Dione: Saturn's Icy Moon with Buried Secrets and Frozen Landscapes

Dione, an icy world with ancient origins

Dione, Saturn's fourth-largest moon with a diameter of 1,123 km, was discovered in 1684 by the astronomer Jean-Dominique Cassini (1625-1712). It orbits Saturn at an average distance of 377,400 km, with a revolution period of 2.7 days. Its surface, primarily composed of water ice, features bright cliffs and chasmata, suggesting past geological activity.

Unlike Enceladus, Dione shows no active cryovolcanism, but data from the Cassini mission (2004-2017) revealed traces of a subsurface ocean beneath its icy crust, about 100 km deep. This ocean, kept liquid by Saturn's tidal forces, could harbor conditions suitable for prebiotic chemistry.

Physical characteristics and composition

With a density of 1.48 g/cm³, Dione is mainly composed of water ice (60%) and silicate rocks (40%). Its surface is marked by:

• Impact craters such as Evander (350 km in diameter),
• Tectonic fractures (e.g., Janiculum Dorsa),
• Deposits of organic matter detected by spectroscopy.

Surface temperatures hover around -186 °C, but thermal models suggest that its subsurface ocean could reach 0 °C near the rocky core.

Origin and geological evolution

Two hypotheses dominate to explain the formation of Dione:

1. Accretion in the Saturnian nebula: Dione may have formed 4.5 billion years ago from the gas and dust surrounding Saturn.
2. Giant impact: A celestial body may have collided with Dione, creating its subsurface ocean (hypothesis proposed by Francis Nimmo (1970-) in 2010).

The lineae visible on its surface indicate possible past tectonic activity, linked to gravitational interactions with Saturn and Rhea.

Exploration and future missions

The Cassini probe made 5 flybys of Dione between 2005 and 2015, revealing:

• A thin exosphere (10^-10 bar),
• Energetic particles trapped in Saturn's magnetosphere,
• Indirect evidence of a global ocean (studied by Mikael Beuthe (1980-)).

The Dragonfly mission (NASA, planned for 2034) could include observations of Dione from Titan, while the ESA is considering a dedicated mission to Saturn's icy moons (Enceladus Orbilander).

Sources: NASA - Dione Overview, ESA - Cassini-Huygens Mission, Nimmo et al. (2016) - Icarus.