Alex McColgan

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 Mimas 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.

A notion 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 Valérie Leynet analysed the drift of MIMAS's orbit using data from the entire 13-year Cassini mission.

Just as MIMAS pushes on the rings, the rings and Saturn push back, causing MIMAS's orbit to precess.

Essentially, the oval of its orbit slowly rotates around Saturn.

The rate of this precession 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.

MIMOS 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 kilometres thick.

What's particularly interesting is that the researchers believe this ocean is incredibly young.

The models suggest it formed between just 2 and 25 million years ago.