Alex McColgan

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.

Mimas 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 Mimas so wrong, and just what lies beneath its surface?

I'm Alex McColgan and you're watching Astrum.

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 Mimas means for the rest of the Saturn system.

Mimas 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, Mimas doesn't seem that special.

It's not overly dissimilar to many of Saturn's other mid-sized moons like Tethys, Dione and Rhea.

But what makes it stand apart is the fact that Mimas 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.

Mimas is dominated by the Herschel crater, an impact basin 130km wide.

To put that in perspective, the crater covers nearly one-third of the moon's diameter.

If a crater of the same relative scale existed on Earth, it would be more than 4,000km wide, larger than the entire continent of Australia, with walls towering more than 150km into the sky.