Saturn’s “Death Star” moon has a hidden secret

Written by Will Dunham

WASHINGTON (Reuters) – Mimas, a moon of Saturn, is famous for its uncanny resemblance to the terrifying Death Star in the original “Star Wars” movie. But it also has another interesting distinction, according to the researchers: a subterranean ocean hidden beneath its icy, cratered outer crust.

Astronomers said on Wednesday that data obtained by NASA’s Cassini spacecraft on the rotational motion and orbit of Mimas confirm the existence of an ocean of liquid water under an ice layer 20 to 30 kilometers thick. They said this ocean appears to have formed recently, in cosmological terms, less than 25 million years ago, and likely between 5 and 15 million years ago.

The results make Mimas an attractive location to explore the conditions that could lead to the formation of life, considering that the first organisms on Earth arose in our planet’s primordial seas billions of years ago.

“At first glance, it seems to be the most likely place in the solar system to look for liquid water,” said Valery Liny, an astronomer at the Paris Observatory and lead author of the study published in the journal Nature. “It looks old and inactive – a huge amount of craters. Nothing that shows the presence of the ocean at the moment.”

Lainey said that Mimas will be the smallest of five moons in our solar system with compelling evidence of the existence of subsurface oceans, along with Enceladus and Titan of Saturn, and Europa and Ganymede of Jupiter. There are also suspicions that subsurface oceans exist on a few other moons, including Jupiter’s Callisto.

Cassini ended 13 years of study of Saturn and its moons in 2017 with its plunge into the massive ringed planet’s atmosphere. Cassini’s observations of Mimas’ icy surface did not detect any distortions indicating an ocean below. But researchers have determined that certain aspects of its orbit can only be explained by the presence of an inner ocean, rather than a solid interior.

Mimas is the seventh largest moon of Saturn, which has more than 100 moons ranging from Titan – larger than the planet Mercury – to some only the size of an apartment building.

Mimas, which is not perfectly circular, has an average diameter of about 250 miles (400 km). It is tidally locked, meaning it always shows the same side toward Saturn, as our Moon does toward Earth. Mimas’s most iconic feature is Herschel Crater, which extends a third of the way up its face and makes it resemble the Death Star.

Earth’s moon is about 2,000 times larger than Mimas.

The presence of an inner ocean indicates that there was a strong heat source within Mimas that turned the ice into an ocean. Mimas follows an elliptical orbit around Saturn at an average distance of about 115,000 miles (186,000 km). As the distance from Saturn changes along its orbit, the gravitational and tidal forces exerted by Saturn also change.

“This leads to a periodic deformation of the interior of Mimas, and part of the energy involved in these deformations is converted into heat,” said planetary scientist and study co-author Gabriel Toubi of the French Scientific Research Agency and the University of Nantes.

Liquid water within Mimas represents more than half of this moon’s total volume, although it is only 1.2-1.4% of the amount found in Earth’s oceans. They added that the fact that water is in contact with Mimas’ rocky core may facilitate the kind of complex chemistry that could pave the way for life.

Scientists believe that the basic ingredients for life – warmth, water and organic compounds – are present in the Saturnian system on the moon Enceladus, from whose surface huge plumes emanate. Even if Mimas also contains these components, the fact that its ocean is so small “may pose a challenge to the evolution of life,” Tobey said.

“However, no one knows how long it takes to live out of a suitable environment,” Toby added. “Mimas may offer a unique opportunity to explore the first stage of life’s evolution.”

(Reporting by Will Dunham, Editing by Rosalba O’Brien)

(Tags for translation)Liquid water

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