Saturn has some famous moons, like Enceladus, a mysterious plume-spewing moon, and Titan,
the intriguing target of NASA's upcoming Dragonfly mission.
But what about Mimas, a moon best recognized for its resemblance to the Death Star from
Star Wars?
It turns out it might be hiding an ocean just beneath its surface, furthering the quest
for possible life in our solar system.
The potential discovery came about as researchers attempted to prove the opposite; that Mimas
was nothing more than an inactive, frozen rock.
After NASA's Cassini probe spotted an unusual wobble in the moon's rotation, researchers
believe Mimas, Saturn's smallest and innermost moon, offered the first clue that it could
be a stealth ocean world.
New research proposes that the wobble could be caused by the sloshing of a liquid ocean
trapped just beneath the icy surface of the moon.
If this is the case, researchers presume Mimas is an entirely different world.
The discovery of the tiny moon's secret ocean suggests that water, and the life it may support,
could be far more plentiful in our solar system than previously assumed.
If Mimas has an ocean, it represents a new class of small, 'stealth' ocean worlds with
ocean-free surfaces.
Scientists are familiar with Interior Water Ocean Worlds (IWOWs), such as Saturn's Enceladus
or Jupiter's Europa, but inner tidal processes tend to fracture their surfaces, displaying
other signs of geological activity.
On the other hand, Mimas appeared to be just a frozen block of ice at first sight.
It turns out its surface has been deceiving us all along, and our new understanding has
significantly broadened the definition of a potentially habitable world in our solar
system and beyond.
In 2020, scientists uncovered evidence of an underground ocean on Ceres, a dwarf planet
orbiting the sun between Mars and Jupiter.
Salt deposits on Ceres' surface appear to have risen from the liquid water beneath it.
These ocean worlds could have deep-sea hydrothermal vents that create energy for living organisms.
If Mimas has an ocean, it might be able to support such ecosystems too.
To investigate the possibility of a hidden ocean under Mimas' frozen surface, the researchers
created a model to test if Mimas' gravitational interactions with Saturn might produce the
tidal forces needed to heat the moon's interior.
The model is based on the fact that tidal processes keep the moon heated inside.
But it has to contend with two facts.
One is that the tidal heating has to be big enough to keep the ocean in liquid form, and
two is that it has to be small enough to keep the surface unfractured.
When the researchers plugged in figures that are reasonable for a world like Mimas, they
discovered that it is possible for a thick shell of 24–31 kilometers to exist over
a liquid ocean.
Most of the time when scientists create these models, they must fine-tune them to produce
what they observe.
This time, evidence for an internal ocean was provided by the most realistic ice-shell
stability scenarios and observed planetary wobbles.
One crucial result from the model is that the heat flow from the surface was linked
to the thickness of the ice shell.
When Juno flies by Europa in September, or when future missions to Europa and Ganymede
arrive there within the next decade, they would be able to make such measurements.
This could provide more circumstantial evidence of an ocean on Mimas.
However, it won't be confirmed until scientists return there.
Although the findings suggest the existence of a present-day ocean on Mimas, reconciling
the moon's orbital and geologic properties with our current understanding of its thermal-orbital
development is challenging.
Evaluating Mimas' status as an ocean moon would serve as a testbed for its formation
and evolution models.
This would aid our understanding of Saturn's rings and mid-sized moons, as well as the
presence of possibly habitable ocean moons on Uranus.
Mimas is an intriguing target for further research.