The gas giant Saturn has 292
moons.
Yet within this spectacularly vast
system, there's one moon that's
instantly recognizable.
With a colossal impact crater dominating
its surface, Mimis looks more like the
Death Star than a natural icy satellite.
Its ominous crater was first spotted
when the Voyager probe sped past in
1980.
Almost instantly, scientists attributed
it to an ancient collision of epic
scale. But since then, the moon has
hardly changed. Compared with the likes
of the active moon Enceladus and Titan,
Mimis almost seems boring, relegated to
just another cosmic fossil. A simple
reminder of how violent the early solar
system must have been.
That is until 2024 when scientists found
that hidden beneath its ancient cratered
shell lies something that simply
shouldn't exist.
Mimis is not dead. It's an ocean world.
This discovery has transformed the Death
Star Moon from a celestial curiosity
into one of the most important objects
in our search for life beyond Earth. But
how did we get Mimis so wrong? And just
what lies beneath its surface?
I'm Alex Mccoan and you're watching
Astramm. Join me today as we peel back
the icy crust of Saturn's innermost moon
to reveal a secret ocean hiding in the
dark and delve into what this maritime
discovery on Mimis means for the rest of
the Saturn system.
Mimis is a tiny moon. It orbits around
186,000 km from its host planet Saturn.
And with a mean diameter of just 400 km,
it's actually the smallest astronomical
body we know of to have pulled itself
into a spherical shape due to its own
gravity.
For the most part, Mimis doesn't seem
that special. It's not overly dissimilar
to many of Saturn's other midsized moons
like Teis, Thion, and Ria. But what
makes it stand apart is the fact that
Mimis is a world whose history is clear
to see on its surface. And that history
is one of unimaginable violence.
The surface is heavily cratered, but one
feature is particularly prominent. Mimis
is dominated by the Hersel crater, an
impact basin 130 km wide.
To put that in perspective, the crater
covers nearly 1/3 of the moon's
diameter. If a crater of the same
relative scale existed on Earth, it
would be more than 4,000 km wide, larger
than the entire continent of Australia
with walls towering more than 150 km
into the sky.
To help you imagine just how crazy that
is, think of this. Mount Everest doesn't
even quite stretch to 9 kilometers.
The impact that created this epic crater
was a cataclysmic event that brought
Mimis to the very brink of destruction.
Simulations suggest that if the impact
had been just a little bit larger or
moving slightly faster, Mimis would have
shattered completely, reduced to just
another ring of debris orbiting Saturn.
Luckily for us though, the moon
survived.
Shock waves from the impact traveled
through the moon's core and focused on
the exact opposite side, the antipodal
point, where they ripped the crust
apart, creating a complex network of
canyons and fractures known as kasmata.
For decades, planetary scientists looked
at this battered surface and concluded
that Mimis must be frozen solid. A warm,
slushy interior would have relaxed over
time, smoothing out the crater walls and
erasing the scars. The fact that the
Hersel crater still stands with walls 5
km high and a central peak reaching up
to 6 km suggests that the ice shell was
rigid and immensely thick, supporting
these massive structures for billions of
years. Essentially, everything was
pointing to Mimis being a geologically
dead world. That is until scientists
found something strange happening on the
surface.
Their clue that they'd got something
wrong came later through the Cassini
mission. This makes sense. Reviewing
information is the best way to see if
your initial assumptions are mistaken.
to all Brilliant has on offer. Plus, there's a 30-day free trial. For Mimis
though, there was no learning platform
to show scientists where they'd gone
wrong. They had to figure it out with
Cassini.
In 2010, NASA's Cassini spacecraft swept
past Mimis and turned its composite
infrared spectrometer towards the moon
to map its surface temperature.
Scientists expected to see a smooth
gradient, warmest at the equator where
the sun was overhead and fading to cold
near the poles.
Instead, they found Pac-Man. Yes, you
heard me correctly. The thermal map
revealed a sharp V-shaped boundary on
the moon's leading hemisphere. Inside
the mouth of the shape, the temperatures
dropped to an icy - 196° C, while the
surrounding regions were a relatively
balmy - 181°.
It was a thermal footprint that looked
exactly like the 1980s arcade icon
eating a dot. The dot in this case being
the Hersel crater. Even more surprising
when you look at Mimis invisible light.
This difference completely disappears.
The surface looks totally uniform.
Nothing at all correlates to the anomaly
we observe. This led astronomer Dr. John
Spencer to playfully joke that Mimis
might actually be the Death Star and
that Darth Vader simply applied a clever
coat of paint to fool our visible light
cameras, hiding the truth from
everything except the prying infrared
eyes of Cassini.
Now, this thermal anomaly wasn't exactly
a sign of internal heat, but rather a
sign of how Mimis interacts with its
environment. You see, Mimis orbits right
inside Saturn's radiation belts. The
moon is constantly bombarded by high
energy electrons trapped in Saturn's
magnetic field. These electrons slam
into the leading face of Mimis, welding
the powdery snow regalith into hard
packed ice crystals, a process
calledining.
Now, this hard ice has high thermal
inertia. That means that just like a
dense rock on Earth, it absorbs heat
during the day and conducts it deep
underground, leaving the surface cool.
At night, that stored heat radiates back
out. The mouth of Pac-Man on the
trailing side is sheltered from this
bombardment, so the surface there
remains relatively fluffy and powdery,
acting like an insulator that heats up
quickly in the sun, but holds no heat at
night.
This solved the mystery of the thermal
map. But in doing so, it also reinforced
the idea that Mimis was a passive
object. The anomaly was due to surface
effects rather than anything internal.
But whilst Mimis is known for its
surface features, it's governed by
something else. Its gravitational dance
with Saturn. An interaction that creates
one of the most famous features in the
solar system.
If you look at Saturn through a
telescope, you'll see its glorious ring
system. And if your telescope is good
enough, you'll see a dark gap splitting
the rings in two. This is the Cassini
division, a 4,800 km wide chasm
separating the A- ring from the B- ring.
This gap exists largely because of
Mimis.
Mimis orbit Saturn once every 22 hours
and 36 minutes and particles orbiting
within the inner edge of the Cassini
division circle Saturn exactly twice for
every one orbit of Mimis. This is a 21
orbital resonance. Every time Mimis
completes an orbit, it tugs on these
ring particles at the exact same point
in their path. It's like pushing a child
on a swing. If you push at the right
moment every time, the swing goes higher
and higher. Mimis' gravity adds energy
to these particles, stretching their
orbits into ellipses until they collide
with other particles or are ejected from
the gap entirely.
For years before Voyager flew past in
1980, revealing the Hershel crater, this
resonance was the primary claim to fame
for Memphis. It was the Shepherd of the
Rings, the gravitational influence that
kept the Cassini division clear. But
recent simulations have suggested that
Mimis didn't just clear a pre-existing
gap. Instead, it may have acted more
like a snow plow.
Standard planetary system models suggest
moons migrate outward over time. But in
2019, when researchers Kevin Bailey and
Benoir Noyel were attempting to explain
why the Cassini division was so wide,
they found this couldn't be the case
here. Their theory instead suggests that
Mimis migrated inward towards Saturn
somewhere between 4 and 11 million years
ago. As it did so, its resonance moved
with it, pushing ring particles aside
and carving out the division over
millions of years. This interaction is
crucial because it tells us that Mimis
is not dynamically static. Its orbit
changes. And it was this orbital
movement, specifically the way Mimis
wobbles, that led to the most recent
revelation about this tiny world.
Before we get into exactly what
scientists found about Mimis, we have to
address an elephant in the room.
Enceladus.
Enceladus is Mimis' neighbor. It's
roughly the same size, composed of
similar materials, and orbits just
outside Mimis, but Enceladus is
spectacular. It has tiger stripe
fractures at its south pole that blast
gizes of water vapor into space,
creating Saturn's earring. And we now
know Enceladus has a global subsurface
ocean that is warm and potentially
habitable. I have another video on the
latest discoveries here if you want to
take a look. The problem is that
according to standard tidal heating
models, Mimis should be the active one.
Mimis is closer to Saturn than Enceladus
and its orbit is much more eccentric.
Both of these factors mean that tidal
heating, the friction generated inside a
moon as it gets stretched and squeezed
by gravity should be much stronger. The
math says that Mima should be
experiencing significantly more tidal
heating than Enceladus.
So why was Enceladus melting while Mimis
looked like a frozen block of ice? This
is what researchers have long called the
Mimis paradox. The prevailing theory was
that Mimis was simply too cold. If a
moon freezes solid, its ice becomes
rigid and it can't flex. This means that
it doesn't generate friction. Instead,
entering a state of high Q or quality
factor, which implies low dissipation.
Imagine hitting a bell. The energy isn't
absorbed. Instead, the bell rings.
Enceladus, for whatever reason, stayed
warm and slushy, allowing the tides to
keep adding energy to its interior.
Mimis, on the other hand, we assumed,
missed its window. It froze early and
stayed frozen.
But that isn't the end of the story. In
2014, researchers noticed something
wasn't quite right. They measured the
vibration of Mimis, the slight wobble it
experiences as it rotates. This wobble
was almost twice as big as it should
have been for a solid body. Now, at the
time, there were two possibilities.
Either Memphis had a strangely shaped
core, elongated like a rugby ball, or
its ice shell was physically
disconnected from its core, sliding
around on top of a liquid layer. Most
scientists bet on the rugby ball, an
ocean just didn't make sense given the
thick ancient crust. But then came 2024.
In February of 2024, a study published
in Nature Astronomy put the debate to
rest. A team led by Valerie Le analyzed
the drift of Mimis' orbit using data
from the entire 13-year Cassini mission.
Just as Mimis pushes on the rings, the
rings and Saturn push back, causing
Mimis' orbit to process. Essentially,
the oval of his orbit slowly rotates
around Saturn. The rate of this
procession depends heavily on the
distribution of mass inside the moon.
When the researchers plugged the data
into their models, the rugby ball core
theory fell apart. It couldn't explain
the so-called orbital drift I just
described. There was only one solution
that fit the data perfectly. Mimis has a
global subsurface ocean, but this isn't
an ocean like the one on Europa or
Enceladus. It's what I'm going to call a
stealth ocean. It lies beneath an ice
shell that is 20 to 30 km thick. What's
particularly interesting is that the
researchers believe this ocean is
incredibly young. The model suggests it
formed between just 2 and 25 million
years ago. This young age of the ocean
is crucial. In fact, it explains
everything. The ocean is so new that the
heat hasn't had time to radiate through
the thick ancient crust. Essentially,
the moon is melting from the inside out,
and features that formed long before the
ocean formed, like the Hersel crater,
haven't yet been impacted. This
discovery resolves the Mimis paradox.
Mimis isn't dead. It's just late to the
party. You might even say it's an
Enceladus in waiting. Given enough time,
the ocean will continue to eat away at
the ice shell. In a few million years,
the stresses will become too great. The
crust will crack and Mimis might begin
to spray its own gizers into the
Saturnian skies.
Now, the realization that Mimis has a
young ocean leads to all sorts of
fascinating possibilities. But there is
one gaping question. Where did it come
from?
It turns out that the most likely
culprit is actually its orbital
instability. At some point in the recent
past, perhaps just 10 million years ago,
Mimis likely entered a chaotic resonance
with another moon, possibly Teisi.
This interaction would have pumped up
Mimis' orbital eccentricity, stretching
its orbit into an even more extreme
oval. As Mimis swung closer to and
further from Saturn, the immense tides
were now powerful enough that they began
to flex the moon's solid interior. The
friction generated heat, enough heat to
melt the ice and create a global sea.
And then there's the even bigger
question I just know you've all been
waiting to ask me. Could this be another
potential place beyond Earth to look for
life? Sadly, this ocean is likely to be
a fleeting phenomenon. The very act of
sloshing water around inside the moon
dissipates energy. This acts as a break,
circularizing the orbit. As the orbit
becomes more circular, the tidal
stretching stops and the heat source
turns off. This is also how we know that
the ocean must be new. If it was
ancient, Mimis' orbit would already be
circular. Our best guesses put this cool
down happening somewhere on the scale of
millions of years. But who knows that
may or may not be long enough for life
to get going.
We are lucky to be living in the brief
window where Mimis is an ocean world. As
its orbit settles down, the ocean will
eventually begin to refreeze and this
refreezing will be destructive. As water
turns to ice, it expands. This expansion
will push outward on the crust, likely
shattering it and creating massive
canyons similar to those we see on
Sharon or Dioni.
It turns out that the mimis we see today
is a snapshot of a world in transition.
A change that will eventually destroy
the moon as we know it.
The story of Mimis is a humbling lesson
in planetary science. For centuries, we
saw a cratered gray wasteland and
assumed it was devoid of activity. We
saw the Death Star and assumed it was a
bringer of destruction, not a cradle of
potential habitability.
But nature is far more creative than our
assumptions. The confirmation of an
ocean on Mimis suggests that liquid
water might be far more common than we
ever dared to dream. If a small
unassuming world like Mimis can hide a
global ocean, what about the moons of
Uranus? What about the objects in the
Kyper belt? Mimis has taught us that
even the dead worlds might have a
heartbeat. We just have to look close
enough to find it.