Saturn’s rings and one of its moons, Dione (Cassini).

Even the little frozen satellites of the outer solar system harbor interesting secrets.

Saturn has more than 80 frozen moons that are not particularly striking at first glance. After all, most are clumps of ice and rock with a diameter of, at most, several tens of kilometers. However, the largest satellites of Saturn still hide very interesting secrets … And one of them is Dione.

A frozen moon
Dione is a world about 1,100 kilometers in diameter that is made up of a rocky core surrounded by a crust of ice so thick that frozen water is estimated to account for two-thirds of the mass of this satellite of Saturn. In addition, there are also indications that there could be an ocean of liquid water hidden under the frozen crust of this satellite. The existence of this inner ocean is deduced both from the peculiarities of Dione’s gravitational field and from the “aura” that has been detected around it, which suggests that the moon is surrounded by a fine haze of ice particles spitting out from one or more points on its surface.

Although it may seem far-fetched to propose that there is an ocean inside a satellite whose surface temperature is around -186ºC, it is a perfectly possible phenomenon that has been detected in other celestial bodies. One of them is Enceladus, another moon of Saturn in which the water that hides under its crust is ejected into space by cryovolcanoes on its surface. In fact, the ice and salt particles that this satellite ejects are the main source of material for one of Saturn’s rings.

However, the evidence available so far did not seem robust enough to say with any certainty that Dione has an ocean under her frozen crust. But a new study has provided new evidence that supports this satellite’s hidden ocean hypothesis.

Bright grounds
Despite Dione’s small size, its crust presents a highly varied geography full of canyons, ridges, depressions and craters. But a particularly striking feature of this satellite are irregular “lines” that are present on the opposite side of the direction of its movement around Saturn and that reflect much more light than the surrounding terrain. In addition, there are two details that suggest that these clear lines must have formed recently: they “pass through” other landforms (such as craters) and their light color indicates that their material has only been exposed to space radiation for a short time.

In the 1980s it was postulated that these lines were material that had been expelled from within Dione and that had fallen to the surface as if it were snow. However, the most recent observations suggest that we are before a slightly more exotic formation scenario.

Inner water movement
The constant bombardment of space radiation produces a curious effect on the ice on the surface of Dione: when cosmic particles collide with their water molecules at high speed, the collision releases one of the hydrogen atoms of this substance and it diffuses through the ice. The result of this constant irradiation is that the crystalline structure of Dione’s ice becomes “out of order” over time.

Well, the authors of the study in question have managed to measure what fraction of the ice is crystalline and amorphous along the surface of Dione and have discovered that the frozen water that makes up the bright lines of this satellite is highly ordered compared to that of the terrain that surrounds them. That can only mean that it is a material that has been exposed to space radiation for a short time. Less than 200 million years, to be more specific.

According to this new study, the hypothesis that best seems to explain the existence of these clear lines is that hotter material from the depths of the satellite has risen to the surface. By increasing the temperature of the ice, the water molecules that compose it would have rearranged and recovered their original crystalline structure, “undoing” the imperfections produced by radiation over millions of years.

This type of phenomenon can only occur if there are heat transport mechanisms taking place under the frozen crust, which, in turn, would be an indication in favor of the existence of a possible large ocean of liquid water within it. In this case, the heat needed to keep that groundwater in a liquid state likely comes from gravitational interactions between Dione, Saturn, and the closest moons.