Saturn is easily the most recognizable
planet in our solar system and some of
its moons are the most famous too. Titan
and Celadus and Ria all move like
clockwork in an elegant dance around
this ringed giant. But out in the far
reaches of Saturn's gravitational
influence, millions of kilometers from
the planet prowls an object that does
not belong. It is dark, scarred, and
solitary. It moves backwards, crashing
against the flow of the rest of the
system. It is a time capsule from an era
when the giant planets migrated and the
solar system tore itself apart.
This is a world of landslides, of frozen
carbon dioxide, and a hidden ring of
dust so massive it dwarfs Saturn itself.
It is a moon that shouldn't be there. A
visitor snatched by Saturn's gravity and
held prisoner for 4 billion years. This
is Phoebe.
I'm Alex Mccoan and you're watching Astramm. Join me today as we journey to
the edge of Saturn's domain to unravel
the secrets of its darkest, most
mysterious moon.
The story of Phoebe begins with a quiet
revolution. For thousands of years,
astronomy was limited by what is
possible to see with the human eye.
First, astronomers used the naked eye,
and then in the early 1600s, the
invention of the telescope
revolutionized what it was possible to
see. But it wasn't until the 19th
century that the dry plate photographic
revolution changed everything.
For the first time, astronomers could
leave a camera shutter open for hours,
allowing light to accumulate on glass
plates, revealing objects thousands of
times fainter than what Galileo or
Cassini could have ever dreamed of
observing themselves.
In 1898, a team from the Harvard College
Observatory led by William Henry
Pickering set up an outpost in the thin
dry air of Arkipa, Peru. Using the 24-in
Bruce telescope, they began a deep
photographic survey of the southern sky.
The work was tedious. The glass plates
were shipped back to Cambridge,
Massachusetts, where they were poured
over by computers, human analysts using
magnifying loops to spot new objects on
the plates. Then in 1899, Pickering was
examining plates taken the previous
August when he found a speck. It was
faint, magnitude 15.5,
roughly 4,000 times fainter than the
limit of the naked eye. But what was
most fascinating about this object was
that it moved. Pickering traced its path
across multiple nights. The stars stayed
fixed. Saturn moved, but this spec moved
with Saturn, yet not like the other
moons we already knew of. On the 18th of
March 1899,
he announced the discovery of Phoebe. It
was a landmark moment, the first natural
satellite in history to be discovered
not by direct observation, but by an
image on a photographic plate.
As astronomers tracked Phoebe in the
early 20th century, the excitement of
discovery turned into confusion. Phoebe
wasn't just far away. It was wrong. The
solar system has rules. Because
everything formed from the same spinning
disc of gas, the planets in their moons
almost universally spin and orbit in the
same direction, counterclockwise or
prograde.
Phoebe breaks this law. It orbits Saturn
clockwise in the opposite direction to
the planet's rotation and the other
moons. This is known as a retrograde
orbit. Now, when we find a moon orbiting
backwards, we know one thing with
absolute certainty. It did not form
there. If Phoebe had formed from the
dust surrounding the infant Saturn, the
drag from the gas cloud would have
forced it into a prograde orbit. A
retrograde orbit is a smoking gun
pointing to a very violent history. It
means Phoebe is an immigrant, a captured
object that formed elsewhere and was ins
snared by Saturn's gravity.
And the distance of this moon is
staggering. Phoebe orbits at a mean
distance of nearly 13 million km from
Saturn. That is nearly 4 times further
out than Napotus and almost a quarter of
the distance from the sun to Mercury. It
takes 550 days, about 18 months, for
Phoebe to complete a single lonely lap
around the ring planet. It's so far out
that from the surface of Phoebe, you
would not be able to see the rings of
Saturn with the naked eye. All you would
see is the glare of a tiny but bright
planet in the night sky.
Because it is so far from its host
planet, Phoebe went uninvestigated for
nearly a century. Even Voyager 2, which
flew through the system in 1981, only
saw it as a jagged, dark blob from 2.2
million km away. We knew it was there.
We knew it was weird, but we didn't know
what it was. That changed with Cassini.
When mission planners were designing the
trajectory for the Cassini spacecraft,
they realized they had a unique problem.
To enter orbit around Saturn, Cassini
had to break. This meant approaching the
planet from the outside in. They
realized that on this arrival leg before
the critical engine burn that would trap
the probe in Saturn's gravity, they
could pass by Phoebe. It was a oneshot
chance to get a close-up look at this
elusive moon. The encounter had to
happen on the 11th of June 2004, 19 days
before orbital insertion. Because once
Cassini fired its engines and settled
into the inner system, that was it. It
would never have the fuel to go back to
Phoebe or match its backward speed. For
a few frantic hours, the dark moon
filled the cameras, transforming from a
dot into a complex world. The relative
velocity of the flyby was a staggering
5.8 km/s.
And then Phoebe was gone, receding into
the darkness.
videos. Hopefully that's at least nearly as exciting as these pictures of Phoebe.
The images Cassini beam back revealed a
world that looked nothing like its
siblings. Phoebe is roughly spherical
about 213 km across but it looks
absolutely beaten. It is dark with an
albido of just 10% making it as black as
asphalt.
Cassini's cameras mapped the surface in
detail, revealing the crater saturation
and brightness variations. Spectrometers
analyzed the surface composition,
detecting water ice, carbon dioxide,
ironbearing minerals, and signatures
consistent with primitive carbonatous
materials found in meteorites expected
in outer solar system bodies. Dominating
the surface is the Jason crater, a
massive impact basin 101 km wide, nearly
half the diameter of the moon itself.
The fact that Phoebe survived such a hit
is not only astonishing, it tells us it
is a solid body and not a loose pile of
rubble like some asteroids we've been
visiting. But the most striking features
were the landslides.
Gravity on Phoebe is weak, but strong
enough that when meteorites strike, they
destabilize the crater walls. Cassini
saw bright white streaks where the dark
surface material had slumped away.
Phoebe isn't a rock. It's a dirty ice
ball. It is an ice rich body coated in a
thin veneer of dark dust, perhaps only a
few hundred meters thick.
The true nature of Phoebe was hidden in
its density. By measuring how much the
moon tugged on Cassini during the flyby,
scientists calculated its density to be
about 1.63 g per centime cubed. This is
the key. Saturn's regular moons are
mostly pure porous ice, about 1 g per
cm. Phoebe is much denser, implying it
is around 53 to 67% rock. This density
is similar to Pluto and Triton. It
suggests that Phoebe is a planetary
embryo, a world that started to
differentiate, separating into a rocky
core and an icy mantle, but was stopped
sometime in its development. The
chemical fingerprint confirmed it.
Cassini spectrometers detected carbon
dioxide trapped in the surface rocks. On
the inner moons, volatile CO2 would have
boiled away eons ago. Its presence here
confirms that BB formed in the deep
freeze of the outer solar system far
beyond the orbit of Neptune, a survivor
from the Kyper belt that found its way
to Saturn and got caught up in its
gravitational pull. And Phoebe's story
doesn't end with his capture.
For billions of years, this dark moon
has been subjected to a relentless rain
of micrometeoroids.
Every tiny impact blasts a little bit of
that dark surface material into space.
Because Phoebe's gravity is so weak,
this dust escapes easily, entering a
retrograde orbit around Saturn. Over
billions of years, this dust has
accumulated into a structure that
remained hidden from humanity until
2009.
Using the Spitzer Space Telescope,
astronomers discovered the Phoebe ring,
and it is enormous. This ring is
invisible to the naked eye, but in
infrared, it glows.
It spans from 6 million to 16 million km
from the planet. You could fit roughly 1
billion Earths within its volume. And if
you could see it from Earth, it would be
the width of two full moons in our sky.
It is the largest ring in the solar
system. And Phoebe orbits right in its
heart, but the dust particles within it
are spread extraordinarily thin, making
it virtually invisible in reflected
sunlight.
Spitzer could only detect it by sensing
the faint thermal glow emitted by the
sparse dust grains themselves.
Crucially, this gigantic ring is tilted
by 27° relative to Saturn's main flat
ring plane, perfectly matching the
inclination of Phoebe's own orbit. It
also shares Phoebe's retrograde motion.
The ring's composition inferred from its
infrared signature is consistent with
the dark primitive material making up
Phoebe's surface. The source was
confirmed. Micrometeoroid impacts on
Phoebe are in fact generating this vast
diffused halo of dust.
This discovery solved one of the oldest
mysteries in astronomy. In 1671, Giovani
Cassini discovered the moon Apotus and
noticed it had a yingyang appearance.
One hemisphere as white as snow, the
other black as coal. Well, now we know.
Phoebe orbits retrograde or clockwise
while
prograde or counterclockwise.
As the dark dust from the Phoebe ring
spirals inward towards the planet, it
slams head on into the leading face of
Apotus. Phoebe is effectively spray
painting its neighbor from millions of
kilometers away like an interstellar
bangy, creating the stark contrast that
puzzled astronomers for 300 years.
But how did this dark dusty moon born
far beyond Neptune end up at Saturn so
much closer to the sun? The leading
theory known as the Niss model suggests
that 4 billion years ago the giant
planets migrated violently disrupting
the primordial Kyper belt. In that
gravitational chaos, countless icy
bodies were thrown into the sun or
ejected into interstellar space. But a
lucky few like Triton and Phoebe were
captured and became moons.
Phoebe then is more than just a moon.
It's a relic. It is a surviving piece of
the building blocks that formed the
outer solar system, preserved in deep
freeze of Saturn's gravity. It tells us
of a time when the planets moved,
flinging rocks around the solar system
to be captured by the huge gas giants
that dominated. And while we've learned
much, questions remain. What lies
beneath the dark surface layer? Does it
have a differentiated internal
structure? Perhaps a rocky core
surrounded by an icy mantle? Can we
definitively pinpoint its origin region
within the Kyber belt or scattered disc?
Phoebe remains an object of intense
interest. A primordial messenger holding
clues to the materials, processes, and
dynamics that governed the early solar
system. It's a reminder that planetary
systems are not static, but dynamic
places where objects can wander, be
captured, and leave their imprint far
from where they originated. Phoebe, the
dark intruder, continues to orbit
Saturn, a lonely sentinel that has left
a permanent mark on the Saturn system.
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