Saturn’s moon Titan is perfectly placed
A safe spot in orbit prevented the mammoth moon from being swallowed whole “Finishing with just a single large moon, however, seems much more difficult and leads to the question of how Titan formed”
As gas giant planets in our Solar System, Jupiter and Saturn are pretty similar to each other. They have similar compositions and are both several hundred times more massive than Earth, and so very much in a class of their own even compared to the ice giants Uranus and Neptune.
When it comes to their moons, however, the Jupiter and Saturn systems are wildly different from each other. While both planets have a family of around 80 moons overall, the mass distribution is very different. Jupiter has the four large Galilean satellites – Io, Europa, Ganymede and Callisto – all roughly the same size, whereas Saturn has the single giant moon, Titan. As the largest moon in the Solar System, Titan’s even bigger than the planet Mercury.
What determines whether a gas giant gets a family of sizeable moons or a single jumbo satellite? Some moons – like Neptune’s Triton, or Phoebe around Saturn – are captured objects, but the majority of satellites around the giants are believed to be born in circumplanetary discs. These arise when an infant star is creating a new planetary system from the large disc of gas and dust swirling around it. Embedded within as the disc disperses. How many moons survive, of what size, and at what orbital distance from the central planet?
Saved in a safe patch
Their simulations showed, as had been expected, that large moons like Titan mostly lose orbital energy in the dusty disc and spiral in towards the planet to be devoured. For particular combinations of moon mass and orbital radius, however, the overall balance of forces causes the moon to instead drift slowly outwards, or even hover at the same orbital distance; these are like safe patches, and a single giant moon is able to survive destruction. What appears to have happened with Saturn is that several inner moons may have spiralled all the way in to be destroyed, but Titan formed in an outer orbit and migrated inwards until it settled in one of these safe patches. Once the disc dissipated, any further migration ceased and Titan has stayed put ever since. To find out more about Jupiter’s moon Io, turn to page 26.
was reading… Formation of single-moon systems around gas giants by Yuri I Fujii and Mashahiro Ogihara. Read it online at https://arxiv.org/abs/2003.05052
is an astrophysicist and co-presenter of The Sky at Night shown much more clearly in this careful treatment of the data than it has before.
The explanation for this pattern – old stars in the middle and young further out – is that star formation seems to have happened in an ‘inside out’ fashion, working its way from the denser centre of the galaxy to its outskirts. This is particularly true for the more massive spirals, where perhaps the differences between central and outlying regions are most profound.
Underlying issues
Interestingly, the story told by the star formation histories may not reflect what’s going on underneath.