|Atlas and Pan, Saturn's most intriguing 'moons'|
Both the moons are shepherd satellites, part of a small group of moons that are said to maintain Saturn's rings' sharp and well-defined edges. But the appearance of these two moons is not at all what you would expect if they had formed naturally. They look extremely unnatural, in fact.
They probably are.
Both moons are likely to be the long abandoned remains of two advanced generation ships. These vast vessels, capable of supporting hundreds of thousands of individuals for tens of thousands of years, were probably in the Saturn system attempting to harvest anti-matter to use as fuel, perhaps for their Alcubierre propulsion system. Such as propulsion system for such large spacecraft would require a significant amount of anti-matter, but obtaining this amount would be feasible at Saturn (or indeed Jupiter). It's estimated that up to a kilo of anti-protons enters the Solar System every second, which will become concentrated in the magnetic fields of the giant planets. Despite this, it does seem that Atlas and Pan were not able to collect enough in time to escape.
Over many millions of years the spacecraft have attracted a lot of dust and debris from Saturn's rings, which is why their appearance is now moon-like, at least in their bulging central sections.
The diagram below shows the possible interior of Atlas (click to enlarge). Pan is no doubt similar:
A mission to those moons is long overdue. A lander is required, one that can drill deep into the crust down to the hull beneath. And later a manned mission to enter one of the giant ships. Only then will we begin to understand why they visited Saturn, and why they had to stay. Perhaps we will find the occupants, some even surviving in deep hibernation awaiting rescue. And we may be able to reverse engineer the technology we find to accelerate our own advancement towards interstellar travel.
It's entirely possible, of course, that the ship, although stranded, is otherwise functioning perfectly normally (the anti-matter it managed to acquire could be more than enough to run the ship's systems for millenia, even though the Alcubierre propulsion system is not able to function). It's occupants could be living out their lives, generation after generation, totally oblivious to their predicament, trapped in the 'planned ignorance' phase of their journey by the ship's controlling intelligence (such a phase would continue to avoid the risk of some occupants developing the desire to escape. Only when the destination system was reached would the current generation receive education on the ship's purpose and their colonisation mission). Finding such an orbiting colony would be a momentous discovery.
The overriding reason for such a mission is that it may be essential to our immediate survival. If the 'Atlas' and 'Pan' generation ships have indeed collected some anti-matter their storage facility may have degraded and be at risk of failure. Such a failure would set off an anti-matter explosion that could be catastrophic, even here on Earth, and prematurely end our civilisation.
It's a chilling prospect, and one that should not be ignored.