Saturn’s moon hides an underground ocean


A team of researchers from the universities of Arizona and Colorado at Boulder have found “compelling evidence” that Mimas, Saturn’s small moon known as ‘the Death Star’ for its resemblance to the imperial station of Star Wars, hides an ocean beneath its icy surface. In this way, the innermost moon of the ringed planet, almost 400 km in diameter, joins the list of satellites of Saturn and Jupiter in which life could have developed.

As the scientists explain in an article published in Icarus, the first clues that Mimas could be a «Secret Ocean World» it came years ago, when NASA’s Cassini spacecraft detected a slight wobble in its rotation.

An oscillation that could be due to the movement of a large mass of water trapped under the icy surface of the small moon. If confirmed, the researchers say, Mimas will be an entirely new type of world, and could mean that water, and thus the possibility of life, may be much more common in the Solar System than previously thought.

“If Mimas has an ocean -says Alyssa Rhoden, first author of the article- represents a new class of small ‘stealth’ ocean worlds with surfaces that do not reveal the existence of the ocean”.

live without photosynthesis

Here on Earth, life depends primarily on sunlight to survive, but there are some places where organisms can thrive in total darkness. A good example is the bottom of the oceans, where numerous organisms gather around hydrothermal vents that release heat and nutrients from the planet’s interior.

In those places, life does not depend on photosynthesis, but on chemosynthesis, taking advantage of chemical reactions to synthesize food. A fact relevant to our search for life beyond Earth, especially when it was discovered that Jupiter’s moon Europa and Saturn’s moon Enceladus harbored liquid oceans under their icy crusts.

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One of the greatest discoveries in planetary science of the last 25 years, in fact, is that our planetary system abounds with worlds with oceans hidden under surfaces of rock and ice. Examples range from the icy satellites of the giant planets, such as Europa, Titan, and Enceladus, to planets as distant as Pluto.

the sun is not necessary

Worlds like Earth, with oceans on the surface, must perforce reside at certain distances from their stars so that their temperatures allow the existence of liquid water.

But so-called ‘inner water ocean worlds’ (IWOWs) are found over a much greater range of distances, vastly expanding the number of potentially habitable places that exist in the galaxy. On those worlds, the energy needed to keep the water liquid does not come from the sun’s heat, but from the gravity of the planets they orbit.

Gravitational tides, in effect, dissipate the moons’ orbital and rotational energy as heat. And to match the interior structure inferred from the Mimas wobble, tidal heating inside the moon must be large enough to prevent the ocean from freezing, but small enough to maintain a thick icy layer.

moon of saturn

Using models of tidal heating, the team of scientists developed numerical methods to create the most plausible explanation for a 22-32 km thick steady-state ice sheet on top of a liquid ocean. According to Rhoden, “Evidence for an internal ocean simply arose from the more realistic ice sheet stability scenarios and observed oscillations”.

“Because the surface of Mimas has many craters -continues the researcher-, we thought it was just a frozen block of ice. IWOWs, such as Enceladus and Europa, tend to be fractured and show other signs of geological activity. But it turns out that the surface of Mimas was playing tricks on us, and our new understanding has vastly expanded the definition of a potentially habitable world in our solar system and beyond.”.

Fountain: Jose Manuel Nieves / ABC


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