Saturn's small, crater-covered moon Mimas may have a vast ocean under its thick icy surface, according to a new paper.

Why it matters: The discovery suggests the conditions under which oceans — thought to be promising places to look for signs of life — can exist may be more varied than previously thought.

The backstory: In earlier work, study co-author Valéry Lainey, an astronomer at the Observatoire de Paris in France, and his colleagues described two possible explanations for Mimas' odd orbit.

• It either has a very elongated rocky core or an ocean under its surface that allows the planet's outer shell to move separately from its core, resulting in the rocking motion the astronomers observed.

What they found: The team analyzed thousands of images of Mimas and other moons of Saturn collected over 13 years by the Cassini mission and concluded the rotation and orbit of Mimas could only be explained by an ocean hidden beneath the moon's icy shell.

• That ocean, the researchers report in Nature, is about 70 kilometers (or roughly 45 miles) deep and believed to make up more than 50% of the moon's volume.
• They also think the ocean formed within the last 25 million years. "At the beginning of the Cassini mission, we thought this object was 4 billion years old and never changed," Lainey says.

What's surprising about Mimas is that it "doesn't look at all to have a global ocean," Lainey says.

• "It doesn't look like Enceladus. It doesn't look like Europa or Ganymede," he says, listing moons of Jupiter and Saturn, along with Titan, known to have oceans. "It looks like a very old, heavily cratered, geologically inactive satellite."
• The finding "makes you think that maybe you have habitability conditions in many, many, many other objects in the solar system," he says.