,
so it doesn't have much of an atmosphere. But these volcanoes are
continually providing material that could be an atmosphere. But you might ask,
well, why doesn't it have a tremendous atmosphere
with all the volcanoes that have been going on for eons? Well, it's because
the material escapes. NARRATOR: The key gas is sodium. Even though it only makes up
5% of Io's ejected materials, it's easily detectable
by telescopes on Earth. Sodium emits an orange glow. In fact, sodium is commonly
used to illuminate street lights in many cities across the world. The sodium and the
other gas molecules are pelted by light from the
Sun and electrons in Jupiter's powerful magnetic field. Electrons and protons
are knocked off some of these particles and ionized. Now in plasma form, these
ions are taken on a ride by Jupiter's powerful
magnetosphere. Its speed increases dramatically
because it's been picked up by the magnetic field. It's like they're taking a
ride on a cosmic carousel. The magnetic field
lines are the poles here, and then every now
and then a sodium atom gets picked up along
with its electron. And now I'm in the
Jupiter plasma torus with all the other
ions and electrons that have been
captured previously. And the ions and the
electrons recombine. The neutral is not confined
by the magnetic field, and it goes off at
a much higher speed. And that's enough to
escape from Jupiter. And they form the largest
visible cloud of gas that's permanently
in the solar system. NARRATOR: If we could see the
nebula with the naked eye, it would be the size of
12 moons in the night sky. It is so enormous that to
view it Mendillo and his team created their own
specialized telescope. Well, as it turns out, to
get a big field of view all you need is a small lens. Like a pair of binoculars
gives you a big field of view. NARRATOR: Even in
1991, the telescope may have seemed
ordinary, but its camera was highly sophisticated and
at the time revolutionary-- the digital camera. Well, once you've got a
picture that's in numbers, you can do all kinds
of things with it. NARRATOR: Mendillo and his
team knew that sodium existed in the nebula. Sodium also exists in
the Earth's atmosphere. They were able to compare
digital photographs of both Jupiter and
Earth's atmospheres and bring forth an
image of the nebula. Well, that was very
difficult to do if you just had two photographs
and pieces of paper. But now that we have
these digital cameras, we've revolutionized the way
that we can process images. NARRATOR: Though Io would be
a fascinating scientific and aesthetic destination,
its hostile environment probably precludes that. Even landing an unmanned probe
would be difficult among Io's convulsing fissures. But there's another moon
orbiting Jupiter which may not only support human exploration,
but possibly support its own alien life forms. Europa is one of the most
fascinating and enigmatic objects in our solar system,
really unlike anyplace else in the solar system and for
that matter unlike anything on Earth. The surface features
are such that there are cracks in the surface. There's mottled terrain. There's chaotic terrain. And it looks like
icebergs in some areas. NARRATOR: We know
Europa is an alien moon. Could it be home to
alien life forms as well? [music playing] NARRATOR: Europa orbits 400,000
miles from Jupiter's surface, about double the distance of Io. And like its convulsive
cousin, it too is molded by
gravitational tides. Jupiter has the greatest effect. Its mass, like a
persistent lover, pulls the reluctant
moon toward its surface. Europa resists with
its own gravity, and they form a kind of
symbiosis hundreds of thousands of miles above the gas giant. But it's not just Jupiter
tugging at Europa. Little Io and larger
cousin Ganymede pull at Europa from
different directions. It's the same orbital resonance
that has such a dramatic effect on Io. However, the results are
far different on Europa. Europa's surface is cold,
minus 550 degrees Fahrenheit in some places, yet
there is heating. And what rises to the surface
of Europa is also what makes the moon so exciting-- water. This water, actually
a kind of glacial ice, is rising from an
underground ocean and oozing out onto the
surface, repaving it as a Zamboni does an ice rink. Europa's ocean is thought
to be shallow, only about 10 or 15 miles below the surface. NARRATOR: Water was the
cradle of life on Earth. Could the same be true
on Europa or other moons? Icy satellite oceans could
be the most common habitat that exists in the universe. Earth might be relatively rare,
but icy satellites are probably plentiful. [music playing] NARRATOR: In February of 2007,
the New Horizons spacecraft, on its way to Pluto
and the outer reaches of the solar system, managed
to fly close enough to Europa to send back some
startling pictures. Seen only as the sun is rising
or setting behind Europa are enormous geological
patterns that have been dubbed crop circles. They are very large. If you were to, you know, try to
drive across one of the circles you would very, you know,
gently go in and travel down to a location that's a
few hundred feet lower than the surface you came up
from and then rise back up. NARRATOR: The resemblance
of Europa's crop circles to the mysterious ones that dot
the countryside here on Earth ends when you
consider their size. Each one is 2,000 to
3,000 miles in diameter. They're too shallow and
uniform to be impact craters. Asteroids and comets come in
different sizes and shapes. Europa's crop circles
are remarkably similar geologically. Although nothing
has been proven, it seems the great
mass of Jupiter may once again be the culprit. The speculation is that the
icy covering surrounding Europa is not tethered to
the core of the moon. Rather, it's floating
above the subsurface ocean like a spherical iceberg. The polar region may be
somehow shaped by Jupiter and then over hundreds
of thousands of years slowly tugged
toward the equator. This forms a line of small
circle depressions dropping from the polar region
toward the equator. If Europa's ice crust
has similar structure to icebergs on Earth,
most of it would be under the ocean's surface. This has huge implications
for future exploration. It means there has to be
something like eight or nine times that amount of ice
underneath them to allow that kind of a large-scale
topography to exist. NARRATOR: The iceberg theory
lays to rest the belief that Europa's subsurface
ocean can be easily tapped through a thin crust. Radar mapping and
ultraviolet data will prove to be even more
important before a Europa lander can make its way
down to the surface. Future explorers will have to
search out hotspots and places where this mysterious ocean has
welled up through the surface and from there try to
find a way to dip into it. [music playing] Those explorers
may choose instead to set up a base of operations
on Ganymede, Jupiter's largest moon. As Phobos might serve Martian
exploration as a waystation, Ganymede might do the same
for the Jovian system. Larger than the planet
Mercury, its gravity is closer to that of Earth's
than any of Jupiter's moons. And though it's in orbital
resonance with both Io and Europa, it's far
enough away from Jupiter to be less affected by the
gas giant's relentless tides. On Ganymede, you could say
park in some nice, big crater, and build your domed,
protected region protected from the charged particles
in the Jovian system, and make a pretty safe place to
study not just Ganymede itself, and its magnetic field, and
its interior, and its geology, but the Jupiter
system as a whole. NARRATOR: Ganymede is the
only moon in the solar system with its own magnetic field. To have this
distinction, Ganymede must have sufficient mass
and a hot inner core. Its mass is obvious, but
where the heat is coming from is a bit of a mystery. Ganymede is affected by both
Io and Europa's tidal forces, but measurements on its orbit
indicate that it's round enough to avoid the squashing and
stretching that its smaller cousins endure from Jupiter. The thought is maybe something
happened in Ganymede's past to change its orbit slightly. And maybe its eccentricity
got kind of haywire for a little while and
generated a lot of heat within Ganymede, caused
the core to be hot. We don't really know. NARRATOR: What we do know is
the New Horizons recent flyby of the Jovian system gave
us a tantalizing glimpse of the wonders that await
us on Jupiter's alien moons. Scientists look
forward to the day a lander touches down
on one of these moons and starts to
uncover the secrets of these mysterious worlds. [music playing] Another icy moon orbits
the gas giant Saturn. It's too small to hold
onto its own atmosphere, but that doesn't stop it
from sapping the atmosphere of its parent planet. Enceladus, even
though quite small, is named after a tribe of
giants in Greek mythology. Like Io and Europa, Enceladus
has an eccentric orbit around its parent planet Saturn. The tidal forces of
Saturn squeeze and knead this tiny moon and
create heat at its core. But unlike Io, it doesn't
regurgitate molten material that coalesces into
a massive gas cloud. Water doesn't well up to
the surface as it does on icy Europa. No. Enceladus actually spits
plumes of icy water into the atmosphere of Saturn. So we don't call it a volcano. It's more like a geyser. The water vapor is then in orbit
around the little, tiny moon. Or because it's near
Saturn, Saturn's gravity can pull it into the planet. NARRATOR: Interested by their
work on the torus of Io, Michael Mendillo and his
team at Boston University began to consider Enceladus's
effect on Saturn's atmosphere. And it turns out that
water is a wonderful catalyst to have the ions and
electrons recombine. NARRATOR: Before
Cassini, scientists relied on computer
models to determine atmospheric conditions
surrounding Saturn. They indicated that Saturn
should have a very robust ionosphere. Surprisingly, Cassini's
data indicated that Saturn's
ionosphere was only 10% of what computer
models had predicted. It seems that the icy water
ejected from Enceladus is neutralizing the charged
particles in Saturn's ionosphere. MISSION CONTROL: Commit. Lift off. [music playing] NARRATOR: Scientists had learned
quite by accident the effect water can have on
Earth's atmosphere. In 1973, when NASA
launched its Skylab workshop, it launched its last gigantic
Saturn V rocket, the moon rockets. And it had never had a launch
that allowed the space vehicle to keep its engine burning
as high as the ionosphere. Well, this gigantic
engine dumped a ton per second of water vapor, which
comes out of a giant rocket motor, into the ionosphere. And the ionosphere nearly
vanished on that day. NARRATOR: It blew a gaping
hole in Earth's ionosphere, the top layer of the atmosphere,
a hole that took the sun's ionizing radiation
24 hours to repair. However, on Saturn, where
Enceladus continuously dumps 6 tons of water per
minute into its atmosphere, the long-term effects
have been significant. There's no worry that Enceladus
will strip away its parent planet's ionosphere
completely, but this tiny moon only 300 miles in
diameter has gotten the attention of the scientific
community and Saturn itself. While we prepare probes
to Phobos and Europa and study data from
Enceladus and Io, an entirely new set of
moons has literally just come into the picture. [insects chirping] NARRATOR: Before the
1990s, most astronomers agreed that there were only
34 moons in the solar system. Most of those were regular moons
like our own, spherical bodies that orbit their host planet in
the same direction it rotates. But a handful of
these satellites were what's known
as irregular moons. These freakish moons
follow elongated orbits. Their orbits are
often tilted, and they rotate in the opposite
direction of their host planets. They look like flying
potatoes, or splinters, or misshapen lumps. They've been hard
to find before now because they're very small, and
they're also usually very dark. [music playing] NARRATOR: The advent
of digital photography and the use of
light-sensitive optics changed the lunar
terrain within a decade. Dr. Brett Gladman from the
University of British Columbia discovered his first
irregular moon in 1997 at the Palomar Observatory. Since then, Dr.
Gladman has brought to light 17 previously hidden
objects in the solar system. So you detect objects
in the outer solar system by observing the move relative
to the background stars and galaxies which
are stationary. So if you take a picture of
the sky and you wait an hour, and you take another
picture of the sky, none of the stars in the
galaxies will record. But distance objects in
the outer solar system will displace by a visible
amount between the two pictures. And so by comparing
the two pictures you can see, as we have
here, a moving target. [music playing] NARRATOR: The object could
be a comet or an asteroid, or if it orbits a planet
in a retrograde fashion, a new irregular moon. Another important distinction
between regular moons and their irregular
counterparts-- irregular moons are
captured, that is, they formed independently
of their host planet and most likely were part of the
debris that originally formed our solar system. Phoebe, the largest irregular
moon orbiting Saturn, is a classic example. Phoebe orbits
Saturn very far out. It has a very elliptical
and very inclined orbit. It orbits in a
retrograde direction. Voyager images suggested
that this thing looks like it could be an asteroid. And so people thought maybe
it is a captured asteroid. Now we know from Cassini it's
a very waterized, rich body. That pretty much rules out the
asteroid belt. The thinking is is that Phoebe could very well
have come from the Kuiper Belt way out in the outer
reaches of the solar system. NARRATOR: The Kuiper
Belt is thought to be the debris left over
after the solar system formed. It revolves around the Sun
beyond the orbit of Pluto. However, another theory suggests
something altogether different. It's much more
likely that Phoebe formed in an independent
orbit around the Sun and then was captured
into orbit around Saturn, whereas most of the
other objects that formed near Saturn's distance were
either created by Saturn or ejected from
the solar system. NARRATOR: Phoebe would then be
made of the planetary debris that was floating around Saturn
at the time of its birth, and possibly it's made
up of different material than some of the irregular
moons orbiting Jupiter. If this is the case,
astrogeologists may be able to
discern and compare the different ingredients that
birthed these two gas giants. Three main theories
currently exist as to how Phoebe and its
other irregular counterparts lost their independence. Two suggest the irregulars were
captured as the solar system was still forming and the
planets were still an accreting blob of gas and debris. The gas drag theory is
the most straightforward. Thick gases were swirling
around the accreting planet when a comet, asteroid, or a
shattered combination of both passed through the
gaseous mixture. We know that the giant planets
built their regular satellite systems in a large
accretion disk around each of the planets, sort
of like a mini, little solar system forming
around each planet. And the gas and dust
that was in that disk can also serve in
its outer regions as a source of friction, where
passing planetesimals formed independently are
slowed down a little bit and captured into orbit. NARRATOR: The second
theory is really a variation on the first. It's sometimes called
the pull-down theory. Here, instead of an object
being caught by simply passing through the accreting
gases, it is unsuspectingly pulled into the
forming planet's orbit by its growing
gravitational pull. The gas drag and pull-down
theories of capture work well for both
Jupiter and Saturn because their mixture
of ingredients was massive enough to slow
down these passing objects. But what about the icy
giants, Neptune and Uranus? Because of the extreme cold,
they formed much more slowly. And it's difficult to believe
that their icy accretion mixture contained enough
mass to snare a passing piece of the solar system. Yet both icy giants have their
own irregular moons, hence, a third theory-- three body interaction. You discover that
many of the objects are actually more
than one object. They're usually
two objects often, that because they're both
more or less the same size as the other in a
binary relationship, instead of being a big object
with a small object going in orbit around it like
this it's two more or less similar-sized objects going
around a common orbit. Between the two of them
is called the Barycenter. NARRATOR: A binary
pair exists when two objects of the same size are
tight enough to the Barycenter to prevent a third larger object
from splitting them apart. But when one of the binary
pair is significantly larger than the other, the more
massive third object has a greater chance
of separating them. The smaller one will tend
to have the much bigger orbit and swing out further. NARRATOR: This brings the
smaller object close enough to the planet to be captured,
while its partner is slung out into an independent orbit. One bizarre moon seems
to defy classification. Triton orbits Neptune
in a retrograde fashion, counter to Neptune's rotation. That would make it an irregular
moon, except that it's spherical and orbits
close to the equator with an almost perfectly
round circumference-- a classical description
of a regular moon. [music playing] [booming] It also spews out
mysterious, icy plumes with some indications that
it once was or possibly still is volcanically active. [music playing] NARRATOR: Before
Voyager 2 ventured into the outer solar system,
Neptune's moon Triton was assumed to be a
geologically dead ball of rock about the size of our own moon. When Voyager beamed back
photographs revealing a world with mountains, fault lines,
and fissures indicative of tectonic movement, as
well as a surprisingly thick atmosphere,
scientists were amazed. Geologic forces
usually associated with much warmer
and larger planets might be occurring on a
frozen moon slightly smaller than our own. Voyager detected no active
volcanoes in 1989, however, like Saturn's moon Enceladus,
geysers periodically erupted from the
planet's surface. What's really
stunning about Triton is not just that it has some
unique geological processes occurring, but the fact
that they're happening even though Triton is irregular moon. Most large moons
in the solar system are regular satellites, with
the very important exception of Triton, Neptune's
largest moon, which orbits the wrong way. So Triton is thought to
have been a captured object. NARRATOR: A captured moon
that acts like a regular one. How did an object
the size of Triton slow down enough not to
either pass through Neptune's atmosphere or collide
directly into the icy planet? [jazzy music] There's no sure bet, but some
theories carry better odds. Much like gamblers at
the roulette wheel, Triton and Neptune played
the odds and trusted to luck. DEALER: Place your bets. Get lucky now. NARRATOR: In roulette, there
are several ways to bet. Each one carries different odds. And like most games of
chance, the longer the odds, the greater the payoff. DEALER: Money
black, money black. NARRATOR: There are at
least three possible ways Triton could have been
captured by Neptune. [music playing] All three hypotheses
are physically possible, but the first one, the idea of
gas drag, is the least likely, simply because the period of
time which Neptune had a disk of gas and dust which could have
captured a proto-Triton object was a very short period of time. And so the window of
opportunity was very small. So that's like betting on the
green zero on the roulette table. More likely is the possibility
that the proto-Triton, sometime in solar system history,
crashed into a set of regular, middle-size,
icy satellites. And it was the collision
which gave us Triton. And that's like betting on
the first third of the numbers on the roulette table, so you
have like a one in three chance of that taking place. Your best bet is to bet
on the even numbers. There you have a one in two
chance of things happening. And the best bet for
the capture of Triton right now is this binary catcher
hypothesis, because there we know there are probably
thousands of objects that have existed in the Kuiper Belt
that would have the right size and be partnered with
another even larger object. And Neptune could
capture one of them. NARRATOR: No one knows
for sure which number paid in the early days
of the solar system when Neptune captured Triton. However, once Triton
began orbiting Neptune in an irregular fashion, it
started obliterating anything that got in its way. Neptune doesn't have a very
regular system of satellites. It's thought that the capture
of Triton disrupted what would have otherwise been a
nice, regular system like the other
large planets have. NARRATOR: It's as if Triton
was angry at losing its freedom and took it out on Neptune's
other hapless moons. But where did this
headstrong moon come from? Data from Voyager 2 indicates
that Triton's density nearly matches Pluto's. This suggests a kinship that no
other regular moon can claim. It's suspected that Pluto and
Triton are both objects that originated in the Kuiper Belt. The outer solar system
consists of these large objects going every which
way, essentially. Some of them form giant planets
themselves, and some of them were tossed out
further where they sit today in the Kuiper Belt. NARRATOR: Collisions,
accretions, and even captures have diminished what was
once a major thoroughfare of planetary building materials. Early in solar system
history, the Kuiper Belt had far more larger objects. It may have once had a
cumulative mass of 50 Earths, whereas the current
Kuiper Belt mass is much less than one Earth. NARRATOR: What's left
of the Kuiper Belt is as old as the
solar system itself. The material that makes
up the binary objects-- shards of collisions and
even some alien moons-- hasn't changed in over
four billion years. It's amazing how
really different all the moons of the
outer solar system are. I first got interested
in astronomy as a kid in the 1960s. We hadn't seen any
of these moons. They were little dots
in your telescope. And so we had no idea
how radically different they could be. I think the most shocking thing
was how much variety there is in the solar system. I think that blew me and
everybody else away who lived through that period. NARRATOR: As we travel back
home from the frigid outskirts of our solar system, awed by
the vastness of the universe and the majesty of
the planets, it's worth it to pause and take
notice of the small worlds in the shadows. Those alien moons that
were once considered afterthoughts hold
mysteries just waiting for human curiosity to solve.
