[Music]
You might not know much about Tetus, but
you should know this. The moon called
Teetis 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 Teis,
Saturn's scarred survivor.
Teis's 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 customer of the time,
these were initially dubbed the Siderea
Lordier or stars of Louie for King Louis
the 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. Tethus,
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 Teetus is
composed almost entirely of water ice
with a small amount of rocky material
integrated within. It suggests Teus
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 Tethus a
quintessential example of an outer solar
system ice world. Physically, Tethus
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 Ipotus
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 Teis 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 Tetus' internal evolution,
suggesting that it may have formed early
in the moon's geological history. As TE
is 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 ally 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. Iithacasma therefore
isn't just a spectacular site. It's a
record of Tetus' 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 magnetosphere.
Only not all of those micrometeoroid
impacts were micro. One was large enough
that it nearly shattered frozen Tethus
like a bullet through a snow globe. It's
time to take a look at the Odysseyus
crater.
If Ithaca Kasma speaks to internal
forces, the Odysseus crater screams of
external violence.
Located on Teas' leading hemisphere,
Odysseus is the moon's largest impact
basin, measuring approximately 450 km
across. That's more than 2 fths the
diameter of Tethus 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 Teus' 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 Teetus' 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 Teetus' 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.
through my link in the description. And now back to Teis.
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 Tetris 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 Teis' 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.
[Music]
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' 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.
Tethus 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 Teis, and studying the
two helps us better understand these
dynamic interactions.
Sadly, there are no new missions to
Tetis currently planned. But Tetis 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. Teetus 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.