Can a moon vanish? It turns out it is
possible. In the early days of
astronomy, Zatnian moon Apotus routinely
appeared to vanish from sight entirely.
Some days it was there and the next it
was gone. This was the ultimate
magician's trick. But as we figured out
how it was done, Apotus' mysteries only
grew deeper. That is because Apotus is a
place of profound contrast. One half
plunged into near absolute darkness and
the other half gleaming with the
brilliance of reflective ice. around its
equator, a colossal mountain range, a
stony girdle wrapped precisely around
its equator whose origins defy
explanation.
Why does this moon present such a
dramatic twofaced characteristic? What
cataclysmic event or slow, inexorable
process could have sculpted a mountain
chain taller than Mount Everest in a
place that defies explanation?
And what secrets does this distant world
hold about the turbulent youth of our
solar system? I'm Alex Mccoan and you're
listening to the Astramm podcast. Join
me today as we journey to Arapitus,
Saturn's third largest moon and arguably
its most enigmatic.
Our story begins with the keen eye of
Giovani Dominico Cassini. In 1671, using
the refracting telescopes of the Paris
Observatory, Cassini spotted a new point
of light orbiting the ringed giant. But
this new moon behaved strangely. When it
was on the western side of Saturn in its
orbit, it was reasonably visible. But as
it traveled to the eastern side, it
seemed to vanish, becoming incredibly
faint, often disappearing entirely from
view in those telescopes. Reflecting on
his observations, Cassini remarked that
sometimes after having begun to
disappear with the 32- ft telescope, it
has been sought in vain the next day
with one of 45.
Cassini concluded that this moon must
have one hemisphere that is
significantly darker than the other and
it must be tidily locked to Saturn
always keeping the same face towards its
planet just as our own moon does towards
Earth. This meant that as Apotus orbits
Saturn it alternates in presenting its
bright face and its dark face towards
Earth. When the bright side faced us on
the western side of his orbit from our
perspective, it was visible. When the
dark side faced us on the eastern side,
it faded from view. It was an
astonishing observation for the 17th
century and marked the first surface
property ever detected on a planetary
moon outside the Earth moon system.
Cassini named the four moons he
discovered Sidiraa loyicia or the star
of Louie in honor of King Louis I 14th
but these names didn't stick. Later John
Hershel suggested names from the Titans
brothers and sisters of Saturn in
mythology.
Ayapotus, the father of Atlas and
Prometheus, and by extension, father of
Prometheus's creations, mankind became
the moniker for this peculiar world. For
centuries, Apotus remained little more
than a point of light exhibiting this
bizarre brightness variation. Even the
Voyager probes, which revolutionized our
understanding of the outer solar system
in the late 1970s and early 1980s, only
managed distant flybys. Voyager 1
captured images showing the dark
brightness difference, confirming
Cassini's deduction, but the resolution
wasn't high enough to reveal the
intricate details. Voyager 2 fared
slightly better, passing closer and
giving us our first tantalizing glimpse
of another astonishing feature, a hint
of the massive ridge near the equator.
But the full picture, the breathtaking
weirdness of Apotus would have to wait
for the arrival of a dedicated Saturn
explorer.
Enter the Cassini spacecraft, named
fittingly after the moon's discoverer.
Arriving at Saturn in 2004, the Cassini
mission would spend 13 years unveiling
the secrets of the ring planet and its
diverse family of moons, and Ipotus was
high on its list of targets.
Cassini's observations, particularly a
close targeted flyby in September 2007
that brought the probe within just 1,640
km of the surface of Apotus transformed
the moon from a distant glint into a
complex world we could finally study.
Apotus is a moon with a low density,
just 1.2 2 times the density of water,
suggesting that it's composed of up to
75% water ice. Its shape is not
perfectly spherical, partially due to
the equatorial ridge, giving it that
distinctive oblate walnut-like
appearance. It likely doesn't possess a
subsurface ocean like Enceladus or
Europa as there's no evidence of recent
geological activity or the tidal heating
needed to maintain liquid water deep
beneath its icy shell. Its surface is
ancient, cold, and heavily cratered,
particularly in the bright regions of
Ron Terra, testifying to billions of
years of bombardment.
Its orbit is also unusual. Apotus orbits
much farther out than other Saturnian
moons at an average distance of about
3.5 million km, more than twice the
distance of the next major moon, Titan.
This large distance means it takes 79.3
Earth days to complete one orbit around
Saturn. Crucially, its orbit is also
slightly inclined, tilted by about 15°
relative to Saturn's equatorial plane,
where most other major moons orbit. This
distance means Ipotus is less affected
by Saturn's magneettosphere and the
intense radiation belts closer to the
planet. Its orbital inclination might be
a clue to its formation or early
migration history, possibly indicating
it didn't form with the same equatorial
disc as Saturn's closer regular moons.
But let's turn to the aspect of Aapotus
that makes it so fascinating to look at
its surface.
Iapotus is cleanly divided into two
distinct regions. The leading
hemisphere, the side that faces forward
as Ipotus orbits Saturn, is incredibly
dark. This region is known as Cassini
Regio, and it covers about 40% of
Ipotus' surface. Its albido, the measure
of how much light it reflects, is
extremely low, somewhere between 0.03
and 0.05.
That's as dark as asphalt or coal dust.
It absorbs almost all the light that
falls on it.
In stark contrast, the trailing
hemisphere, the side facing away from
the direction of orbit along with the
poles, is incredibly bright. This region
encompassing Monavu Terra to the north
and Sarraosa Terra to the south boasts
an albido of 0.5 to 0.6, six as bright
as freshly fallen snow or brilliant
water ice. This almost tfold difference
in albido is the greatest interheismic
albido contrast that we know of in the
solar system. Imagine seeing this world
up close. One side a deep reddish brown
or black, the other a dazzling white. In
some areas, particularly near the
equator, the boundary between those
halves can appear remarkably sharp, as
if painted on. In other areas,
especially at higher latitudes, the
transition is more gradual with bright
raid craters splattered across the dark
terrain and dark streaks dusting the
bright terrain like splashes of mud on
snow. This image from Cassini vividly
shows this contrast. It is a world
literally split down the middle. So what
causes this yingyang appearance? For a
long time, the leading theory proposed
an internal origin, perhaps some form of
dark cryovcanic material erupting from
Ioptus' interior and coating one side. A
paper reviewing the Voyager data from
Bradford Smith and his colleagues in
1981 argued for this theory, identifying
this ring shape as a crater partially
flooded with dark eruptive material as
supporting evidence. However, Cassini's
radar and imaging data revealed that the
dark material forms a relatively thin
layer, perhaps only tens of cm to a
meter thick in most places. Furthermore,
impact craters within Cassini Regio
often excavate bright icy material from
underneath, punching through the dark
veneer.
This strongly suggested the dark stuff
wasn't welling up from below, but was
being deposited from above.
A paper in 1982 made spectrophototric
observations of the dark leading surface
and found that the composition of
material was carbonacious.
similar to that of the Merches Sun C2
meteorite which fell to Earth during a
shower in 1969.
To explain this finding, the authors
looked to potential culprits close to
Ipotus within the Saturn system. The
prime suspect, dust from Saturn's outer
irregular moon, Phoebe.
Phoebe is a relatively large dark moon
captured by Saturn's gravity, orbiting
the planet in the wrong direction. A
retrograde orbit at a great distance.
Over billions of years, impacts on
Phoebe and other small, dark, retrograde
moons have kicked up a vast, tenuous
ring of dark dust particles. The sun's
radiation exerts pressure on these dust
particles causing what is known as the
pointing Robertson effect. As a result,
this dust spirals inwards towards
Saturn, also moving in a retrograde
direction. Now, back to Apotus. It
orbits Saturn in a prograde direction,
the same direction as Saturn's rotation
and most other moons, but its orbit is
significantly further out than the main
rings and closer moons. As it journeys
around Saturn, its leading hemisphere
essentially plows headon into this
incoming stream of retrograde dark
particles originating largely from the
Phoebe group. It's like driving through
a swarm of insects. They mostly splatter
on your front windshield. The trailing
hemisphere, shielded by the moon itself,
collects far less of this dark material.
But deposition alone doesn't fully
explain the intensity of darkness we see
on the leading side of Aapotus or the
complexity of the border between dark
and bright. This is where thermal
segregation and water ice sublimation
come into play. The dark material
deposited on the leading hemisphere
absorbs more sunlight than the
surrounding bright ice. This causes the
surface temperature to rise slightly.
Ahus is incredibly cold. But even a
small temperature increase is enough to
cause the volatile water ice to
sublimate, turning directly from solid
ice into water vapor gas. This
sublimating water vapor doesn't just
disappear because Apotus rotates very
slowly. Its day is about 79 Earth days
long, matching its orbital period.
Daytime temperatures are higher and the
ice has plenty of time to sublimate away
from the warmer dark areas. In fact,
sublimation on Apotus is so fast, if its
surface were pure ice, we would see over
100 m of ice lost to sublimation in a
billion years. But that's still faster
than any Saturnian satellite.
But where does the ice go? It
preferentially recondenses and freezes
onto the coldest available surfaces.
These are the already bright icy regions
on the trailing hemisphere and near the
poles which reflect more sunlight and
stay cooler. Scientists suggest that due
to its negligible atmosphere,
sublimating molecules follow ballistic
trajectories, jumping impressive
distances to the trailing hemisphere in
the style of a missile.
This creates a runaway positive feedback
loop. Dark areas get slightly warmer,
lose their ice, become even darker. As
the underlying dark dust concentrates,
they absorb more heat and sublimate more
ice. Bright areas stay cold, collect
migrating frost, becoming even brighter,
reflect more sunlight, stay even colder,
and collect more frost. Over millions
and billions of years, this process has
dramatically sharpened the boundary
between the dark leading hemisphere and
the bright trailing hemisphere, leading
to the extreme contrast we see today.
So, the great dicty seems to be a
combination of external pollution from
retrograde moons and an internal sorting
mechanism driven by sunlight and ice
sublimation. A truly remarkable example
of planetary processes at work.
But the two-tone coloration is only part
of Aapotus' strangeness. As Voyager 2
hinted at and Cassini confirmed in
stunning detail, Aapotus possesses
another unique feature in the known
solar system, a colossal equatorial
ridge. This ridge is effectively a wall
of mountains stretching for around 1,600
km at least halfway around the moon's
circumference. On average, it is about
200 km wide at the base and reaches
towering heights of up to 20 km above
the surrounding terrain. That's
significantly taller than Mount Everest.
And the most baffling part, it follows
the equator almost perfectly, primarily
confined within the dark Cassini Regio,
although more equatorial mountains
identified by Voyager on the trailing
hemisphere could be a continuation of
this structure. In images like this one
taken by Cassini during its close flyby,
the scale is breathtaking.
It gives Ayafetus a distinct walnut
shape.
What could have possibly formed this
belt of mountains? This is one of the
biggest ongoing mysteries of Aapotus.
Several hypotheses have been proposed,
each with its own strengths and
weaknesses.
One leading idea suggests the ridge is a
relic of a much faster spinning past.
Recent models have suggested that early
in its history, shortly after formation,
might have been rotating much more
rapidly, perhaps completing a rotation
in around 16 hours. This rapid spin
would have increased oblateness, causing
the moon's equatorial regions to bulge
outwards due to a central fugal force
while the moon was still young and
relatively plastic or partially molten.
As Apotus cooled and solidified, this
equatorial bulge froze in place. Later,
gravitational interactions with Saturn
slowed the moon's rotation down over
millions of years until it became tidily
locked, rotating only once per orbit, or
79 days. As it slowed, the central fugal
force supporting the bulge diminished,
and the now solid crust couldn't fully
relax back into a sphere. The material
essentially slumped downwards, piling up
near the equator to form the massive
ridge we see today. This theory fits
well with the ridg's equatorial location
and its ancient heavily cratered
appearance, suggesting it formed very
early on. However, it requires Apotus to
have spun incredibly fast, possibly
close to its structural breakup speed
and to have solidified while maintaining
that bulge.
Another intriguing hypothesis proposes
that the ridge is the result of material
falling onto an external source.
Specifically, the collapse of a
subsatellite or a ring system that once
orbited itself. If Apotus possessed a
debris ring early in its history,
perhaps sourced from a captured body or
a large impact, this material would
naturally orbit above its equator. If
this ring became unstable, or if IUS
migrated slightly in its orbit, the ring
material may have been pushed just
beyond the rush limit, which is the
point at which the tidal effects of a
large body overwhelm the internal
gravity of a smaller one. This may have
caused the ring material to slowly rain
down onto the surface of Apotus, piling
up preferentially along the equator.
This could potentially explain why the
ridge seems most prominent on the dark
leading hemisphere if the ring material
itself was dark or its deposition
coincided with the influx of dark dust
from Phoebe. Some models suggest that
even a small moonlet breaking up within
Ioptus' rush limit could provide enough
material. While elegant in explaining
the location, the exact mechanisms for
forming and then collapsing such a ring
are still debated.
Other theories such as tectonic
processes or convective upwelling from
the interior struggle to explain why the
activity would be so narrowly confined
to the equator over such a vast
distance.
A giant impact is also unlikely to
produce a long, linear, and perfectly
equatorial feature.
Cassini's detailed images show the ridge
is heavily cratered, confirming its
ancient origins. It appears to be
structurally composed of icy, crusty
material. Some sections show evidence of
landslides and collapse features.
Whatever the process, it must have
happened very early in's history,
perhaps within the first few hundred
million years of the solar systems
formation. The ridge stands as a
monumental momento of Apotus' dynamic
youth, a frozen record of processes we
are still striving to understand.
So, what do we know about?
More than we used to, but less than we'd
like. The Cassini mission fundamentally
changed our perception of this
fascinating moon. Its close flyby
provided the data that solidified the
exogenic dust and sublimation theory for
the planet's two faces. It gave us
astonishing highresolution views of the
equatorial ridge, allowing detailed
study of its structure and morphology,
even if its origin remains debated.
Cassini measured Aapotus' shape, gravity
field, and surface composition with
unprecedented accuracy. Without Cassini,
Apopetus would have largely remained the
faint, perplexing point of light it was
for centuries. And yet, there are still
so many mysteries. What is the true
origin of Avatus' ridge? What is the
dark material of Apatus' mysterious face
actually composed of beyond something
simply carbonacious?
Unless we go there, it will be difficult
to know for sure. No missions are
currently planned to sample or even
circle this perplexing place. So, for
now, we remain in the dark, which feels
appropriate. After all, as I said at the
beginning, Apotus was once the ultimate
vanishing act, and a magician never
reveals his secrets.
Well, that's all we have time for today.
I hope you've enjoyed listening to this
podcast on Apotus. If you've enjoyed
what you've heard, please feel free to
follow us for more podcasts on other
fascinating space topics. But for now,
I'm Alex Mccoan, and this has been
Astramm. All the best and see you next
time.
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