Space: Discovery of the ocean beneath the surface of Mimas, the real Death Star

When Star Wars, the first installment in the original Star Wars trilogy, was released in 1977, science fiction fans were reeling from the attacks of the Death Star, a spherical ship that owes its destructive power to a superlaser located in the vast cavity of its northern hemisphere, of course. its most distinctive feature.

Surprise! Three years later, in 1980, the Voyager 1 space probe discovered a disturbingly similar hole on the surface of Saturn’s moon Mimas… and it’s not a movie anymore. What ? The Empire Strikes Back in Our Solar System too? Fortunately, there are no destructive weapons here, but a giant impact crater with a diameter of 139 km, soon named Herschel (after the man who discovered this moon of Saturn in 1789). Let’s put it this way: the blow hit Mimas, and not vice versa, and this could have been fatal for her.

If its resemblance to the fictional vessel has become notorious, today’s discovery has brought some clarity to this small celestial body. We learned Wednesday in the journal Nature that beneath this moon’s thick icy crust lies an ocean of liquid water. Instead of causing death, Mimas would actually have an important resource, albeit insufficient, to create life. “Ultimately, this is an object that looks more like Genesis Planet. Star Trek “, says Valerie Laney, an astronomer at the Paris Observatory and lead author of the study.

Riddle of the Rings

At the heart of this discovery is a mystery, the source of which is also located in the 14th arrondissement of the capital. It was in 1675, when the first director of the Paris Observatory, Jean-Dominique Cassini, discovered a darker cut in the rings of Saturn, clearly dividing them into two parts. “This is a 4,500 km wide structure with very few ice particles,” explains Valerie Laney.

In recent years, a hypothesis has become widespread to explain the division of Cassini: it was Mimas, playing “gravitational snowplow”, that repelled the particles. However, there is one catch: over time, the rings should have regained a more uniform structure. If this is not the case, it is because Mimas came close to Saturn in the relatively recent past. Understand: over the last ten million years.

A migration that could cause a thermal shock sufficient to melt the star’s internal ice. However, Mimas’s profile does not fit this scenario: the cratered moon does not suggest any activity beneath its surface. Do you have a very young ocean? It is the study of its rotation that will gradually lead scientists along this path. By studying hundreds of images from the Cassini probe (him again), they will discover changes in its trajectory that could only be caused by a hidden ocean.

Mimas thus joins a small family of five moons of Jupiter and Saturn (along with Enceladus, Europa, Ganymede and Titan) that have liquid water beneath their surface, here at least 20 km deep. On the seafloor, the interaction of water and a rocky core may have contributed to the emergence of life.

Valerie Laney’s team is now calling for a space mission to be sent there to find out for sure. “This is a very good, very exciting result,” enthuses Fabien Casoli, president of the Paris Observatory. And, like a thriller, the adventure is full of unexpected twists.