This perspective illusion must therefore be due to the different distance at which the Cassini probe was from the two moons at the time of shooting. The distance between Titan and Cassini was indeed 1.1 million km, while that between Dione and the probe was just 130,000 km.
The two moons, in addition to being so different in size, also differ greatly in their physical nature.
Dione is essentially an icy body, with a surface shaped by the numerous meteorite impacts that have occurred throughout its history. It is believed that in the past Dione was geologically active, as evidenced by some fractures of a cryovolcanic nature, but today there are no longer any signs of geological activity.
Titan, on the other hand, is a unique body in the Solar System. Composed of water ice and rock material, it is surrounded by a thick atmosphere composed mainly of nitrogen which prevented the observation of its surface until the arrival of the Cassini mission.
The spacecraft found that its surface is geologically very young, with many mountains and few craters. Furthermore, lakes of liquid hydrocarbons have been discovered: the temperature on Titan is in fact very close to that of the triple point of methane, which therefore can exist simultaneously in the liquid, solid and gaseous state (similarly to what water does on Earth).
Credit: NASA, JPL, SSI.
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