In comparison with other large moons of Saturn (and Jupiter), the satellite Mimas is not replete with cracks and breaks, reminiscent of our Moon with its craters. Thus, it should be a dry world of rocks, but this seems to be the case differently. Mimas has a strange orbit, as if it has something sloshing around inside it, or its core has an unusually elongated shape. As modeling has shown, everything speaks in favor of a hidden ocean, and this is a godsend for scientists.
Artist's representation of Saturn's moon Mimas. Image source: Observatoire de Paris
Detailed data on the Saturn system was collected by the Cassini automatic station from 2004 to 2017. A team of scientists from the Paris Observatory used this information to reassess the orbital parameters of Mimas, which resembles the Death Star thanks to the huge impact crater on its surface.. They wanted to rule out one scenario that would make the moon's orbit look unusual for a monolithic, rocky celestial body.
According to one option, Mimas may contain a highly elongated core, which causes it to undergo oscillatory movements as it passes through its orbit.. In the second case, a global water world may be hidden under its rocky surface, the flows of which also cause changes in the orbital motion of the satellite.
Modeling has shown that the existence of an elongated core seems to be the least likely scenario. Given the dynamics of Mimas's orbital motion under the influence of the gravity of Saturn and its other largest moons, the orbital parameters of the suspicious moon are most likely explained by a liquid subsurface ocean.
Calculations show that the liquid ocean on Mimas is relatively young — it is only 2-3 million years old. Most likely, shortly before its appearance, the orbit of this moon changed from a stable circular one to an elongated one, which is considered normal in a system with many moons. The gravity of Saturn began to have an intermittent effect on the bowels of Mimas, and this led to gravitational heating of its core and internal structure. Water began to be released in liquid form and gradually a global subsurface ocean formed there, which has now approached the surface of Mimas at 20–30 km.
Clockwise from top left: Enceladus, Europa, Ganymede and Titan. In the center is Mimas. Image source: Observatoire de Paris
From the appearance of this moon you cannot tell that masses of water are splashing under its crust, much more than in the earth’s oceans. Mimas does not have cracks and geysers, like the moons Enceladus, Europa, Ganymede and Titan, so it kept its secret for a long time. Equally important, if there really is a global ocean out there, its youth is a way to peer into the past of the other moons of Saturn and Jupiter to understand the evolutionary development of subsurface water worlds. The early geological history of these worlds can literally take place before the eyes of scientists, which scientists are incredibly happy about.