In the vast expanse of our cosmic
neighborhood, orbiting the majestic
ringed planet Saturn, lies a world of
stark contrasts and scientific
mysteries. Dioni,
named after the titanus of Greek
mythology, Dion's bright reflective
surface hides ancient scars and
geological wonders. It's the fourth
largest in a system of over 200 known
satellites. By diameter, a mere 1,120
km wide, smaller than the width of
Texas. Yet, despite its modest size,
this frozen world holds secrets that
continue to captivate planetary
scientists and astronomers alike. Today
we journey to this distant moon. A place
where icy cliffs rise above crated
plains. Where mysterious wispy terrains
streak across the surface. And where the
forces of orbital resonance have shaped
a world unlike any other.
I'm Alex Mccoan and you're watching
Astramm and this is Dioni, Saturn's
fractured moon.
Like many of Saturn's moons, Dion's
discovery dates back to the dawn of
telescopic astronomy. It was first
spotted by the renowned Italian
astronomer Geioavani Cassini on the 21st
of March 1684.
Cassini, working from the Paris
Observatory, had already discovered
Saturn's moon Apotus 2 years earlier and
would go on to discover two more of
Saturn's moons, Rya and Tethus.
Initially, Cassini simply called it
Saturn 4, denoting it as the fourth moon
of Saturn in order of distance. It
wouldn't be until 1847
that English astronomer John Hershel
suggested using the names of Titans from
Greek mythology for Saturn's moons, and
thus Dioni received its mythological
name. The name is fitting. In Greek
mythology, Dioni was a Titanus, often
described as the daughter of Oianis and
Tethus and sometimes considered one of
the consorts of Zeus. Some myths even
suggest she was the mother of Aphroditi.
Like its mythological namesake, the moon
Dion would turn out to have surprising
connections and relationships to other
bodies in the Saturnian system. But more
on that later. For nearly three
centuries after its discovery, there
only remained little more than a point
of light in our telescopes. A mysterious
world about which we knew virtually
nothing. Even with Earth's most powerful
observatories, this distant moon
revealed few of its secrets. All of this
would change dramatically with the dawn
of the space age. Our first close
glimpse of Dioni came from NASA's
Voyager 1 spacecraft which flew by the
Saturnian system in November 1980.
As Voyager approached, Dion only began
to transform from a mere dot of light
into a world with distinct features and
character. The Voyager images, though
limited by the technology of the time,
revealed a moon with a complex surface.
Dionia appeared heavily cratered on its
trailing hemisphere, the side that faces
away from its direction of orbit. The
largest of these impact craters is
Iander, a basin approximately 350 km in
diameter. Other significant craters
include Enas and Daido, named after
characters from Virgil's Eniard. But
perhaps most intriguing were the strange
bright streaks that crisscrossed
portions of its surface. These features,
which scientists dubbed wispy terrain,
appeared unlike anything seen on other
worlds at that time. But with the
limited flyby and imaging capabilities,
Voyager could only wet our appetite for
knowledge about this enigmatic moon. The
spacecraft measured Dion's diameter at
about 1,120 km, making it Saturn's
fourth largest moon after Titan, Ria,
and Apotus. Voyager also determined that
Dioni has a density of about 1.48 g per
cubic cm, suggesting a composition
primarily of water ice with a modest
rocky component, likely containing a
small silicut core. But the brief
encounter left planetary scientists with
more questions than answers. What were
those mysterious wisps? How did Dion
surface evolve? What forces shaped this
distant world?
It would take another spacecraft bearing
the name of Dion's Italian discoverer
from 400 years ago to truly begin to
unravel these mysteries.
In July 2004, NASA's Cassini spacecraft
entered orbit around Saturn, beginning
what would become a 13-year mission of
discovery throughout the Saturnian
system. This sophisticated spacecraft
carried a suite of scientific
instruments that would revolutionize our
understanding of Saturn and its moons.
And Cassini conducted several close
flybys of Dioni during its mission,
coming as close as 99 km to the moon's
surface.
These encounters provided unprecedented
highresolution images and a wealth of
scientific data about this mysterious
world. One of the most significant
discoveries came when Cassini revealed
the true nature of the mysterious wispy
terrain first glimpsed by Voyager. What
had appeared as bright streaky deposits
from a distance were actually exposed
ice cliffs. The faces of enormous faults
cutting through Dion's icy crust. These
features now called kazmata are in some
cases hundreds of kilome long and
several km deep. The most prominent of
these features is palatine kasmata, a
massive system of fractures stretching
more than 600 km across Dioni surface.
Other major fracture systems include
Padua Kazmata and Carthage fossai. In
some places, these fractures cut through
craters and other surface features,
indicating they formed relatively
recently in Dion's geological history.
Dion's density measurements from the
Cassini mission confirmed that it is
composed primarily of water ice with a
significant rock component, roughly 40%
rock and 60% ice by mass. This gives
Dioni a mass of about 1.1 * 10 ^2 kg.
Based on this density and mass,
scientists confirm the belief Dioni
likely has a differentiated interior
with a modest rocky core surrounded by a
thick mantle of ice. The surface
temperature of Dioni is extremely cold,
averaging about - 187° C. At those
temperatures, water ice behaves like
rock does on Earth, allowing for the
formation of solid geological features
like mountains, craters, and canyons. As
mentioned, half of Dion's surface is
pocked with craters, signs of ancient
impacts. But perhaps more intriguing
than the craters are the areas where
craters are notably absent. Certain
regions of Dioni show evidence of
resurfacing. Geological processes that
have erased older features and created
smoother planes. These areas primarily
on the leading hemisphere suggest that
Dion has experienced periods of internal
activity that has refreshed parts of its
surface. But let's talk more about
Dion's Kazmata.
This bright ice stands in stark contrast
to the surrounding older surface
material which has been darkened by
exposure to radiation and micrometeorite
impacts over billions of years. But what
caused these enormous fractures?
The leading theory is that they formed
in response to global stress patterns in
Dion's icy crust. As Dioni cooled and
evolved over billions of years, its
crust would have experienced tension and
compression forces that ultimately led
to fracturing.
Some scientists believe these fractures
may be evidence that Dioni once had a
subsurface ocean that froze over time.
As water expands when it freezes, the
growth of ice would have placed enormous
stress on the outer crust, potentially
causing it to crack in the patterns we
see today. Others suggest that tidal
flexing, the stretching and squeezing of
Dioni due to Saturn's gravitational
pull, may have generated enough heat and
stress to create these features. We
still don't have the full answers.
Dion orbits Saturn at a distance of
about 377,400
km, completing one orbit every 2.7 Earth
days. Dioni is tidily locked to Saturn,
meaning the same side always faces the
planet, just as our moon always shows
the same face to Earth. Like many moons
in the Saturnian system, Doneyi
participates in a global dance of
orbital resonances that has profound
implications for its evolution. Perhaps
the most significant of these
relationships is the 21 mean motion
resonance that only shares with
Enceladus.
This means that for every orbit Dion
only completes around Saturn, Enceladus
completes exactly two orbits. This is
not merely a coincidence. It's a
gravitational relationship that has
locked these two moons into a
synchronized pattern that affects both
worlds.
This resonance creates tidal forces that
generate heat within both moons.
However, the effects are much more
dramatic on Enceladus, which shows
extraordinary geological activity,
including its famous southern polar
geysers that continuously erupt water
ice into space. Dioni being larger
experiences proportionately less tidal
heating from this relationship. And did
you know Dioni also has its own small
satellites?
Two tiny moons named Helena and Polyuses
orbit Saturn at the same distance as
Dioni. These moons occupy what is known
as Lrange points in Dion's orbit. Stable
gravitational points that precede and
follow Dioni as it circles Saturn. These
tiny moons, sometimes called Trojan
moons, are just a few kilometers across
and are likely captured or formed from
debris in Dion's orbital vicinity.
But what about that subsurface ocean?
The possibility is certainly intriguing.
Scientists are always fascinated with
any such location around the solar
system due to all oceans potential to be
the birthing place of life. So the
existence of an ocean would be
significant on Dioni and it's not
without evidence.
Cassini's magneettometer detected a weak
interaction between Dioni and Saturn's
magnetosphere, which some scientists
interpret as evidence for a conductive
layer beneath the surface, possibly a
salty subsurface ocean. The extensive
fracture systems on Dion surface also
suggest significant geological activity
in its past, which could have been
driven by the presence of liquid water
below. Models of Dion's internal heat
budget, considering radioactive decay in
its rocky core and tidal heating from
its orbital resonance with Enceladus,
suggest it may have generated enough
heat throughout its history to maintain
a liquid layer between its core and icy
crust. If Thioni does harbor a
subsurface ocean, it would join a
growing list of ocean worlds in our
solar system. Places like Europa,
Enceladus, and possibly Titan that could
potentially provide habitable
environments for simple forms of life.
However, unlike Enceladus with its
dramatic plumes of water vapor erupting
into space, Dioni shows no current signs
of geological activity that would
provide easy access to any subsurface
ocean. If an ocean exists within Dioni,
it remains well hidden beneath
kilometers of ice.
Since the end of the Cassini mission in
2017, when the spacecraft was
deliberately plunged into Saturn's
atmosphere, no other spacecraft has
visited the Saturnian system. This means
our current understanding of Dioni is
based entirely on the data collected
during these limited encounters with
Cassini and Voyager. However, the
scientific community has proposed
several concepts for future missions to
the Saturn system that could include
further exploration of Dioni. These
include orbiter missions that would
study multiple moons of Saturn, landers
that could touch down on Dion's surface,
and even specialized missions to search
for signs of a subsurface ocean. One
particularly ambitious concept is the
Enceladus orbiter, which would primarily
study Enceladus, but could potentially
make multiple flybys of Dioni as well.
Another proposal called the Journey to
Enceladus and Titan would similarly
focus on those two moons, but include
observations of Saturn's other major
satellites, including Dioni.
While none of these mission concepts
have been approved for development, the
scientific questions raised by our
limited exploration of Dioni ensure that
it remains a compelling target for
future study.
As we draw back from our close
examination of Dioni, we're left with a
portrait of a world of fascinating
contrasts. A moon whose seemingly static
icy surface tells a story of a dynamic
past. The extensive networks of bright
fractures that crisscross its surface
speak to powerful forces that have
shaped and reshaped this world
throughout history. The striking
difference between its crated trailing
hemisphere and smoother leading face
insert ongoing processes that continue
to sculpt its features and subtle clues
in its gravitational and magnetic
signatures whisper the possibility of a
hidden ocean below its frozen exterior.
In many ways, the only exemplifies the
surprising complexity we've discovered
throughout Saturn's system of moons.
What once appeared as mere points of
light in our telescopes have been
revealed as unique worlds with their own
geological characters and evolutionary
histories. These discoveries challenge
us to rethink our understanding of what
drives geological activity in the outer
solar system where the heat of the sun
is too distant to power the processes we
see on Earth. In the vast symphony of
Saturn's moons, Dioni plays its own
distinct melody. One that continues to
fascinate and inspire us to look deeper
into the mysteries of our solar system.
Can a moon vanish? It turns out it is
possible. In the early days of
astronomy, the Saturnian 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
inexurable 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 watching
Astramm. 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
Iapotus 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 its
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 Sidira lossier 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.
Apotus, 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 Apotus 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
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 Ronvu 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 Apotus 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.
Apotus 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 dichotomy 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 Cini 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 Ayapotus 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 Apotus from 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 ifus
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
Ipotus' 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 Aapotus' 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 diapus? 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 Apopetus' shape,
gravity field, and surface composition
with unprecedented accuracy. Without
Cassini, Apotus 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 Apatus' 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.
In the vast expanse of our solar system,
orbiting the ringed giant Saturn, lies a
world of ice and mystery.
This is Ria, Saturn's second largest
moon. Although you're more likely to
have heard of its largest sibling,
Titan, Ria is a fascinating place in its
own right and harbors many secrets and
wonders.
Ice cliffs hundreds of meters high, an
internal structure that defies
scientific models, and the enigmatic
discovery that lingers around this moon
that's equally as strange as the thing
we didn't find.
It's time for this lesserknown moon to
step out from its larger sibling shadow.
Let's learn all there is to know about
Ria.
I'm Alex Mccoan and you're watching
Astramm. Join me today on a voyage of
discovery to the Saturnian system and
the moon Ria to see what scientists have
discovered there and to witness its
beauty for ourselves.
Ria was discovered on the 23rd of
December 1672
by Giovani Dominico Cassini, the Italian
astronomer who made numerous
contributions to our understanding of
Saturn and its moons. Named after the
Titan goddess Ria in Greek mythology,
this moon has been a subject of
fascination for astronomers and
planetary scientists for centuries.
Let's start with the basics. Ria is the
second largest moon of Saturn after the
mammoth Titan with a diameter of 1,528
km. It's roughly half the size of our
own moon. If we were to place Ria next
to Earth, it would stretch from London
to Warsaw.
Ria orbits Saturn at a distance of about
527,000
km, completing one orbit every 4.5 Earth
days. It's locked in a synchronous
rotation with Saturn, meaning the same
side always faces the planet, a common
feature among large moons in our solar
system.
The surface of Ria is one of the most
reflective in our solar system with an
albido of 0.7.
This means it reflects 70% of the
sunlight that hits it, appearing bright
white against the backdrop of space.
This high reflectivity is due to its
composition. rear surface is primarily
made of water ice.
But this doesn't make it a featureless
ice sheet. Far from it. The moon is
heavily cratered, telling a story of
ancient impacts and a long violent
history. The largest of these craters,
named Maldi, spans an impressive 480 km
in diameter, about the distance from
Paris to Amsterdam. These craters aren't
just scars on rear surface. They are
windows into the moon's past and
composition.
Observations have shown that the bottoms
of some craters appear darker than the
surrounding terrain. This could indicate
that impacts have excavated material
from beneath Ria's bright surface,
possibly exposing darker amorphous water
ice and impact melt. In this photo, you
can see a crater in the southern
hemisphere of Ria surrounded by bright
ejector.
But one of the most intriguing features
of RIA is its system of bright wispy
streaks that crisscross its surface.
These were first observed in detail by
the Voyager space probes in the early8s,
but their true nature wasn't understood
until the Cassini mission provided
highresolution images decades later.
Initially thought to be deposits of
frost or ice, these wisps were revealed
to be ice cliffs, some hundreds of
meters high. They are likely the result
of tectonic activity in Ria's past where
the icy crust fractured and some
sections were thrust upward. These
features give us clues about the moon's
geological history and the processes
that have shaped its surface over
billions of years.
But Ria's mysteries don't end at its
surface. In 2010, data from the Cassini
spacecraft led to a surprising
announcement. Ria appeared to have a
tenuous atmosphere or exosphere. A small
amount of gas around Ria is expected,
but it was unexpected that there is
enough to be measurable. The detected
atmosphere consisted primarily of oxygen
and carbon dioxide with the oxygen being
about 5 trillion times less dense than
Earth's atmosphere at sea level. While
incredibly thin, the mere presence of
this atmosphere raised intriguing
questions about its origin and
sustainability.
One theory suggests that the oxygen is
produced by the decomposition of water
ice on Rio's surface when it's struck by
ions from Saturn's magnetosphere. The
carbon dioxide, on the other hand, is
thought to be the result of organic
molecules on the surface being oxidized
by the oxygen in the exosphere. This
discovery made RIA the first moon known
to have an oxygen atmosphere other than
Jupiter's moons Ganymede and Europa. It
also opened up new avenues for research
into the complex interactions between
Saturn's moons and the planet's powerful
magneettosphere.
Ria's exterior is fascinating, but what
about its interior? Let's dive deeper,
literally.
While we can't directly observe what's
beneath Ria's icy surface, scientists
have used various methods to infer its
internal structure. Based on Ria's
average density of 1.236
g per cubic cm, which is only slightly
higher than that of water, we can deduce
that Ria is composed primarily of water
ice, about 75%,
and a smaller fraction of rocks, the
remaining 25%.
Ria has often been described as a frozen
dirty snowball due to this structure.
This puts Ria in stark contrast to moons
like our own moon, which are primarily
rocky bodies. When Cassini visited Ria,
the prob's instruments also provided
valuable data about Ria's composition.
Spectroscopic observations confirmed
that water ice is the dominant component
of Ria's surface, but they also detected
the presence of other materials. One
surprising discovery was the detection
of carbon dioxide trapped in the surface
ice.
This finding suggests that Ria's ice
isn't pure water, but contains other
compounds, possibly including complex
organic molecules due to its higher
moment of inertia. Most scientists
believe that Ria is not differentiated
into layers. Instead, it is thought to
be a homogeneous mix throughout with the
exception of its core. Some scientists
challenged this model though. In 2006, a
paper was released in the journal Icarus
that suggested that Ria might have a
layer of liquid water under its surface,
a subsurface ocean. While this idea is
still highly speculative and not as well
supported as the subsurface oceans on
moons like Europa or Enceladus, it's an
intriguing possibility that future
missions might investigate. The presence
of such an ocean would have significant
implications for Ria's potential
habitability and of the conditions
necessary for liquid water in the outer
solar system. Ria's interior contains
mysteries, but one of the puzzling
features of Ria is its shape. While many
large moons and planets tend towards a
perfect sphere due to their own gravity,
Ria is noticeably non spherical. It's
slightly flattened at the poles and
bulges at the equator, a shape we call a
traial ellypoid. What's strange is that
Ria's shape doesn't quite match what
we'd expect based on its rotation rate.
This discrepancy has led scientists to
propose that Ria might have a nonuniform
internal structure with denser material
concentrated away from its center.
Now, let's talk about one of the most
exciting moments in RIA's recent
history. The Cassini missions close
flybys. The Cassini spacecraft, a joint
project of NASA, issa, and the Italian
space agency, spent over a decade
studying the Saturnian system, including
several close encounters with RIA. The
closest of these flybys occurred on the
11th of January 2011 with Cassini
passing a mere 76 km above Ria's
surface. The last close flyby was on the
9th of March 2013 when Cassini passed
just 997 km above Ria's surface. This
close approach allowed the spacecraft to
capture incredibly detailed images of
Ria's cratered terrain and gather
valuable data about the moon's
composition and determine it does not
have an internal magnetic field. These
flybys provided us with our best look
yet at rear surface features. They
imaged a landscape dominated by two
types of terrain. heavily crated regions
and younger, smoother planes first shown
in earlier Voyager missions. The heavily
crated areas we mentioned earlier are
thought to be extremely old, possibly
dating back to the early days of the
solar system. These ancient surfaces
provide a record of the intense
bombardment that occurred in the solar
systems youth. The smoother planes, on
the other hand, are likely the result of
a resurfacing event, such as from
material ejected from large impact
events. This material would have fallen
back to the surface, filling in older
craters and creating smoother regions.
But perhaps one of the most intriguing
results from the Cassini mission was
what it didn't find. Prior to Cassini's
arrival, some scientists had proposed
that Ria might have a system of rings.
This idea was based on unusual
measurements of electron densities
around Ria made by Cassini in 2005.
However, despite careful searches during
its flyby, Cassini found no evidence of
rings around rear.
This nondetection doesn't completely
rule out the possibility of some kind of
debris disc around Ria, but it does make
it much less likely.
Another intriguing aspect of RIA is its
place in Saturn's complex system of
moons. Ria orbits Saturn in what is
known as the E-ring, a diffused ring of
ice particles thought to be supplied by
the cryovcanic eruptions from Enceladus.
While Ria itself doesn't seem to be
geologically active like Enceladus, its
position in this ring means it's
constantly interacting with these ice
particles. Although Ria is in the
extremities of the E-ring beyond where
it is dense, over time, this interaction
could affect Ria's surface properties
and contribute to the moon's bright
appearance.
As we reflect on what we've learned
about Ria, it's clear that this moon,
often overshadowed by its more famous
siblings, is a fascinating world in its
own right. From its heavily created
hemisphere to its relatively smooth
side, from its tenuous atmosphere to its
place in Saturn's complex system of
moons, RIA continues to intrigue
scientists and spark our imagination.
But our exploration of Ria is far from
over. Many questions remain unanswered.
What is the exact nature of its interior
structure? How has it evolved over the
billions of years since its formation?
Could it harbor simple forms of life in
a hypothetical subsurface ocean? These
questions and more await future missions
to the Saturnian system. While there are
currently no planned missions
specifically targeting RIA, the moon is
likely to be observed by any future
spacecraft sent to study Saturn and its
moons.
As we conclude our journey to RIA, let's
take a moment to appreciate the
diversity of worlds in our solar system.
From the scorching surface of Venus to
the icy plains of Pluto. From the
towering volcanoes of Mars to the
potential subsurface oceans of icy moons
like Enceladus, our cosmic neighbor is
filled with wonders waiting to be
explored. Ria with its icy surface
reflecting the distant sunlight stands
as a testament to the beauty and mystery
of our universe. As we continue to
explore and learn, who knows what
secrets this distant moon may yet
reveal.
You might not know much about Teis, but
you should know this. The moon called
Tethus is a survivor. An icy world
orbiting Saturn at 295,000
km. It may appear unremarkable at first
until you learn what it has endured and
become amazed that it's even here at
all. Take a closer look at its scarred
surface and you'll see what I mean. Not
many moons house a gargantuan trench
system that wraps around 34 of their
body. A tectonic rupture so vast it
beggars belief for a world this size.
And opposing it, a gigantic impact basin
stares back. Evidence of a collision so
violent it should by all rights have
blasted this moon into fragments.
What did this? And how did Saturn's
fifth largest moon survive the
onslaught? The answers lie in its frozen
icy shell, and today they are coming to
light.
I'm Alex Mccoan, and you're watching
Astramm. Join me today as we delve into
the history and geology of Tethus,
Saturn's scarred survivor.
Tethus' first observation came from
Giovani Dominico Cassini. Following his
arrival at the Paris Observatory,
Cassini, the man who centuries later had
a well-known probe named after him,
identified Tetis in 1684 alongside
Dioni, adding them to his count of
Saturnian moons that already included
Ria and Apotus.
Following the Custo, these were
initially dubbed the Siderea Lordier or
stars of Louie for King Louis I 14th.
But the name didn't stick. And in the
19th century, John Hershel proposed a
naming convention drawing on the Greek
Titans to tie in with Saturn or Cronis,
the Titan's king. Teis, fittingly for a
water ice world, was named after a
Titanus and Sea Goddess. And it is
indeed icy. A key characteristic
revealed by gravitational measurements,
primarily from the Cassini spacecraft,
is Teas' remarkably low density,
approximately 0.97 g per cubic cm, or
0.97 times that of liquid water. As a
result of this, Tetus' mass is less than
1% of that of our own moon, despite
spanning a third of its diameter. This
extremely low value strongly indicates
that Tetis is composed almost entirely
of water ice with a small amount of
rocky material integrated within. It
suggests Tessus likely never fully
differentiated into a distinct rocky
core and icy mantle. Or if it did, the
core is exceptionally small. The density
might also imply some degree of internal
poricity. meaning the ice within might
not be perfectly compacted, potentially
containing voids or less dense ice
structures. This composition makes Teis
a quintessential example of an outer
solar system ice world. Physically, Teis
presents a typically bright icy
satellite. Its high bond albido
reflecting around 60% of incident
radiation from the sun confirms a
surface dominated by water ice.
relatively free from significant amounts
of darker rocky or carbonatous
contaminants seen on moons like Apotus
or Phoebe, Teis has an irregular shape
resulting from the gravitational
influences of its neighbors. The
classical sphere shape has been squashed
into a triacal ellypoid, meaning that it
has three axes.
However, perhaps the most arresting
feature on Teesus is one of its
landmarks, Ithaca Kasma.
This enormous canyon system dominates
the moon's topography. It measures up to
2,000 km long, 100 km wide, and 5 km
deep. Now, I really want you to take a
moment to grasp the size of this
feature. Imagine a crack running from
New York City almost to Denver,
Colorado, and you start to grasp the
scale relative to the moon size. It
dwarfs Earth's Grand Canyon and is
proportionally one of the largest canyon
systems in the entire solar system,
rivaled perhaps only by Valis Marinerys
on Mars.
The existence of such an imposing
feature on a seemingly simple icy moon
begs the question, how did it form?
Several theories have been proposed, but
the leading hypothesis connects Ithaca
Kasma to Teetus' internal evolution,
suggesting that it may have formed early
in the moon's geological history. As
Tethus formed and subsequently cooled,
any liquid water within its interior
would have begun to freeze. And unlike
most substances, water expands when it
freezes. If Tethus once possessed a
substantial subsurface ocean, or even
just a partially liquid interior, the
gradual solidification of this water
would have caused the moon's volume to
increase like an inflating balloon. This
expansion would have placed immense
stress on the rigid outer icy crust.
Ithaca kasma according to this theory is
the result of the crust cracking and
splitting apart under this relentless
internal pressure a global scale stretch
mark signifying the moon freezing solid
from the inside out the sheer scale of
the kasma suggests that Tetis might have
expanded laterally by around 5% during
this process which is crazy to think
about an entire moon swelling up until
it literally literally began to burst.
Other theories have considered links to
the giant impact crater Odysseus,
suggesting the shock waves might have
fractured the opposite hemisphere.
However, the kasma is thought to be
older than Odysius basin and detailed
modeling suggests the expansion freezing
model provides a better fit for the
observed geology. Ithaca Kasma therefore
isn't just a spectacular site. It's a
record of Teetus' thermal history, a
dramatic indicator that this now frozen
world may once have undergone growing
pains on a cosmic scale.
Surface temperatures on Tetis are
incredibly low, averaging around -
187° C, ensuring water ice remains
brittle and stable against sublimation,
except over very long geological time
scales. It's cold enough there that the
water ice on Tetis can often act like
rock. Teis lacks any detectable
atmosphere, leaving its surface directly
exposed to the harsh environments of
space. Micrometeoroid impacts, solar
radiation, and charged particles within
Saturn's magneettosphere.
Only not all of those micrometeoroid
impacts were micro. One was large enough
that it nearly shattered frozen Tetus
like a bullet through a snow globe. It's
time to take a look at the Odysseyus
crater.
If Ithaca Casma speaks to internal
forces, the Odysius crater screams of
external violence. Located on Tethus'
leading hemisphere, Odysius is the
moon's largest impact basin, measuring
approximately 450 km across. That's more
than 2 fths the diameter of Teis itself.
The impact responsible for this enormous
feature was thought to have occurred
between 400 million and 1 billion years
ago. And judging by the size of the
crater compared to its moon, if Tethus
had been icy and hard like it is now,
there is a significant chance Teis would
have been broken into pieces. But it
seems the impact occurred when Tethus'
interior was still partially molten or
slushy, allowing it to absorb and
dissipate the impact energy more
effectively than a completely solid,
brittle body could have. an unlucky
break.
Interestingly, Odysius is remarkably
shallow for its diameter. Unlike the
sharp ball-shaped craters we see on
rocky bodies like our moon or Mercury,
Odysseus is relatively flat, its rim
subdued, and its central peak complex
largely collapsed. This is not how it
would have looked immediately after the
impact and suggests a phenomena known as
viscous relaxation was at play over
geological time scales. Even solid ice
behaves somewhat like a very thick
fluid, especially when topography is
significant. The immense depression
created by the impact and the uplifted
rim would have slowly slowed and slumped
under Tetus's own gravity, gradually
smoothing out the crater's profile. The
degree of relaxation observed in Odysius
provides further clues about the thermal
state and composition of Teas' crust at
the time of the impact and in the eons
since, which makes sense. like a nuke
suddenly detonating. Such a large impact
tends to release a lot of energy, and it
would seem logical that not everything
would remain icy. The impact likely
resurfaced a significant portion of the
surrounding terrain, either obliterating
all the craters or smothering them with
its ejector.
So, Tethus has taken some hits over the
years, both from within and without. But
that's not all there is to this moon
made of ice. Another notable discovery
from Tetis was its unusual thermopysical
properties, which amusingly tied it to a
1980s computer game character. In 2015,
Cassini's composite infrared
spectrometer made a series of
observations of Teetus' daytime
anti-Saturn hemisphere over a 9-hour
time period. This produced data to
confirm an anomaly that had previously
been identified in 2012 that the lower
latitudes of Tetus' leading hemisphere
appear cooler in the day and warmer at
night compared to the surroundings. This
area of Tetus also appeared darker in
infrared and UV color ratio maps when
compared to the rest of the leading
hemisphere. funnily enough, the pattern
when mapped out made the moon look like
a giant Pac-Man, which gave scientists a
good chuckle and proving their sense of
humor, they gave this phenomenon the
same name. Still, the Pac-Man heat map
on Teis and a similar one on neighboring
Mimis needed an explanation.
Tethus orbits Saturn at a distance of
about 295,000
km, making it the fifth largest moon in
the Saturnian system. Its orbital period
is short, just under 1.9 Earth days. And
like most major Saturnian moons, it is
tidily locked. Why does this matter?
Well, this gravitational interaction
ensures Tethus perpetually presents the
same hemisphere towards Saturn, dividing
its surface into a leading hemisphere
facing the direction of orbit and a
trailing hemisphere. This helps keep
Pac-Man facing the same direction, which
also brings us to its color. The leading
hemisphere facing the direction of orbit
appears to be between 10 to 15% brighter
than the trailing hemisphere and its
near infrared spectrum suggests that it
has more pure water ice on its surface.
This pattern is generally attributed to
external factors. The leading hemisphere
is the side that gets exposed the most
to weathering by particles from Saturn's
E-ring and Enceladus's plumes. This
weathering will sand blast material onto
the surface of the leading hemisphere,
which tends to consist of submicronsized
icy particles. Smaller water ice
particles reflect more light than larger
particles, therefore giving the leading
hemisphere a higher albido.
The trailing hemisphere, conversely, is
more heavily bombarded by energetic
plasma particles trapped within Saturn's
magneettosphere. This high energy
radiation can sputter surface molecules,
break chemical bonds, and implant ions,
processes collectively known as space
weathering, which tend to darken and
reen icy surfaces over time.
Contamination by micrometeoroids
carrying darker material could also play
a role. In other words, we now think
that Saturn's rings are to blame for
feeding Pac-Man as the differential
bombardment from E-ring particles across
the moon's leading hemispheres modify
the surface. This will give certain
regions a higher thermal inertia,
meaning they store and release more heat
effectively than their surroundings,
therefore creating the observed
temperature differences. It's a pretty
fun finding, one that makes you realize
just how much these moons are shaped by
their unique interactions with Saturn's
rings.
Teis may lack the atmospheric complexity
of Titan or the obvious present-day
activity of Enceladus, but it is far
from a boring inert world. It is a body
profoundly shaped by extreme events. Its
story is writ large across its surface
in the form of Ithaca Kasma and the
Odysius Crater. It is also an incredible
example of how worlds and moons are
influenced by their wider neighborhoods.
Teis is icy, but the subtle variations
in its surface texture and color hint at
ongoing interactions with the Saturnian
environment. Traveling around Saturn and
its rings changes Teetis, and studying
the two helps us better understand these
dynamic interactions. Sadly, there are
no new missions to Teetis currently
planned. But Teis has survived worse
fates. It has been squeezed. It has been
sandlasted with icy dust. And it has
been struck from above with forces that
could have shattered it. Teis is a
survivor. And although its crust is made
of ice rather than rock, it is
unapologetically hardy. It has weathered
anything the universe has thrown its way
so far and will keep going strong.
Well, everyone, here it is, your most
asked for video, Titan.
And to be fair, I can understand the
curiosity towards it. It is the only
moon with a substantial atmosphere.
There's clear evidence of stable bodies
of surface liquid on it. And best of
all, mankind has visited it, so I will
be able to show you a lot of real photos
and video footage. I'm Alex Mccoan, and
you're watching Astramm. And here is
everything you could want to know about
Saturn's biggest moon, Titan.
But let's start from the beginning and
give some context. this remarkable
planet-like place.
Titan is the sixth spherical moon from
Saturn. And unlike Jupiter's four
Galilean moons in the Saturn system,
Titan is all by itself in its size. The
rest of Saturn's moons are pretty small
in comparison.
To give some idea of how big it is,
Titan's diameter is 50% larger than
Earth's moon and is 80% more massive. In
fact, it's the second largest moon in
the entire solar system after Jupiter's
moon Ganymede. It does actually appear
slightly bigger than Ganymede if you
were to put them directly side by side,
but this is caused by Titan's thick
atmosphere, which extends its apparent
diameter.
Even so, Titan's real diameter is still
larger than the smallest planet,
Mercury, but it's only 40% as massive.
As its density is quite low for its
volume, its gravity is reasonably weak
at only 0.14gs
or 1.35 m/s squared, which is even less
than our moon.
Due to Titan's low density, it is
thought that its composition is half
water ice and half rocky material.
Unlike other celestial objects this
size, it is believed Titan has a
differentiated interior. This means it
has layers. And like a lot of other
large moons, one of those layers is
thought to be a liquid ocean comprised
of water and ammonia under the moon's
crust. This liquid ocean is comparable
to Earth's magma layer situated between
the core and the crust and has been made
liquid due to heat, pressure, and to a
certain degree tidal forces. The
existence of this liquid layer was
proven more likely when Cassini, the
spacecraft orbiting Saturn, discovered
extremely low frequency radio waves in
Titan's atmosphere. Titan's surface is
not known to be a good reflector of low
frequency radio waves, but the liquid
interior would be. Another reason is
that the surface features on the moon
have shifted by up to 30 km since
Cassini started observations, which
could imply that the surface is not
attached to the core, but is rather
floating on this liquid ocean layer. And
while there is no evidence of life on
Titan, scientists do speculate that the
conditions would be right for there to
be life in this subsurface ocean.
Unfortunately, if there was life to be
found on the surface or below, we'll
have to wait a while as there no planned
missions to check out this possibility.
And Jupiter's Europa is a more likely
candidate to be investigated for life in
the foreseeable future. The
differentiated interior of Titan does
not produce a magnetic field. Titan is
still quite protected from the solar
wind though as 95% of its orbit around
Saturn is within Saturn's own
magneettosphere.
Titan orbits Saturn once every 15 days
and 22 hours and has a rather large
orbital eccentricity, which means the
orbit isn't so circular. Its orbital
plane is also at an angle. But that
doesn't mean that Titan is likely a
captured object. Rather like Jupiter's
Galilean moons, it is thought that
Saturn also had several large moons in
the past, but most of these have been
destroyed through big collisions which
left Titan the lone victor. Saturn's
medium-sized moons like Ayetus and Ria
are thought to be the remnants of this
tumultuous beginning. Titan's day, like
the day on our moon, is identical in
length to its orbital period. This means
Titan's rotation is tidly locked to
Saturn and only ever shows one face to
the planet. Not that visually it makes
any difference. Titan's hazy atmosphere
completely blocks the view of the
surface from an outside perspective. On
the other hand, you might just be able
to see Saturn while standing on Titan,
although the view would be significantly
obscured. This does mean, however, that
if you were to stand on one spot on
Titan, Saturn would never move in the
sky. Removing Titan's haze, this is what
it would look like.
This leads us on to one of the topics
that truly sets Titan apart from the
rest of the moons in the entire solar
system, its substantial atmosphere.
I remember the first time I ever saw a
photo of Titan, I was truly blown away,
as it never occurred to my young self
that the moon could even have an
atmosphere.
I thought it must have been a new planet
they discovered recently or something.
To me, what looks odd about the
atmosphere is how far it stretches into
space.
When you see a picture of Earth, you see
that the atmosphere has quite a tight
fit around the planet. Titan, on the
other hand, looks like it has a thick
blanket all over it. There are a number
of reasons for this. The first one is
that Titan is a lot smaller than Earth,
but its atmosphere is 1.9 times more
massive than Earth's or 7.3 times more
massive on a per surface area basis. The
second reason is that Titan's gravity is
a lot weaker than Earth's, meaning it
doesn't pull it down as strongly. The
mass of the atmosphere actually means
the pressure at the surface is 1.45 45
atmospheres or 45% more than the
atmospheric surface pressure on Earth.
And comparing the two, you can see the
extent of how far Titan's atmosphere
stretches into space. 600 km high is
only the limit of the meosphere.
Earth's meosphere, on the other hand,
stops at 120 km.
Even at the distance of 975 km, the
Cassini spacecraft had to make
adjustments to maintain a stable orbit
against atmospheric drag when it made
its closest approach.
Like Venus, Titan is a super rotator,
meaning its atmosphere rotates faster
than the rotation of the planet. This
can especially be seen at the poles on
the moon. Each pole has a polar vortex
that rotates once every 9 hours compared
to the moon's rotation of 16 days. The
vortexes on each pole seem to be like
permanent hurricanes.
So, what does the atmosphere consist of
and why is it orange in color? Well, the
atmosphere is 98.4%
nitrogen, the remainder being mainly
methane and small amounts of hydrogen.
There are also trace amounts of
hydrocarbons from the break up of
methane in the upper atmosphere due to
UV light. And it is these hydrocarbons
that are thought to give Titan its
orange hue. This constant breakup of
methane to hydrocarbons should have
meant the moon ran out of methane within
50 million years. A very short space of
time compared to the age of the solar
system. This means there must be a
source that replenishes the methane. The
most likely candidate being cryov
volcanoes, although biological life has
not been ruled out. The methane in the
atmosphere creates a greenhouse effect
without which the temperature on Titan
would be a lot lower. Conversely,
however, the haze also reflects a lot of
the sunlight, creating an anti-
greenhouse effect, which cancels out
some of the potential greenhouse effect
from the methane. Now, while Titan's
upper atmosphere gets 1% of the sunlight
Earth does due to the distance from
Titan to the sun, another result of this
reflection of sunlight means the surface
of Titan only gets about 0.1% in the
end. The Huygens's team liken the
difficulty of taking photos at this
light level to taking pictures of an
asphalt parking lot at dusk.
All these things combined means that
while it would be dark, a human would
only need an oxygen mask and to wrap up
extremely warm to be comfortable while
standing on the surface of Titan.
It really is cold on Titan, - 180°
centigrade on average. This means any
water on Titan remains solid and doesn't
ever melt, evaporate, or sublime. Then
why are there sometimes clouds on Titan?
Well, these are not water ice clouds,
but rather methane clouds, which means
yes, it can rain methane on Titan. In
fact, the temperature on Titan is just
right for methane to be liquid. Methane
freezes at - 182.5° centigrade and boils
at - 161.5°
centigrade. The temperature combined
with the surface pressure got scientists
very excited at the prospect of there
being a hydrocarbon lakes or seas on the
surface of Titan. Similar to water lakes
and seas on Earth.
If there really were lakes on this moon,
it would be the first time this had ever
been observed outside of Earth. This was
actually one of the main driving forces
behind Cassini Huygens to see what there
was under that thick atmosphere. The
Huygens's probe, named after the
astronomer who discovered Titan in 1655,
was designed to enter Titan's atmosphere
and land on the surface. The possibility
of even landing on an ocean was taken
into account during its design process.
As the probe descended, its parachute
was pulled and after an almost 3hour
journey, it finally rested on the solid
surface of Titan. Sadly, it wasn't able
to see any legs, but what it did see
confirmed that methane lakes once
existed as Huygens landed on what
appeared to be a dried up lake bed.
These stones you see from the surface
photos are rounded stones much like
pebbles found in a river or a lake on
Earth. Cassini, from the perspective of
space, was able to confirm that methane
lakes are still found on Titan today.
Near the South Pole, Cassini observed an
area which was later confirmed to be a
lake called Ontario Lacis. It is 20%
smaller than its North American
namesake, Lake Ontario. So, in other
words, it is still pretty big at 15,000
km. On this side of the lake, you can
see a smooth shoreline eroded by waves.
On the west side, you can see the first
evidence of a river and delta on Titan,
meaning that liquid hydrocarbons flow
down higher plains to the lake, leaving
delta deposits in much the same process
you would find on Earth. Ontario Lacus
is extremely shallow, only estimated to
be between 40 cm and 3 m deep, the
deepest point likely to be just over 7
m. As Cassini's radar mapped this lake,
it found that the lake did not have
waves bigger than 3 mm, meaning the
surface would appear like a sheet of
glass or a mirror. This doesn't mean
there can't be bigger waves, unless this
liquid is particularly viscous. But the
likelihood is that it was simply not a
windy day as the observations were
taken. The atmospheric density and
gravity on Titan should mean that waves
would be bigger on Titan than they would
be on Earth. As the north pole of the
moon began to come out of a 15-year
winter, another lake was discovered,
Jingpo Lacis. As Cassini was passing by
the moon, sunlight was reflected off the
surface of Jingpo Lacis like a mirror
directly into the view of Cassini. Upon
further observation, Cassini was able to
detect further evidence of moving liquid
on Titan, as can be seen by these rivers
flowing into the lake.
The second biggest lake on Titan to be
discovered is Lijia Mari. Found in the
north pole region of Titan, it is bigger
than Lake Superior on Earth with a
surface area of 126,000 km.
While parts of this lake are reasonably
shallow, the average depth is a lot
deeper than Ontario Lacas at 50 m and
some parts of it could reach depths of
over 200 m.
Plenty of rivers can be seen flowing
into the lake,
and there are large islands found around
this area here. A particularly curious
observation Cassini made, dubbed the
magic island, is the appearance and
disappearance of what appears to be an
island. Although scientists are unsure
exactly what happened here, the theories
are that it could be silt suspended in
the lake, bubbles, or subsurface ice
rising to the surface as the lake warmed
during the moon's spring. But still very
curious.
The largest lake on Titan at 400,000
km is the Kraken Mari. As you can see,
the lake is split up into two main
parts, separated by a small stretch
quite similar to Earth's straight of
Gibralar. Its nickname is the throat of
the Kraken. Because of tidal forces and
the size of the lake, it's thought the
tide changes by about 1 meter and so
this straight may have strong currents
and even whirlpools. The Kraken Mar is
also quite deep in comparison to Ontario
Lacis, but isn't any deeper than 170 m.
So, we know about the lakes on Titan
now, but what other interesting surface
features might it have? Well, plenty,
actually. Titan's surface is quite
young, as young as 100 million years
old, which means its surface must be
geologically active. Some scientists
believe the dirty ice crust is
substantially rigid. Although there is
some evidence to suggest that there's
tectonic activity on the moon, possibly
caused by tidal forces with Saturn. The
main factor of a renewed surface,
however, is likely to be the same thing
that produces methane in the atmosphere,
cryovcanos.
Now, this is pretty interesting. You
know how magma on Earth is pretty hot,
but when it comes out of the ground, it
freezes? Well, Titan has the same thing,
except its magma is water and ammonia.
And when it comes out of the cryo
volcanoes and spills over the land, it
too freezes to renew the surface.
Because a water and ammonia blend is a
lot less viscous than lava, it flows
further than lava on Earth. This means
mountains are more flat and will never
reach the heights of volcanoes on Earth.
While it is hard to confirm specific
cryo volcanoes on Titan due to the
obstruction caused by the atmosphere,
the most likely candidate is Sautra
Patera found on the southern hemisphere.
In this image, height has been
exaggerated by a factor of 10, but it
gives a good idea of the sides of the
dome and the 1.7 km deep pit, the
largest that we know of on Titan.
The force necessary for this to erupt
would have had to have been incredible.
And while it doesn't appear to be active
now, it is being actively monitored.
The fact that lava on Titan is a mix of
water and other minerals means the
surface could be compared to dirty ice.
Because we know the Huygens bounced and
wobbled in a certain way as it landed on
Titan, we have a rough idea of what the
consistency of the surface could be
like. Scientists have referred to it as
a soft, damp sand. Another theory is
that it's like snow with a thin crust on
top. Imagine walking on frozen snow. If
you're careful, you can walk on a solid
surface, but if you stomp too hard, you
will sink in quite deeply. And Titan's
believed to be something like this.
Titan's highest mountains come in the
form of ridge belts like the Rockies in
America.
These ridge belts could also be a form
of cryov volcanoes.
The largest mountain on Titan can be
found in one of these mountain ranges
known as Mithrim Monz and is 3,337
m high. Interestingly, mountains this
tall are thought to be topped with
methane snow. Also found on Titan are
many gorgees, valleys, and dunes using
infrared cameras to see the surface from
space. What can also be seen quite
prominently are these large patches of
dark terrain.
Originally, these dark patches were
thought to be seas until the Huygens's
probe landed on one of those areas known
as Shangria.
They could well have been seas in the
past, but now they are plains of dark
mineral deposits similar to the Namib
Desert on Earth where they appear as
windswept dunes in some places.
Overall, Titan can be compared to Earth
in lots of ways. Scientists think that
Titan shows signs of what early Earth
could have been like, only much colder.
It's fair to say that Titan is
remarkably interesting, and I can only
hope it gets its own mission one day.
Cassini has done a great job, but it was
never a Titan orbiter, and its mission
will soon be over anyway. What I would
find extremely fascinating is to explore
more of its surface. And hopefully it
won't be long until a mission to do that
will be approved. Until then, here was
everything you could want to know about
Titan.
It's been 3 years since Cassini ended
its mission by diving into the
atmosphere of Saturn. Its mission was
tremendously successful, and it was, in
my opinion, one of the greatest missions
of all time. The data it collected over
the 13 years in orbit around Saturn is
still being poured over today as there
are still a lot of unsolved mysteries
about Saturn and its moons.
One of the most impressive discoveries
of Cassini was confirming the existence
of liquid methane lakes on the solar
system's most unusual moon, Titan. But
knowing they are there didn't help
answer how they came into being. But now
scientists may have come up with an
answer.
I'm Alex Mccoan and you're watching
Astramm. And today we will explore
everything we know about Titan's legs.
What Cassini has seen and discovered and
how we now think they came into being.
There are only two places in the solar
system that have stable bodies of liquid
on their surfaces. Earth and Titan. That
doesn't mean they are very similar
places, though. Earth is a rocky planet
with an iron core and silicut mantle.
Titan is an icy world. Its crust and
mantle are made up of mainly water ice
with a subsurface ocean of water
surrounding a rocky core.
This means Earth is much denser and
coupled with its bigger size means the
gravity on Earth is a lot stronger than
on Titan. Both worlds have dense
atmospheres, but due to their
differences in gravity, Earth's
atmosphere hugs the planet tightly.
While Titans, although a little more
massive in total than Earth's, stretches
hundreds of kilometers above the moon's
surface. Their compositions are also
different. Earth is much closer to the
sun, meaning it is much warmer than
Titan, which allows there to be an
active water cycle.
Water is frozen on Titan. However, the
temperature is just right for liquid
methane to be present on its surface.
That means yes, Titan experiences a
methane cycle in a similar fashion to
Earth's water cycle.
Methane clouds have been spotted in
Titan's atmosphere, and these clouds
rain liquid methane onto Titan's frigid
surface. This process carves out rivers
through the surface ices that flow into
small seas or lakes. Radar data from
Cassini visualized in these images
clearly show various river beds and
valleys cut away by the flowing methane.
The most prominent of these flow
channels are found around Titan's second
largest lake Mar.
These channels dubbed Vid Flumina are
special because they look to be much
older than any other flow channel on the
moon. The canyons have walls hundreds of
meters tall with very steep slopes.
Liquid methane is visible in the canyon,
but right now it isn't flowing. Rather,
it is likely to be a drowned river
valley due to the level of Liam Mari,
although some higher altitude
tributaries are likely to still be
flowing down into the canyon.
The lakes on Titan can be huge. The
biggest one, the Kraken Mari, is bigger
than the Caspian Sea on Earth at roughly
400,000 km.
The radar data suggests that a lot of
these lakes are very shallow, some of
them only a few meters deep. Some of the
larger ones, however, can have depths of
over 200 m. All in all, this puts the
total amount of potential fossil fuels
on Titan to over 300 times the proven
amount on Earth.
The legs of Titan are surprisingly
still. Some legs do not have ripples
larger than about 1 mm, meaning it would
appear like a glass or a mirror.
This came as a surprise to scientists
who expected waves of up to a meter in
height due to the low gravity and dense
atmosphere blowing over the surface.
This could mean that there just so
happened to not be any wind as Cassini
passed over. the lakes are a lot more
viscous than expected or that they
occasionally freeze over. One other
theory has been proposed and that is
that dense aerosols may sit on the lake
surface creating a film which prevents
waves from forming.
The detection of lakes on Titan couldn't
be possible without the aid of Cassini's
radar instrument and infrared camera to
see through Titan's thick haze. These
instruments use a process called
specular reflection to make
observations. The radar instrument
detected large dark patches across the
surface which were later confirmed to be
lakes by the infrared camera when
sunlight reflected off the lake and into
Cassini's view. A bit like what you
sometimes see of Earth from space.
Specular reflection with the infrared
camera and the sun ended up being used a
lot to confirm what the radar secular
reflection was seeing. Interestingly,
clouds and the subsequent effects of
their rain were also detected. In the
left image, you see a light patch
indicating dry ground, but a year later,
the area was imaged again and this time
darker patches can be seen indicating a
very flat surface, highly likely to be
shallow and small lakes.
The lakes aren't distributed evenly
across the moon. Rather, most of them
are found near the poles.
However, the Huygens's probe, which was
part of the Cassini mission and landed
on Titan's surface near the equator, did
see flow channels around the equator,
too. But where are the lakes these
channels would flow into? Well, this is
thought to be the result of seasonal
variations like summer and winter
producing different types of weather on
Earth. At certain points in Titan's
year, rain would fall in these parts and
pool on the surface. However, these
probably evaporate during the rest of
the year and the rain migrates to the
north and south, which is what Cassini
and Huygens would have observed. Even
though Cassini was in orbit around
Saturn for 13 years, that's not even 1/
half of a Saturnian year. Saturn takes a
full 30 Earth years to orbit the sun
once. So Cassini didn't capture a
complete picture of all the seasonal
variations.
The lakes likely vary in depth
throughout the year with some
potentially drying up completely.
There is one other mystery that was
thrown up by the Cassini data to do with
Titan's Lake that may have recently been
solved. Some of Titan's smaller lakes
have very steep rims that tower above
the lake level. This is unusual as legs
on Earth typically form from erosion
processes and dissolving limestone,
meaning the rims are shallow and smooth.
These are called castic legs. But this
process doesn't add up for a number of
Titan's legs. Scientists now believe
that these could be explosion craters
that have since filled up with liquid
methane. Models suggest that Titan was
once colder with nitrogen rain and snow
instead of methane. This liquid nitrogen
would have seeped into the crust,
creating pockets of liquid nitrogen. As
the greenhouse gas methane was pumped
into the atmosphere by cryov volcanoes
in Titan's past, the world warmed and
would have caused the liquid nitrogen
now trapped under the surface to
evaporate and expand, building the
pressure in the pocket until the lid
basically blew off, leaving an explosion
crater.
There is still a lot to be learned from
the Cassini data. The lakes of Titan
were just a small aspect of the mission
and scientists are going to be piecing
these puzzles together for years to
come. Better understanding the Saturn
system as a whole. I really hope future
space agencies will be as ambitious and
as successful as the Cassini mission.
There is still an awful lot to discover
out there.
Enceladus,
perhaps one of the most intriguing
objects in the entire solar system.
And yet, it is only the sixth largest
moon of Saturn, and in natural light, it
looks very unassuming.
However, there's a lot more Twins
Enceladus than meets the eye.
It's an active icy world with jets of
water vapor pouring out from its
southern hemisphere.
Thanks to the remarkable Cassini
mission, we have studied and observed
Enceladus in exquisite detail and
perhaps know more about it than some of
the closer and bigger Jovian moons.
However, although we've seen a lot from
the outside, it's the inside of the moon
that still holds so many mysteries.
I'm Alex Mccoan and you're watching
Astramm. And together we will explore
some of the most fascinating details of
Enceladus, piecing together photos and
data from a variety of missions to find
out almost everything you could want to
know about this special moon. So stick
with me on this journey of discovery.
Let's first of all discuss where
Enceladus fits into our solar system.
Enceladus is currently Saturn's 14th
closest moon. I say currently as Saturn
has some tiny moonlets hidden in its
rings that may or may not be classified
as moons in the future. It is the second
closest major moon though, second only
to Mimis. That means that its orbit
takes it just outside of Saturn's major
rings. Its orbit follows the planes of
the rings very precisely, and it only
takes 33 hours to orbit Saturn once.
Interestingly, it is in a 21-1 orbital
resonance with Dioni, Saturn's fourth
closest major moon. In other words, it
orbits twice around Saturn in the time
Dioni orbits once. This orbital
resonance is believed to prevent
Enceladus' orbit from ever becoming
perfectly circular, which causes
Enceladus to undergo tidal deformation.
This is significant as these tidal
forces heat up Enceladus' core.
You see, as far as we can tell,
Enceladus' surface is predominantly made
of clean water, certainly with little to
no rocks or much else there. Because
Saturn is situated so far away from the
sun, it means the outer layer of water
on Enceladus has frozen over. Enceladus
is essentially a frozen ocean world, a
giant ball of water ice.
Because it is free from other materials
on the surface, the moon is one of the
whitest objects in the solar system with
a bond albido of 0.81,
which is pretty much as high as snow. As
such, it is one of the coldest
satellites of Saturn with a noon
temperature of -200°
as the white color of its surface
reflects a large percentage of the
sunlight reaching it back into space.
However, about 30 to 40 km down under
the surface of Enceladus, pressures
start building and heat energy generated
from the tidal defamation of its orbit
has increased the temperature of the
water ice to the point where the water
at this depth can exist in a liquid
form. It could well be that there is an
entire mantle or a global ocean of water
that the ice crust is resting upon, very
much like the magma mantle that our
rocky crust on Earth rests upon. At the
very least, scientists expect there to
be a huge pocket of water under the
moon's south pole. How do we know this?
Well, the most obvious indication are
the huge plumes of water being ejected
from the cracks in the crust. Something
referred to as water or cryo volcanism.
These jets are really active,
consistently blasting around 250 kg of
water into space every second at speeds
exceeding 2,000 kmh.
This is powerful enough that most of the
water vapor particles escape Enceladus's
weak gravity, and they end up in orbit
around Saturn, forming Saturn's E-ring.
This ring around Saturn is very diffuse,
and so isn't really visible unless it is
backlit by the sun.
From this angle, the light shining
through the water particles make the
ring appear exceptionally blue. In fact,
this ring is considered the bluest
object in the solar system, even more so
than Neptune due to the ring's
uniformity.
The E-ring is Saturn's second outermost
ring, and it is 2,000 km wide. Its shape
is also heavily influenced by the orbit
of Enceladus. Enceladus's plumes create
tendril shapes in the rings as more
material erupts out of it. However,
these sections of the ring tend to
smooth off as Enceladus moves further
away along its orbit.
During the course of Cassini's mission,
Cassini was able to pass through these
plumes to detect the substances being
ejected from them.
Cassini wasn't designed with this in
mind. Scientists didn't know about the
plumes until Cassini got there. However,
Cassini was equipped with an instrument
called the cosmic dust analyzer designed
to detect what the tiny dust grains in
orbit around Saturn are made of. And it
was able to use it for Enceladus'
plumes, too.
As it wasn't specifically designed with
this in mind, it might not have given us
the full picture of what's in these
particles. But while water was the
predominant substance detected, amino
acids, carbon dioxide, nitrogen, and
methane were also found. Amino acids are
significant as they are the building
blocks of life and can be found around
the thermal vents at the bottom of
Earth's oceans. Does this mean Enceladus
has thermal vents of its own? And if so,
do they have ecosystems of life around
them?
While evidence for an underground ocean
is abundant, scientists still aren't
completely sure about Enceladus's
internal structure. At some points in
the past, scientists believe that
Enceladus was water all the way through.
However, data from Cassini suggests that
Enceladus's mass is in fact greater than
previously thought, meaning it must have
some amounts of iron or silicut material
in its core. Scientists are starting to
lean towards the theory that the
internal structure is differentiated,
meaning it's a celestial body with
defined layers within it. An object of
this size really doesn't have to be
differentiated. In fact, it's so small
at only about 500 km across that it is
right on the borderline of being in
hydrostatic equilibrium, or in other
words, being rounded by its own gravity.
There are a number of objects out there
of similar or smaller sizes that are not
in hydrostatic equilibrium, like
Neptune's Proteus. In any case, assuming
it does have a differentiated interior,
this core is likely to be predominantly
rocky. This is important as thermal
vents in Enceladus' water ocean would
have to come from a rocky core. A rocky
ocean floor would also provide nutrients
and minerals essential for what we
believe life would need to form and
evolve. Thermal activity clearly does
exist due to the way Enceladus has
plumes in the first place and the amino
acids detected in the plumes suggest a
rocky core.
As Cassini passed over Enceladus, it
also mapped out the thermal emissions
from the moon. It turned out that the
jets line up with what has come to be
known as Enceladus's tiger stripes.
These are large depressions roughly 130
km long, 2 km wide, and 500 m deep. It
is believed that these are tectonic
fractures in the moon's icy crust.
What is really interesting about the
surface features of Enceladus is that
there are virtually no impact craters at
all over the southern hemisphere and not
many anywhere else.
This implies that Enceladus's surface is
very young. As while it does have a thin
atmosphere made up from the ejected
water from the plumes, this isn't nearly
enough to burn up asteroids before they
hit the surface. Some water from the
plumes obviously settles again on the
surface, which smooths it off over time.
This is another reason why Enceladus is
so round for such a small celestial
body.
In fact, apart from the tectonic
fractures and few craters, Enceladus'
topographical variation is really quite
minimal. There are no mountain ranges to
speak of, although there is what you
might call a rough terrain around the
South Pole if you zoom in far enough.
This is perhaps the highest resolution
image we have of its surface. And as you
can see, it really does appear like a
giant glacier. It's interesting to note
that even in this small view, there are
some smooth sections of ice, but also
jagged regions.
Over the north pole, the clear
difference is the number of craters
present there. While there are tectonic
fractures here, too, there are no plumes
on this side of the moon. So, the
surface here is clearly a lot older than
around the south pole, which is why it
has more of a crated surface. As Cassini
flew through the plumes around the South
Pole, it also took the opportunity to
image the surface closely around the
tiger stripes.
The surface here is pretty incredible,
unlike anything you would have seen on
Earth. It's like the surface has been
folded, squashed, and shifted around,
leaving these remarkable fracture lines
and formations in the surface ice. The
fact that Cassini was able to get so
close to Enceladus is a feat in and of
itself. It's fantastic that we can have
such a closeup view of something so far
away. Unfortunately though, since the
Cassini mission has ended, we no longer
have anything in orbit around Saturn
that can study Enceladus further. There
have been plenty of mission proposals in
the past, but all of them were cancelled
before they came to fruition. What would
be incredible is a probe that could
either make its way into Enceladus's
ocean somehow or at the very least
search the plumes for signs of life.
There is already a mission called
Dragonfly going to the nearby moon of
Titan. However, this won't have anything
to do with Enceladus.
So although a couple of mission
proposals are currently under review to
go there, that means we are
unfortunately still a couple of decades
away at least, which is a shame because
who knows what secrets lie in wait under
that crust.
So there we have it, a look at the
intriguing little moon of Enceladus.
I'm happy to announce we have a weekly
newsletter to keep up with all the
discoveries in our cosmos. And our
designer, Peter, has made the most
beautiful email you'll ever receive.
Sign up with the link down below. It's
the best way to stay connected between
videos, short, focused updates on what's
new and fascinating in space each week.
No spam, no filler, just the good stuff.
You'll get the latest news, visuals, and
insights delivered straight into your
inbox. If you enjoy Astramm videos,
you'll love this. Join the newsletter
and stay curious with us.